hopwood

Hi all,

I took my 928 back to my local dyno after having had the 86.5 manifolds fitted (thanks Rog). The car felt much better right at the top end - previous dyno run had power peaking at 287bhp (239rwhp) at 5250rpm so I knew the car had breathing difficulties at high revs. NB: The scan of the previous run is shown.

As soon as I can get my scanner working I will post up the new run. But the news is:

Power and torque are about 20ft/lb down below 4000rpm. Peak power is up to 300bhp, power at 5500rpm is up by 30bhp. Interesting to see the deficit at low-mid revs.

I am still running the original front centre box with single outlet, single pipe all the way back and original muffler (ie the original exhaust system from manifolds back; no cats). I believe that since Porsche switched to a dual design with the release of the 300hp S, that the exhaust is still restricting me and I'm hoping there is another 20-30bhp on the table to take me up to S4/GT power in the old bugger.

I have found a custom exhaust shop who are going to make me a new system - at the moment I'm leaning towards a Y-pipe as per design on Louie Ott's site, 3" single pipe with intermediate and final silencers. All opinions welcomed - I don't want to lose any more torque.

Cheers,

Joel

PS: This 928 is a 1979 with 1986 4.7 16v motor, 10.4:1 compression, original K Jet injection and Euro "S" intake manifold. S4 brakes and suspension. Stock appearance.

Dave928S

You'll find that Greg Gray, who posts here, did a lot of work on a twin 2 1/2 setup some time back .... but I can't find it right now. That's what I'm going for ... it's an opened up version of the original system fitted to euro S cars, with an x pipe, and the size is about right for the 86.5 manifolds that you've fitted. He's gone way further than that by fitting his custom extractors, which are a work of art, and is doing heaps of research on 16V mods.

I'll keep looking on my computer .. it's here somewhere.

Meanwhile ... do a search for some of Gregs posts on the issue and you'll find not only his comments, which are specific to 16V, but a few others who have done lots of work in that area as well.

Fabio421

I hate to be the bearer of bad news but 300BHP on a Euro S is nothing to brag about.

Rick Carter

Is this wheel horsepower or corrected flywheel horsepower?

hopwood

Thanks Dave. I'm intending to keep single pipe as I like the smoother sound. I'm hoping that Greg will comment on whether it should be 2.5" or 3".

Sorry Fabio, wasn't intending to brag, just share. This is a 32 year old 928 that I bought for £2.5k, I think the fact that it's putting out 300hp is worth sharing.

Rick, the 300 is corrected, I will post the wheel number when I put the new scan in.

Tough crowd.

Joel

hacker-pschorr

Don't let it get to you, we mean well

I just overlayed your dyno to mine, the torque / hp curve are identical. The upward torque curve at the same RPM, same for the peak.
The HP drop-off point is also the same.

I wonder what cams are in your engine.

blown 87

I would have thought that the 30 PSI of boost would have got you more than 300.

Lizard928

Yah, but hacker, your running a 4.5 with what 8.5-9:1 cr? Granted you now have LH.

I feel that there is more to be had from this engine for sure. An exhaust will help, but I would also be looking at a different injection system.

hacker-pschorr

Wow, looks who's up before the crack of noon today.

Yup, that's why I'm scratching my head. Scale this down a few HP it's an exact copy of mine. Other Euro engines I've seen had different curves, at least a little bit.

Smaller valves, way less aggressive cams (assuming he has Euro's) and lower compression ratio by over a point, similar exhaust........exact same power curves?

I'm not being critical of his car / engine, just curious.

toofast928

Nice number Joel. Engine is 32 years old and never been rebuilt. Open the exhaust and pump 5 psi then go back to the dyno!

Fabio421

Joel, I'm sorry if I came across as abrasive. That wasn't my intention. I was just commenting that you should have seen 300BHP before adding the 86.5 manifolds.

123quattro

When I put the Euro cams/heads on top of my US 4.7 nothing changed under 4000 rpm. It's just over 4000 rpm torque didn't drop away nearly as quickly. With heads/cams, headers, a Y and a full 3" exhaust mine did 271whp.

AO

271 with a correction factor of 15% yields 318.

I agree with previous comments that this thing should have been putting those numbers down in stock form. At least you know where you stand and can work from here.

z driver 88t

Just wondering if he's testing on a Dynojet or Mustang chassis dyno. Generally the Mustang dynos tend to read a little lower than Dynojet IIRC.

hopwood

Hey guys,

I think I explained myself badly. The scan is from PRE manifold. I will upload the new one in the next 24 hrs, the top end curve is very different and peak power is up to 300.4 crank hp. The differential between the graph shown and the post manifold graph at 6000rpm is 30crank hp.

Hacker, the cams in my engine are the most aggressive you can get for 16v (see attached table), a Euro S2.

So, from the top: the graph you see is what you get if you drop a 310bhp Euro S2 motor into an OB, feed it with K Jet and restrict it with the original single pipe exhaust system from a 4.5L 240bhp Euro. As you can see, the exhaust is completely throttling the top end.

The graph you are about to see is what happens when you add 86.5 manifolds to the mix - answer: you lose torque in the midrange and gain 30crank hp at the top end.

The graph you will see in a couple of weeks is what happens when you add a 3" exhaust to the mix.

Colin, one day you may see what happens with electronic injection too but not yet :-)

Joel