paul r

Hi,
Recently had my 86 951 reprogrammed on a rolling road dynamometer. It made 246bhp at the wheels. Can anyone tell me what percentage transmission losses there are on this car so that I can calculate the flywheel power. Note my car has a limited slip differential.

At the end of the full throttle acceleration run the reprogrammer lifted off the gas and let the car coast down. He said that the power curve resulting from the coast down period was in fact the transmission loss curve. Can anyone comment on this statement also. If it is true would adding this loss be a more accurate than the percentage loss method.

Finally my car had completed about six full throttle runs in a space of about twenty minutes. Had the car been given longer to cool down before the final run would the power have been greater, if so by about how much?

Thanks Paul R.

Mike B

I've been told 20 to 25 % which would put you around 295 to 307 at the flywheel...Of course those loss figures are subjective and depend on a host of factors...What modifications have you made...

adrial

I've been told 10% and have seen numbers to back it up...
It was someone on here that has a 500HP John Milledge 951...dyno'ed the engine then dyno'ed the car...was a 10% difference...

--Adrial

Danno

Rather than going by what "I've been told", how about looking at the empirical data yourself. Check out Farzaan's dyno-chart comparisons at: Farzaan's Dyno Day

Look under Paul's '89 944 Turbo at the "Stock chips @ 11.2psi (stock boost)" chart. Well, I'll just put it here:


Knowing that the stock '89 Turbo makes 247hp, we can compute the drivetrain loss:

(247 -212.5) / 247 * 100 = 14%

For a non-S turbo, I've got a chart on my website:[u]951 RacerX website[u] and look at the chart for Tony Garcia's car vs. stock '86 & stock '89. Here's the chart:

In this case, the non-S turbo makes 180.6hp at the rear wheels. Knowing non-S Turbo has 217hp, computing drivetrain loss we get:

(217-180.6) / 217 * 100 = 17%

Both of these are roughly centered around the 15% figure that "I've been told" is pretty much standard for most cars. I think any Porsche engineer that lets a car out of factory with a 20-25% drivetrain loss would probably get fired! . Anyway, assuming 15% loss, you can go back and forth like this:

rear-wheel HP = crankHP * 0.85
crankHP = rear-wheel HP / 0.85

EDIT: Fixed broken links & pictures.

Russ Murphy

Howdy all,
Great info Danno. I wonder if losses are static as a percentage of hp or change percentage wise under greater loads?
I trying to envision where all the heat would go(like the "lost" 75 hp of the 500hp Milledge engine). the most thorough discussion on the topic I've read is here: Dyno Thoughts and HP Losses

Danno

My take on frictional losses through the drivetrain is that it's linear. Comes from the friction equation f=Mu. Basically, the more load you put on a part (hp & tq), the more friction you'll have. You jack up the car and can take the 944 tranny and spin it by hand. You're putting a very, very light load on it (weight of wheels) and the frictional loss is very low. Now put the car down and try to put 500-lb*ft of torque through that gearbox and the frictional losses will increase.

Similar example with frictional losses is to drag a house brick across your garage floor. The load on that brick is its weight, its friction is defined by f=Mu and it'll take a certain amount of force to move it at 1m/sec across the floor. Put two more bricks on top of that first brick and you'll see the load triple, and the friction triples (f=Mu) and the force required to move it across at 1m/sec also increases.

In this house-brick example, nothing has changed in the friction-related interfaces. The same side of the brick is sliding across the floor at the same speed, yet friction has increased by three times because you have three times the load. Same thing with gearboxes and bearings. The mating surfaces are the same, but because they are grinding against each other harder, friction increases.

That's why you need a beefier gearbox for the Turbo cars (and even stronger for TurboS). Even though it's moving the same mass of a car, it's doing it quicker with more power. Thus the friction on those gears is higher. Higher friction means more heat and more wear. Thus you have the hardended 1st & 2nd gears in the TurboS tranny. Also anyone who've tried the folly of putting a non-Turbo tranny (like a 944S) into a Turbo car will tell you how long those things last under the load that a Turbo motor can apply.

So in summary, the frictional losses through the drivetrain is based upon how much force you are trying to transmit through it. A 500hp motor is definitely going to be putting a lot more force through the tranny, and provided the tires don't slip, the friction will increase roughly proportionally to the load.

