Horsepower: pre/post '72 and net rwhp

[Charles F.]

I know this has been discussed before, but I am looking for an explanation of how things changed. For example, assuming an L36 is rated in 1968 at 390 HP and 460 torque, what would be the equivilent rating under the post-1972 system ? Secondly, I am under the belief that the "ballpark" reduction to estimate rwhp is about 15%. Is this reasonable ?

Thanks.

Chuck Faillace

[William C.]

Re: Horsepower: pre/post '72 and net rwhp

The NCRS spec guide is likely the easiest way to answer your questions, the '71 lists both SAE Gross and Net for example the 350/270hp engine (Gross) is 210hp Net. the 330hp LT-1 is 275hp net. The same engines for '72 were rated at 200hp and 255hp (net) so you would be dropping about 25% from the Gross on the Base engines, and a similar amount for the high perf versions.

[Larry M.]

Re: Horsepower: pre/post '72 and net rwhp

Chuck:

Agree with Bill Clupper regarding SAE GROSS AND SAE NET horsepower. The 15% (additional) reduction that you mentioned from engine horsepower to rear wheel horsepower is also a generally accepted and reasonable number to use.

Larry

[Chuck S.]

Re: Horsepower: pre/post '72 and net rwhp

I know this has been discussed before, but I am looking for an explanation of how things changed. For example, assuming an L36 is rated in 1968 at 390 HP and 460 torque, what would be the equivilent rating under the post-1972 system ? Secondly, I am under the belief that the "ballpark" reduction to estimate rwhp is about 15%. Is this reasonable ?

Thanks.

Chuck Faillace

The most dramatic REAL change was from 70 to 71, and resulted from reduction in compression ratio; the LT1 went from 11:1 compression ratio (370 ghp) to 9:1 compression ratio (330 ghp) to permit the use of unleaded regular gasoline.

Then, from 71 from 72, there was an apparent further reduction in horsepower, but this was primarily due to the change in the rating method to net horsepower at the rear wheels; the LT1 went from 330 ghp to 255 nhp...since that reduction is about 23% compared to the 15% noted above, I'm thinking maybe they were also still tinkering with the tuning to meet emissions standards. Compression ratio remained the same in 72 at 9:1, but I didn't check to see if the timing was changed for 72.

[Larry M.]

Re: Horsepower: pre/post '72 and net rwhp

Chuck:

SAE NET horsepower and SAE GROSS horsepower are both measured at the flywheel. There is then a further reduction of horsepower from the flywheel to the rear tires. This reduction is due to friction and other mechanical losses, and consumes an additional 15 to 20 percent of the flywheel horsepower. It is called "powertrain loss". For manual transmissions this is around 15%; for automatic transmissions the higher 20% number is used.

Hope this helps. Larry

[Michael M.]

Re: Horsepower: pre/post '72 and net rwhp

Its really funny, when you think about it. My '70 L-46 is 350 HP, 11:1 CR 3.36 rear end, my '71 LT-1 is 330 HP 9:1 CR 3.70 rear. There is no way that the L-46 will keep up with the LT-1, the difference between the two as far as power is striking.

I have often heard and read about car manufacturers "fudging" the HP ratings on their high HP cars to appease the government standards, I wonder if this is the cause. I should just take both cars down and havew them dynoed.

[Jim B.]

Re: Horsepower: pre/post '72 and net rwhp

As I understand it, HP ratings pre '72 were gross in that the HP was measured with the engine in a stand, fuel, coolant, and air fed to it, no water pump, no radiator fan, no transmission and no acessories, thus a gross HP number for the engine alone which was noticeably higher than that produced by the engine in the car by some 15%. In the pre '72 time frame this is the "advertised" HP numbers the manufacturers used.
The net HP ratings were done with the engine having its normal loads, fuel pump, water pump, alternator, Fan, etc Thus a rating net of any normal loads on the engine. There is an SAE standard for this measurement to reduce fudging by the car makers. I believe that Chevrolet uses the SAE method today.
Both of these HP and TQ measurements were done at the flywheel not the rear wheels.

The rear wheel measurement could be taken with a dyno but only the newer corvettes (C5 and C6 for sure) had this test as a part of their maunfacturing process. I saw it done in Bowling Green as a validation test. The rear wheel measurement was usually 15-20% lower than the SAE Net measurement and a whole lot lower than the Gross measurement. eg: a 405 HP (SAE) C5 Z06 would produce about 350HP at the rear wheels.

Looking at the numbers on the 70 and 71 I would surmise that some of the performance difference you are seeing is due to the gear ratio's. A 3.70 rear axel will produce quiite a bit more accelleration than a 3.36 but a lower top speed even with the same engine. You also don't mention what transmissions are being used because that too will vary with the differing gear ratios producing mixed results.

Both great cars though. Enjoy the ride.

[Alan S.]

Re: Horsepower: pre/post '72 and net rwhp

Following along with Michael's thought... I traded a 68 GTO (400ci/350hp4speed) on my 71 Coupe (350/270/4speed). My recollection is that there was not as great a difference in street performance as one may have expected. The GTO had a very high torque # and that difference was noticeable, as it would spin the tires with the least encouragement. So, were the numbers being played with? That would be a real shock!
Regards,
Alan

[Joe C.]

Re: Horsepower: pre/post '72 and net rwhp

As I understand it, HP ratings pre '72 were gross in that the HP was measured with the engine in a stand, fuel, coolant, and air fed to it, no water pump, no radiator fan, no transmission and no acessories, thus a gross HP number for the engine alone which was noticeably higher than that produced by the engine in the car by some 15%. In the pre '72 time frame this is the "advertised" HP numbers the manufacturers used.
The net HP ratings were done with the engine having its normal loads, fuel pump, water pump, alternator, Fan, etc Thus a rating net of any normal loads on the engine. There is an SAE standard for this measurement to reduce fudging by the car makers. I believe that Chevrolet uses the SAE method today.
Both of these HP and TQ measurements were done at the flywheel not the rear wheels.

