is there actual a formula for % lost at RW
is there actual a formula for % lost at RW I keep hearing different thing about % of hp lost from trans the the rear wheel.I heard or read 12-15 or even 20% depending on the trans.Can someone give me the truth of what it is for an m6 or a4.
Originally Posted by Magnet
ive always heard 20 for an A4 and 15 for a M6 a, but the exact numbers dont even matter its just to get a ballpark idea
On The Tree
Joined: Sep 2004
Posts: 159
Likes: 1
From: Cape Coral, Fl
There is no formula due to variations in losses from different components, frictions and drags. The only real way to get close is put the engine on an engine dyno, then a chassis dyno. That will show you the losses for that setup under those conditions(fluid temps, tire pressures, etc...) The 15% and 20% losses that are thrown around are rough estimates.
I remember reading something about this a while back I think it was more to the tune of 13% for a 6 speed and 15% for an auto on a stock rear end. It is a couple percentages more with a 9" or a 12 bolt.
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My question is what are these percentages relevant to, a stock setup?
I only ask because if you increase hp using a percentage is going to create a higher differential between flywheel and rear wheel hp. To me is seems like it only takes X amount of horsepower to turn the clutch, tranny, driveshaft, rear, wheels, and all that. You may be able to develop a percentage from the stock setup because you know the rated flywheel and tested rear wheel power numbers but they percentage may not necessarily apply to all power levels.
Here's an example:
Flywheel hp * percent loss = estimated rwhp loss
350 * 12% = 42 (or 308 rwhp)
450 * 12% = 54 (or 396 rwhp)
550 * 12% = 66 (or 484 rwhp)
Basically using the percentage method says that as flywheel hp goes up you're losing more from drivetrain losses.
If you haven't changed anything in your drivetrain why would you be losing more power just for making more power at the flywheel?
I only ask because if you increase hp using a percentage is going to create a higher differential between flywheel and rear wheel hp. To me is seems like it only takes X amount of horsepower to turn the clutch, tranny, driveshaft, rear, wheels, and all that. You may be able to develop a percentage from the stock setup because you know the rated flywheel and tested rear wheel power numbers but they percentage may not necessarily apply to all power levels.
Here's an example:
Flywheel hp * percent loss = estimated rwhp loss
350 * 12% = 42 (or 308 rwhp)
450 * 12% = 54 (or 396 rwhp)
550 * 12% = 66 (or 484 rwhp)
Basically using the percentage method says that as flywheel hp goes up you're losing more from drivetrain losses.
If you haven't changed anything in your drivetrain why would you be losing more power just for making more power at the flywheel?
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Originally Posted by My90Iroc
Basically using the percentage method says that as flywheel hp goes up you're losing more from drivetrain losses.
If you haven't changed anything in your drivetrain why would you be losing more power just for making more power at the flywheel?
If you haven't changed anything in your drivetrain why would you be losing more power just for making more power at the flywheel?
the more power you have from teh engine. the faster you turn the parts of the rest of the drivetrain, so the more friction there is, resulting in more drivetrain loss.
i'll use the same example as before, if you have a drive train and it takes a 350hp engine to turn it, and you only put 290 to the ground. so that's 60hp lost in a drive train.
ok now you take that same drive train, and hook up a 60hp motor. would it put any power to the ground? i think it would. i would only put about 45hp or so, but some would still make it to the wheels.
in response to the original post. i've always heard 20-25% for an A4 and 15-18% for a M6
All the dyno people I talk to say 15% for an M6 & 20% for an A4. That is an ball park based on testing. I can tell you my car lost another 7.5% thru a 9" & stall converter. But it also runs 6/10ths quicker! 
Someone at Yank told me if you lock the converter you can get an auto to dyno with a manual. Otherwise you have all that loss of the converter "slipping". So without that, they won't be the same....
Teching In
Joined: Oct 2005
Posts: 20
Likes: 0
I have been analyzing dyno numbers for a long time and have collected a lot of data.
I have hundreds of dyno tests for stock cars, all performed on Dyno Jets.
Almost every car I have data for shows about a 12% drivetrain loss for a manual transmission. One thing to remember is the OEMs do not rate their engines the same way as aftermarket dyno's do. The OEM rating is a little more conservative, so the actual drivetrain loss is actually a little more than 12% but this is what you will see if you use the official rated engine power.
What I did was average all of the dyno runs for each kind of car. I then caculated the percent of drivetrain loss using the factory rated engine power.
The cars I have are 1986-2005 Mustangs, 3rd and 4th gen F-bodies, 4th, 5th, and 6th gen Corvettes, and the new GTO.
The only cars that do not show about a 12% loss are the LS1 F-body, The 03-04 Cobra, and the 2003 Mach I. We know these cars are under-rated (Especially the LS1 F-body)
The F-body would be about a 3% loss if it was rated correctly.
The 03-04 Cobra would have about a 7% loss.
The 03 Mach I would have about an 8% loss.
All the others are right around 12% give or take a percent.
I don't have a lot of dyno info on autos except for the LS1 F-bodys. It looks like the A4 has about a 15% loss.
I saw a post from a dyno shop a while back (can't remember who) that stated, from what they have seen, the manual cars show about a 12% loss.
This backs up what I have found so far.
I have hundreds of dyno tests for stock cars, all performed on Dyno Jets.
Almost every car I have data for shows about a 12% drivetrain loss for a manual transmission. One thing to remember is the OEMs do not rate their engines the same way as aftermarket dyno's do. The OEM rating is a little more conservative, so the actual drivetrain loss is actually a little more than 12% but this is what you will see if you use the official rated engine power.
What I did was average all of the dyno runs for each kind of car. I then caculated the percent of drivetrain loss using the factory rated engine power.
The cars I have are 1986-2005 Mustangs, 3rd and 4th gen F-bodies, 4th, 5th, and 6th gen Corvettes, and the new GTO.
The only cars that do not show about a 12% loss are the LS1 F-body, The 03-04 Cobra, and the 2003 Mach I. We know these cars are under-rated (Especially the LS1 F-body)
The F-body would be about a 3% loss if it was rated correctly.
The 03-04 Cobra would have about a 7% loss.
The 03 Mach I would have about an 8% loss.
All the others are right around 12% give or take a percent.
I don't have a lot of dyno info on autos except for the LS1 F-bodys. It looks like the A4 has about a 15% loss.
I saw a post from a dyno shop a while back (can't remember who) that stated, from what they have seen, the manual cars show about a 12% loss.
This backs up what I have found so far.