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Discussion Starter #1
I put a post on the C4 forum similar to the one here about modding your Vette and my son’s LS1 engine. The C4s have a lot of years worth of low power 350s too, and the later ones had the LS1 engines. It was mentioned that the C3s went from the 300/350hp plus engines & how by 73 the hp ratings really started to plummet. One reply was that it was changed from flywheel hp to rear wheel hp.

fishslayer143 I appreciate your information on correcting that thought. I had heard that often enough to not question it, but also knew different facts where the fly to rear didn’t make sense. You got me researching, and here’s what I got. All you real knowledgeable people out there, throw your 2¢ in anywhere you see me messing up!



The pre 73-72 cars hp was measured at the flywheel, like in the Nelson & Lingenfelter dyno videos, headers hooked up to a bare engine, SAE Gross. It was also greatly exaggerated both up and down according to the whims of the manufacturer. 73 and up were measured at the flywheel, but with full accessories, alternator, a/c, water pump, fuel pump and exhaust, more like the actual setup, still exaggerated around, SAE Net . . . a question is coming on this too.

Then there’s the number that to me counts the most, RWHP.

From reading, the average percent of hp loss from gross to net is 20%. I do have questions about this #.

The average loss of hp net to rwhp is 15%. I’m guessing they mean stock.

That means the 200-225 SAE Net hp C3s & C4s are only putting down 180-190 rwhp. That sounds a bit low, but the 70s & 80s are all kinda blurry anyway . . .

The 60s & early 70s 350/350hp SAE Gross Vettes were putting down about 230 rwhp. That doesn’t seem quite right . . . in fact it seems real low . . .

Also, fish, if your car puts down . . . trying to remember . . . 525rwhp? . . . then that engine you built is about 750 SAE Gross? WOAH !! Really one helluva build even at 600 . . .

OK, now to some conclusions I’ve gathered. I know just by putting the same size strip radials on the back end, the percentage net to rwhp drops a lot. That % number is very debatable, and easily lowered. Also, an alternator, a/c compressor, water pump, oil pump, power steering pump, etc. take the same amount of hp to run, no matter how powerful the engine. If an a/c compressor takes 7hp to run, that’s a much higher percentage of hp on a 300hp sae net than on a 600hp sae net engine. That 20% doesn’t seem to be very accurate.

With the pictures and videos I’ve seen from everyone from NRE, Lingenfelter, the crate engine companies, they all do their engines dyno’d SAE Gross. You see the engine on a stand with headers bolted up & the dyno charts.

A 500/525 rwhp Vette should run in the low 11s. Anyone with real world experience, I’d appreciate your input a lot more than average percentages. How much engine hp do I need to put 500+ rwhp on a C3?
 

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Horsepower Loss To Rear Wheels

The 15% to 20% figures relate to an average 327" to 350" V8 engine that puts out 250 to 300 horsepower. But when the engine's power gets increased the percentage of loss decreases. So it would be a lot better to use a fixed loss like 75 to 80 horsepower.
 

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yeah, I agree with toobroke.. the parasitic drivetrain loss is pretty much a constant number at a given rpm, regardless of the engine turning it. example.. a 300net hp engine ,lets say loses 60hp, or 20% to parasitic loss. if you throw 600hp in that same car, its still only losing 60hp to turn the drivetrain at the same rpm, or 10%. a 2009 ZR1 was dyno d stock...638 SAE net rated hp..it produced 567rwhp.. thats a loss of only 71 hp.. just over 10%.. it does not take more power to spin the transmission of a car , simply because you give it more power. now, that said, an automatic car will lose slightly more to parasitic loss than a manual . there are some that believe otherwise,20% on all.. that a 2000hp car will lose 20% or 400hp to turn the drivetrain.. I say thats rediculous........ oh ,and Brian, mine is not rwhp...but I assure you, its a wild ride!
 

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yeah, I agree with toobroke.. the parasitic drivetrain loss is pretty much a constant number at a given rpm, regardless of the engine turning it. example.. a 300net hp engine ,lets say loses 60hp, or 20% to parasitic loss. if you throw 600hp in that same car, its still only losing 60hp to turn the drivetrain at the same rpm, or 10%. a 2009 ZR1 was dyno d stock...638 SAE net rated hp..it produced 567rwhp.. thats a loss of only 71 hp.. just over 10%.. it does not take more power to spin the transmission of a car , simply because you give it more power. now, that said, an automatic car will lose slightly more to parasitic loss than a manual . there are some that believe otherwise,20% on all.. that a 2000hp car will lose 20% or 400hp to turn the drivetrain.. I say thats rediculous........ oh ,and Brian, mine is not rwhp...but I assure you, its a wild ride!

Exactly my point. You should never use a percentage when estimating power loss. Let's say it eats up 20% of a stock engine's horsepower (or about 65-70 horsepower). Now replace that engine with a 14-cylinder 108,000 horsepower Wartzilla ship engine and the same percentage would show a 21,600 horsepower loss! Do you see the problem with using a percentage?
 

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Discussion Starter #5
Toobroke and fish, that’s pretty much how I see it too. Those percentage numbers seemed to relate to a much lower hp engine. I couldn’t see how an increase in hp would equate to an increasing parasitic loss in rwhp. 70 to 85 hp loss from engine to rear wheels sounds a lot more reasonable than a 35% loss in power (20% sae gross to net + 15% net to rwhp). By the hp numbers vs % the 350/350 Vette engines put out about 265-280 rwhp. That seems more reasonable. By using the hp numbers, and compared to the example of the Z06, to get my 500+rwhp, I need an engine that puts out about 600 or more sae net hp. Any gains in engine hp over that would more directly add to rwhp, having the constant parasitic loss already covered.
 

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Toobroke and fish, that’s pretty much how I see it too. Those percentage numbers seemed to relate to a much lower hp engine. I couldn’t see how an increase in hp would equate to an increasing parasitic loss in rwhp. 70 to 85 hp loss from engine to rear wheels sounds a lot more reasonable than a 35% loss in power (20% sae gross to net + 15% net to rwhp). By the hp numbers vs % the 350/350 Vette engines put out about 265-280 rwhp. That seems more reasonable. By using the hp numbers, and compared to the example of the Z06, to get my 500+rwhp, I need an engine that puts out about 600 or more sae net hp. Any gains in engine hp over that would more directly add to rwhp, having the constant parasitic loss already covered.

The biggest portion of the loss is in the rear end as right angle gearboxes typically consume about 10-11%. The lower the gears the higher the loss and the higher the gears the lower the loss. If the drive shaft is dead straight it consumes almost nothing. And then the automatic transmission consumes the rest.
 

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i think ( no, really, occasionaly i do...) that i also depends on what type of powertrain your running, if i remember correctly a th400 eats about 30hp more to turn than a th350...or something like that...did someone say eat ? d*mmit, now i gotta go to taco hell.....
 

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A good rule of thumb..well documented...is manual transmission cars, (especially the C-5 and C-6, which have low drag brakes and C/V joints instead of u-joints) will consume between 10 and 15%. Auto trans. from 18 to 20%. I also feel that higher horsepower engines will lose a small additional conversion loss due to the increased friction produced by the faster acceleration that the stronger engine allows. The new Z-06 has manual transmission and differential oil coolers just to prove my point. Under high loads, the gear friction (horsepower loss) creates excess heat.
P.S. It is said that my supercharger at the boost levels I run...about 20psi...consumes about 90 hp. I am not happy about that...
 
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