Mathematical equation for HP VS. RWHP?

[jibbby]

I have always used the Hp at the flywheel in terms of measuring horse power as do the manufactures on their new cars...Apparently everyone on the highperformance forums etc..use WHP/RWHP and not HP (crank/fly) Is there a mathematical equation to measure and calculate the difference between the two?.... I really need to get with the program....

Example: 300 HP at the flywheel would be how much RWHP?

or

Example 2: 220RWHP would be how much HP at the FLY?


Can anyone offer up a comparison chart or mathematical equation to break it down... I would be so very appreciative... I cannot figure out how to differenciate the two exactly... I can roughly from looking at many dyno charts but not exactly....

 

[Peewee]

Depending on where you read, the results will differ.

Personally I use a 20% drivetrain loss for a manual, and 25% for an auto.
I think JBrady has 'worked out' the 98 LS400 Auto to be about 22% loss.

There are many factors to influence this figure, flywheel weight, gearbox weight, oil weight/condition, driveshaft weight, diff weight, CV's etc etc.
So stating one figure for all is not possible.

But, given a 25% drivetrain loss on 300 fly hp.
This means that 75% of the power reaches the wheels
300 x 0.75 = 225rwhp.

Going the other way
220rwhp, given a 25% drivetrain loss
220 / 0.75 = 293.33hp at the fly.

 

[Zuffen]

Just to clarify manufacturers HP readings they usually quote figures that do not include any accessories on the engine.

This means their figures could be overstated by 10hp to what your engines actual output is.

 

[Nick M]

Just to clarify manufacturers HP readings they usually quote figures that do not include any accessories on the engine.

I am pretty sure that is incorrect. At one time is was correct. That was the old SAE gross horsepower rating. The SAE net horsepower rating included the things the engine needs to operate, such as alternator, waterpump, and it included exhaust. The newer yet SAE rating makes even more standardized testing. They have to use the recomended oil, not leaving it low to reduce windage. And if you have power accessories, they must be included. Such as the power steering.

Using a dynojet or similar device is certainly a better way to determine power in the aftermarket world.

 

[Toysrme]

It'll differ for every transmission. The "normal" way is to have someone calculate drivetrain loss on a dyno & it's expressed as a %.
It really depends on a lot of things. For Toyota's transverse transmissions. It's 17% loss for an E-15(#)/E-5(#) manual transmission & 22% loss for anything in the A54(#) or U1(#)1 family.


15-25% is the "general" range you'll find most 2wd systems in.

 

[jibbby]

Thank you guys for the expanation that does help me alot...Thank you!!!... I can defenitely see how the drivetrain weight on a car can infuence horse power measuring at the wheels...Piont well taken...

Approx. 20% loss on a manual tranny...

Approx. 25% on a automatic tranny..

Got it now for measuring rwhp!!!!!

 

[WDoherty]

i know a lot of domestic guys go with 15% manual and 20% auto, like they said it all depends, also front wheel drive cars have less powertrain loss than rear wheel drive cars.

 

[cowboy bebop]

I never quite understood the 15% and 20% drivetrain loss theory. The losses would result mainly from friction, I would assume, so what would be the differences in friction between say a 500rwhp pull and a 1000rwhp pull? True there is some drivetrain loss, but I have a feeling that it's more a flat figure after a certain point, rather than a pecentage.

Eric

 

[jibbby]

That is what I was thinking Cowboy/Eric...that is why I started this thread and listed the term equation rather then percentage... Percentage is the consensious though....

 

[Toysrme]

The often quoted %'s are like the cc/5 = rough bhp rule for fuel injectors. It's "close enough" so often that few people ever go to the trouble to do a mathmatical formula. I'd rather know the hardcore be-all end-all equation. I've just never seen anyone express it as anything other than a %.

 

[WDoherty]

for an exact formulation you would have to have the exact drag co-efficient for the the powertrain of every car and this drag varies with the power level, because hp is based on how fast you can move something, that's why the loses are greater at high hp and people use a percentage as it takes greater force to accelrate something so fast even though it weighs the same, kinda like diminishing returns

 

[verylost]

for an exact formulation you would have to have the exact drag co-efficient for the the powertrain of every car and this drag varies with the power level, because hp is based on how fast you can move something, that's why the loses are greater at high hp and people use a percentage as it takes greater force to accelrate something so fast even though it weighs the same, kinda like diminishing returns

An easier but more expensive way would be to engine dyno the car and the dyno the car with the transmission and the rest of the drivetrain intact.

