Generally people add 20% drive train loss to rear wheel dyno numbers to get the estimated motor power. Is this for both the hp and torque or just hp?

 

since horspower is a function of torque (and angular velocity), its for both.

 

Yes it's for both, but the percentage depends on your drivetrain... with a very good tranny, clutch and rear end you should be able to get down to 10% for most stick ****s its about 15-20% and or most automatics its about 20-25%



LOL, my "F" key don't work that well... that was supposed to be "stick shifts"

 

I ran my car on a chassis dyno one time; the guy that owned it had a close friend who had an engine dyno. The lowest difference he said they ever saw when they ran a motor on the engine stand and then dropped it into a chassis and ran it that way, was 22%. A more typical reduction in readings between the 2 measurements was closer to 25% for a manual and 28% for an auto.

It's probably true that in fact only 15% or so of the engine's power output gets wasted in the drivetrain, but there's alot of other things that aren't taken into account in that figure. There are more things that subtract from the engine's power that it might make on the stand, to what appears at the wheels. Things like the losses from an alternator, power steering, A/C, water pump; the fact that most dyno cells supply essentially unlimited ambient-temp air to the intake, unlike a car where you have to deal with underhood temps; most dyno exhausts are far better then anything that it is possible to install on a car; tires have friction on the dyno rollers; etc. etc. etc. So the "loss" number is kind of meaningless, because you can't use it for anything. Even though it may be strictly accurate, it doesn't describe what really happens, or what you'll really measure if you do what I described above.

Add to that, your typical engine builder who uses engine stand HP figures as the proof of his build is going to do everything in his power to make that number as high as he can, regardless of whether those things that he does have anything to do with how the engine will actually perform once installed. Typically a large, single-plane intake is used; a large carb; racing gasoline, or other high-quality fuel; slight positive air pressure in the dyno cell room; etc. etc. That further distorts what people think is "drivetrain loss", because those are all things that go away when that motor is installed in a chassis. It's a numbers game. Even if, say, the barometric pressure in the dyno cell is given, how are you the purchaser supposed to know that it's a bit higher than the pressure at the other end of the exhaust? It would be somewhat like measuring your ¼ mile ET on a track with a down grade. If somebody told you what the elevation is at only one end, you'd never know the track was tilted.

HP is not directly measurable on most chassis dynos. It is calculated from torque, which is the quantity that is measurable. So the torque is the thing that's reduced, and the calculated HP figure follows by an equivalent proportion. A few of the old electrical dynos do measure HP directly (since the generator's voltage output times its current output equals power) but very few dynos in use today are that type.

 

How true.

Also if the motor which may have a impressive peak hp reading on a dyno, makes that hp figure at a high rpm say 7200 rpm, the drive line is going to absorb a bigger percentage of that power wonce it's in the car. Drive line power absorption goes up a lot past 5200/5500 rpm. Accessory power absorption goes up even more at higher rpm.

thats why drag racers try to eliminate accessories and run an electric water pump to save on lost hp.

 

but then again if hp is based of torque what if peak torque hp was at 2000 rpms vs 7000.

that would make a chnage in percentage for the hp reading