I was at the Atlantic City Corvette show on Sunday and saw a 1967, 435 hp, L71 big block on a stand. It had an information plaque that said the L71 horsepower was underrated by the factory for insurance purposes and that it actually developed somewhere close to 500 horsepower. Last December a member posted an official 1971 Chevrolet chart that rated their engine lineup in both gross and net horsepower. The 350 cu. in. (RPO LT1) was rated at 330 gross and 275 net horsepower which gives a conversion factor of .8333 (275/330). The 454 cu. in. (RPO LS6) was rated at 425 gross and 325 net horsepower which gives a conversion factor of .7647 (325/425). Using the more conservative factor of .8333 the LS2 calculates to 480 gross horsepower (400/.8333). Using the big block conversion factor of .7647 the LS2 calculates to 523 gross horsepower. I believe if the LS2 were to have a gross hp rating for comparison purposes its rating would be somewhere between 480 and 523. Going back to that information plaque I saw in Atlantic City, it seems that the LS2 has comparable power to the 1967 L71 engine. If my logic is wrong let me know. Getting back to the thread title, can torque be converted just like horsepower or is it a different animal? If the same conversion can be made then can I conclude that the LS2 has a torque rating somewhere between 480 and 523 ft. lbs. using the old gross method?

 

Since there is a linear relationship between tq and hp,
adjusting one is equivalent to adjusting the other by the
same amount. In other words, I think the gross tq rating
of the LS2 is in the neighborhood you suggest.

Pat

 

Well, this might not be much help but here is the formula.

RPM x Torque / 5252 = Horsepower

 

There's no mathematical formula for conversion from gross to net for torque or horsepower, because every engine is different and will react differently to the net testing process versus the gross testing procedure. However, your figures are probably in the ballpark.

Remember that gross HP is measured with the engine on a stand with no accessories, no mufflers, etc. Net is measured with all accessory drives installed and intake and exhaust restrictions matching the backpressure/restriction from the as-installed systems. I'd assume that modern accessories, exhausts and intakes are more efficient/less restrictive than the ones in the days of old, so there's probably less of a difference between gross and net today than there was in the '60s.

 

i don't believe you can make that type of comparison. the ve of an engine has alot to do with it's ability to produce hp/trq. if the same tricks that are used on the current 350 (364 in the vette - ls2) were used on the 454 or even 503 there wouldn't be anything on the planet that would be able to touch it (production vehicles and some exotics).

let gm lose on a 454 and tell them to produce a limited production supercar (has to be a special SS version vette) give it a $100k interior, sound system and charge $85k you would see the bottom drop out of these other performance machines.

 

After reading all of the thoughts on my post, it seems that the best way to compare the output of both engines is to do it as net horsepower. Using the 1971 gross and net horsepower ratings for the 454 big block and the resulting conversion factor to calculate net horsepower for the 1967 L71 engine seems to be the best way to compare the LS2 and L71. Multiplying 500 gross horsepower, as suggested by the engine plaque I saw in Atlantic City, by .7647 results in a net rating of 382.35 horsepower for the L71. The L71 had it's peak horsepower rating at 5800 rpm compared to the LS2 reaching peak horsepower at 6000 rpm. The L71 reached a peak torque of 460 ft. lbs. (gross) at 4000 rpm compared to the LS2 reaching it's peak torque of 400 ft. lbs. (net) at 4400. rpm. Using these calculations and torque numbers it looks like the LS2 would be a better performer on the street than the L71.

Any other thoughts or comments would be appreciated.

 

Take a look at the new Vette magazine. They test a C6 Z51 against a 71 LS6 454 (425hp/475tq net). The 71 even had Dunlop 255/60 radials. The 71 got a best run of 13.58 at 107, while they got 12.89 at 108.9 out of the C6.

BTW- The LS6 454 was rated at 450hp gross in 1970 (Chevelle only, no vettes) and 425hp net in 1971. I agree with your numbers on the LT-1.

 

Just a minor correction - I'm quite sure the LS6 hp
rating in 1971 was a gross rating, not a net rating.

Pat

 

Corvettes in every performance category and serve as a daily driver at the same time. The good thing about the oldie is the styling which is personal preference and the fact it is appreciating in value where the C-6 is depreciating like any other new car.

 

The 425 hp rating for the LS6 was gross hp. The 1970 version of the LS6 in the Chevelle was rated at 450 hp because the compression ratio was higher. The 1971 version of the LS6 was originally going to be rated at 460 hp but when the compression ratio was reduced it also reduced horsepower.

In my spare time I made the following comparison. If you are intersted read through it and comment.


