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I know about the switch from gross to net hp in 1972. My question is a 1970 corvette with 300hp was gross ratings, what would the net ratings be on that 1970. Also, my 1978 has 185hp net, what would that be gross hp? I understand mufflers, belt driven accessories and other variables change hp, but how much?
consider this
1971 base L48 rated at 270 GROSS HP
1972 base L48 rated at 200 NET HP
there ain't no significant difference in those two motors' internals, intakes or exhausts; only HOW they're rated. Both have same heads, dish pistons, low-lift hyd cam & QJet.
71 hydralic cammed LT-1 was rated at 330 Gross HP and a 72 Hydralic cammed LT-1 was rated at 255 NET HP-Same motor-Difference of 75 HP just on the ratings. Figure about 60-75HP between Gross and Net HP! A 185 Net Hp L-48 would probably rated about 250 Gross HP.
net-to-gross difference is in the 15-25% range, depending on accessories [ie, auto tranny absorbs more hp than a manual one]. Roughly, use 20% loss as a good 'guess'timate.
71 hydralic cammed LT-1 was rated at 330 Gross HP and a 72 Hydralic cammed LT-1 was rated at 255 NET HP-Same motor-Difference of 75 HP just on the ratings..
A hydraulic cammed LT-1?
Originally Posted by 7T1vette
net-to-gross difference is in the 15-25% range, depending on accessories [ie, auto tranny absorbs more hp than a manual one]. Roughly, use 20% loss as a good 'guess'timate.
Both net and gross HP are taken at the flywheel, no transmission.
Both net and gross HP are taken at the flywheel, no transmission.
Yep.
I'd like to say (again) that this whole gross/net HP "conversion factor" is a wild goose chase. There is no conversion factor. It all depends on the exact motor you're working with. Anyone with just a little bit of knowledge of engine theory should be able to figure that out.
An L48 "gross vs net" factor would be extremely different from an LT1 or any other high power motor.
The only numbers that are important are what your engine is putting out in your configuration, and even then, HP is only as useful as the capability of the driver and the chassis.
There may be a bit more info here than was asked for, but some may find it interesting.
From time to time, folks try to determine what their engine HP is by back calculating from the Rear Wheel HP data they obtained from a Chassis Dyno. Let’s take a look at what it takes to make sense of that.
First we need to look at the 3, count ‘em 3, different correction factors in use.
1. SAE J607 also called SAE STD, which is the classic Hotrod and Racing Engine correction factor used by most folks on an engine dyno. So, if you plan to compare Hotrod or Racing Engine dyno figures between various engines across the nation, you must use this correction factor in order to be on the same page with most everyone else. It is corrected to 60*F, zero % humidity, and 29.92” hg. This one gives GROSS HP, and excludes the use of accessories, full exhaust system, full air cleaner, or any emissions equipment. Since this correction factor has the most favorable correction conditions, it will of course provide the highest numbers of all the correction factors shown here.
2. SAE J1995 also called SAE GROSS, was used by the OEM’s through ’71. It is corrected to 77*F, zero % humidity, and 29.234” hg. This one gives GROSS HP, and excludes the use of accessories, full exhaust system, full air cleaner or any emissions equipment. The results using this one, are usually somewhere around 20% higher than SAE NET HP figures.
3. SAE J1349 also called SAE NET, has been used by the OEM’s since ’72. It is also corrected to 77*F, zero % humidity, and 29.234” hg. But this one gives NET HP, and DOES include the use of accessories, full exhaust system, full air cleaner, and any emissions equipment.
Since each correction factor will provide different HP results, when it comes to trying to compare and/or calculate one way or the other, between engine dyno numbers and chassis dyno numbers, you MUST use the SAME correction factor for both the engine dyno and the chassis dyno. This will keep things an apples to apples comparison and/or calculation.
If you don’t use the same correction factors, you end up with, at best, an apples to oranges comparison, or at worst, an apples to elephants comparison. Neither one of these is much good for back calculating engine HP from rear wheel HP.
