I got asked a question on this and when I thought about it for a few moments I was unsure of what the answer was. We were talking about converting RWHP to crank HP. As we did we talked about the conversion factor. It being commonly anywhere from 15% to 17.5% depending on the type of transmission the car had. The low number for manual boxes and the high one for automatics. My basicly stock except for the exhaust 1980 Euro S showed 263 hp at the rear wheels. Using the 15% conversion factor that means it generates 309 hp at the crank. (263/.85 = 309.41). Did Porshe under rate these cars? Meaning that the driveline mechanicals absorb 46 hp. on my car. So if I decided to upgrade to headers or make any mod that added HP why wouldn't the driveline hp loss still be 46 hp? After all nothing has changed in the driveline. Using the conversion factor at a constant percentage would seem to understate hp and the understatement increases as the hp does? Am I missing something here?

VT928

1980 Euros Red/Blk Lea

 

VT the rule of thumb is just that and most thumbs vary significantly. And you are correct that the assumption that drag and mechanical losses occur in a linear fashion constant 15%- 20% is flawed. But nearly everyone doing dyno runs and performance "upgrades" will measure the additional power at the rear wheels and ADD in the extra 15-20% to state that crank horsepower is now that much higher. So in your example if you add 30 hp at the rear wheels some would say you added 35 hp at the crank. The result is typically an overstatement of what the crank horspower is based on added RWHP then adding the constant "correction factor". Things a simple as changing the air pressure in the tires can change the "horsepower" . Temperature barometric pressure altitude .........

 

What's your exhaust setup, VT?

 

The muffleris a MagnaFlow dual inlet/dual outlet with a flow path like an X-pipe. It is 20" long including the inlet/outlet extensions. My exhaust comes off the factory manifolds and goes into the inlet side of the muffler. The muffler is positioned where the factory placed the front muffler on Euro models or the Cat on US models. From the outlet side of the muffler twin pipes run back following the factoury routing locations. There is no center muffler or rear muffler in my system. Just twin pipes with gentle bends and a only one weld joint per pipe. The fit is close under the rear crossmeber but does not rattle ot make contact. I reccomend thet the pipes have a fes cross braces welded to them to keep them constant in location to each other. I makes hanging them a snap and really cuts down on vibration. the cange in performance over stock is very noticable. It revs much quicker and the sound is an angry snarl at 6k rpm. I have to be much more attentive to the revs way to easy to hit the limiter @ 63 rpm. The whole system cost me less than 350.00 dollars

VT 928 1980 EuroS Red/Blk Lea

 

give you an actual idea of the exact losses we are talking about, I saw 20hp loss at 6000rpm on a coast down (car in neutral) and from 4000rpm equivilant speeds, it was 10hp. (140mph to 70mph is)
now, this is just the rolling friction and some friction in the drive bearings. 20hp as a cost on a 295 rear wheel hp , would put 7% as a loss. Now, count the driveline friction losses and its probably good for another 7%. of course, you can see that the rolling friction losses are not going to change with a set of headers or even a stroker 6.5 liter. so, there is no linear relationship, but what you can estimate, is about 14% loss for our cars (most round up to 15%) and this is for the high speeds . bring the speeds down to speedlimits, and the looses are probably in the 10% range if i was to guess.

in the industrial gear box world, a single stage gear box gets about 90%, and a 3 stage gear box gets more like 70% eff. so, a lot depends on the design of the gear box, number of stages, types of gears, materials, clearances, fluids, etc.

bottom line, take the 15% loss number as thats what everyone generally uses.

mk

 

VT - can you shoot some pictures of your exhaust setup and post them for us?

Why are crank HP numbers important? I often wonder why anyone is concerned with the power their engine is making, since they can't use that power without that darn power-stealing drivetrain.

Crank HP numbers are virtually always guesstimates, since very few of us go to the trouble of putting our engines on engine dynos.

Isn't the HP number that matters (if any HP number matters) the one that you measure where the rubber meets the road?

 

Sorry I don't have a photo of the exhaust, and since the car is in storage at the fairgrounds it will be April till I see it again. The Vermont winter is here all cold and snow.

 

Crank numbers matter to the manufacturers since it is something they can compare each other against and because us poor shmoes without engine dynos cannot verify. Wasn't there a little Mustang Cobra incident a few years back? I hear some M5 owners were looking deeply into this as well...

 

VT... sorry you're "separated" for another 4 months.

Be sure you're not comparing euro apples to
USA oranges- - the HP units could be DIN or SAE(~2% diff.).
But, yes, where rubber hits the road is the biggy.

G'luck.

 

Understanding Horsepower loss through drive train