L71 & L72 Gross & Net HP
09-17-2007, 09:06 AM
#1
Burning Brakes
Thread Starter
L71 & L72 Gross & Net HP
I'm sure this topic has been discussed many times on the C2 board, but I wanted to get your thoughts on some of my determinations. I normally am active in the C6 section so I hope you don't consider this post as coming from an intruder. My favorite Vette has always been the 1967 L71 and have wanted one since the day I heard those side-pipes. It was never to be, but I now own a 2006 Coupe.
Two years ago I had a little project going comparing my all-time favorite to the C6. It eventually ended up as a magazine article, but over the past two years I realized some of my figures may have been a little off. Below is what I have come up with. Not sure if all my thoughts are correct so that is why I decided to let you C2 guys look at this for a critical analysis. If you have the time, read it and if you think it is wrong, tear it apart.
Thanks much and Best Regards,
Phil
L71 & L72 True Gross & Net Horsepower
Early 1966 L72-427 engines were rated 450 horsepower at 6400 rpm. Not long after production began, these engines were given a lower rating of 425 hp at 5600 rpm. The lower rated engine was the same identical engine that had previously been 450 horsepower. Chevrolet must have realized that the 450 hp rating would attract the attention of safety minded politicians and insurance companies. Also, there seemed to be an unwritten agreement between Ford, Chrysler and GM that no engine would be rated over 425 hp no matter how powerful it was. In 1967, Chevy broke that rule by rating the L71 (triple 2-barrel carb, solid lifter 427) engine at 435 horsepower. The L71 was identical to the 1966 L72 except for the carburetor set-up. L72 had one 4-barrel carb.
Over the years there has been much speculation about the actual horsepower output of these two engines. Some people believed the L71 made nearly 500 gross horsepower. However, performance and dynamometer numbers tell a different story. All known magazine quarter-mile times for L71 and L72 equipped Corvettes averaged 13.42 seconds at 107.62 miles-per-hour. For equal weight cars, quarter-mile trap speeds are an excellent indicator of horsepower. Magazine tested LS1 C5 Corvettes had the same trap speeds as the L71 and L72 cars. Because high performance big-block C2 Corvettes are nearly the same weight as a C5, we can assume they equal the 345 to 350 net horsepower that LS1 engines make.
Corvette Fever magazine had an article titled “Big-Block Power Tune” in their November 2006 issue where a correctly restored 1966 L-72 Corvette was meticulously dyno tuned in order to determine horsepower gain from the tune. Here are the numbers they came up with: Before dyno tune – 289 hp @ 5000 rpm, 331 lb.-ft. torque @ 3800 rpm; After dyno tune – 301 hp @ 5000 rpm, 342 lb.-ft. torque @ 3800 rpm. This car was restored showroom original for acceptable street operation and was equipped with the more restrictive “N14 Side Mount Exhaust System”. The less restrictive standard under car exhaust would most likely have resulted in somewhat higher numbers and probably at a higher rpm. Maximum horsepower was reached at 5000 rather than closer to 6400 rpm where an unrestricted exhaust would have produced peak power. On average, the 345 horsepower LS1 engine also produces 300 rwhp. Equal rear wheel horsepower for both engines translate to 345 net horsepower for the L72.
For the 1970 model year, Chevrolet developed a line-up of high-performance 454 cu. in. engines to replace the 427. Here are the high-performance 427 engines with their 454 replacements: L72 – LS-6, 450 hp; L71 – LJ-2, 460 hp; L88 – LS-7, 465 hp; ZL-1 – LT-2, 465 hp. All but the LS-6 engine were cancelled due to GM’s de-proliferation program to eliminate costly options as well as high insurance rates and top management’s desire to tone down horsepower. The only engine to make production was the LS-6 in the 1970 SS454 Chevelle and then a lower compression 425 horsepower version in the 1971 Corvette. For 1971, Chevrolet reported horsepower in both gross and net ratings. The Corvette LS-6 received a 425 gross and 325 net rating. The gross to net conversion factor for that engine was .7647 which can be used to convert 345 net horsepower to a gross rating. Using that factor (345/.7647 = 451.16 hp) we come up with a gross rating of 451 horsepower for the L72, which for all practical purposes is exactly the same rating Chevy initially gave the engine.
The 1970 SS454 Chevelle, with a 454 cu. in., solid lifter, high compression (11.25:1), LS-6 engine, was rated 450 gross horsepower at 5600 rpm and 500 lb.-ft. torque at 3600 rpm. That engine had the same internals (cam shaft, heads, compression, etc.) and 780 cfm carb as the L72 427. Chevrolet rated the L72 and LS-6 correctly at 450 gross horsepower. Some thought LS-6 was the more powerful engine, but Tom Langdon, a Corvette powertrain engineer had this to say, “Increasing the stroke without enlarging the bore doesn’t translate into a real increase in power. Some of that extra power is eaten up by increased friction. A good 427 (L88) would put out about 600 horsepower. The 454 pulled more torque, but power was just about the same as the L88.” The same goes for L72 and LS-6. They were identical engines except for stroke derived cubic inches and they put out identical horsepower at 450. I wish I could verify the source, but somewhere I did see that a correctly restored 1970 SS454 Chevelle made the same chassis dyno-300 rwhp as the previously mentioned 1966 L72 Corvette.
