# Advanced Engineering Tech - Crank hp gain=whp gain?

View Full Version : Crank hp gain=whp gain?

capn smokey
11-02-2008, 10:52 PM
I have a complicated question to ask, me and my friends have debated this many times. Do gains at the flywheel carry straight over to the wheels or is there some reason why they shouldnt. The way I understand is that a driveline takes a set amount of power to turn regardless of fwhp, if i gain 5hp at the crank will i gain 5hp at the wheels. I know there are other factors such as parasitic losses and increased friction when more power is applied. And i know that the whole percentage thing is dumb because a driveline doesnt lose efficiency just because more power is applied. Please provide some insight so i can convince my friends.
Thanks-Jeremy

1989GTA
11-02-2008, 11:19 PM
"parasitic losses"

Say the you lose 20% to make things easy. Then that 5 hp at the flywheel will be 4hp at the rear wheels.

mtuggle86
11-03-2008, 12:13 AM
from my understanding if you add crank horsepower to a car the dravetrain still robs the engine of the same percentage of the added power so just like he said if your drivetrain loss was 20% and you added 5 hp you would only see 4 at the rear wheels

Old SStroker
11-03-2008, 09:05 AM
from my understanding if you add crank horsepower to a car the dravetrain still robs the engine of the same percentage of the added power so just like he said if your drivetrain loss was 20% and you added 5 hp you would only see 4 at the rear wheels

It's probably not that simple. Let's just look at the rear end. Most of the losses are from friction between the gear teeth rubbing and sliding on one another. Additonal power creates additional friction approximately at the same ratio as the power increase. Bearing friction and windage will not increase at the same rate as the power, and the inertia losses from accelerating the driveline will remain pretty much the same for a small power increase. The same thing applies to the transmission and tire deflection.

In other words, the marginal or incremental increase at the wheels should be a greater percent of the flywheel increase than the overall rear wheel to engine percent. How much? That would be a guess, but if there was a 20% loss (just using the same example) for the entire system before the gain, the 5 fwhp might get about 4.5-4.6 to the wheels. There will be some loss, but not the full 20%.

Measuring all of this accurately would be an expensive proposition, but I suspect the better Cup teams and certainly the F1 teams have the ability to do it accurately. They are also working with WAY less than 20% driveline losses. It's probably considerably less than 10%, but not many teams will give actual numbers.

Sooner Todd
11-03-2008, 01:46 PM
Wouldn't Crank Horsepower be higher than Flywheel Horsepower?

Stang's Bane
11-03-2008, 01:47 PM
It's probably not that simple. Let's just look at the rear end. Most of the losses are from friction between the gear teeth rubbing and sliding on one another. Additonal power creates additional friction approximately at the same ratio as the power increase. Bearing friction and windage will not increase at the same rate as the power, and the inertia losses from accelerating the driveline will remain pretty much the same for a small power increase. The same thing applies to the transmission and tire deflection.

In other words, the marginal or incremental increase at the wheels should be a greater percent of the flywheel increase than the overall rear wheel to engine percent. How much? That would be a guess, but if there was a 20% loss (just using the same example) for the entire system before the gain, the 5 fwhp might get about 4.5-4.6 to the wheels. There will be some loss, but not the full 20%.

Measuring all of this accurately would be an expensive proposition, but I suspect the better Cup teams and certainly the F1 teams have the ability to do it accurately. They are also working with WAY less than 20% driveline losses. It's probably considerably less than 10%, but not many teams will give actual numbers.

I agree. I like to think about it like this for ease of digestion:D

Take a 100 hp 4 cyl and hook it up to a rear end and spin it up on the dynojet. Would probably spit out around 75 rwhp or so. REmove 4 cyl and hook up a ls7. Chances are it will not spit out 375.

I think that the higher the engine power output is the lower the percentage is, even though it is still losing more power to the driveline than the lower Hp engine would..

blackz93
11-03-2008, 02:15 PM
It's probably considerably less than 10%,

Yes it is.

Alvin
11-03-2008, 09:35 PM
Measuring all of this accurately would be an expensive proposition, but I suspect the better Cup teams and certainly the F1 teams have the ability to do it accurately. They are also working with WAY less than 20% driveline losses. It's probably considerably less than 10%, but not many teams will give actual numbers.

The cup guys are have typically 20-25 HP drive line losses on a super speedway car.

foreverzero
11-03-2008, 09:59 PM
I was wondering the same thing, Im glad you guys cleared it up

Old SStroker
11-03-2008, 10:10 PM
Wouldn't Crank Horsepower be higher than Flywheel Horsepower?

They are one in the same as long as the flywheel is attached dierctly to the crank. Generally the engine dyno absorber attaches directly to the crank/flywheel, with perhaps a short driveshaft on some dynos.

The cup guys are have typically 20-25 HP drive line losses on a super speedway car.

That would be right around 5% which pretty much agrees with some numbers I have heard. It's amazing what extremely precise machining, superfinishing, coatings and perfect setup and the right lube can do. All it takes is the knowledge, and of course lots of \$.

capn smokey
11-04-2008, 03:04 PM
Thanks, guys. I knew it was more complicated than just a simple percentage. There should be a formula for calculating it but it prolly varies between different drivelines so figuring it out wouldnt be worth the time or money.

hsutton
11-06-2008, 09:06 PM
The cup guys are have typically 20-25 HP drive line losses on a super speedway car. Alvin, these cars have an unbelievable amount of friction reducing coatings and parts lightening. Ring gears in the rear end are milled to death. They also use standard transmissions with straight cut teath for minimum losses. Put an automatic behind the engine and i guarantee you the 20-21% loss factor will be almost dead on. The higher the power output of the engine, the more your going to loose at the rear wheels. A torque converter will also show more losses the faster you try to accelerate it. Superflow did a engine dyno vs. chassis dyno test some years back, (1999), the engine was a 476 that produced 685 fwhp and when installed in a 3000 lb. Malibu wagon it had 504 rwhp.

1989GTA
11-06-2008, 09:41 PM
We just engine dyno'd a 505 BBC motor going into a 1972 Corvette. When the motor is installed we will put it on a chasis dyno and see what the drop is. The transmission is a built 200-R4. Or is it 200-4R?

Sammyboy
11-08-2008, 02:50 AM
20% is a bit high...For estimation purposes, I would use 15%

1989GTA
11-10-2008, 07:02 PM
Here is the follow up to my above post. On the engine dyno the motor made 525hp. On the chasis dyno the motor made 423rwhp. Both dyno's made by the same manufacturer and the same facility and operator was used for both.