3.4L powertrain efficiency loss - 15%?

Anyone know where the 15% number comes from with loss of power through the powertrain to the rear wheels? Is this an empirical number? Why isn't it 14.7% or 15.3%? If empirical, then the 15% has an accuracy of +- 10% at best because of the varying dynos. Usually 20% numbers are used ala S2000 , is the lesser number for Porsche due to the fact that the engine is so close to the rear wheels? There should be a definitive documented answer somewhere but the search on CC is resistant to "15%" for several reasons.

I don't know where the 15% comes from. I just have used it for years. Everyone I know that tunes cars or races. Uses 15% drivetrain loss for manuals and 20% loss for automatics for most all makes and models of cars. It is just a rule of thumb, not an exact number. You have to actually dyno the engine on a bench dyno to get the exact hp and torque the engine is producing.:2cents:

I remember seeing an earlier post mentioning an 11 percent loss for a CS and thought it was a low number. This latest thread shows the drivetrain loss, for what there is of a drivetrain, at 10 - 11 percent too. http://www.planet-9.com/cayman-tech/13989-dyno-jet.html

Also, if you look at K-man S' hp numbers in his post and picture. He claims 366 FWHP and 311 RWHP. He is using a 15% drivetrain loss on his CS numbers.

Stock 2006 Porsche Cayman S Stock w/ Factory Air-Restrictor Dyno Sheet Details - DragTimes.com According to what he posted above, K-man S' car is 269 rwhp stock...that’s an 8.8 percent drive train loss on the Mustang dyno. His latest numbers came off the same dyno so his 311 rwhp should be 341 hp at the crank. All the dynos sheets on dragtimes are his so you can follow the gains.

Only if the actual flywheel HP is 295HP and the Mustang dyno was spot on. The FWHP wasn't directly measured, and I suspect Ken's FWHP after break in (when that Mustang dyno run occured) was significantly greater than 295, possibly over 300, and that the Mustang was a little optimistic. If we assume FWHP of 300 and 4HP of optimism in the Mustang, we're at 12% drivetrain loss, which sounds more likely. This would mean Ken's actual FWHP right now is 353HP. Or maybe Ken's drivetrain has flubber in it .

It would be ideal if manufacturers posted rwhp and never mentioned fwhp, which is meaningless. But since they only publish fwhp we have to convert back to it from rwhp (which we can measure cheaply) to compare cars. That being the case, you should assume 15% for all cars for purposes of comparison. This gives an EFFECTIVE fwhp to compare. Over the population of cars, that's probably accurate. If one car actually has only 12% losses, that's equivalent to having more fwhp anyway. Mine ran 277 stock on a mustang dyno, or about 326 effective fwhp given 15% losses.

My FWHP is actually infinite, but since that allows me to travel anywhere in the space/time continuum any time I want, I try to keep it to something more reasonable for daily driving. I used a 15% estimate that's true, but I have no idea if it is accurate or not. In fact I would say it probably isn't, most 997 C2S's dyno at 305-310 hp on the IA mustang dyno and Porsche says they have 355 stock, if that's true then they are losing 13-14%, if Porsche is a little conservative and the FWHP is actually 360 or 365 then yeah the loss is around 15%. As someone said, only one way to know and that is to bench dyno the engine and compare to the wheels number to get the drivetrain loss.

:naughty:I have heard that the 2.7 is 100 percent efficient.

The half dozen or so recently posted stock CS dyno runs all seem to be around 263 give or take a couple of HP or a 10-11% loss from Porsche 's published 295.

The reason this fuzzy estimate drives my OCD crazy is that how do you measure performance of a drivetrain sans engine? Is there any particular advantage with having essentially eliminated a long drive shaft and not having to turn the powered wheels as a front wheel drive? So, who from Porshe can (or would they?) give us a definitive engineered, no guesstimates, answer? We should have a Porsche customer rep dedicated to CaymanClub.

Even "bench testing" a motor on an engine dyno can leave engineers (& manufacturers) with wiggle room in coming up with a published power number for an engine, much like EPA estimated vs. real world fuel mileage figures are calculated and, I think "fudged" for sales purposes. Back in "the days" of bhp races between the various American manufacturers, the SAE standards for setting and measuring horsepower had specifications for if the numbers should be published "as the engine would be fitted to the car" i.e., with all of the accessory drives, alternator, power steering pump etc. fitted or as a bare engine, devoid of any item, including perhaps even the clutch or flywheel, that would affect spin up, ergo horsepower. That standard , the SAE specification for measuring horsepower, continues to be revised from time to time Depending on the dyno you use, the software can have a "tcf" setting (aka "Transmission Correction Factor") into which you plug your estimate of driveline losses. Over time 15% has been used as a general rule of thumb (aka "ROT"); 20% is also often used for AWD vehicles. Depending on how "accurate" one believes their Kentucky windage adjustment abilities are, some dyno bench racers will also throw out refined adjustment guess based on whether the engine is longitudealy vs. transversely mounted in the car etc., etc.

You can also do a reverse pull on some dynos to ascertain loss. Not done in reverse gear, but is run up to redline then off the gas and the dyno measures the time against the rate of loss. The dynojet I use can do this, not sure about mustangs.

Forty years (or more) ago people assumed a 15% drive-train loss. I'd hope they'd be more efficient by now. Soon after the CS launched in '06, I read from two sources that the drive-train loss for both the 987 and 997 was about 11%. As Dan said above, an 11% loss would put those frequent 263 rwhp dyno estimates right around Porsche 's advertised 295 fwhp.

The Sun dyno at Emerald (Norfolk, UK, Rolling Road - Emerald M3D.com homepage ) that I've used in the past does a coast down run from peak RPM all the way down to idle to measure transmission losses. It then uses these measurements to derive the flywheel HP. They seem to give peak power figures remarkably close to manufacturers quoted power. Not had my CS on their rollers though...