G-50 transaxle power loss/

Dwight

RCR GT 40 Gulf Livery 347 Eight Stack injection
Supporter
I know in a Cobra we have a 15% to 20% horsepower / torque loss though the drivetrain and side pipes. Cobra side pipes are a big power loser.

My question is how much loss can I expect with a G50 in a GT40?

Dwight
 

Howard Jones

Supporter
The only way to know this is to run a car on a chassis dyno, then take a engine out of a car, put it on a engine dyno, and record the power output. But then you have another problem. Both dynos have been calibrated to different specifications with no COMMON reference. I have never seen an example of this being done. The car manufacturers may do this but nobody else would spend the money to do it aside from professional race teams like NASCAR for example. Those guys would never divulge that information.

Dynos are really a tuning tool that is used to compare output before and after tuning adjustments.

Considering the huge difference in chassis dyno power results between dynos I really have no idea how much difference there would be. But my completely wild guess is about 50 HP. I think it would be a fixed power loss for a given car, not a percentage of crankshaft power. Ya I know it can be calculated but I also don't believe that if you doubled the engine power output the losses would also double if you left everything as before the power increase.

My GT40 has a 345-50HP 302/R21 Renault in it and it made 296HP on a chassis dyno. My SLC has a 450hp 350 SBC G50 Porsche in it. It feels like it has about another 100Hp than my GT40 has.

If you Google this question...............well it's the internet............numbers are all over the map. 50-200HP and 2-25%. By the way, do you really think a 10,000 HP top fuel car has 20% losses in the power train? That's 2000HP or well over a MEGAWATT ! If you sink that much heat into the few quarts of oil in the differential it would flash into fire and the ring and pinion would fuse together. It can't be that much.
 
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Terry Oxandale

Skinny Man
Nice post Howard. So I'm sure there are loads that are a percentage of power output (supercharger for example?), but your thoughts on transaxle (drive train) power loss are interesting, so perhaps the reference of percentage is more based on RPM (higher HP generally associated with higher RPM). And even then, the assumption is linear, which I highly doubt. It appears there are several attempts on the web to distill this to the most easily understood value, without putting too much thought into what that value means.
 

Dwight

RCR GT 40 Gulf Livery 347 Eight Stack injection
Supporter
Keith Craft told me he dyno several motors and then installed them (it) into Cobra. Then chassis dyno it. He typical saw a 15% loss.
I would like to know if a transaxle is more efficient than a 5 speed.
 

Terry Oxandale

Skinny Man
I would imagine the only difference between the two is insertion of any losses from the driveshaft/angles of U-joints, etc. Outside of that, I don't know if there is any difference in that everything else would be pretty similar (gear/oil/friction loses). Using the common representation of "percentage", I would think maybe a point or two at best between the same engine in front of a T-5 vs ZF, with the transaxle getting the benefit. I would also think the additional mass of the split system (transmission/rear differential) would play more significantly in "losses" than comparing the efficiency between the two systems.
 

Howard Jones

Supporter
What I am trying to say is this: the power losses are the friction losses internal to the transaxle. These friction losses are more or less fixed regardless of the power input. If it is indeed about 50HP then it's 50Hp if you plug a 500HP motor or a 200hp motor into it. Now as the heat builds in the pieces that are rubbing against each other losses will increase but I don't believe it would be very much if everything is lubricated correctly.

Where average plays into this is gear speed inside the gearbox. So in this, I agree with Terry. It would be an average of friction losses over the rev range of the gearbox. This average loss would be the same regardless of power input but affected by input shaft speed over time.
Sort of a duty cycle loss average if you get my drift.

So that's my story and I'm sticking to it.
 
I have had quite a lot of experience with 911s on a chassis dyno and the G50 5 speed gearbox losses were normally about 33 to 35 bhp or about 11 to 12%. That was measured as negative horsepower on an inertia type dyno. There is both a fixed and a proportional component to the losses of which the proportional element is often overstated, I suspect, to excuse poor engine performance viz the Euro version of the Dodge Viper which had somehow lost 150 bhp through its transmission (it was the Euro spec exhaust system).
 
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