Thread: Drivetrain loss for dummies!!!!

So ive been noticing lately that people are taking this drivetrain loss % like its a golden rule or something. One guy on a forum made a comment about if a buggati veryon made 987awhp that it would mean its making 1184 crank hp because of a 20% drivetrain loss.

Do people not get that is not a true statement when it comes to modified cars or getting up there in hp.

Its basicly like saying a car that came 140 crank hp from the factor and makes 115whp is all of a sudden gonna lose 75whp from that same drivetrain and all when its at 550whp. Wrong answer people......lol Your drivetrain is not gonna get any more restrictive or hold you back when you go higher in power.

The only things that will change how much loss in your drivetrain there is, is things like lighter clutch and flywheel assemblies (which will actuall decrese the amount loss in the drivetrain) or different tires at different pressures (higher pressure reducing drag meaning less loss or lower tire pressure increasing drag making more loss) Thicker gear oil can have an affect on drivetrain loss as well. But we are talking massive amounts here that people are just taking the % as the golden rule

Adding things like bigger heavier brakes will change things as well and so on.

Even big car mags go off this stupid thing.

Example: nissan gtr 480 crank hp rating/average dyno 430-440awhp. This is all across the board. Thats about 40hp loss through the drivetrain which i can easily see. But motortrend goes onto say because it dynod soo much awhp that taking a conservative 15% drivetrain loss it would make it 510 crank hp and with an normal 25% drivetrain loss because of awd that means the gtr can be packing 580 crank hp. So your meaning to tell me the gtr is losing 150hp just through its drivetrain. hahahaha what crap are they smoking. Thats like saying a stock sr20 cant even get the car moving if it was placed in the engine bay because of the loss.

This does not work out. Im sure nissan engine dynod their engines before putting them in the cars so they know what they make.

Rant over!!!!

So your telling us a transmission will not work any harder at 550whp then it does at 115whp?

We've had this discussion before, too late for me to find it. BenFenner should be able to chime in though.

Not that it doesnt do more work, but using a percentage rule is ludicrous. It's making a horrible assumption that it always will work as it does now and using that to try to get real answers. Def'n 'shot in the dark'

It's a parasitic drag that impedes on the delivery of power. There is no SET hp or tq level that any given drivetrain will absorb, because it's a variable ratio. Percentage is faily accurate, but a SET percentage is just crazy. There are efficient designs, and not so efficient. But you saying that it takes "x" amount of power to "power" the drivetrain shows your lacking grasp of mechanical physics.

Yes we've had this discussion before. There are constant losses (all the mass) and there are variable losses (sheering strength of the gear oil goes up with power). A fixed percentage loss nor a constant loss are accurate. It really is a combination of the two.

Of course when power goes up, power consumption increases as well. Like stated above, affixing a SET number on any vehicle is uninformed. But the percentage rule will get you ALOT closer to actual losses than a certain number per drivetrain. But the variables are constant and constantly changing, and differ from each and every car, down to the lubricant used, and even the age and breakdown of that lube. Too many factors to consider to use any rule and be accurate.

But in reality, who really cares about BHP

Someone should get an engine dyno, on a modified SR20 and compare the two side by side.

I almost had that data for a BMW e30. There was a guy making 420 HP or something at the crank (engine dyno) and I'm still waiting for him to give wheel output numbers from a chassis dyno (he probably never will). That would give stock flywheel and wheel numbers and highly modified flywheel and wheel numbers and a good formula could be made for actual drivetrain loss on the e30 and could carry over to other RWD cars maybe in general.

I'm still hoping some day to see good data.

^Best post so far.

Acutally if you go look at stock evo and stock wrx and stock gtr dynos they all seem to lose around 35-45whp from what is rated at the crank to what its actually putting down.

Again if an awd system is sucking up 35-45hp on average that does not equate in any way shape or form to a % As a standard wrx was rated at what, 260hp, and puts down about 220awhp, an evo 8 puts out 300 at the crank and from what ive seen puts down about 255-265awhp. Same goes with the STI, i believe import tuner's sti put down right at 260awhp stock form back then. Again, pretty consistent if you ask me.

Again if both systems lost say 40 hp through the drivetrain a 260 crank hp lost more of a % of its hp than the 300 crank hp car or the 480 crank hp from the GTR. Because as its shown the gtr puts down between 430-440 so roughly a 10% loss. Which is nowhere near the so called 20-25% they say an awd drivetrain takes up. But for a wrx to lose 40hp thats almost a 20% loss. Do you see where im going with this.

The % just cant be taken as a golden rule. You will be closer to actual numbers if you had some stock numbers prior to modding and take the factory crank hp rating and subtract the numbers it lost going to the wheels. Take that number and add that to whatever your new modified hp level is and as long as your oils, clutch and flywheel weight and so on stayed the same then you should be safe saying that your new crank hp is your actual whp plus what the car initially lost through its drivetrain using the stock numbers.

So its pretty safe to say if our cars lose about 20-25hp through our drivetrain that our same car using the same drivetrain now making 300whp would be making roughly 325 crank hp. That will get you a lot closer than a percentage, A percentage would say your making 350 crank hp. A 25 hp difference is a big difference. Again as the numbers go higher this gets exagerated even more. Which isnt correct.