When Engine and Chassis Dyno numbers are compared properly, by using the same correction factor, the most widely accepted drive train loss figures for non-IRS cars have typically been around 12 to 15% for stick cars, and around 25 to 30% for automatics.
So, let’s look at some real world Engine Dyno vs Chassis Dyno tests, to see how things typically shake out:
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From June 2011 Popular Hotrodding Magazine
‘70 Chevy Nova
555ci BBC
675 HP from Engine Dyno using SAE J607/SAE STD correction factor
9” rearend (not IRS)
Powerglide automatic
1. On a Dyno Dynamics Eddy Current chassis dyno with 2 smallish 12” diameter rollers, using SAE J1995/SAE GROSS correction factor. This is an apples to oranges comparison because of incompatible correction factors, and Rear wheel HP = 487, for a 28% drive train loss. The loss was so high here, not only because of the incompatible correction factors, but also because under load, the 2 smallish dyno rollers caused MAJOR out of shape, HP robbing, tire distortion.
2. On a Dynojet Inertia chassis dyno with 1 large 24” diameter roller, using SAE J1349/SAE NET correction factor. This is an apples to elephants comparison because of REALLY incompatible correction factors, and Rear wheel HP = 564, for a 16% drive train loss. The larger dyno roller did not cause any HP robbing tire distortion here.
The results between these two chassis dyno’s varied by a whopping 77 Rear wheel HP, or 12%. This leaves you with absolutely no chance of back calculating engine HP with any degree of accuracy. Even if you threw out number 1 with all the tire distortion, and looked only at number 2, you still couldn’t accurately back calculate engine HP. Because that 16% loss is considered to be more in line with stick drive train losses, when compatible correction factors are used, and NOT automatic drive train losses when significantly incompatible correction factors are used.
The results here just leave you scratching your head. So, you can’t accurately back calculate engine HP and you can’t even feel confident about how much HP you are actually putting to the ground either. This certainly questions the value of using a Chassis dyno at all.
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From February 2011 Super Chevy Magazine
’72 Corvette
383ci SBC
426 HP from Engine Dyno using SAE J1995/SAE GROSS correction factor
IRS rear end
4 speed stick
On Super Chevy Magazine’s brand new Dynojet inertia chassis dyno, and using the SAE J1349/SAE NET correction factor. This is an apples to oranges comparison because of incompatible correction factors, and Rear wheel HP = 283, for a whopping 34% drive train loss.
’70 Chevelle LS6
454ci BBC
450 HP from factory Engine Dyno using SAE J1995/SAE GROSS correction factor
Solid rear end (no IRS)
TH400 automatic
On Super Chevy Magazine’s brand new Dynojet inertia chassis dyno, and using the SAE J1349/SAE NET correction factor. This is an apples to oranges comparison because of incompatible correction factors, and Rear wheel HP = 285, for a whopping 37% drive train loss.
’69 L72 Yenko Camaro
427ci BBC
425 HP from factory Engine Dyno using SAE J1995/SAE GROSS correction factor
Solid rear end (no IRS)
Stick tranny
On Super Chevy Magazine’s brand new Dynojet inertia chassis dyno, and using the SAE J1349/SAE NET correction factor. This is an apples to oranges comparison because of incompatible correction factors, and Rear wheel HP = 288, for a whopping 32% drive train loss.
’57 Chevy BelAir
283ci SBC
245 HP dual quad, from factory Engine Dyno using SAE J1995/SAE GROSS correction factor
Solid rear end (no IRS)
Powerglide automatic
On Super Chevy Magazine’s brand new Dynojet inertia chassis dyno, and using the SAE J1349/SAE NET correction factor. This is an apples to oranges comparison because of incompatible correction factors, and Rear wheel HP = 158, for a whopping 36% drive train loss.
