200 Cell Cats

coullstar · 11 Oct 2018

Smoother and slightly higher pitch with the X-pipe.

Martin996RSR · 11 Oct 2018

Dammit said:
48mm ITB's fed from a GT3-RS plenum, driving the resonance flaps from Syvecs.

Crikey! That should really breath well. I was going to comment that raising the rev limit by itself would probably on hurt engine longevity without an upside, as I've seen plenty of dyno charts in stockish M96s where it's all over by 6.5k. With that induction set up and capacity increase you will probably be looking at GT3 engine power levels(as I'm sure you know).

Martin996RSR · 11 Oct 2018

Dammit, my top tip for the best all around exhaust is the After Hours style. It's probably loud and uncouth, but i would bet it's the least restrictive. Here is a video of a dyno comparison (done by the manufacturer so obviously to be wary of) https://www.youtube.com/watch?v=N3od84Ps3uU

It shows a torque advantage of just over 10 lb/ft.

The best exhaust would be a custom made one that had long collectors and then fed directly into an after hours style loop exhaust without any cat pipes running across the back of the car. This would maximise weight losses and minimise exhaust pipe losses. It probably wouldn't cost much more than a Cargraphic set up either.

Jamesx19 · 11 Oct 2018

Without back to back testing of manifolds or X-pipes Vs crossover pipes on the same engine, on the same Dyno, on the same day, it's impossible for anyone to be sure what works best.

I understand that scavenging the exhaust system is as much about low pressure waves creating a depression at the back of the exhaust valves (at a useable RPM) as it is about gas velocity and exhaust pulse's.

High end, very expensive computer software can get you close to the right dimensions of primary and secondary pipes but often as already stated by Dammit, the physical fit may cause compromise.

I suspect that as a general rule, based on nothing more than reading about the subject in some books by reputable people like David Vizard, longer primaries are probably better than short. Larger ID primaries will work better for higher reving engines. Too large and low end torque may suffer, but I bet it's not as much with modern fuel injection as it would have been in the past with carburettors.

All complete supposition of course as I don't have a Dyno of my own!

Martin996RSR · 11 Oct 2018

Jamesx19 said:
Without back to back testing of manifolds or X-pipes Vs crossover pipes on the same engine, on the same Dyno, on the same day, it's impossible for anyone to be sure what works best.

I respectfully disagree. Try this thought experiment: Take a straight pipe and test the flow rate. Now take a pipe of the same diameter and put a 180 degree bend in it with a five inch radius, and make the total length of that pipe another 10 inches longer. Do you think the second pipe will flow better or worse than the first?

Of course the second pipe will flow worse. The only thing we can't know without testing is how much worse - and it's there that the dyno testing will help us. It could be that the x-pipe robs you of 10lb/ft or it could be that it robs you of only 3, but I guarantee you that it doesn't add any power and I guarantee it's heavier.

maldren · 11 Oct 2018

For what it's worth, not sure I agree about Fuel Injection vs Carburettors either, the exhaust side is all about getting spent gasses out, FE and Carbs are at the other end. There's a bit of overlap of course but efficient exhaust systems matter to all engines.

There have been enormous improvements in computer modelling and measurements in recent years but Anand and Roe's Gas flow in the internal combustion engine was an excellent guide in my student days (and they were my lecturers in Manchester). No longer available but you may get it through your library.

Jamesx19 · 11 Oct 2018

Hi Martin,

It's an interesting subject isn't it. I am aware of the view that states there is flow loss in a pipe with a bend that has a tighter inner radius than 2x it's diameter.

Question is,
1.) How much flow do you need at useable rpm's. ( There maybe greater restriction in the CAT or silencer box)

2.) When the exhaust gases encounter the X, instead of having to continue following the same pipe, they are able to split into the second half of the system, hence there is now double the exit volume available.
Also, when this occurs there will be a change of cross sectional area that sends a negative pressure wave back up the system to the valve which causes a depression at the back of the valve, scavenging more exhaust gases from the cylinder.

Instead of each Bank of 3 cylinders acting independent of each other (almost 2 engines joined together at the crank) all 6 exhaust gas pulse's affect each other, not just 3.

IF this all occurs at a useable RPM, then an X pipe MAY produce a worthwhile result.

You can have an educated guess, but Idon't think you can say with certainty that one system is conclusively better than another until you test them back to back in the circumstances I described earlier.

Any sort of pressure waves in the exhaust will affect air going in as well. At low rpm's large bore pipes designed to work at high rpm dont have enough gas going through them to get the required velocity and may actually contribute to a flow reversal of air, leading to "standoff" above the mouth of the carburettor. This is caused by air going in and out of the carb, before finally being sucked into the engine. Each time the air goes through the carb in which ever direction, it picks up fuel from the jet, leading to overfiueling and consequent loss in torque/power. Fuel injection limits this effect by only injecting the right amount of fuel regardless of how many times the air has pulsed in and out of the Venturi before being sucked into the engine.

Again, I'm happy to be corrected. These thoughts are only my understanding of the real experts work on the subject.

asterix_the_gaul · 26 Oct 2018

So, I'm in two minds at the moment, I've been thinking about the 200cell straight cats, and you are supposed to get a mild performance increase but I'm assuming that's going to be at the top end and you would loose torque lower down the rev range? I guess without back to back dyno you wouldn't know?

Martin996RSR · 27 Oct 2018

By having less of a blockage in the exhaust (200cell vs 600cell) you should achieve a performance gain. It stands to reason. If you go for the bigger than standard bore pipes then you may see a decrease in low end torque as past the optimum point, the bigger the diameter pipe, the slower the exhaust gasses travel. It's the slowing down of the gasses that costs you torque.

So in short; get the 200 cell cats and stay with the standard bore ones if losing low end torque is a concern (which it probably should be on a road car).

If anyone offers you advice on this topic and uses the word 'backpressure' then ignore them. 'Backpressure' is to exhaust engineering as cheese is to moon geology.

Phil 997 · 27 Oct 2018

I had the topgear big bore on my gen1 , it worked very well, but agree with Martins theory about torque loss in low mid range . Now I never noticed it on the butt dyno as so much had been done to the car v stock but I would have loved to try the std bore 200 cells for a direct comparison :thumb: :thumb:

asterix_the_gaul · 27 Oct 2018

Martin996RSR said:
If anyone offers you advice on this topic and uses the word 'backpressure' then ignore them. 'Backpressure' is to exhaust engineering as cheese is to moon geology.

:floor: :floor:

200 Cell Cats

coullstar

Singapore

Martin996RSR

Montreal

Martin996RSR

Montreal

Jamesx19

Hockenheim

Martin996RSR

Montreal

maldren

Albert Park

Jamesx19

Hockenheim

asterix_the_gaul

Barcelona

Martin996RSR

Montreal

Phil 997

Le Mans

asterix_the_gaul

Barcelona

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