Official motor-to-wheel hp/tq difference
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Staging Lane
Joined: Feb 2010
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Official motor-to-wheel hp/tq difference So ive been wondering for a while now how much power the accessories and drive line rob from the power at the motor number? Is there a big difference from auto to manual or is it more hype?
is it a percentage like 30%? is it a number like 40-50hp?
this way, i can better determine whp for current and new cars as manufactures usually do power at the motor (and yes i know its usually underrated) any thoughts?
is it a percentage like 30%? is it a number like 40-50hp?
this way, i can better determine whp for current and new cars as manufactures usually do power at the motor (and yes i know its usually underrated) any thoughts?
Launching!
Joined: Mar 2010
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So ive been wondering for a while now how much power the accessories and drive line rob from the power at the motor number? Is there a big difference from auto to manual or is it more hype?
is it a percentage like 30%? is it a number like 40-50hp?
this way, i can better determine whp for current and new cars as manufactures usually do power at the motor (and yes i know its usually underrated) any thoughts?
is it a percentage like 30%? is it a number like 40-50hp?
this way, i can better determine whp for current and new cars as manufactures usually do power at the motor (and yes i know its usually underrated) any thoughts?
They say the manuals lose 15% and an automatic will lose 18% from parasedic loses. It depends on more than just the driveline and accesories. It is based on gear ratio, tire size, stall size and other factors.
I dont buy the fixed percentage stuff. I believe that as you make more power the percentage would be less. if you have the same exact drivetrain in a 400 whp car and an 800 whp car the drivetrain isnt gonna magically eat up double the hp...
Launching!
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I made 705 std corrected hp at the crank with no accessories, and 599 sae corrected rwhp with all accessories including stock water pump and the converter locked, so that's 15.0%
If the data was sae on both sides it would have been less than 15%
If the data was sae on both sides it would have been less than 15%
Launching!
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from what ive seen, the vette has a more efficient drivetrain. Or at least shows up more on the dyno. Could it be because the car weighs less?
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Staging Lane
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im aware it does. stall size and your other factors will change the whp. if a manual car has a 400hp motor, stick that in an auto with a stall, beefy tranny, heavy rims, etc and the dyno will have to be different but does that mean one will be faster then the other? i guess that depends on the race type (drag vs. roll)
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Well here is an example for ya. I had 676 at the crank on my 441 LS7 and after putting it in my 2001 Trans Am with a 4L60E and a 9 inch rear i had 566 at the wheel............. so you figure it out. 
The simple and short answer is that resistance and heat increase when you increase the force on an object and when you spin it faster. That is why it is not a fixed number.
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Staging Lane
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bout 16% for ya. im thinking if not fixed then a variable range of 14-17%. See? not fixed! just fixed-ish
the amount of force the drivetrain uses is a fixed number based off of rpm. It has nothing to do with the amount of power going into it. I have engine dynoed well over 100 engines and then installed them into the car and chassis dynoed them. It's always thae same hp loss on certain cars no matter what the crank hp was.
the reason alot of people believe it is a percentage is because alot of lower hp engines run lower rpms. Well at 5000 rpm the drivetrain uses less power than 6500.
500 hp 5000 rpm 15% equals 75hp
650 hp 6500 rpm 15% equals 97.5hp
it's not that the drivetrain is absorbing more power,it's that your turning it higher rpm and in fact it takes more hp to turn at a higher rpm.
the other thing that alot of people forget about is exhaust. as the hp increases the same exhaust system will become more restrictive creating a greater loss of power. On the engine dyno we usually aren't hindered by exhaust, but in the car most of the time we are.
shawn pretty much described my thoughts way better than I could.
Thanks Shawn.
Last edited by DynoDR; Jan 1, 2011 at 10:53 AM.
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From: Turnin' Wrenches Infractions: 005
Did you dyno it unlocked at any time? Without that, we are missing a piece of the puzzle. Most street cars here are gonna run unlocked at WOT and on the dyno.
Yes it does because you are accelerating that same drivetrain at a much faster rate, therefore increasing the work load. Think about a car with the back wheels of the ground with the ds disconnected. Now spin the pinion gear so the wheels spin. The faster you desire to accelerate the wheels, the more energy you will need to apply. Drivetrain loss is not static, it is very dynamic.