The drive train losses for this group of 4 cars ranged from 32% to 37%, and were high due to incompatible correction factors. On top of that, there was no clear distinction at all between stick cars, automatic cars, and IRS cars, even though stick cars typically only have about half as much drive train loss as automatic cars. So, none of these figures are worth much either, in terms of accuracy or usefulness.
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From March 2011 Hotrod Magazine
2011 Shelby GT500 Super Snake Mustang
5.4L Supercharged V-8
750 HP from Engine Dyno using SAE J607/SAE STD correction factor
solid rearend (not IRS)
6 speed stick
1. On a Dynojet Inertia chassis dyno using SAE J607/SAE STD correction factor. Rear wheel HP = 654, for a 13% drive train loss. Being that this is an apples to apples comparison because of the same correction factors being used, it makes sense to see a 13% drive train loss, which is in the range of what would be expected for a non-IRS stick car.
2. On a SECOND Dynojet Inertia chassis dyno at another shop, which also used the SAE J607/SAE STD correction factor. Rear wheel HP = 652, for a 13% drive train loss. Being that this is an apples to apples comparison because of the same correction factors being used, again it makes sense to see a 13% drive train loss, which is in the range of what would be expected for a non-IRS stick car.
3. On a Superflow Auto Dyn eddy current chassis dyno using SAE J607/SAE STD correction factor. Rear wheel HP = 630, for a 16% drive train loss. Being that this is an apples to apples comparison because of the same correction factors being used, it still makes sense to see a 16% drive train loss, which is just on the outer edge of the range of what would be expected for a non-IRS stick car.
4. On a Mustang brand eddy current chassis dyno using SAE J1349/SAE NET correction factor. This is an apples to oranges comparison because of incompatible correction factors, and Rear wheel HP = 568, for a 24% drive train loss. These numbers are too far off to be of any use for comparison or for even determining how much HP is put to the ground.
5. On a Dyna Pack hydraulic type chassis dyno that bolts directly to the hubs, and using SAE J1349/SAE NET correction factor. This is an apples to oranges comparison because of incompatible correction factors, and Rear wheel HP = 585, for a 22% drive train loss. These numbers are too far off to be of any use for comparison or for even determining how much HP is put to the ground.
Overall, these 5 chassis dyno’s ranged a whopping 86 RWHP, or 11% on the SAME car, with the exact SAME setup. Good luck trying to ever back calculate engine HP from these numbers, or even trying to determine how much HP is actually delivered to the ground.
However, if you only look at the 3 dyno’s above that used the same correction factor as the engine dyno, you’ll see that they produced reasonable numbers that actually are usable. They ranged only 24 HP or 3%, which is about as good as you are ever going to get. And this backs up the statement that you MUST use the SAME correction factor for both the engine dyno and the chassis dyno.
Also, new cars since ’72 have been rated in SAE NET engine HP. So, you can fairly reasonably use a chassis dyno that uses the same SAE NET HP correction factor for comparison there. But even that is a little iffy because the OEM’s often underrate their HP levels, so that introduces more error back into any comparison/back calculation.
If you don’t use the same correction factors for comparison, then all chassis dyno’s are really good for is to compare back to back changes you make while on that dyno. That way you are only looking at the delta’s, and not caring about what the absolutes truly are. If used in this manner, any random chassis dyno “can” be a useful tool for modifications (though it’s wise to make a few back to back pulls with no changes to see if the dyno is repeatable, because some are not).
And if you are using mismatched correction factors for comparison, don’t even bother trying to back calculate your engine HP, because you won’t get valid results. In addition to that, with all the other variables between dyno’s (due to different makes and models, strap tension, tire pressure, tire rubber compound, dyno cooling fan airflow, etc, etc), don’t put much stock in the amount of HP they claim you are putting to the ground either, since the numbers will be all over the place.
So, at the end of the day, you have to decide for yourself if you think a Chassis Dyno session is even worth the cost and effort. Joe Sherman, the Godfather of Engine Building, has told me that he thinks Chassis Dyno’s are a complete joke and not worth the effort. And after reviewing the data above, I’d have to agree with him.
