Calculating Horsepower???
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Racer
Joined: Aug 2002
Posts: 409
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Calculating Horsepower???
Is the best estimate(ie most accurate) to add 20% HP to the rwhp or subtracting 20% from the flywheel HP(for a c5 automatic)??? Thanks!!
Last edited by painexpert; Oct 27, 2004 at 01:01 PM.
Melting Slicks
Joined: Dec 2000
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Originally Posted by painexpert
Is the best estimate(ie most accurate) to add 20% HP to the rwhp or subtracting 20% from the flywheel HP(for a c5 automatic)??? Thanks!!
Pro
Joined: Jan 2004
Posts: 749
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From: Las Cruces New Mexico
Originally Posted by HIGHRPM
Divide your rwhp by 0.80 to get fwhp.... or multiply your fwhp number by 0.80. You get the same result either way. Adding 20% to the rwhp number gives an incorrect number...while subtracting 20% from the flywheel number gives the correct number.
20% I believe is typically used with A4 while 15% is generally accepted for 6 speeds.
Race Director
Joined: Mar 2001
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From: Whippany NJ
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St. Jude Donor '08-'09
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Originally Posted by HIGHRPM
Divide your rwhp by 0.80 to get fwhp.... or multiply your fwhp number by 0.80. You get the same result either way. Adding 20% to the rwhp number gives an incorrect number...while subtracting 20% from the flywheel number gives the correct number.
Racer
Joined: Mar 2003
Posts: 279
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I think the 13% to 15 % lost is about right for stock motor base line. However, I am not sure this calculation is right for additional hp/tq from bolt ons. Let’s say a stock Z06 has 350 rwhp, that’s 55 hp or 14% loss. If we apply the 14% loss to a turbo Z06 with 580 rwhp, we would come up 674 hp at flywheel. That’s a loss of 94 hp. We all know the turbo is driven by exhaust gas. Now the question where does the 39hp go? Therefore, I think the percentage of hp/tq loss should not be a constant at 13% to 15%. Instead the percent loss should drop with additional hp/tq from bolt ons. Please comment.
Pro
Joined: Jan 2004
Posts: 749
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From: Las Cruces New Mexico
Originally Posted by atz06
I think the 13% to 15 % lost is about right for stock motor base line. However, I am not sure this calculation is right for additional hp/tq from bolt ons. Let’s say a stock Z06 has 350 rwhp, that’s 55 hp or 14% loss. If we apply the 14% loss to a turbo Z06 with 580 rwhp, we would come up 674 hp at flywheel. That’s a loss of 94 hp. We all know the turbo is driven by exhaust gas. Now the question where does the 39hp go? Therefore, I think the percentage of hp/tq loss should not be a constant at 13% to 15%. Instead the percent loss should drop with additional hp/tq from bolt ons. Please comment.
Safety Car
Joined: Apr 2004
Posts: 3,738
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From: Austin, Texas
Originally Posted by eamador11
hmmm...I divided 412 by .80 and I get 515HP. Is that right?
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Instructor
Joined: Oct 2001
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From: Mesa AZ
Actually, the loss does stay pretty consistant on a manual transmission at about 15-16% no matter what the HP is. I know this from seeing engine dynos and then the chassis dyno right afterwards. This number is correct for Dynojet dynos, Mustang dynos and others vary.
An automatic car will actually loose MORE than 20% on huge HP cars. This is due to torque converter slip. The slip on a low HP car may be 300RPM (as an example), but on a high HP car, it can be as high as 700RPM at WOT (unlocked of course). I have seen this very often with every brand of converter -- it is just a function of the converter. This is why the "stall speed" varies on vehicles -- the HP difference can change it drastically.
On an automatic (as stated) dividing by .80 is a safe bet and on a manual transmission, dividing by .85 is also a safe bet. Remember though, chassis dynos are only tuning tools, and cannot take exact measurements. If you divide by .85 on a Mustang dyno (which are normally much lower) you may only get 300 motor HP out of a stock car instead of 350HP!
Mike
An automatic car will actually loose MORE than 20% on huge HP cars. This is due to torque converter slip. The slip on a low HP car may be 300RPM (as an example), but on a high HP car, it can be as high as 700RPM at WOT (unlocked of course). I have seen this very often with every brand of converter -- it is just a function of the converter. This is why the "stall speed" varies on vehicles -- the HP difference can change it drastically.
On an automatic (as stated) dividing by .80 is a safe bet and on a manual transmission, dividing by .85 is also a safe bet. Remember though, chassis dynos are only tuning tools, and cannot take exact measurements. If you divide by .85 on a Mustang dyno (which are normally much lower) you may only get 300 motor HP out of a stock car instead of 350HP!
Mike
Le Mans Master
Joined: Sep 2004
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From: Los Angeles
Just a theory here so don't flame me. It might be that the manual transmission and drivetrain require a certain amount of horsepower to crank them through the rpm range. If you modify your engine to do 6500 rpm instead of 6000 or change gears or transmissions, you will eat up more heat and energy. If the rpm range remains the same the minor differences in loss due to heat will be negligible, therefore leaving a fixed amount of 50 or 60hp loss on the manual transmission setup. This of course does not apply to the automatic as mike stated because of the variable amounts of energy a torque converter will absorb under different loads. With the automatic, it's probably a bit more constant percentage as opposed to constant hp loss.
Just a thought.
If anyone has ever made any real world observations of this, I'd love to hear it.
Just a thought.
If anyone has ever made any real world observations of this, I'd love to hear it.