Notice that the 425 horsepower L72 is rated at 5,600 rpm and so is the 450 hp LS-6. This had to be a carefully considered rpm number for Chevrolet. If they would have been truthful and advertised an rpm rating closer to 6400, the new 454 would have looked no stronger than the 427. I’m sure many L72 owners were aware of the early production 427 being rated 450 hp at 6400 rpm. The 454 did develop more torque, but torque doesn’t sell fast cars—horsepower does. Chevy had to make their new engine look more powerful than the old one. As far as horsepower ratings are concerned, it seems Chevy took the gloves off in 1970. I’m sure they knew the era of hi-performance engines was nearing its end, so apparently it was time to tell the truth. I think it is to Chevy’s credit they were being truthful in rating the 454 and am sure if they could have advertised the LS-6 at more than 450 horsepower they would have. LJ-2 (L71’s triple-carb 454 replacement), was going to be advertised at 460 horsepower, ten more than the LS-6 (L72’s replacement). Horsepower ratings for performance 454 engines tell us what the true ratings were for the L71 and L72 as they made equal power. We have not yet considered the LS-7 and LT-2 which were the 454 equivalents to the L88 and ZL-1. Both should have been rated at over 500 horsepower. I suppose Chevy wanted to be honest, but not that honest. There would have been some explaining to do if the horsepower prohibition advocates realized street going rocket ships were being sold and powered by L88 and ZL-1 engines.
I think we have determined that the L72 engine rated 450 SAE Gross and 345 SAE Net horsepower. Now let’s look at the L71. The only difference between the two engines is carburetion. L72 had a Holley 4-barrel and the L71 had three 2-barrel carburetors often referred to as tri-power. The L71 was rated 10 horsepower higher—435 hp @ 5800 rpm. The three 2-barrel carburetors on L71 allowed for more air flow (1,000-cfm) than the L72 4-barrel (780-cfm) which supposedly resulting in increased power. L71 most likely produced the same 460 SAE Gross horsepower that it’s tri-power 454 cu. in. counterpart LJ-2 did and 352 SAE Net horsepower (460 x .7647). Tom Langdon’s statement that a 454 developed more torque but not more horsepower than an equivalent 427 is the basis for assuming the L71 made equal power to the LJ-2.
Even though the L71 engine received a higher power rating, actual performance between L71 and L72 was nearly equal. In fact L72 had the best magazine quarter-mile time, albeit not by much (L71 – 12.9 sec. @ 111 mph vs. L72 – 12.8 sec. @ 112 mph). The Holley 4-barrel on the L72 may have given it an advantage. Those familiar with both intake systems claim the L72 intake provides better power than the the triple carb set-up. Tri-power L71’s operated on the primary center carburetor only, until you put your foot into it, then the two secondary carbs, controlled by a mass-air vacuum signal from the center carb’s venturi, would open up for full power. Those progressive, vacuum controlled secondary carbs may have been a slight disadvantage in the quarter-mile, but when working properly made for a smooth build-up of power. If driven conservatively, the primary center 2-barrel provided better gas mileage than the 4-barrel equipped L72.
Magazine performance numbers and chassis dyno figures make a compelling argument that Chevrolet did have their horsepower ratings correct for early production L72’s and the 1970 and ’71 LS-6, but in-between, advertised L72 and L71 ratings were on the low side. I am confident that as delivered horsepower numbers for both these engines are equivalent to the 345 and 350 net horsepower rated LS1 engines. Whether the L71 actually produced more power than L72 is debatable, but what is presented here is in keeping with Chevrolet's advertised ten horsepower difference.
L72 – 450 SAE Gross HP, 345 SAE Net HP
L71 – 460 SAE Gross HP, 352 SAE Net HP
Two years ago I had a little project going comparing my all-time favorite to the C6. It eventually ended up as a magazine article, but over the past two years I realized some of my figures may have been a little off. Below is what I have come up with. Not sure if all my thoughts are correct so that is why I decided to let you C2 guys look at this for a critical analysis. If you have the time, read it and if you think it is wrong, tear it apart.
Thanks much and Best Regards,
Phil
L71 & L72 True Gross & Net Horsepower
Early 1966 L72-427 engines were rated 450 horsepower at 6400 rpm. Not long after production began, these engines were given a lower rating of 425 hp at 5600 rpm. The lower rated engine was the same identical engine that had previously been 450 horsepower. Chevrolet must have realized that the 450 hp rating would attract the attention of safety minded politicians and insurance companies. Also, there seemed to be an unwritten agreement between Ford, Chrysler and GM that no engine would be rated over 425 hp no matter how powerful it was. In 1967, Chevy broke that rule by rating the L71 (triple 2-barrel carb, solid lifter 427) engine at 435 horsepower. The L71 was identical to the 1966 L72 except for the carburetor set-up. L72 had one 4-barrel carb.
Over the years there has been much speculation about the actual horsepower output of these two engines. Some people believed the L71 made nearly 500 gross horsepower. However, performance and dynamometer numbers tell a different story. All known magazine quarter-mile times for L71 and L72 equipped Corvettes averaged 13.42 seconds at 107.62 miles-per-hour. For equal weight cars, quarter-mile trap speeds are an excellent indicator of horsepower. Magazine tested LS1 C5 Corvettes had the same trap speeds as the L71 and L72 cars. Because high performance big-block C2 Corvettes are nearly the same weight as a C5, we can assume they equal the 345 to 350 net horsepower that LS1 engines make.
Corvette Fever magazine had an article titled “Big-Block Power Tune” in their November 2006 issue where a correctly restored 1966 L-72 Corvette was meticulously dyno tuned in order to determine horsepower gain from the tune. Here are the numbers they came up with: Before dyno tune – 289 hp @ 5000 rpm, 331 lb.-ft. torque @ 3800 rpm; After dyno tune – 301 hp @ 5000 rpm, 342 lb.-ft. torque @ 3800 rpm. This car was restored showroom original for acceptable street operation and was equipped with the more restrictive “N14 Side Mount Exhaust System”. The less restrictive standard under car exhaust would most likely have resulted in somewhat higher numbers and probably at a higher rpm. Maximum horsepower was reached at 5000 rather than closer to 6400 rpm where an unrestricted exhaust would have produced peak power. On average, the 345 horsepower LS1 engine also produces 300 rwhp. Equal rear wheel horsepower for both engines translate to 345 net horsepower for the L72.