I agree with generally a 20-25% difference, and starting in 1971, GM lowered compression ratios to accomodate no lead low octane fuel dropping power even further. As mentioned, the power ratios are really a black science, as some of the high hp big blocks(although gross), were actually advertised under rated for insurance reasons.
From: Who says "Nothing is impossible" ? I've been doing nothing for years.
I just took my motor out and put it on an engine dyno then put it back in and put it on a chassis dyno, result for mine was 21-23% loss from gross to RWHP I like to see things and do them not read the mucho trash out there
I'm trying to navigate this gross v net discussion, and I'm drawn back to jackson's post
1971 base L48 rated at 270 GROSS HP
1972 base L48 rated at 200 NET HP
so when I read about C3 HP being terrible because of the way they measured it, did it really change that much??? If they continued to use Gross HP as a gauge through the 70's would the numbers still have been respectable, or was it the emissions issues that actually did tank the HP?
Jedi, you may not have spotted it but you've replied to an ancient post.
However, your question is still relative. This query/debate has been going on for years. The example you shared from jackson's post is a good one. It shows a 26% difference in hp ratings but the major difference between these two years was the yardstick by which they measured.
The answer to your question is "a combination of both". Had they continued to measure gross hp through the '70's the numbers would still have dropped due to lower compression and poorer heads, etc. but the hp numbers would not have sounded nearly as bad as they did for the era.
Example: the '75 L-48 rated at 165 net sounded pretty poor although the car was a good performer for its era when compared to everything else offered in 1975. If you add 26%, a gross hp reading may have been a little over 200 hp and that doesn't sound quite so bad.
Cheers, Greg
Jedi, you may not have spotted it but you've replied to an ancient post.
However, your question is still relative. This query/debate has been going on for years. The example you shared from jackson's post is a good one. It shows a 26% difference in hp ratings but the major difference between these two years was the yardstick by which they measured.
The answer to your question is "a combination of both". Had they continued to measure gross hp through the '70's the numbers would still have dropped due to lower compression and poorer heads, etc. but the hp numbers would not have sounded nearly as bad as they did for the era.
Example: the '75 L-48 rated at 165 net sounded pretty poor although the car was a good performer for its era when compared to everything else offered in 1975. If you add 26%, a gross hp reading may have been a little over 200 hp and that doesn't sound quite so bad.
Cheers, Greg
oh yeah, its pretty ancient, but I was hoping it would still be relevant.
Thanks for the reply, that's what I was thinking!! I figured it might be a combo of the two factors, new yard stick and the actual performance.
For 1971 only, GM rated them both ways, gross and net.
3 of the 4 show a 22% drop, but not the LT-1, it only showed a 17% drop. Why?
The gross HP dropped was roughly 30 HP on all 4 engines, vs 1970, due to lower CR & distributor tuning.
If you want to go down from gross to net, you would subtract 22%. If you want to go backwards, from net to gross, you would need to add 28%.
The only differences between the SAE STD (Hot Rod engine dynos) and SAE Gross (GM) correction factors (post #12) are in temperature and air pressure. Both of these favor the SAE STD factor (Hot Rod Engine Dynos) by packing in 10% more cooler & denser air (Gas Laws). So a 450 GM rated LS6 would be about 10% more HP or 495HP on a typical Hotrod engine dyno. That is almost exactly what I have seen in dyno tests run both on-line, or in person. Most of the current engine dyno runs add a little power to the old Mfgrs ratings. And the editors always say the mfgrs were just sand-bagging! Maybe .... the editors just don't understand the math.
I never thought that part was a big deal. It was the chassis dyno's I didn't understand. Now I know why. They are a hot mess, just too many variables. Great thread.
Last edited by leigh1322; Jan 17, 2023 at 11:40 PM.
there were some noticeable power differences from 71 to 75. almost entirely due to retarding the timing. a little bit of jetting and the crappy exhaust.
Difference between gross and net hp