The rear wheel measurement could be taken with a dyno but only the newer corvettes (C5 and C6 for sure) had this test as a part of their maunfacturing process. I saw it done in Bowling Green as a validation test. The rear wheel measurement was usually 15-20% lower than the SAE Net measurement and a whole lot lower than the Gross measurement. eg: a 405 HP (SAE) C5 Z06 would produce about 350HP at the rear wheels.

Looking at the numbers on the 70 and 71 I would surmise that some of the performance difference you are seeing is due to the gear ratio's. A 3.70 rear axel will produce quiite a bit more accelleration than a 3.36 but a lower top speed even with the same engine. You also don't mention what transmissions are being used because that too will vary with the differing gear ratios producing mixed results.

Both great cars though. Enjoy the ride.

Jim,

I agree with everything that you said, but would like to add a few more things:

1. Gross HP was measured without an air filter; SAE net HP used a filter.
2. Gross HP was measured with stock manifolds into an unrestricted pipe; SAE net used an exhaust system to match its application.
3. Gross HP was measured at 60 degrees Fahrenheit; 29.92 in-hg baro.; SAE net was at 77 degrees, 29.6 in-hg.

Joe

[Dave C.]

Re: Horsepower: pre/post '72 and net rwhp

So...if I'm following correctly, my 66 427 has a rating of 425hp. Not taking into consideration that GM lowered the hp for insurance purposes, I should reduce the 425 by 15% to get net HP and another 15% to get an estimated rear wheel HP?

[Charles F.]

Re: Horsepower: pre/post '72 and net rwhp

Dave:

I have followed the thread to the same conclusion as you; hope we have it right.

The thread has focused on HP. Does the analysis work the same way for the torque values or is it handled differently ?

Thanks to all for the education (once again) ! I love learning; always a good day when you learn something new.

Chuck Faillace

[Duke W.]

Re: Horsepower: pre/post '72 and net rwhp

... a few of observations based on my database of simulation data, lab dyno tests, and real wheel dyno tests:

1. The SAE gross ratings were highly optimistic - both peak torque and peak power, as were the specified compression ratios. No OE built engine can achieve anywhere near the SAE gross ratings on a lab dyno. The SAE gross ratings are a good 10 percent high, and the as built CRs are typically 0.5 less than advertised. Net ratings beginning in '71 were fairly honest, but compression ratios were still over specified by up to 0.5 point. Early net ratings are fairly honest. but are not directly comparable to today's net ratings as SAE air density was reduced somwhat at some point during the eighties.

2. Massaging the heads will achieve close to the SAE gross HP ratings with no other changes, but will only have a minor impact on the overrated SAE gross peak torque. (Peak torque is primarily a function of displacement and compression ratio, but head flow and valve timing have a major impact on where in the rev range peak torque occurs. Note that peak gross torque ratings of all 327s from 250-375 HP are in the range of 344-360 lb-ft)

3. The difference between SAE gross and net is a combination of air density correction, front end accessories, and exhaust system backpressure. The air density correction alone is about 4.5 percent.

4. The "ballpark" difference between advertised gross and net is typically taken as about 20 percent, but when you take into account the 10 percent SAE gross overratings, the true difference is much less. My data indicates a true difference of about 11 percent with nearly half of this being air density, and most of the rest is exhaust system back pressure. Fan loss is less than 2 HP if the viscous clutch does not engage, but 10-15 HP if the viscous clutch fully engages.

5. Big blocks will show a bit more difference due to greater exhaust system backpressure because of higher exhaust system flow.

The following are my estimates for true OE 327 power - SAE gross, SAE net, and RWHP with SAE air density correction on a Dynojet inertia dynamomenter, the most common type in use. (Other manufacturers chassis dynos will often give different results.)

Config....SAE gross....SAE net....RWHP

327/300..250............225..........190
SHP/FI....300-325......260-280...220-235

The generally accepted drivetrain/tire loss for a front engine-rear drive configuration with manual trans in direct drive is 15 percent (20 percent with a vintage auto trans), so a good estimate of true SAE net is RWHP (with SAE air density correction) divided by 0.85.

A basic pocket porting/port matching/chamber relieving job with multiangle valve seats and a true CR half a point less than OE specified, all other things OE, will increase power about 10-15 percent without any loss of low end torque or change in idle characteristics and add 500-1000 revs to the useable rev range. Optimizing valve timing for actual head flow can further increase output.

OE Vintage engine output range was greater than modern engines due to less precise machinging. The standard deviation of modern engines is probably no more than one percent. Vintage engine output standard deviation was probably at least double this amount.

So if you want to get a ballpark estimate of what a OE engine in good running condition will do on a chassis dyno with SAE air density correction, multiply the (overrated) advertised SAE gross HP by 0.8 then again by 0.85, but don't be surprised if it comes out a little less, especially for big blocks.

Duke

[Charles F.]

Re: Horsepower: pre/post '72 and net rwhp

Duke:

Thanks for the excellent information. How does it work with regard to torque ? Is there a similar reduction from Gross to Net to the rear wheels or is the math different.

Chuck

[Duke W.]

Re: Horsepower: pre/post '72 and net rwhp

The driveline loss is essentially the same at any RPM point and since torque and power are linearly related, the approximate driveline efficiency factors apply to torque and power at all engine speeds.

Duke