For the 1970 model year, Chevrolet developed a line-up of high-performance 454 cu. in. engines to replace the 427. Here are the high-performance 427 engines with their 454 replacements: L72 – LS-6, 450 hp; L71 – LJ-2, 460 hp; L88 – LS-7, 465 hp; ZL-1 – LT-2, 465 hp. All but the LS-6 engine were cancelled due to GM’s de-proliferation program to eliminate costly options as well as high insurance rates and top management’s desire to tone down horsepower. The only engine to make production was the LS-6 in the 1970 SS454 Chevelle and then a lower compression 425 horsepower version in the 1971 Corvette. For 1971, Chevrolet reported horsepower in both gross and net ratings. The Corvette LS-6 received a 425 gross and 325 net rating. The gross to net conversion factor for that engine was .7647 which can be used to convert 345 net horsepower to a gross rating. Using that factor (345/.7647 = 451.16 hp) we come up with a gross rating of 451 horsepower for the L72, which for all practical purposes is exactly the same rating Chevy initially gave the engine.
The 1970 SS454 Chevelle, with a 454 cu. in., solid lifter, high compression (11.25:1), LS-6 engine, was rated 450 gross horsepower at 5600 rpm and 500 lb.-ft. torque at 3600 rpm. That engine had the same internals (cam shaft, heads, compression, etc.) and 780 cfm carb as the L72 427. Chevrolet rated the L72 and LS-6 correctly at 450 gross horsepower. Some thought LS-6 was the more powerful engine, but Tom Langdon, a Corvette powertrain engineer had this to say, “Increasing the stroke without enlarging the bore doesn’t translate into a real increase in power. Some of that extra power is eaten up by increased friction. A good 427 (L88) would put out about 600 horsepower. The 454 pulled more torque, but power was just about the same as the L88.” The same goes for L72 and LS-6. They were identical engines except for stroke derived cubic inches and they put out identical horsepower at 450. I wish I could verify the source, but somewhere I did see that a correctly restored 1970 SS454 Chevelle made the same chassis dyno-300 rwhp as the previously mentioned 1966 L72 Corvette.
Notice that the 425 horsepower L72 is rated at 5,600 rpm and so is the 450 hp LS-6. This had to be a carefully considered rpm number for Chevrolet. If they would have been truthful and advertised an rpm rating closer to 6400, the new 454 would have looked no stronger than the 427. I’m sure many L72 owners were aware of the early production 427 being rated 450 hp at 6400 rpm. The 454 did develop more torque, but torque doesn’t sell fast cars—horsepower does. Chevy had to make their new engine look more powerful than the old one. As far as horsepower ratings are concerned, it seems Chevy took the gloves off in 1970. I’m sure they knew the era of hi-performance engines was nearing its end, so apparently it was time to tell the truth. I think it is to Chevy’s credit they were being truthful in rating the 454 and am sure if they could have advertised the LS-6 at more than 450 horsepower they would have. LJ-2 (L71’s triple-carb 454 replacement), was going to be advertised at 460 horsepower, ten more than the LS-6 (L72’s replacement). Horsepower ratings for performance 454 engines tell us what the true ratings were for the L71 and L72 as they made equal power. We have not yet considered the LS-7 and LT-2 which were the 454 equivalents to the L88 and ZL-1. Both should have been rated at over 500 horsepower. I suppose Chevy wanted to be honest, but not that honest. There would have been some explaining to do if the horsepower prohibition advocates realized street going rocket ships were being sold and powered by L88 and ZL-1 engines.
I think we have determined that the L72 engine rated 450 SAE Gross and 345 SAE Net horsepower. Now let’s look at the L71. The only difference between the two engines is carburetion. L72 had a Holley 4-barrel and the L71 had three 2-barrel carburetors often referred to as tri-power. The L71 was rated 10 horsepower higher—435 hp @ 5800 rpm. The three 2-barrel carburetors on L71 allowed for more air flow (1,000-cfm) than the L72 4-barrel (780-cfm) which supposedly resulting in increased power. L71 most likely produced the same 460 SAE Gross horsepower that it’s tri-power 454 cu. in. counterpart LJ-2 did and 352 SAE Net horsepower (460 x .7647). Tom Langdon’s statement that a 454 developed more torque but not more horsepower than an equivalent 427 is the basis for assuming the L71 made equal power to the LJ-2.
Even though the L71 engine received a higher power rating, actual performance between L71 and L72 was nearly equal. In fact L72 had the best magazine quarter-mile time, albeit not by much (L71 – 12.9 sec. @ 111 mph vs. L72 – 12.8 sec. @ 112 mph). The Holley 4-barrel on the L72 may have given it an advantage. Those familiar with both intake systems claim the L72 intake provides better power than the the triple carb set-up. Tri-power L71’s operated on the primary center carburetor only, until you put your foot into it, then the two secondary carbs, controlled by a mass-air vacuum signal from the center carb’s venturi, would open up for full power. Those progressive, vacuum controlled secondary carbs may have been a slight disadvantage in the quarter-mile, but when working properly made for a smooth build-up of power. If driven conservatively, the primary center 2-barrel provided better gas mileage than the 4-barrel equipped L72.
Magazine performance numbers and chassis dyno figures make a compelling argument that Chevrolet did have their horsepower ratings correct for early production L72’s and the 1970 and ’71 LS-6, but in-between, advertised L72 and L71 ratings were on the low side. I am confident that as delivered horsepower numbers for both these engines are equivalent to the 345 and 350 net horsepower rated LS1 engines. Whether the L71 actually produced more power than L72 is debatable, but what is presented here is in keeping with Chevrolet's advertised ten horsepower difference.
L72 – 450 SAE Gross HP, 345 SAE Net HP
L71 – 460 SAE Gross HP, 352 SAE Net HP
Last edited by Marina Blue; 09-20-2007 at 10:24 AM. Reason: Final edit
09-17-2007, 11:31 AM
#3
I appreciate your effort and research... but I'm tenative in calling
what you've presented here reliable conclusions..
1. Your Trap Speed analysis... there's no mention or apparent concern
over traction... ask a L71 member what happens to his skinny
stock bias ply tires when the hammer gets dropped.