Extrapolate the data and voila, formula. Hire a college intern and he should have it finished in a week.

 

[Lextreme II]

LOL..... I might do a engine dyno in the next few months...

 

[Warpspeed]

The actual transmission loss is definitely in the 15% to 20% range for most transmissions, that is, engine to rear hubs, but it is not that simple.

The losses arise from two main causes, sliding friction in the helical cut gears producing oil shear, and oil churning/windage losses. Forget about ball bearings and uni joints, their losses are insignificant.

Oil churning losses are going to be highest at high Rpm and very low loads. Oil shear from the continuous transmission of very high torque heats the oil signifying a definite frictional power loss. Anyhow, the average at full power is in the range of 15% to 20% for most transmissions.

The biggest problem with roller dynamometers are the tyres. Even if there is not much slip, power transmission by friction into a relatively small diameter roller through a small arc of contact is not going to be very efficient. So very much can depend on tyre pressure, wheel diameter, and how hard you tie down the car. Losses of 25% to 30% are sometimes claimed between flyweel power and measured rear wheel power. Hub dynos have none of these tyre related problems, and always read consistently higher than roller dynamometers.

Transmissin losses also vary with engine Rpm and gear ratio, and how the extra power is obtained. Suppose you modify two identical production engines. One is n/a and spins to silly Rpm and develops 400 Hp. The other runs a supercharger at enormous boost pressure and also develops 400 Hp, but at half the Rpm.

Put through identical transmissions and both run in the same gear, the rear wheel Hp on a roller dynamometer will quite likely be very different, even though the flywheel power of both is identical. The n/a car will have twice the roller speed at half the torque. Try changing tyre pressures, or tie down both cars differently, and everything changes.

That is probably why Fred's dyno reads differentkly to Joe's dyno, the rollers are different diameters, and Fred has a different system of holding the car down on the rollers. Joes rollers have a non slip surface. Both dynos are perfectly accurate, but read differently. So whose dyno is right, Fred's or Joe's ?

Don't worry about it. If it reads more after you have modified it, be happy.

 

[jibbby]

I brought this thread back up as found it very insightful back in the day when I was still learning how to walk...


Curious, does anyone know if the drivetrain loss on front wheel driven car as apposed to a rear wheel driven car is less on average when measuring for horsepower at the wheels?...Is there less power loss on front wheel drive cars?

Awd - I would think you would lose as much as 30% or more of the power when you have turn and deliver power to all four wheels, does that sound about right?.....

 

[crushu]

Also, hi stall convertors loose a lot of RWHP, but gain a lot of RW Torque.

We dyno a lot of cars, basicly, find somewhere, and keep going back there, all dyno's read different, and all dyno operators do their thing different, so you will have more consistancy with the one place.
As was said before, if you do something, and go back, and it reads higher, then all good, if not, you know you went wrong.
Also, take the figures it gives you with a grain of salt, my brothers car reads anywhere from 680rwhp - 740rwhp, depending on the day, dyno, and operator.

 

[cjsupra90]

In the sence that you are asking, yes typically FWD's loose less then RWD's and AWD's loose more then any.

But lets look at an MR2 or an NSX, they would have the same lose as a typical FWD even though they are RWD, but lets change the example to a Porsche or Ferreri or Lambo, those would typically have the same as most RWD's.

The question should be actually asked, Would a transverse drivetrain setup loose more or less the a longitudinal drivetrain setup, and the answer would be the transverse setup would loose less and this is because the power is always traveling in a straight line where as a longitudinal setups, the power has to make a 90deg turn at the differential and this adds more friction.

 

[XR8tt]

The closer the engine is to driven wheels usually means less weight in transmission.. Which limits power levels.. Put heavier wheels /tires on your car while at dyno?? Even going to lighter drive shaft as in alloy makes a small difference... I find rwhp is "about" fwkw....

 

[cjsupra90]

Something else that I would like to add is that the % of lose that most people assume is not even close. In reality the typical is really in the area of 8 to 10 percent for transverse engine and drivetrain setups and 10 to 12 percent for longitudinal setups.

Here is some good reading about dyno's and true losses through the drivetrain. Also at the end of the artical at this link is links to a few more articals that should be read also. http://www.sdsefi.com/techdyno.htm
Happy reading.....

 

[XR8tt]

Have gone to a few dyno days and its interesting to compare known engine power to rwd.. If I remember correctly the 2000 Mustang Cobra was caught out big time with its manufacturers power ratings...
The Monaro Aus, GTO to U.S is another where power ratings don't equal rwkw/ hp...