Is the driveline power loss between the crank and the wheels equal to a percentage of engine hp or is it an absolute amount of hp regardless of engine output? I had thought that driveline loss was a percentage of crank hp output, but it seems like all the driveline mechanical components are fixed and so the hp power loss should be the same whether you are running 287hp at the crank, or 500hp at the crank. The driveline loss would thus be around 45-50hp for both the 287hp motor and the 500hp motor even though on the stock motor it equals 17% while on the high hp motor it would equal 10%. Is this correct, or is it an equal percentage loss, in which case the stock motor loses 45-50hp and the boosted motor loses 85-90hp?

 

The percentage should be fairly constant as it is caused by losses outside the engine (transmission, rear end, wheels & tires).

 

Varies with rpm so it's not a constant anything. Also varies with tires [brand, model, size, inflation] and the temperature of tire road interface.
Varies with fluid temperatures oil, ATF/MT, diff, bearing grease often overlooked is any drag in rotor pad clearances as rotors heat up and expand.

No 2 identical cars can have exactly the same power loss curve.........they maybe tweeked to within 1-2% with every component identical.

Obviously the tightness of tiedown straps and the tire tread depth must be identical.

Why chassis dyno numbers must be though of as approximations and only accurate between identical cars not closer than +- 2% at very best.

 

well said

 

So based on what Q45 Tech said, what would be the better way to represent power #'s. With the understanding that everything changes, dyno's read differently etc.

Or is this all really a moot point and I might as well make up whatever I want. I say this because if someone wants me to give them a crank HP #, I only have my 502 whp (or 492 SEA corrected). I am just making up #'s if I use the divisor of 17% and say my crank HP is 595 when in fact I don’t know of all of the factors that affected my dyno run

Just curious really.

 

While its true that the drivetrain resistance will vary with RPM, HP is always specific to RPM. (HP = torque X RPM).

So drive train loss, expressed in HP is pretty much a specific number, not a percentage.

"Drive train loss" is the energy required just to turn the drivetrain components and overcome the frictional resistance. If that number is 30 HP, it will be 30 HP whether the input power comes from a 100 HP engine, or a 500 HP engine.

bill

 

I don't think that exactly answers the question the OP was asking. Yes, external conditions do affect the percentage of loss. But all things being equal, the loss through a drive train is going to be the same percent (+- a small amount) regardless of whether the engine is a 300hp N/A or 450hp F/I.
In other words, the percent loss is not a function of HP but on conditions outside the engine (tires, transmission and rear end gears and oils, etc.).

 

The losses thru a gear mesh are variable with the applied force.
The oil gets thinner as the temperature rises and the reason the temperature rises is the frictional losses............

The largest loss component is the tire road interface.
Next comes the diff gear set because of 90 degree transfer in rotation, then each gear mesh in tranny.


To measure the frictional losses all one needs are temperature sensor [lots of them] to measure the instantaneous temp rise of every single and minute component after the flywheel.

If I hade to create a translation number for a manual tranny and average tires on average road surface I would use 16.5% and add 4-5% for an AT/TC.

 

i think its like 17% loss for manual... 18% for auto.. roughly in 1:1 gear...

on a Z with stock tire size / wheels on a chasis dyno...

 

The best torque convertor is 3-4.5% more than manual [assuming direct gear] depending on ATF viscosity [temperature] and stall speed and differential speed [slip amount vs rpm].

http://fluid.power.net/techbriefs/hanghzau/5_3_3.pdf

 

when the TC is "locked" (above stall speed, or in 4th or 5th), then that loss disappears though - correct?

The formula in that PDF no longer applies then...

What I'm curious about is how the torque multiplication that occurs on launching an AT car offsets the drivetrain loss? The stock TC has a 2.0 multiplication factor for torque, but you still have to consider drivetrain loss (expressed now in torque), so the net torque to the drive shaft is actually less that 2.0 the engine torque because of slippage, correct? In fact, when the torque multiplication falls to 1.0, then drivetrain loss from the TC should fall to 0.

 

A general rule of thumb for drivetrain loss on a front engine rear wheel drive vehicle is about 20% and about 15% for transaxle vehicles (where the tranny and the diff are in the same case so no torque is lost through a driveshaft).

 

This thread seems to have split into trying to cover several different ideas. To the original poster, parasitic loss will ALWAYS be a percentage. I have given up on trying to explain basic physics on this forum, so if you really want to know why that is I suggest hitting up Wikepidia and going from there. The percentage of loss will always be the same even if the power output changes, given 500 hp or 50 hp, unless the factors contributing to that loss are changed. Changing oil viscosities and temperatures are some changes that will make a marginal difference at best. Lightening the reciprocating assembly, from the crank to the wheels and everything in between, will make the largest difference. Trying to determine your loss on two seperate dynos will be pointless, because as has been stated to death- different dynos will all read differently. If you have a dyno run stock, and have a new run with your mods, and none of the mods included a lighter flywheel, wheels, etc.. , and both were done on the same dyno, then the parasitic losses will be close enough to be considered identical. So if you lost 15% on the stock run when compared to the factory 287hp, then it will be safe to assume 15% loss with the modified run as well. Any differences in temperatures and such will be too marginal to really care about. Hope that answers your question.
Will

 

except for resolute, nobody has answered the original question. are the losses a percentage, or a fixed value for a particular drivetrain? i always heard that it was a percentage also, but i don't know for sure because my information did not come from an absolute source.

 

It is a percentage. If you think about it, otherwise, as you increase the power of your engine, your drivetrain gets more efficient! And that is impposible, the more power you put through the drivetrain, the more power you will loose.

 

I look at it this way 287 flywheel 233 RWHP = 18.8 %
287 flywheel 218 RWHP. 5 AT = 24.0 %
Dynoed my 5 AT and it make 218 to 219. My stage 3 stillen makes 307 RWHP

 

WTF does that mean? I'm guessing the 18.8% is for a MT and the 24% is for AT, correct? The original question is asking if HP level causes a variation in the percent loss, not different drivetrain or other external conditions. It's a given that different external conditions will cause the percent loss to be different.

 

I would say with the 5 AT the more you make that you lose more at the wheels. Stillen claim 410 crank and 343 wheels which is 16.3% loss. I have yet to see anybody get those Numbers. I have changed my converter and will post friday night to see if any thing changes from the stock converters #s.

 

I'd believe 16% loss for a MT, but certainly not an AT. Is that what they're claiming the HP at crank vs wheels is for an AT?