Porsche Enthusiasts Club Forum

The factory front silencer on my '84 8v has a rather interesting looking cone-shaped piece in it. I assumed it was some kind of resonator fitted both to optimise NVH and help lift the torque curve. Do the aftermarket versions have them...?






The by-pass manifold could be interesting, if you could get it working more for than against..

My understanding was that the 968 inlet developments (bypass manifold, tract length, valve lift and closing timing, valve and port Ø etc.) were all optimised for torque in the higher rpm range 4000-6500rpm. VarioCam was then used to supplement and boost the mid-range volumetric efficiency by correcting the IVC point and encouraging scavenging.

Do you have any pictures of the part of your exhaust you mention?

Porsche tried several different variations of exhaust on the 944 from 1982 through to the last of the s2 and turbo off the production line, often looking to improve performance, but without improving peak power, often to fill troughs in the torque curve.

By the time they got round to intakes on the 968, they were well ahead of the game, with the 993 varioram, they were leading the world in intake and exhaust design.

With the S2, it is clear there are gains to be made, especially with the cat equipped cars, but not how many people would think. I found it interesting but not shocking that a resonator as replacement for a cat would be better than a straight through pipe or cat bypass silencer that many people would automatically opt for.

If you believe everything you read on rennlist the best exhaust on a turbo is a drainpipe, I fitted a straight through pipe and besides sounding like a tractor it dropped bottom end, turbo sounded nice whistling through it though .

Hi Jon - do you know why a resonator is better over straight pipe ? I wanted a bit more grunt out of the exhaust, and was considering ditching the cat ....

Its all to do with pressure waves and how they reflect and react within the exhaust. A resonator at exactly the right position with the right volume causes additional beneficial pressure waves within the exhaust and produce an extraction effect.

In a non turbo car the effect will cause a pulling force which will help draw the exhaust gasses from the engine, which reduce losses or improve efficiency (lots of power is sapped by quickly pushing the exhaust gasses out normally)

On a turbo charged car, the result will be beneficial pressure waves which will reduce the pressure at the turbo discharge and so increase the relative (to discharge) pressure of exhaust gasses entering the turbo, causing a faster spool up and improved efficiency. Which will in turn lower the absolute pre turbo exhaust pressure and reducing losses on the exhaust stroke a little like the non turbo (but to a lesser effect)

Hi Jon
very interesting stuff,
keep working at it, sounds promising for the race car (when you get it, still wating for AMOC)

Dave

Interesting, following on from my previous post I'd ditched the first silencer and fitted the twin silencer setup off WUF, as it was too loud I made up some baffles and the bottom end came back, suprising that a change that far from the exhaust valve made such a difference. Re bike exhaust system valves their usual purpose is to reduce noise at the point noise testing is done, I've removed them on my last 2 bikes with no loss of performance.

Is this WUF as in Ricks old car?

The science of exhaust system design is a very mixed up subject with in recent years the world just going for bore rather then design.

Back in the 60's and 70's it was a different story, with the mini tuning world getting absolutely amazing gains from LCB Manifolds and tuned length exhausts. Yes it was partly down to the siamese designed exhaust ports for number 2 and 3 cylinders, but a good example of tuned lengths being important for exhaust pulse tuning, harmonics as well as a complete other subject of lining up pulses as different cylinders converge in the collectors.

I should also start a bit of a 944 turbo thread as there seems to be the interest, and possibly a thermal management thread, as previous posts from some members have shown that they are confused about managing heat for performance.

Yes I have the twin silencer exhaust off Rick's WUF, I drove the car when Ben had it and thought about buying it but decide to stick with mine. Exhaust technology has always been something of a black art, considering valve overlap, anti reversion, scavenging,shock waves, expanded/reduced pipe, unequal length headers etc, think I remember from the days of CCC magazine someone experimented with exhausts on the original RX7 and the optimum length was something like 27 ft .

Ahhh the RX7 Wankel engine... Witchcraft I tell you, Witchcraft!!!

The good thing is that with a 944 there is nearly always an exhaust valve on one of the cylinders open, so with the resonator at the right place and just right volume, and the tailpipe which is at a fixed distance, the beneficial negative pulses and the hamonics of them caused by the two expansion areas will improve performance across the rev range.

Volumes are very important as well, as the wrong volume will hurt power, so a little like radio mast lengths, the distance from the exhaust valves and the volumes need to be the correct ratio of one another as well as the expected volume of gasflow per second.

So to ask the million dollar question.......

Can you produce an exhaust that will give a proven and noticable improvement to performance for a 944s2?

Just to throw an easier one out there is the jury still out on chips? And what about intake side? I'm quite interested in the intake side especially improvements to the manifold and AFM/MAF conversion options.

At the moment I can provide a cat bypass section, which can also be installed onto cars with a factory decat. So far it is looking like we can do them for £160 each.

All dyno runs has shown the chips to provide the same results as seen before, which is curious as even I am surprised by the gains, which were a little higher than expected, so I did not want to publish the results until I was sure.

The thing is, one car will respond better to a form of modification than others might, and to be honest, as this car was down on factory figures anyway, I was not expecting the gains I got. So I have been checking the car for any abnormalities which might have given it unreasonable benefits from the chips we used.

I could understand if this was the test car we had developed the chips on, as that would point to our development work being too closely honed to this particular car, but the car used for testing the chips on is one of my other S2's currently in the process of having its sills replaced.. So I am tempted to wait for that car to come back and double check the results.

Its funny you should mention MAF kits, we are working on this for the 944 S2 and 944 Turbo (2.5, 2.8 and 3.0), as well as may release one for the 1987 onwards 944 2.5, 2.7 and 2.5S

Are the resulting figures higher then the extra bhp factory specials? 225bhp? S2

I wonder if the 225 bhp was a factory figure or Porsche GB's marketing to shift the last few S2's.

It is difficult illustrate with a photograph, as the perspective is all out, but you can see the complexity of the forms from looking down the up-stream end of the pipe. I love this kind of detail - A helmholtz engineer somewhere has spent some time on this.

When I get a spare minute, I will try to produce a scale drawing of what I can see inside.