Drivetrain loss & WHP vs. CHP calculation discussion (read this if you want to do it right)
10-23-2006, 09:20 AM
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Drivetrain loss & WHP vs. CHP calculation discussion (read this if you want to do it right)
Nearly every post that I read concerning WHP and crank HP incorrectly calculates the drivetrain loss. Drivetrain losses are a constant. Losses do not increase as a percentage of the output of the engine. If a stock AAN makes 227hp/258lb.ft CHP and has an estimated drive train loss of 20% (I picked this number at random) the WHP would be 227x0.8=182WHP and 258x0.8=206lb.ft. This is a loss of 45hp and 52lb.ft.
The losses remain constant even if the output of the engine increases. They do not get larger. The drivetrain loss is still 45hp and 52lb.ft. if the engine is stock or makes 400 or even 600+ WHP. The power required to turn the drivetrain is not a percentage of the total output of the engine it remains constant.
The proper way to calculate CHP from WHP is to add the losses calculated on a stock engine, WHP+drivetrain loss=CHP. It is incorrect to calculate the loss as a percentage of the total output of the engine then add that to the WHP. If you calculate the losses as a percentage of the total output of the engine the losses would increase as output increases.
The problem with this is that actual drivetrain loss is only a guess. To my knowledge no one has done a proper baseline of a stock AAN powered S-car to measure the drivetrain loss. To measure the loss you would need to measure the output of an engine on an engine dyno then measure the output of that same engine installed in a vehicle under the same conditions.
Another way to measure the loss is to run a properly tuned stock car on a chassis dyno and compare the results to the factory claimed power numbers. The difference would be the drivetrain loss. This would be hard to do since AAN's are all over 10 years old and age and mileage has reduced the output of the engine by some unknown amount. At least it would give a better number than a guess. Once the hp and tq losses are known they are simply added to any WHP dyno results of a vehicle with the same drivetrain to get estimated CHP.
I'm sure this will spark many comments, but these statements are based on my education as a mechanical engineer and discussions with powertrain design engineers that I work with. It is also in Corkey Bell's book on supercharging. I hope this will help to correct some of the wildly incorrect CHP calculations I've see on this forum.
The losses remain constant even if the output of the engine increases. They do not get larger. The drivetrain loss is still 45hp and 52lb.ft. if the engine is stock or makes 400 or even 600+ WHP. The power required to turn the drivetrain is not a percentage of the total output of the engine it remains constant.
The proper way to calculate CHP from WHP is to add the losses calculated on a stock engine, WHP+drivetrain loss=CHP. It is incorrect to calculate the loss as a percentage of the total output of the engine then add that to the WHP. If you calculate the losses as a percentage of the total output of the engine the losses would increase as output increases.
The problem with this is that actual drivetrain loss is only a guess. To my knowledge no one has done a proper baseline of a stock AAN powered S-car to measure the drivetrain loss. To measure the loss you would need to measure the output of an engine on an engine dyno then measure the output of that same engine installed in a vehicle under the same conditions.
Another way to measure the loss is to run a properly tuned stock car on a chassis dyno and compare the results to the factory claimed power numbers. The difference would be the drivetrain loss. This would be hard to do since AAN's are all over 10 years old and age and mileage has reduced the output of the engine by some unknown amount. At least it would give a better number than a guess. Once the hp and tq losses are known they are simply added to any WHP dyno results of a vehicle with the same drivetrain to get estimated CHP.
I'm sure this will spark many comments, but these statements are based on my education as a mechanical engineer and discussions with powertrain design engineers that I work with. It is also in Corkey Bell's book on supercharging. I hope this will help to correct some of the wildly incorrect CHP calculations I've see on this forum.
10-23-2006, 10:08 AM
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if you want true power numbers, stop messing with the "drivetrain losses"
pull the lump out and test it. Only true way you will know what kind of power you are making is if you test the engine by itself. IMO, people who care too much about what kind of power they are putting to the wheels use it as an excuse as to why they lost their last stoplight dragrace.
10-23-2006, 10:29 AM
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Actually whp numbers are much better for comparison sake than crank numbers
And to go a step further you should divide actual whp by actual vehicle weight to get the power to weight ratio. With power to weight ratio you can make a very good comparison of which car should be faster. This removes drivetrain losses and vehicle weight from the comparison.
IMO companies that sell power upgrade parts should advertise whp gains not crank. This way you can tell if they meet their claims if you do a before/after dyno test. Even if the dyno is a little high or low the difference should be the same as claimed by the manufacturer.
IMO companies that sell power upgrade parts should advertise whp gains not crank. This way you can tell if they meet their claims if you do a before/after dyno test. Even if the dyno is a little high or low the difference should be the same as claimed by the manufacturer.
10-23-2006, 10:48 AM
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It's all a crap shoot. Dynos read high or low, people will pick/use higher number to inflate their
ego.....It might be "better" for comparision sake, but for accuracy's sake, crank based off of an engine dyno is the way to go.
10-23-2006, 12:52 PM
#7
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I've always thought this was true also.
There has been a lot of discussion both ways about this in the past. Beings friction loss is heat generated if you use the 20% number and multiply by say "500 CHP" thats a lot of heat that has to be somewhere. I just don't see parts heating up that much.
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10-23-2006, 01:37 PM
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Shouldn't there be a difference between drivetrain loss and dyno run hp?
Drivetrain loss is at a given speed, so the drivetrain loss should be based on friction of the gears, tires, etc etc.
Dyno runs calculate horsepower while an engine is accelerating. A more powerful engine will be able to accelerate the drivetrain faster, but the rotational inertia will require more torque, and the difference could would be based on a somewhat set %.
So say we take a measurement right as the engine is accelerating past 5252rpm. A 200lb/ft engine would lose 20lb/ft to inertia, resulting in 180hp.
A 400lb/ft engine would accelerate it faster, but has to overcome more inertia resulting in 40lb/ft loss, so it would come up as 360hp.
It's been a little while since physics, so this may be a bit off.
Dyno runs calculate horsepower while an engine is accelerating. A more powerful engine will be able to accelerate the drivetrain faster, but the rotational inertia will require more torque, and the difference could would be based on a somewhat set %.
So say we take a measurement right as the engine is accelerating past 5252rpm. A 200lb/ft engine would lose 20lb/ft to inertia, resulting in 180hp.
A 400lb/ft engine would accelerate it faster, but has to overcome more inertia resulting in 40lb/ft loss, so it would come up as 360hp.
It's been a little while since physics, so this may be a bit off.