2. If GM list a HP rating at a lower Rpm....BUT doesn't list that
rating as a PEAK hp... have they really played any shell games
except perhaps people who aren't performance minded enough
to look at the dyno sheet at a higher rpm? An engine can make
425hp in the 5k range and 450 in the 6k rpm range... same motor.
3. Listing CFM capacity of carburetion is not indicitavive of
that particular induction systems performance.... a more honest
testing procedure would be to send those systems to Wilson
Manifolds or like facilty for CFM testing through the port.
In small blocks...the highest flow induction set ups are
normally single carb set ups... multiple carb packages normally
don't flow as well as a high flow single.
4. Most Drivetrain and Accesory loss to achieve net HP is
about 15%.... your showing about 22%..doesn't it seem
like loosing 100hp through parasitic loss is a little much??
5. You are making some good points with your research... but
why not present your info as "Interesting Information" rather
than "Reliable Conclusions"..... if your sources were engineering
tech papers rather than magazine articles you'd have a stronger
case.
Tom Langdon has been running Langdon's Stovebolt 6 parts company
for years...
here:
http://www.stoveboltengineco.com/
what you've presented here reliable conclusions..
1. Your Trap Speed analysis... there's no mention or apparent concern
over traction... ask a L71 member what happens to his skinny
stock bias ply tires when the hammer gets dropped.
2. If GM list a HP rating at a lower Rpm....BUT doesn't list that
rating as a PEAK hp... have they really played any shell games
except perhaps people who aren't performance minded enough
to look at the dyno sheet at a higher rpm? An engine can make
425hp in the 5k range and 450 in the 6k rpm range... same motor.
3. Listing CFM capacity of carburetion is not indicitavive of
that particular induction systems performance.... a more honest
testing procedure would be to send those systems to Wilson
Manifolds or like facilty for CFM testing through the port.
In small blocks...the highest flow induction set ups are
normally single carb set ups... multiple carb packages normally
don't flow as well as a high flow single.
4. Most Drivetrain and Accesory loss to achieve net HP is
about 15%.... your showing about 22%..doesn't it seem
like loosing 100hp through parasitic loss is a little much??
5. You are making some good points with your research... but
why not present your info as "Interesting Information" rather
than "Reliable Conclusions"..... if your sources were engineering
tech papers rather than magazine articles you'd have a stronger
case.
Tom Langdon has been running Langdon's Stovebolt 6 parts company
for years...
here:
http://www.stoveboltengineco.com/
09-17-2007, 12:48 PM
#4
Burning Brakes
Thread Starter
I appreciate your effort and research... but I'm tenative in calling
what you've presented here reliable conclusions..
1. Your Trap Speed analysis... there's no mention or apparent concern
over traction... ask a L71 member what happens to his skinny
stock bias ply tires when the hammer gets dropped.
2. If GM list a HP rating at a lower Rpm....BUT doesn't list that
rating as a PEAK hp... have they really played any shell games
except perhaps people who aren't performance minded enough
to look at the dyno sheet at a higher rpm? An engine can make
425hp in the 5k range and 450 in the 6k rpm range... same motor.
3. Listing CFM capacity of carburetion is not indicitavive of
that particular induction systems performance.... a more honest
testing procedure would be to send those systems to Wilson
Manifolds or like facilty for CFM testing through the port.
In small blocks...the highest flow induction set ups are
normally single carb set ups... multiple carb packages normally
don't flow as well as a high flow single.
4. Most Drivetrain and Accesory loss to achieve net HP is
about 15%.... your showing about 22%..doesn't it seem
like loosing 100hp through parasitic loss is a little much??
5. You are making some good points with your research... but
why not present your info as "Interesting Information" rather
than "Reliable Conclusions"..... if your sources were engineering
tech papers rather than magazine articles you'd have a stronger
case.
Tom Langdon has been running Langdon's Stovebolt 6 parts company
for years...
here:
http://www.stoveboltengineco.com/
what you've presented here reliable conclusions..
1. Your Trap Speed analysis... there's no mention or apparent concern
over traction... ask a L71 member what happens to his skinny
stock bias ply tires when the hammer gets dropped.
2. If GM list a HP rating at a lower Rpm....BUT doesn't list that
rating as a PEAK hp... have they really played any shell games
except perhaps people who aren't performance minded enough
to look at the dyno sheet at a higher rpm? An engine can make
425hp in the 5k range and 450 in the 6k rpm range... same motor.
3. Listing CFM capacity of carburetion is not indicitavive of
that particular induction systems performance.... a more honest
testing procedure would be to send those systems to Wilson
Manifolds or like facilty for CFM testing through the port.
In small blocks...the highest flow induction set ups are
normally single carb set ups... multiple carb packages normally
don't flow as well as a high flow single.
4. Most Drivetrain and Accesory loss to achieve net HP is
about 15%.... your showing about 22%..doesn't it seem
like loosing 100hp through parasitic loss is a little much??
5. You are making some good points with your research... but
why not present your info as "Interesting Information" rather
than "Reliable Conclusions"..... if your sources were engineering
tech papers rather than magazine articles you'd have a stronger
case.
Tom Langdon has been running Langdon's Stovebolt 6 parts company
for years...
here:
http://www.stoveboltengineco.com/
That 100 hp loss was not to the rear wheels, but Chevy's own numbers from 1971. LS-6 was rated two ways: 425 gross and 325 net. Drive-train loss to the rear wheels is about 45 hp in my write-up.
As far as rating at different rpms, the 450 hp figure for L72 was at 6400 rpm per Chevrolet, not 5600, and I assume the LJ-2 rating was also done at higher rpm. Chevy did advertise a 10 hp difference between L72 and L71 which seemed to hold out at higher rpms as indicated by ratings on the 454 engines.
Those small tires made a bigger difference in et's than trap speed. It is a commonly held belief that trap speed is a good indicator of horsepower regardless of traction.
What doesn't make sense to me is the rpm at which Corvette Fever reached peak hp. It was 5,000. Also I have seen a test where a 426 Hemi reached peak hp at 5000 rpm.
