Question on drivetrain/HP loss
Thread Starter
Instructor
Joined: Jun 2003
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From: Fort Myers FL
Question on drivetrain/HP loss
If we say that an A4/M6 loses 20/15% respectively, why are these expressed as a % and not a fixed number? Assuming the mechanicals that cause the loss stayed the same during an engine swap, wouldn't the drivetrain eat the same amount of HP no matter what was powering it?
Put another way, in an A4 with 400BHP we might expect a RWHP figure of 320. If we bumped that motor up to 600BHP, then the RWHP should only be 480. Why does the parasitic loss increase relative to the BHP?
Thanks
Put another way, in an A4 with 400BHP we might expect a RWHP figure of 320. If we bumped that motor up to 600BHP, then the RWHP should only be 480. Why does the parasitic loss increase relative to the BHP?
Thanks
Safety Car
Joined: Jun 2004
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From: Witness Protection Program Somewhere, USA
2026 Restomod of the Year Finalist
I've wondered that myself. According to the % reasoning, the more HP/TQ you make, the higher the loss to the rear wheels. 350HP x .15=298HP (52HP). 600HP x .15= 510 (90HP). Huh? 
Le Mans Master
Joined: Sep 2004
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From: Shiloh Ohio
Well I am not sure on this but I have always assumed that the 20%/15% worked well on a vehicle in the 300-400 crank HP range. I think as the crank HP goes up the % of parasitic loss to the rear wheels would decrease. 
Race Director
Joined: Sep 2001
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From: Hockessin Delaware
Originally Posted by shurite44
Well I am not sure on this but I have always assumed that the 20%/15% worked well on a vehicle in the 300-400 crank HP range. I think as the crank HP goes up the % of parasitic loss to the rear wheels would decrease.
That makes sense - perhaps it is within a % range depending on crank hp, i.e. the % goes down as hp increases.
Le Mans Master
Joined: Jan 2005
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From: Dallas/Grand Prairie Texas
I don't know, but I had my stock.....very stock 02 A4 coupe 3.15's dynoed and came out at 308.6RWHP and 321.1RWT???
If there's 18% then I'm over 375HP at the crank?? doesn't sound like there's really 18% loss if you ask me......or why would GM advertise 350HP vette??
If there's 18% then I'm over 375HP at the crank?? doesn't sound like there's really 18% loss if you ask me......or why would GM advertise 350HP vette??
Le Mans Master
Joined: Sep 2004
Posts: 7,027
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From: Shiloh Ohio
Originally Posted by JoshVette
I don't know, but I had my stock.....very stock 02 A4 coupe 3.15's dynoed and came out at 308.6RWHP and 321.1RWT???
If there's 18% then I'm over 375HP at the crank?? doesn't sound like there's really 18% loss if you ask me......or why would GM advertise 350HP vette??
If there's 18% then I'm over 375HP at the crank?? doesn't sound like there's really 18% loss if you ask me......or why would GM advertise 350HP vette??
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Heel & Toe
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Transmission efficiency is defined as the ratio of output shaft power to input shaft power. 85 percent (i.e. 15 percent loss) is very inefficient for a manual transmission but not too far off for total driveline loss from the crankshaft to the road wheels (something like 10 or 12 percent loss might be more realistic). The efficiency of the transmission alone should be around 93 to 95 percent. The other losses are the propshaft bearings, final drive assembly, axle bearings, maybe some brake drag and tire flex. Also keep in mind that it's difficult and expensive to measure these things accurately so don't take the numbers too seriously, especially data from an inertia dynamometer. Steady-state measurement is the best but it requires a more expensive dyno.
Le Mans Master
Joined: Jan 2005
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From: Dallas/Grand Prairie Texas
Originally Posted by shurite44
If you have no power adding mods I would say that is a one in a million C5. Just from following the forum what I have seen with A4's is 287-299. Oh well be happy.
I made sure I changed my gear fluid, trani fluid, oil, new air filter and had everything up to speed so I could get my best #'s as close to "like new" as possible. I'd bet most don't do that though?
....I still don't think it's 18% loss either.....I need to take mine to the 1/4 track and see if I have a freak or what, but it's getting real hot out, performance will be less.
Safety Car
Joined: Oct 2000
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From: Monticello MN
Originally Posted by JoshVette
I've heard of quite a few actually with around 305-310HP stock A4. But most of them are 04's with little miles.
I made sure I changed my gear fluid, trani fluid, oil, new air filter and had everything up to speed so I could get my best #'s as close to "like new" as possible. I'd bet most don't do that though?
....I still don't think it's 18% loss either.....I need to take mine to the 1/4 track and see if I have a freak or what, but it's getting real hot out, performance will be less.
I made sure I changed my gear fluid, trani fluid, oil, new air filter and had everything up to speed so I could get my best #'s as close to "like new" as possible. I'd bet most don't do that though?
....I still don't think it's 18% loss either.....I need to take mine to the 1/4 track and see if I have a freak or what, but it's getting real hot out, performance will be less.
Josh, were your dyno numbers SAE corrected and smoothed? If not, you will see power and torque numbers (higher than actual) not corrected for atmospheric conditions and your peak numbers will be spikes, rather than the smoothed peak. Personally, SAE corrected and smoothed numbers are the only ones I compare anything to. Dyno numbers give ballpark results and a base to look at when adding mods (if they're on the same machine). Just my personal opinion.
Ed
Racer
Joined: Jun 2004
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From: Brookfield WI
Originally Posted by JoshVette
I've heard of quite a few actually with around 305-310HP stock A4. But most of them are 04's with little miles.
BTW, my car had 2,600 miles on it at the time of this original dyno. I brought it to DTE for a cam package and Stage II differential.
Last edited by ChipN; Jul 9, 2005 at 12:17 AM. Reason: Added comment about mileage
Melting Slicks
Joined: Mar 2005
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From: Columbia, SC
My 2000 FRC (MM6) with a 2004 crate LS1 replacement eng with Z06 airbox, K&N, 2000 (smaller 78mm) MAF & TB, stock 26# injectors, LS6 intake, and full '03 Z06 exhaust (cast headers, H pipe, Ti cat backs) dynoed at 324 RWHP/338 RWTQ at A&A. After 5 hours of work, Charlie was able to get it to 330 RWHP/344 RWTQ. Engine had 900 miles on it.