Now this is completely independent from the kinds of speeds you can achieve, of course. Because the air-resistance goes up with the square of the speed and the power requirement to overcome that air-resistance goes up to the third power of speed. So at high-speeds, frictional losses are miminal compared to air drag.

Mike B

Ok, I certainly stand corrected...I'll go as low as 15 %...and not a penny more...Thanks Danno...great info

Danno

Mike, that's a good way to look at it though (being pessimistic). A lot of people under-estimate how much power they are actually losing through the drivetrain.

Following Russ's link, there's an article here that's pretty informative:
http://www.pumaracing.co.uk/power3.htm

Mike B

That's a good article...Thanks Danno

paul r

Hi,
I contacted the guy at Puma Racing and he ran his programme for the standard 220, 250 and my 290bhp 951's for 0-60mph times. His results were 5.9s, 5.5s and 4.9s respectively.
Anyone got actual experience of 0-60 times for their cars, and what mods have you done?

Also what do the manufactures state the accuracy of their chassis dyno's to be?

Paul R

fc-racer

Danno, great way to describe the losses, very well done. For 0-60 times, when I took my 944TS to the 1/4 mile track, I extrapolated from the timeslip and came up with 0-60 in a little over 5 seconds. My car made 300hp. 1/4 mile was 13.8 @ 104mph.

fc-racer

Andrew

I borrowed my buddy's Gtech and the best it gave me was a 4.89 then a 4.92 and 5.00 this is with minimal tire spin. it claims max of 250hp/wheels. my car is mostly stock but i have some chips and a NA tranny in there! this makes the biggest difference! Danno, remember when my brother and you ran them up on Hwy 9? That is just what the Gtech said though.

-Andrew

Robby

Not trying to argue here, but I've talked w/Jon Milledge, and he does contend that 951's lose less than 10%. There are several things to consider w/this... First off, the graphs you are showing (Danno) were, I assume, dyno'ed fairly recently (w/in the last few years?). I think it is fair to assume that MOST of our beloved 951's have lost at least a small percentage of their original power. Also, when people like Jon build these engines, they do much more than just add power- they usually use lighter clutches, flywheels, and other rotational parts. At any rate, I'm not sure what he was referring to (I didn't think about the variables until after the conversation)- whether or not he was referring to new, used, and/or rebuilt and modified engines, etc... I personally, have always assumed, based on what I've read and seen, that a well tuned 951 should lose close to 15%unless reciprocating parts were lightened. Again, this is my opinion, and I really don't have anything real to back it up with. I did dyno my Turbo S right after I bought it- it's compression and leakdowns were pretty good- it dynoed at 198 and 216. The TQ seemed ~5 or so low, and the power, ~10 - 15. I wasn't very impressed, and have had so many little nagging problems (and several large ones) that I haven't returned. I do plan to do it again, after I rebuild the engine and do several little mods (at least several ) I'm just trying to decide how far to go w/the rebuild vs how far in debt I want to (and can afford to) go... I'm having serious problems deciding what to do w/all of it, as I've heard many bad things about some of the 2.8L's and such, not performing any better than stock(!). I'm also not sure if I want to stick w/the stock engine management, but stand alone is a pretty expensive proposition...

Also, I've been told that different types of dynos will show different results, and I understand that Jon's dyno is different from the oft used Dynojet's- not sure that it's necessarily better, but different...

Danno

I think this gets into that debate on whether lightweight parts "fools" inertial dynos. On other dynos, lighter parts don't show up as higher HP. That's because the engine is putting out the same amount of force.

It's like a bar brawl where I punch you with my bare fist or with a 10-lb bottle of tequila. I both cases, I've put the SAME amount of power into my swing, but the extra weight of the bottle stores up energy I was expending before it hits the target.

So the idea of MOMENTUM comes into play. Inertial dynos can be fooled by the weight of parts.

Anyway, does Milledge have TWO dyno runs for his engines? First an engine dyno to measure raw crankshaft power? And a second rolling-road dyno with that SAME engine installed to measure drivetrain loss?

fc-racer

The stock hp and torque numbers as quoted by Danno above using my dyno sheets were on a fresh block and head, all purchased new from Porsche to the tune of $22,000! That is about as fresh as you're gonna get. The lines look great, the torque rolls off exactly how you'd expect a stock car to. If anything, I was a bit surprised to see the torque so high on a stock car, but I watched the boost gauge myself, there was no creep.

fc-racer