Last edited by Marina Blue; 09-17-2007 at 07:56 PM.
09-18-2007, 11:51 AM
#5
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Member Since: Feb 1999
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Like Kyle suggested, they just used the rating at a different rpm. The L72 was originally rated 450 @ 6,400 and then re-rated 425 @ 5600. They wanted to show a power increase for the L71 so it got rated at 5,800. They didn't have to lie, they could just pick a lower or higher rpm and use that figure to suit their purpose.
09-18-2007, 12:14 PM
#6
Burning Brakes
Thread Starter
Like Kyle suggested, they just used the rating at a different rpm. The L72 was originally rated 450 @ 6,400 and then re-rated 425 @ 5600. They wanted to show a power increase for the L71 so it got rated at 5,800. They didn't have to lie, they could just pick a lower or higher rpm and use that figure to suit their purpose.
Another quandary is the Corvette Fever dyno test that showed the '66 L72 reaching peak horsepower at 5,000 rpm. There was another chassis dyno test, but of a '67 Plymouth Belvedere II with 426-Hemi where peak horsepower was reached at 5,000 rpm and maintained to 5,500 rpm before dropping off- www.moparmusclemagazine.com/roadtests/37426/. Does this make any sense?
Not trying to be a pain, just looking for answers.
Last edited by Marina Blue; 09-18-2007 at 06:40 PM. Reason: clarification
09-18-2007, 08:53 PM
#7
Team Owner
Member Since: Nov 2005
Location: Beach & High Desert Southern California
Posts: 25,480
Received 2,339 Likes
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890 Posts
These rpm levels don't make sense to me and point to Chevy using whatever number looked good in advertising or to fudge ratings to make one engine look more powerful than it actually was. That 450 hp LS-6 may not have looked stronger than the old L72 if they would have advertised it was rated at 6400 rpm instead of 5600. I think I may have just answered my own question.
Another quandary is the Corvette Fever dyno test that showed the '66 L72 reaching peak horsepower at 5,000 rpm. There was another chassis dyno test, but of a '67 Plymouth Belvedere II with 426-Hemi where peak horsepower was reached at 5,000 rpm and maintained to 5,500 rpm before dropping off- www.moparmusclemagazine.com/roadtests/37426/. Does this make any sense?
Not trying to be a pain, just looking for answers.
Another quandary is the Corvette Fever dyno test that showed the '66 L72 reaching peak horsepower at 5,000 rpm. There was another chassis dyno test, but of a '67 Plymouth Belvedere II with 426-Hemi where peak horsepower was reached at 5,000 rpm and maintained to 5,500 rpm before dropping off- www.moparmusclemagazine.com/roadtests/37426/. Does this make any sense?
Not trying to be a pain, just looking for answers.
Chevy was good at playing the numbers game, but with the BBC it was usually to discount the advertised horsepower. The most obvious case of numbers gaming was with the L88 rated at 430hp. Compression and cam timing changes (and intake plenum changes, and carb changes, and ignition timing changes, and ...) gaining only 5hp over the L72 (and 5hp less than the L71)? We know the Engineers at GM were honest saints
but we also know everyone at GM had to answer to the marketing department. The advertised hp ratings discounts, and the referenced rpms at the discounted hp numbers, can be honest statements if the engineers offered the marketing department compatible information (even the modern C6-LS7 can be re-rated at a lower hp if we choose to pick 2000 rpm as the resolved hp number to advertise). Reading the article linked informs us that the "stock" 426 hemi was retrofitted with a hydraulic lifter cam ("but it has the exact same specs"). Even with the same "specs" (whatever that was intended to communicate for cams that have to be ground differently) the mass of the large mopar-hemi lifters & valves tend to pump-up hydraulic lifters around 5500 rpm unless the builder takes care & attention with the spring selection. This is the rpm where the hemi test was shut down. The original hemi solid lifters & cam and springs would have allowed for a more reliable valve train well past 5500 rpm, one that would have posted better hp numbers well past the torque/hp break even point.
To get a parallel on how small changes can alter the power levels with the older 60's vintage performance engines, with the older BBC a simple swap from exhaust manifolds to headers would easily drop the ET 3-tenths and pick up over 5-mph in the quarter mile (with minor jet changes). It was not hard to get a L78 or L72 to pull hard past 6400 rpm with headers and a tall open element air cleaner, delivering performance well past the later advertised 5600 rpm rated power point. The stock L71/L72/L78 rear wheel power may not compare well to the modern LS-X engines, with the gross to net discount and the friction loss of the older drivetrain, but with a few minor changes they can produce power levels that exceed the advertised ratings (when the RWhp is corrected to gross hp).
Much of the statistical reverse engineering of 60's performance engines does not make sense unless we all agree the automotive marketing departments had considerably more involvement (and influence) on the hp ratings than today. Your article does communicate what many owners of stock L72 cars know, that they deliver the advertised power ratings assigned in the 60's but they are no match for an efficient modern LS6/7 power train and tire combination (power & delivery systems that benefit from forty years of improvements and improved technology).
09-18-2007, 09:17 PM
#8
Burning Brakes
Thread Starter
Update to original post
Your replies have made me think this over more carefully and I believe I have come up with an even stronger case. If you have time, re-read my original post.
Thanks again.
Phil
Thanks again.
Phil
09-18-2007, 09:36 PM
#9
Burning Brakes
Thread Starter
Chevy was good at playing the numbers game, but with the BBC it was usually to discount the advertised horsepower. The most obvious case of numbers gaming was with the L88 rated at 430hp. Compression and cam timing changes (and intake plenum changes, and carb changes, and ignition timing changes, and ...) gaining only 5hp over the L72 (and 5hp less than the L71)? We know the Engineers at GM were honest saints but we also know everyone at GM had to answer to the marketing department. The advertised hp ratings discounts, and the referenced rpms at the discounted hp numbers, can be honest statements if the engineers offered the marketing department compatible information (even the modern C6-LS7 can be re-rated at a lower hp if we choose to pick 2000 rpm as the resolved hp number to advertise).
