Determining Crank HP From RWHP - Pontiac GTO Forum
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post #1 of 9 (permalink) Old 10-23-2011, 07:33 PM Thread Starter
 
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Determining Crank HP From RWHP

I know this is going to open up a can of worms but here goes. What's the best to way to figure out your engine's crank HP from chassis dyno (rw HP) figures? I've heard take the rear wheel HP numbers and divide by .75, .80, .85 etc. depending on who I've talked to. Today I read and article from the GTOAA that stated the following:

RWHP divided by .85 = "Net" Crank HP (Net meaning alternator, fan. PS, full exhaust and other add ons are on engine).

Also, "Net" Crank HP divided by .80 = Gross Crank HP (as measured in the Sixties).

Thoughts and opinions are welcome.
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post #2 of 9 (permalink) Old 10-23-2011, 07:52 PM
 
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post #3 of 9 (permalink) Old 10-23-2011, 08:52 PM
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Originally Posted by 67HardTop View Post
I know this is going to open up a can of worms but here goes. What's the best to way to figure out your engine's crank HP from chassis dyno (rw HP) figures? I've heard take the rear wheel HP numbers and divide by .75, .80, .85 etc. depending on who I've talked to. Today I read and article from the GTOAA that stated the following:

RWHP divided by .85 = "Net" Crank HP (Net meaning alternator, fan. PS, full exhaust and other add ons are on engine).

Thoughts and opinions are welcome.
The calculations are right. Just depends on what number to use. I've heard you lose anywhere from 15 - 20% from the crank to the rear wheels. So that rules out .75. It sounds like GTOAA is saying you lose 15% which is why they use .85 as the figure... I would think GTOAA would have the more accurate info.



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post #4 of 9 (permalink) Old 10-23-2011, 09:37 PM
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post #5 of 9 (permalink) Old 10-24-2011, 11:51 AM
 
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It also depends on what transmission is in the car...and other things...



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post #6 of 9 (permalink) Old 10-25-2011, 04:53 PM
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Quote:
Originally Posted by 67HardTop View Post
I know this is going to open up a can of worms but here goes. What's the best to way to figure out your engine's crank HP from chassis dyno (rw HP) figures? I've heard take the rear wheel HP numbers and divide by .75, .80, .85 etc. depending on who I've talked to. Today I read and article from the GTOAA that stated the following:

RWHP divided by .85 = "Net" Crank HP (Net meaning alternator, fan. PS, full exhaust and other add ons are on engine).

Also, "Net" Crank HP divided by .80 = Gross Crank HP (as measured in the Sixties).

Thoughts and opinions are welcome.
Real answer? You can't. Not with anything close to 100% accuracy anyway, for two reasons:
1) Every dyno is different - run the same car (or engine) on two different dyno's and you're going to get two different results.
2) Parasitic loss in the transmission/drivetrain/rear axle --- varies too much from car to car, even with "the same components". "My" 3200 rpm stall converter, TH400, ujoints, drive shaft alignment, rear axle combination is going to absorb a different amount of torque than "your" 3200 rpm stall converter, TH400, ujoints, drive shaft alignment, rear axle combination is.

Real value of a chassis dyno session is for fine tuning your fuel system and ignition system so that they're "the best" for your particular car. Value of an engine dyno is for comparing different parts combinations and set ups on the same engine to find out which set up makes the best torque/power on that engine.

For real-world activities, we race cars - not dyno sheets

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post #7 of 9 (permalink) Old 10-25-2011, 05:16 PM
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Absolutely correct ^.

Chassis dyno's are tuning tools, they also help quantify gains (or losses ) from adding aftermarket accessories.

Generally speaking, figure 15% for stick shift cars and 20% for auto's.
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post #8 of 9 (permalink) Old 10-27-2011, 07:35 AM Thread Starter
 
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Thanks guys- appreciate the input. I guess I have no choice other than taking the car to the track and seeing what happens. Damn the bad luck...;-)
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post #9 of 9 (permalink) Old 10-27-2011, 12:23 PM
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Thanks guys- appreciate the input. I guess I have no choice other than taking the car to the track and seeing what happens. Damn the bad luck...;-)
Yeah, the things we endure for these cars - we're all saints, I tell ya...

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