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The mkiv Supra Owners Club

Does anyone truely know transmission loses?


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I attendend a rolling road day at Janspeed in Salisbury on Saturday with a few members on the Lexus owners club,My car came back at just over 337 brake at the wheels which I was happy about as the last time it went on the rollers it was 320,I was even more happy when they said that my car was the most powerful they have had on thier rollers ever as they have only had them in for 3 months,Anyway that was rather short lived as Dangerous Brain turned up and royally pissed on my bonfire and put in a nice respectable figure with comparitively little mods,so at least there is 2 supras at the top of the pile there anyway!

 

Thier claim was that normal transmission loses are at 30% which I thought was way too high as that meant that my car having only a lighter flywheel since last RR has increased by 60 odd BHP,The first time on the rollers at (please dont boo me) Interpro in Bristol came out at 320 at the wheels and 385 at the flywheel,so with the output at 337 at the wheels what would be the true flywheel output?

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Interpro in Bristol came out at 320 at the wheels and 385 at the flywheel,so with the output at 337 at the wheels what would be the true flywheel output?

You're talking about power at the wheels, right? Not at the hubs?

 

320 RWHP and 385 Flywheel means about 17% losses.

 

If you apply the same percentage loss to 337RWHP it works out to about 406 at the Flywheel.

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I've heard it doesn't work out in a percentage.

For example if you had a 100 bhp car and lost 20% its so much, but if you had 800 bhp and lost 20% its going to be sh1t loads more.

Your transmission loses a set amount i.e. 45 bhp and 55lbft no matter what power the car is.

Why would it lose more in one case than another?

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I've heard it doesn't work out in a percentage.

For example if you had a 100 bhp car and lost 20% its so much, but if you had 800 bhp and lost 20% its going to be sh1t loads more.

Your transmission loses a set amount i.e. 45 bhp and 55lbft no matter what power the car is.

Why would it lose more in one case than another?

 

It's a question that's been asked a number of times on here. A while back, I found a forum called 'engineer's edge'. Out of interest, here's the question and the answer that were offered there:

 

 

 

 

When figuring drive-line losses are they a fixed amount or are they on a sliding scale? For instance A 215 HP rated engine register's about 176 rear wheel HP on a dynamometer. The drive-train losses can be assumed to be about 18% or 38 HP. If one were to increase the original 215 HP by 70 HP to 285 HP, shouldn't the dynomometer now read 246 HP? or should it read 246 HP less 18%? I would think that the drive-line losses are fixed, 38 HP in this example. Of course I am assuming that the torque and HP were developed at the same RPM and on the same dyno with the same conditions. Please explain if I am in error.

 

The answer is neither of your answers may be correct. You have to look at what is causing the energy loss. Part of the loss is due to moving the lubrication fluids around. The energy to move the fluid is not a function of power through the system. Part of the loss is due to frictional forces. Frictional forces are proportional to the transmitted force, which is proportional to the transmitted power. So friction forces will be a percentage of transmitted power and fluid movement forces will be fixed. And there are lots of other factors making up your losses.

 

So, the 285HP motor may not lose 18% but is will certainly lose more than 38HP. For the exact same drivetrain at the same rpm. (experience tells me fixed losses are small compared to proportional losses so the practical answer is you lose 18%)

 

Cliff

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Everyone gets that calculation wrong! Wasn't there a mahoosive thread about it including about 4 pages of back and forth arguing over the maths involved! :p

 

20% off 100 is 80

80 + 20% is NOT 100!!

 

 

Ahem, some of us would rather leave that casket buried :blush: :read:

 

Its a complicated subject, made worse by there being no true 100% answer without removing and running the engine properly. Regarding fixed/percentage losses the Puma Racing calculation seems to make sense for an estimate:

 

RWHP + 10 (small fixed loss) /.88 (for RWD, .9 for FWD) allowing in effect 12% for percentage losses.

So using that calc for 2 cars one extreme to the other, a car with only 100 RWHP would end up with approx 20% losses but a car with say 1000 RWHP would have approx 13% ish - if I've worked that out correctly without going to the hundredth decimal point :p

 

Does that make sense to people here?

 

:Popcorn:

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I remember at surrey rolling road Class One put his stock twin turbo on the rollers, it came out almost bang on 320 at the fly if i remember correctly?

 

Now surrey RR quotes rear wheel HP, so whatever percentage they're using for losses can't be too far off?

 

I don't remember what they use though! :drown:

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Losses vary, as you put more torque through a diff and gearbox their losses increase, especially when you start to go OVEr the makers recommended figures, as the gear sets try to move apart and frictional losses increase substantially. Losses through tyres vary enormously. A rolling road, IMO, is a comaprator of performance, on car X, on day X, at temp X, at humidity X, within as short a time interval of measurements as possible. Hub driven "rolling roads" are a lot better, an engine dyno is in a different league to either, IMO.

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I remember at surrey rolling road Class One put his stock twin turbo on the rollers, it came out almost bang on 320 at the fly if i remember correctly?

 

Now surrey RR quotes rear wheel HP, so whatever percentage they're using for losses can't be too far off?

 

I don't remember what they use though! :drown:

 

19% for manuals, 24% for autos (for Supras).

 

Mine made 250 at the wheels at SRR.

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