Reading the article linked informs us that the "stock" 426 hemi was retrofitted with a hydraulic lifter cam ("but it has the exact same specs"). Even with the same "specs" (whatever that was intended to communicate for cams that have to be ground differently) the mass of the large mopar-hemi lifters & valves tend to pump-up hydraulic lifters around 5500 rpm unless the builder takes care & attention with the spring selection. This is the rpm where the hemi test was shut down. The original hemi solid lifters & cam and springs would have allowed for a more reliable valve train well past 5500 rpm, one that would have posted better hp numbers well past the torque/hp break even point.
To get a parallel on how small changes can alter the power levels with the older 60's vintage performance engines, with the older BBC a simple swap from exhaust manifolds to headers would easily drop the ET 3-tenths and pick up over 5-mph in the quarter mile (with minor jet changes). It was not hard to get a L78 or L72 to pull hard past 6400 rpm with headers and a tall open element air cleaner, delivering performance well past the later advertised 5600 rpm rated power point. The stock L71/L72/L78 rear wheel power may not compare well to the modern LS-X engines, with the gross to net discount and the friction loss of the older drivetrain, but with a few minor changes they can produce power levels that exceed the advertised ratings (when the RWhp is corrected to gross hp).
Much of the statistical reverse engineering of 60's performance engines does not make sense unless we all agree the automotive marketing departments had considerably more involvement (and influence) on the hp ratings than today. Your article does communicate what many owners of stock L72 cars know, that they deliver the advertised power ratings assigned in the 60's but they are no match for an efficient modern LS6/7 power train and tire combination (power & delivery systems that benefit from forty years of improvements and improved technology).
but we also know everyone at GM had to answer to the marketing department. The advertised hp ratings discounts, and the referenced rpms at the discounted hp numbers, can be honest statements if the engineers offered the marketing department compatible information (even the modern C6-LS7 can be re-rated at a lower hp if we choose to pick 2000 rpm as the resolved hp number to advertise). Reading the article linked informs us that the "stock" 426 hemi was retrofitted with a hydraulic lifter cam ("but it has the exact same specs"). Even with the same "specs" (whatever that was intended to communicate for cams that have to be ground differently) the mass of the large mopar-hemi lifters & valves tend to pump-up hydraulic lifters around 5500 rpm unless the builder takes care & attention with the spring selection. This is the rpm where the hemi test was shut down. The original hemi solid lifters & cam and springs would have allowed for a more reliable valve train well past 5500 rpm, one that would have posted better hp numbers well past the torque/hp break even point.
To get a parallel on how small changes can alter the power levels with the older 60's vintage performance engines, with the older BBC a simple swap from exhaust manifolds to headers would easily drop the ET 3-tenths and pick up over 5-mph in the quarter mile (with minor jet changes). It was not hard to get a L78 or L72 to pull hard past 6400 rpm with headers and a tall open element air cleaner, delivering performance well past the later advertised 5600 rpm rated power point. The stock L71/L72/L78 rear wheel power may not compare well to the modern LS-X engines, with the gross to net discount and the friction loss of the older drivetrain, but with a few minor changes they can produce power levels that exceed the advertised ratings (when the RWhp is corrected to gross hp).
Much of the statistical reverse engineering of 60's performance engines does not make sense unless we all agree the automotive marketing departments had considerably more involvement (and influence) on the hp ratings than today. Your article does communicate what many owners of stock L72 cars know, that they deliver the advertised power ratings assigned in the 60's but they are no match for an efficient modern LS6/7 power train and tire combination (power & delivery systems that benefit from forty years of improvements and improved technology).
Hydraulic valves in the hemi make a lot of sense in limiting higher rpm performance.
If you have access to that Corvette Fever mag I mentioned, take a look at it. According to the write-up they used identical replacement parts to the originals for the rebuild. There has to be something wrong with the peak hp rating occurring at 5000 rpm. A more knowledgeable observer may pick out something that I missed. Maybe they didn't want to run the engine any higher than the reported 5200, but I would be surprised if that were the case.
Phil
09-18-2007, 11:00 PM
#10
Burning Brakes
Thread Starter
The stock L71/L72/L78 rear wheel power may not compare well to the modern LS-X engines, with the gross to net discount and the friction loss of the older drivetrain, but with a few minor changes they can produce power levels that exceed the advertised ratings (when the RWhp is corrected to gross hp).
Much of the statistical reverse engineering of 60's performance engines does not make sense unless we all agree the automotive marketing departments had considerably more involvement (and influence) on the hp ratings than today. Your article does communicate what many owners of stock L72 cars know, that they deliver the advertised power ratings assigned in the 60's but they are no match for an efficient modern LS6/7 power train and tire combination (power & delivery systems that benefit from forty years of improvements and improved technology).
Much of the statistical reverse engineering of 60's performance engines does not make sense unless we all agree the automotive marketing departments had considerably more involvement (and influence) on the hp ratings than today. Your article does communicate what many owners of stock L72 cars know, that they deliver the advertised power ratings assigned in the 60's but they are no match for an efficient modern LS6/7 power train and tire combination (power & delivery systems that benefit from forty years of improvements and improved technology).
It is nearly impossible to compare net rated to gross rated engines with all of the variables that come into play. Today's accessories as well as intake and exhaust systems are more efficient and now include air conditioning as a standard drag on the engine.
We may never know the complete picture of dyno room methods used 40 years ago, but to me it is looking like they tried to maintain truth in advertising at least some of the time. Calculating in reverse is never accurate but occasionally all the dots, which include after-the-fact dyno tests and perhaps trying to think like marketing departments, seem to point in the same direction here.
