Roygarth

<a href="http://dyno.scoobynet.co.uk/PEindex/jap.htm" target="_blank">http://dyno.scoobynet.co.uk/PEindex/jap.htm</a>

Saw this on another thread and was astonished in a 'not that I know much about this sort of thing' way!

What are the BHP loss figures for the 964 C2 & C4 between flywheel(?) and wheels?

Thanks!
Piers
1990 C4 Coupe

Christer

A figure that is bandied about is around 12-15% for the C2 - no idea on the C4.

Wil R. Ferch

This sort of question really amazes me..no offense intended as I run afoul of this myself...but if you find out what's being produced at the rear wheels...what (really) difference does it make what it makes at the crank other than bragging rights?? The *real* issue is ( always has been) how much hp gets to the ground...no?
---Wil Ferch ( lurker over from 3.2 list).

scotsman60

The figures are measured at the wheels and then the flywheel numbers are calculated. On average the losses between flywheel and wheels are in the high 20 to low 30 percent region which does seem very high. The losses are calculated by the dyno software - no idea what the basis is.

It's interesting because most Porsche owners use a 15% loss figure to get to their flywheel power from the power measured at the wheels. I've no idea where the 15% number came from but it's almost half the power losses that these guys are assuming.

Doug

Roygarth

</font><blockquote><font size="1" face="Verdana,Tahoma,Arial,Helvetica,Geneva">quote:</font><hr /><font size="2" face="Verdana,Tahoma,Arial,Helvetica,Geneva">Originally posted by Wil R. Ferch:
<strong> The *real* issue is ( always has been) how much hp gets to the ground...no?
---Wil Ferch ( lurker over from 3.2 list).</strong></font><hr /></blockquote><font size="2" face="Verdana,Tahoma,Arial,Helvetica,Geneva">...er, Will, that's why I asked the bleedin' question! Do you know the answer?

Yours confused
Piers

scotsman60

Piers,

I think what Wil is saying is that the wheel horsepower is what's measured and is what matters as far as performance is concerned. The flywheel horsepower is derived and is generally less important for most people.

Doug

tonytaylor

Piers,

423 posts and you`re asking the difference between rwhp and fwhp - shame on you

Roygarth

Thanks Christer for the answer re. the C2, does anyone know the answer for the C4?

The reason I asked the question is that I have always assumed that the straight line C2 performance advantage over the C4 is due to the increased weight of the C4 (100Kg according to the handbook.) But I am now led to believe that 'in general' power loss is greater on a 4WD than a 2WD vehicle.

So if C2 and C4 are starting with 250 FWHP but end up with different RWHP (is it called F&RWHP in a C4?) then one could assume the performance differential is due to a combination of increase weight and drivetrain power loss in the C4.

Ref.Wil's 'bragging rights' issues. If I spouted the above in my local pub the landlord would strangle me with the drawstrings from my high performance anorak!

Thanks!
Piers
1990 C4

914und993

I seem to remember the suggestion that the C4 loses an additional 5% over the C2, and that would translate into about a 2-3 mph difference in top speed, according to the old DOS program Cartest.

It wouldn't surprise me if when these cars were built the engines off the assembly line with the slightly higher bench dyno test figures were put into the C4 models, to 'even' things up a little. Anyone ever hear of anything like that?

Chip

PorschePhD

You can do a roll back loss calculation on most dynos. Every 911 I have ever dynoed has shown around 15%. Usually less, more like 12-13. The AWDs will not really see more than 15. My twin recently dynoed 509HP and 589TQ. On the roll back loss it pulled 15%. At these numbers this is 598HP and 692 using a .85 number. Don't let anyone tell you 20-30% on these cars. Simply not true <img border="0" alt="[nono]" title="" src="graemlins/nono.gif" />

Drew_K

I remember that people on this board have dyno'd their cars and reported rwhp of 209. That's almost exactly a 15% loss relative to the 250 (247?) hp reported by Porsche.

Christer

I had mine dynoed recently and started with 207rwhp which I was told was a little low. The figure which is normal for a fresh motor lies somewhere between 212-215rwhp on a C2. I guess there must be more losses in the C4 drivetrain, but unless you swap the same engine into both models and put them on a dyno , it is all guesswork.

Piers, I would say if you are around the 205-210rwhp mark then you don't have anything to fret about. And even if you are lower, there is no point worrying unless you have something obvious to worry about - such as oil leaks or high oil comsumption.