996.2 200 cell cats?!

I currently have a top gear exhaust system, which I had fitted when I had a new engine fitted, its the valvetronic version as mine had the factory fitted option. Is it worth changing the cats? Topgear do a 200 cell for £499 Cheers guys for any input

I have this set up with the 200 cell X-pipe. It's loud with the valves open and your foot down but pretty normal with them closed on a motorway cruise

I would say yes as it improves the throttle response and performance of the engine. The cross-over cats help to improve the low down torque (good for street) The X-pipe increases top end power but does away with a little low end torque (good for track). Here's a video of my install with cross-over cats https://www.youtube.com/watch?v=vmcq9Sd-jAM&t=748s

Excellent thanks 👍

Thanks, very helpful 👍

200 Cell cats are definitely a route to a small power gain. However there is a lot of misinformation out there regarding small vs big bore and X-pipe vs crossover. This is with apologies to 82DVY, for whom I have the greatest of respect, and I'm a big fan of his videos. As an aerospace engineering graduate I hope my opinion on this subject is respected. Engines can be regarded as pumps for moving air. The less restriction there is on the air/gas coming in and going out, the more work that is done by the engine gets to go to the gearbox rather than be spent on sucking that air in or blowing that exhaust out. The best exhaust on a four stroke engine is none at all. The next best is the shortest straightest constant bore pipe you can fit. The longer the exhaust is, the more drag there is on the gasses being pumped through it, add in bore changes and bends and the gas has to do even more work before it comes out of the end. There is no such thing as free energy, so the energy needed to change the direction of the flow of the gas comes at the price of a reduction in speed of that gas flow. Given this information, it's easy to see that adding more bends to the exhaust can only harm the efficiency of that exhaust and therefore power. This is immutable truth, not just my opinion. What is my opinion is this: The 200 cell crossover pipes present the least restriction to the gas flow and therefore give the best power hike. I'm pretty sure that VS a stock exhaust they give as much as 10hp. The X-pipe has a 180 degree bend in it which can only harm gas flow across the rev-range, so it can't give any torque advantage over a crossover at any point in the rev range. Part of the spurious thinking that drives the supposition that the X-pipe gives a torque benefit low down in the rev range is due to 'back pressure'. BACK PRESSURE IS NOT A THING. It's not possible to make an engine give more torque by making the exhaust more restrictive. A moment's thought on this is enough to make one realise that BP being a performance enhancer is about as usesful as tits on a fish. Something that is true though, is that the X-pipe definitely produces a more refined sounding tone.

Hi Martin, All correct and I can't imagine anyone doubt's their level of respect for your opinions. One thing you've failed to consider is the interference/assisted scavenging effect of shared exhaust pipes. With the spacing of exhaust blowdown events from three cylinders (one bank of a 996) and appropriate exhaust valve opening periods and timing, it is possible to minimise the interference of the exhaust events from each cylinder in the secondary exhaust pipe, and maximise the assisted scavenging effect at the end of each primary at certain speeds in the engine RPM range. Typical exhaust valve opening periods are between 240-270deg, meaning it's possible to fit three blowdown events in to a 720deg engine cycle with the 1-6-2-4-3-5 firing order (effectively 1-2-3 down each side). The standard 996 exhaust manifold has very similar length primaries, the secondary is then variable length per cylinder due to how the primaries connect to it but the length of this pipe still has a usable/tunable effect on engine torque. The cats are effectively the end of the secondary because the pressure wave will reflect at the large change in pipe volume at the entry to the cat. The tertiary pipe is therefore the pipe from cat to silencer and unlikely to have much effect on engine torque other than through minimising back pressure, as you elude to. Having a x-over pipe here would increase interference of exhaust blowdown events and therefore average back pressure presented to the upstream exhaust pipes, so agree unlikely to offer any positive benefit to engine torque. Once you have an appreciation for the possibilities of enhancing cylinder scavenging with control of variable exhaust valve timing, and 'tuned' exhaust system lengths it is possible to see that the 'best' exhaust for a variable speed engine is not none at all. This book was key to my learning/application of the above during my education and early part of my career as an engineer who designed exhaust systems for automotive applications, and is a thoroughly recommended read! https://www.amazon.co.uk/Scientific-Exhaust-Systems-Engineering-Performance/dp/0837603099 Cheers, Rob

Thanks, some interesting and good points in there, I'm pretty happy with my power obviously a freshly rebuilt engine is probably running close to the 320bhp and with the better quality exhaust maybe a touch better 😀

Another excellent read, the top gear exhaust provides me with a lovely note, I have the BMC panel filter with resonate delete which I also prefer. I purchased a sprintbooster from a gentleman on here, I can say its the best £100 I've spent, no added bhp but definitely eliminates any throttle response lag.

:thumbs: [/img] Sounds great and a decent weight saving

It better for the cost :lol: I had one on my C4s and it sounded amazing. That was before they basically doubled in price.

Sexyyyy

As Turborob stated (Very well in fact!) I was under the impression that the main advantage of the X-pipe cats was their better 'Scavenging" effect. What I can say from personal experience of having the Top Gear 63mm X-pipe fitted (In addition to Direnza manifolds and Top Gear 'Sports" boxes) is that my car seems to be more 'Rev happy" ie seems to be breathing more happily at high rpm and almost feels like it's encouraging you to rev it out. I know the 3.4 is pretty rev you anyway but with the change of exhaust as above it does seem even more so. Is there a drop in low end torque as tends to be suggested for these X-pipes? Maybe... though my butt dyno isn't sensitive enough to really tell. It is pretty well masked anyway by the engine seemingly demanding to wring out every rev it has. As regards the sound I'd say this is (In my opinion) the biggest change. The exhaust note now seems to have less of a 'Burble" and more of a higher pitched 'Wail", certainly at high rpm. As a whole in normal driving I don't find the exhaust to be overly loud nor drones at motorway speeds (Though again this is an opinion thing, others might disagree) but dear god when you go high rpm and wide open throttle the noise is insane, a real proper banshee like wail. So do I like my exhaust system? Most definitely.

coullstar, luckily that pic is 5 years old so probably a good bit cheaper than now To be fair the exhaust needed replacing and wasn't that much more than the OEM solution, well that's what I told my wife :thumbs:

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