2005 Corvette C6 Z51 vs 1967 Corvette Sting Ray L71

Specifications\ 2005 CorvetteC6 Z51\ 1967 Corvette Sting Ray L71

Wheelbase\ 105.7 in.\ 98.0 in
Length\ 174.6 in.\ 175.1 in.
Width\ 72.6 in.\ 69.6 in.
Height\ 49.1 in.\ 49.8 in.
Weight\ 3179 lbs.\ 3155 lbs.

Engine

Displacement\ 364 cu. in. (6 liter)\ 427 cu. in. (7 liter)
Compression ratio\ 10.9:1\ 11.0:1
Horsepower\ 400 @ 6000 rpm (net)\ 435 @ 5800 (gross)
Torque\ 400 @ 4400 rpm (net)\ 460 @ 4000 (gross)

Transmission\ 6-speed manual\ 4-speed manual

Axle ratio\ 3.42:1\ 3.55:1

Acceleration

0-60 mph\ 4.1 sec.\ 5.0 sec.
¼ mile (sec. @ mph)\ 12.5 @ 115 mph\ 12.9 @ 111 mph

Underrated L71

The L71 was most likely 495 horsepower, 60 higher than the factory rated 435 horsepower. Underrating horsepower was common in the late 1960s to keep insurance rates down on these powerful cars.

1971 Gross and Net Horsepower Ratings according to Chevrolet

350 cu. in. (RPO LT1) – 330 gross hp. \ 275 net hp. 275/330 = .8333 conversion factor
454 cu. in. (RPO LS6) – 425 gross hp. \ 325 net hp. 325/425 = .7647 conversion factor

Factors of conversion for net to gross or gross to net horsepower:

.8333 - Chevrolet factor for 1971 LT1 (350)
.7647 - Chevrolet factor for 1971 LS6 (454)
.8000 - Commonly used factor to convert gross or net horsepower

LS2 and L71 horsepower and torque conversions

Gross horsepower & torque comparisons

LS2
480 hp @ 6000 rpm (400/.8333)
523 hp @ 6000 rpm (400/.7647)
500 hp @ 6000 rpm (400/.8)

480 ft. lbs. torque @ 4400 rpm (400/.8333)
523 ft. lbs. torque @ 4400 rpm (400/.7647)
500 ft. lbs. torque @ 4400 rpm (400/.8)

LS2 Average Gross Ratings
501 hp @ 6000rpm
501 ft. lbs. torque @ 4400 rpm

L71 Gross Rating
495 hp @ 5800 rpm (adjusted rating)
460 ft. lbs. torque @ 4000 rpm (factory rating)*

Net horsepower & torque comparisons

L71
412 hp @ 5800 rpm (495 x .8333)
382 hp @ 5800 rpm (495 x .7647)
396 hp @ 5800 rpm (495 x .8)

383 ft. lbs. torque @ 4000 rpm (460 x .8333)*
352 ft. lbs. torque @ 4000 rpm (460 x .7647)*
368 ft. lbs. torque @ 4000 rpm (460 x .8)*

L71 Average Net Ratings
397 hp @ 5800 rpm
368 ft. lbs. torque @ 4000 rpm (factory rating)*

LS2 Net Rating
400 hp @ 6000 rpm
400 ft. lbs. torque @ 4400 rpm

Conclusion:

The LS2 and L71 are almost equals in power output with the edge going to the LS2. This is proven by their ¼ mile times. The LS2 has a slightly higher ¼ mile speed, 115 mph. to 111 mph, and has a better time of 12.5 seconds vs. 12.9 seconds. Some of the difference is due to tires as evidenced in the 0-60 mph times. The LS2’s time was 4.1 seconds and the L71 was 5.0 seconds. Using the same tires, the ¼ mile performance would probably be roughly the same. The July 2005 issue of Vette magazine compares a 2005 C6 to a 1971 LS6. The results corroborate my conclusion. Note, the LS6 engine was underrated by the factory as was the L71.

*The torque rating of the L71 is as reported by Chevrolet in 1967. However, if that is underrated as was the horsepower and it is equal to the higher horsepower rating then the average torque as calculated would be 495 ft. lbs. of gross torque and 397 ft. lbs. of net torque. Looking at the ¼ mile speed and times, this seems to be a reasonable assumption to make.

 

Well, this caused me to get out my Corvette Black Book, and check for myself. I'm not sure where you got your net ratings, but your 425 gross for the LS6 in 71 is correct. The change from 450 to 425 was because of lowered compression, introduced in 71. The LT1 went from 370 to 330. Net ratings started in 72. For that year, there was no LS6, only an LS5 rated at 270hp net. This was a drop from the 71 LS5 at 365hp gros, so your conversion would be .7398. However, the LT1 was still available, with no significant changes, and rated at 255hp net vs 330hp gross, so your conversion factor would be .7727. Your .7646 factor is definitely in the ballpark.