Dyno hype question

So I'm reading about potential tow vehicles,

With a the 60 mph mark rushing by in less than 4 seconds, the century in 8.6 and a 12.1 second quarter mile run, the EV 12 will thoroughly spank any current Ferrari production model as well as cars such as the Dodge Viper GTS and Shelby Series 1. The EV 12 started its life as a normal E320 before undergoing extensive makeovers, especially in the engine compartment, where Renntech squeezed in a 7.4 litre V12 equipped with titanium connecting rods, ported and polished cylinder heads, bigger valves, moer aggressive camshafts and modified intake manifolds. The result: 481 horsepower and 483 lb-ft of torque at the rear wheels or 620 horsepower at 5800 rpm and 625 lb-ft of torque at 4600 rpm (using an estimated 23% loss through the driveline) plus a very decent 53/47 front/rear weight balance. A heavy duty five-speed automatic (that's right!) transmission is hooked up to the powerplant and power is sent to the rear wheels through a Torsen limited-slip differential.

and it occurs to me: everyone views the dynamometer (or dynonometer, however the heck it's spelled) as a kind of final, scientific arbiter of an engine's performance. It's where all the hype is supposed to give way to hard numbers, right?

But no one, as far I can tell, agrees on what the difference should be between the hard numbers of horsepower at the wheels, and the number you actually tell guys when you're describing your real horsepower -- at the crank. The thing I was reading suggests a 23% loss in the drivetrain, and other sources I've read use a figure from as low as 8% to as high as 27%. Even in articles about Porsches (and the 915 transaxle in particular) I've seen a spread of 11% to 20%.

This makes for sloppy numbers. If my car is generating 235 hp at the wheels, then the range of numbers that I can use for the world as the 'hard and fast results of my dyno' could range from 264 hp (11% loss) to 276 hp (15% loss) to 294 hp (20% loss). Obviously, there's a lot of difference between 264 hp and 294 hp. Especially if you're trying to realistically gauge the effects of upgrades that are on other guys' cars.

Has anyone with a dyno, a scientific mind, (and maybe too much time on his hands), ever tested the real loss of a typical 915 transaxle? Couldn't accurate numbers for the different Porsche drivetrain configurations be collected and shared with dyno operators, or posted on the web?

More simply (for me), does anyone out there have a figure for the power consumed by the 72-73 transaxle?

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Jack Olsen
1973 911 T (3.6) sunroof coupe

Hi Jack,
When I dyno'd my car I was told by the owner of the test facility that the loss from flywheel to the rolling road is about 15% in Porsches. Another race mechanic told me 20%. Hope this helps.

Randy

Haven't tested a 915 box, but we test Hewland and Staffs gearboxes for Formula Fords and Formula Continentals. It is pretty simple. You drive the input shaft with an electric motor and measure the amperage. A "blueprinted" gearbox will have significantly less parasitic loss than a brand new stock gearbox.

Haven't tested a 915 box, but we test Hewland and Staffs gearboxes for Formula Fords and Formula Continentals. It is pretty simple. You drive the input shaft with an electric motor and measure the amperage. A "blueprinted" gearbox will have significantly less parasitic loss than a brand new stock gearbox.

Jack,very good subject,as you know i had my car dyno two weeks ago,and i had the same questions,at the dyno place they claim that is a 12% loss,then i show the results to my mechanic and they claim that is 16% so i still have my question.
In my case it dyno 276 at the rear wheels so what is at the flywheel? i was told that it 320?
Can Anyone come up with the proper numbers?
and on what are your numbers base on.
Regards Juan

This is EXACTLY why I have said that only rear wheel numbers matter. You can't run the quarter sitting on an engine!

The Porsche shops I have dealt with average 15% loss for the 915. NONE of them rely on crank horsepower. If it doesn't get to the road it doesn't matter.

Need to add this - the factories like to blurr the horsepower line by using different rating systems. Brake HP, DIN, SAE, etc. Based on the accessories the engine has, the smog equipment, etc. All that is bull**** to confuse you. What do you think the parasitic loss is for, say a motorcycle vs an all wheel drive smogged SUV?

[This message has been edited by RarlyL8 (edited 01-05-2001).]

I agree with Rarely... what is at the wheels is what matters. The loss from the crank to the ground will differ between the types of vehicles as well as, (but not as significant) between different vehicles of the same type. It's tough to guess what your loss is.

Well for that matter then what about aero losses? These are the biggest of them all and you're completely ignoring them! This is why everyone should just road dyno. But road dyno is no good because it gives low numbers. Engine builders love to overestimate the power output. For accurate road dyno all you need is accurate weight. It takes into account all losses.