The LS1 engine that I used in this treatise, would never make 450 or 460 hp if rated using gross procedures. It just doesn't have the same potential as the the old iron. But we aren't discussing potential here, the concern is accurate showroom stock numbers and I believe we have made a reasonable case for what they really were. I don't believe they were close to 500 as I have seen stated in various magazines and believe it or not on a L71 engine display with the Corvette Heritage Tour.
Last edited by Marina Blue; 09-18-2007 at 11:16 PM.
09-19-2007, 12:53 AM
#11
Le Mans Master
the car and driver 12.9 et was bogus, it was a chevy prepped car and not a stock factory tune, also never happen on skinny street tires, been there done that with 67 L71, turned ,with g70/15 tires and a bad driver(me at 20), 13.8 @108mph. side pipe car -with the restrictive side pipes- which have smaller internal diameter than an undercar exhaustcar and driver numbers for the people who didn't know and liked to brag, I know someone like that.
which have smaller internal diameter than an undercar exhaustcar and driver numbers for the people who didn't know and liked to brag, I know someone like that.
09-19-2007, 06:43 AM
#13
Le Mans Master
Hi MB
I thought your work was pretty spot on other than not noting the sidepipe restriction issue. In other words a L72 with sidepipes would get beat every time by one with an under carriage exhaust all other things being equal.
Seat of the pants is also close and I rated my 385 hp C5 Z06 just a tad quicker than my L72. (never could race them because I was the only one allowed to do that LOL)
However unless I missed it you omitted that with or without sidepipes that just made a bad thing worse for the L72,71, their exhaust manifolds were performance anchors. On the other hand the LS 1 has a very good overall exhaust system.
Soooooo to make all things equal in measuring available power at the flywheel between mid 60 technology versus nearly 50 years later put headers on both of them.............. then the the little motor does what any properly prepped BB owner knows what it will do....... fall behind the BB either on the dyno or on the race track....(and may I add by a considerable margin)
Doug
I thought your work was pretty spot on other than not noting the sidepipe restriction issue. In other words a L72 with sidepipes would get beat every time by one with an under carriage exhaust all other things being equal.
Seat of the pants is also close and I rated my 385 hp C5 Z06 just a tad quicker than my L72. (never could race them because I was the only one allowed to do that LOL)
However unless I missed it you omitted that with or without sidepipes that just made a bad thing worse for the L72,71, their exhaust manifolds were performance anchors. On the other hand the LS 1 has a very good overall exhaust system.
Soooooo to make all things equal in measuring available power at the flywheel between mid 60 technology versus nearly 50 years later put headers on both of them.............. then the the little motor does what any properly prepped BB owner knows what it will do....... fall behind the BB either on the dyno or on the race track....(and may I add by a considerable margin)
Doug
09-19-2007, 10:25 AM
#14
Burning Brakes
Thread Starter
Hi MB
I thought your work was pretty spot on other than not noting the sidepipe restriction issue. In other words a L72 with sidepipes would get beat every time by one with an under carriage exhaust all other things being equal.
Seat of the pants is also close and I rated my 385 hp C5 Z06 just a tad quicker than my L72. (never could race them because I was the only one allowed to do that LOL)
However unless I missed it you omitted that with or without sidepipes that just made a bad thing worse for the L72,71, their exhaust manifolds were performance anchors. On the other hand the LS 1 has a very good overall exhaust system.
Soooooo to make all things equal in measuring available power at the flywheel between mid 60 technology versus nearly 50 years later put headers on both of them.............. then the the little motor does what any properly prepped BB owner knows what it will do....... fall behind the BB either on the dyno or on the race track....(and may I add by a considerable margin)
Doug
I thought your work was pretty spot on other than not noting the sidepipe restriction issue. In other words a L72 with sidepipes would get beat every time by one with an under carriage exhaust all other things being equal.
Seat of the pants is also close and I rated my 385 hp C5 Z06 just a tad quicker than my L72. (never could race them because I was the only one allowed to do that LOL)
However unless I missed it you omitted that with or without sidepipes that just made a bad thing worse for the L72,71, their exhaust manifolds were performance anchors. On the other hand the LS 1 has a very good overall exhaust system.
Soooooo to make all things equal in measuring available power at the flywheel between mid 60 technology versus nearly 50 years later put headers on both of them.............. then the the little motor does what any properly prepped BB owner knows what it will do....... fall behind the BB either on the dyno or on the race track....(and may I add by a considerable margin)
Doug
Actually I didn't realize that side-pipes were more restrictive than under carriage. Thanks for the info.
There has been alot to read here so you probably did miss my comments on BB potential:
Last edited by Marina Blue; 09-19-2007 at 10:28 AM.
09-19-2007, 10:53 AM
#15
Racer
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All the tech info is great, but back in the day the L 72 was king at the strip. All the tri-power did was put lipstick on the pig. Talking pure stock.
Why do you think Chevy engineers went with 4 barrel on the L 88?
They knew tri-power was just a marketing eye appeal tool just like it is now. People still think they are the best because they look good. Put an L 71 next to the L 72, exact same cars in color cond etc. The L 71 will get the most attention. Why? The visual of the tri-power.
These days I like them all.
Why do you think Chevy engineers went with 4 barrel on the L 88?
They knew tri-power was just a marketing eye appeal tool just like it is now. People still think they are the best because they look good. Put an L 71 next to the L 72, exact same cars in color cond etc. The L 71 will get the most attention. Why? The visual of the tri-power.
These days I like them all.
09-19-2007, 11:39 AM
#16
Burning Brakes
Thread Starter
Seat of the pants is also close and I rated my 385 hp C5 Z06 just a tad quicker than my L72. (never could race them because I was the only one allowed to do that LOL)
However unless I missed it you omitted that with or without sidepipes that just made a bad thing worse for the L72,71, their exhaust manifolds were performance anchors. On the other hand the LS 1 has a very good overall exhaust system.
Soooooo to make all things equal in measuring available power at the flywheel between mid 60 technology versus nearly 50 years later put headers on both of them.............. then the the little motor does what any properly prepped BB owner knows what it will do....... fall behind the BB either on the dyno or on the race track....(and may I add by a considerable margin)
However unless I missed it you omitted that with or without sidepipes that just made a bad thing worse for the L72,71, their exhaust manifolds were performance anchors. On the other hand the LS 1 has a very good overall exhaust system.
