David J. Harrington

I always here people talk about estimated crank horsepower. I don't really care too much about this since it is the power at the wheels which moves you. However, some of the things I hear seem ludicrous. For instance, I usually hear people assuming 12-15% driveline loss. Could someone explain how the driveline loss could be a function of you engine's power? This would say that my car stock, made 217 crank HP. Assuming only a 10% loss, it would be a little under 200RWHP. Then, if I mod that same car to put out 500HP at the crank, does that mean I am now losing 50HP through the driveline instead of the original 22??? This seems like bad science to me. Should we all go aroung saying something like, "my car loses 20HP through the driveline", instead of, "my car loses 15% though the driveline." If anyone has a valid answer to this question, I'd like to hear it.

Thanks.
Dave(confused)

aka 951

Think about it this way...if you unplugged a couple of plug wires in you car and put it on the dyno and made a whole 50hp at the rear wheels...there'd be no way you lost the 45hp or so a 300 rwhp 944 looses. Another example...stock 951s dyno at around 190 to the rear wheels...and are rated at 217 hp. With your 45 rwhp car above, there is now way you losing 37 hp in the drivetrain.

In other words...you're driveline will always take a percentage of the power, not a fixed figure.

Erick

Dave

Both / Neither statment is completly accurate. The best way to view it would be "a drivetrain losses x hp plus y% of the rest. Keeping in mind that some transmissions / rearends / tranfer cases / transaxels are more efficient than others. Audi's quattro system is considered to be one of the most efficient drive trains in the world. A manual is more efficient than an automatic by design.
As far as where that power goes, if your transmission is sitting on your workbench, and you turn the input shaft by hand/breaker bar, did you generate any heat (friction), yes. Enough to measure? NO. bet it gets warm when you put 217 crankshaft hp into it. HTH

new2944

Whoa!

The first chap is on the ball about it being a (roughly) constant.

Whether a car is driven by a 30hp engine, or a 300hp engine, the driveline (internal friction and tire) losses will be the same at any given speed!

Now, the faster the wheels turn, the more resistive friction there will be. It's probably a logarithmic curve with high initial losses followed by a diminishing/constant loss as speed increases.

Most good tuners will do an initial dyno run that called a "coast-down". They run the car up to 115MPH on the dyno, then take it out of gear and let it coast all the way down to about 50MPH. This will give you a pretty good idea of your average driveline loss. (including tires, very important)

Once you have this coast-down value it will stay constant for whatever engine, or mods, you throw into the car, as long as you change nothing downstream of the clutch and the weight stays about the same.


Hope this helps.

fletch.

I was watching speedvision the otherday (like I watch something else) and they had a show about building the race engines for the Vettes that ran at le mans. The guys were saying that not only do you have HP lose in the drive train, but the engine itself has a huge amount of loss. They calculated it by turning the engine over without starting it. They claimed to lose 130HP internally in the race engine they built for lemans.