Soooooo to make all things equal in measuring available power at the flywheel between mid 60 technology versus nearly 50 years later put headers on both of them.............. then the the little motor does what any properly prepped BB owner knows what it will do....... fall behind the BB either on the dyno or on the race track....(and may I add by a considerable margin)
09-19-2007, 01:08 PM
#17
Le Mans Master
As I indicated before you IMO have done your homework well
Also I believe GM is to be commended for how well they have utilized every cubic inch of the LS series motors. It is my understanding that even the angle approach to necessary bends along the entire exhaust system was calculated to the best possible position within the given options for the C5 & C6. If I could afford one without having to sell my 66 there would be either a new Z06 or at least a new 436 hp Z51 6 speed sitting in my driveway.
Until the ZO6 of 01 the L72 remains the production king for all of the 20th century in magazine performance comparison tests and that includes the ZR1. There is a nice article about that in the 66 section of the C2 registry..... it is a real good read
Doug
09-19-2007, 02:16 PM
#18
Team Owner
Member Since: Nov 2005
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A fast pig 
I was going to post the following in an earlier L72 thread, but work delayed the effort and the thread slipped off the list. My experience with the L71/L72 intakes still applies.
In the late 70’s and early 80's a buddy and I put considerable work into his SS396 68' Camaro in an effort to match popular magazine ET time slips. The original automatic trans oval-port L35 396 ran mid-14's. The oval-port 396 however was faster than my stock 70' GTO (running high-14's @ 93 mph) but not producing the 11-second time slips claimed in magazines. During a period of evening fun we swapped a 65’ L78 396 into the car with hopes of an 11-second runner (what the magazines claimed). The resulting L78 396 ran 13.20's @ 112 with headers, 9x25.5 slicks and an open 3.08 differential. The result was not bad for a night’s work, but far from the magazine article time slips.
He later blew up the 396 block and sleeved it to build into 427 L72 specs, in a further quest to run sub-11-second ET’s (and later into L88 specs, sort-of, and then modified a little more). The process to get this Camaro into the low-10's took over three years and we ran against many claimed L72 and L88 configurations in corvettes and chevelles. Sandbagging the engine assembly details was as common as cooked coolant overflowing in the hot pits at the end of the track turn around.
We talked to everyone to gain knowledge and cheap cast-off parts. It's hard to judge what was original with the cars we ran against but all the cars ran axle gears in a 4.56 or lower ratio (mostly lower). The cars never had exhaust manifolds (good headers were the first mod and easily 3-tenths of ET improvement). A re-curved distributor was good for a tenth and pocket porting and a three-angle valve job was good for three more tenths. The cars running ET’s close to the magazine reports were not stock.
In our tests the L72 manifold ran better than the L71 tri-power, but both GM manifolds choked the intake more than a Tarantula (the old Edelbrock single-plane manifold with the rotated carb pad). We were offered many L72/L71 intakes to use (just like the GM cams we were offered for free, indicating the cars we ran against had already swapped intakes and cams). We ran a tri-power set up a few times but never stocked up on L71 tri-power intake & carb sets as they were slower, and always good for a few dollars in trade. At one time we had three or four factory L78/L72 intakes, as they were popular and inexpensive, some with milled plenum dividers like the L88. Nobody expected these 4-barrel intakes to be worth much money with faster after market manifolds available. I offer this information with a word of caution that a claim that the L71 was more powerful than a L72 due to improved airflow would be counter to my experience running both setups on the same engine.
09-19-2007, 02:57 PM
#19
Burning Brakes
Thread Starter
63 340HP, your caution is well received. Thanks for taking the time to share your experience.
Note: I have changed my original post to reflect what you have said. Again, thanks for the input.
Note: I have changed my original post to reflect what you have said. Again, thanks for the input.
Last edited by Marina Blue; 09-19-2007 at 03:52 PM.
09-19-2007, 03:10 PM
#20
Burning Brakes
Thread Starter
Hello again Phil
As I indicated before you IMO have done your homework well
Also I believe GM is to be commended for how well they have utilized every cubic inch of the LS series motors. It is my understanding that even the angle approach to necessary bends along the entire exhaust system was calculated to the best possible position within the given options for the C5 & C6. If I could afford one without having to sell my 66 there would be either a new Z06 or at least a new 436 hp Z51 6 speed sitting in my driveway.
Until the ZO6 of 01 the L72 remains the production king for all of the 20th century in magazine performance comparison tests and that includes the ZR1. There is a nice article about that in the 66 section of the C2 registry..... it is a real good read
Doug
As I indicated before you IMO have done your homework well
Also I believe GM is to be commended for how well they have utilized every cubic inch of the LS series motors. It is my understanding that even the angle approach to necessary bends along the entire exhaust system was calculated to the best possible position within the given options for the C5 & C6. If I could afford one without having to sell my 66 there would be either a new Z06 or at least a new 436 hp Z51 6 speed sitting in my driveway.
Until the ZO6 of 01 the L72 remains the production king for all of the 20th century in magazine performance comparison tests and that includes the ZR1. There is a nice article about that in the 66 section of the C2 registry..... it is a real good read
Doug
I am in the same boat. I have a 2006 but those '08 436 hp cars look tempting. As usual the situation is, "unlimited wants but limited means." Both the '08 and Z06 are out of the question for me. However, my C6 is a great car to drive. Drove it from Pennsylvania to Sebring, FL in March 2006 for the 12 hour race. Believe it or not, after a whole days drive I still felt good. It was the most comfortable trip I ever had. This car is a keeper, but I wish it had a C2 brother for company.
I am somewhat concerned that once again we are in the same situation as 1970. If thats the case we had another nice run, but who knows what the future holds.
Do you have the address for the C2 Registry?
Thanks again.
Phil