Help Me Improve My Exhaust

[mystery machine]

Car is a MY06 UK STI 2.5L.

I would like to replace one of my cats with a 'cat delete' pipe. I'd be most grateful if any of the resident exhaust gurus could answer a few questions about this mod...

• Which cat should I remove?
  
• Does the car have to be remapped? (for continued smooth running).
  
• Will it pass the MOT test?
  
• Can you recommend a good replacement pipe?
  
• Will this release more burble? (the 2.5L does NOT have equal length headers, BTW! ).
  

Thanks

[marko72]

With limited knowledge....

You could remove both cats or remove the second cat and fit a sportscat downpipe. Ive heard you really need the 100cell type. With that it should pass the mot or for the former a friendly mot place may be required. Or else just remove the second cat and keep the downpipe cat for mot purposes. This is what I have.

Andy F advised me when removing cats it is remap time. So at least you will end up with more power

I reckon there are lots of quality exhausts out there. I have Hayward and Scott and like it. Others will have there recommendations.

As for more burble Im not 100% on the equal, unequal thing.

Cheers.

[arch]

From memory your car will have two cats one in the downpipe and one between the downpipe and centre section. If you remove a cat you really should be off through to Andy to have it remapped so best to do everything in one hit and then get it mapped. A 100 cel sports cat downpipe while a good idea is not a total necessity, but will flow better than a standard cat. If your budget is tight take the car down to Meercat and get them to remove the cat between the downpipe and the centre section and replace it with a link pipe, easily and cheaply done. I am not sure if the Hawkeye is any different from the earlier Blobeye STI in terms of fitment but the Prodrive sports cat dwonpipe can be a cheap way of removing the restrictive standard cat downpipe. I have a prodrive sports cat downpipe on my blobeye WRx, its the only cat fitted and passes the MOT with no issues. So really it comes down to budget.

Couple of pics below showing what comes with the STI PPP and a pic of the WRX intake and exhaust system, the only real difference on the STI is it does not have a cat in the uppipe. A good cheap upgrade is a 2nd hand Prodrive cat downpipe, the prodrive 3rd cat delete pipe which is basically a straight pipe that removes the cat between the downpipe and the centre exhaust section, a straight through pipe to remove the centre resonator box and a prodrive back box. It will remove a lot of restriction, give a nice exhaust note without being stupidly loud and best of all the parts are all available pretty cheap second hand. Throw in a et of -7 temp rated NGK plugs and a panel filter and Then get Andy to remap it. Hope this helps.

[mystery machine]

Guys, thanks for both your replies

Arch, that diagram is brilliant - thanks very much for posting! You seem to be suggesting that a 'turbo back' system would be the least restrictive option, which makes sense. In that case, why do you suggest replacing the rear cat with a through pipe and replacing the front cat with a sports cat? (and not the other way around - especially as that rear cat seems to have some kind of oxygen sensor). Is the rear cat the more restrictive one then? And is there any value in simply replacing the rear cat with a cat delete pipe (and leaving everything else standard)?

Thanks again

Edited May 29, 2010 by Mystery Machine

[arch]

Guys, thanks for both your replies

Arch, that diagram is brilliant - thanks very much for posting! You seem to be suggesting that a 'turbo back' system would be the least restrictive option, which makes sense. In that case, why do you suggest replacing the rear cat with a through pipe and replacing the front cat with a sports cat? (and not the other way around - especially as that rear cat seems to have some kind of oxygen sensor). Is the rear cat the more restrictive one then? And is there any value in simply replacing the rear cat with a cat delete pipe (and leaving everything else standard)?

Thanks again

No massivbe advantage really other than higher flowing catted downpipes are easier to come by cheaply. There is a lambda sensor in the rear cat, the prodrive cat delete pipe come fitted with a boss to refit the sensor. Should have added that the standard cats are much more restrictive than say a prodrive sports cat downpipe or certainly a 100 cel cat.

Edited May 29, 2010 by Arch

[arch]

http://bbs.scoobynet.com/private-for-sale-...ge-hawkeye.html

[sparky_sj04]

And is there any value in simply replacing the rear cat with a cat delete pipe (and leaving everything else standard)?

Yes. Before I got the full lot done on my Hawkeye I started by just changing the centre section, then went on to replace the 'cat' pipe and the backbox - all for Hayward & Scott. That's what gave it all the noise.

I later added a Meercat 100 cell Downpipe and had it mapped, but if you're just after a bit of noise and attitude then fire in a new cat pipe, centre section and backbox and you'll be sorted. Strictly speaking should be no dire need for a remap as you still have your main cat (and therefore your main restriction) in the downpipe. Then again if you later go to add e.g. a Meercat DP then that will join on to the centre section (and remove the 'cat' pipe section).. so might be as well to get the full lot done at once and get it mapped.

[arch]

If your going to do it, do it all in one go and have it mapped as soon as possible after the swap. I personally woouldnt remove a cat and then not have the car mapped.

[Richie Cross]

I later added a Meercat 100 cell Downpipe

My blob WRX came with PPP. I started off with a Meercat 100 cell sportscat/wide mouth downpipe.

Got a Meercat straight through centre section as well but kept my Prodrive backbox.

Ran the car for about a year without a remap and had no problems. When I was doing other mods

I then got a Meercat up pipe and then remapped

[mystery machine]

Thanks for your reply Richie.

OK, as two of you have replaced/removed cats without remapping and without any apparent problems, and that I've found at least one replacement pipe for the rear cat at under £120, I think this might be a worthwhile, if subtle, mod

Archie, I appreciate your advice re mapping... I must admit I'm a little sceptical though. From all that I can gather, these ECUs have some degree of adaptability to minor changes in the system. And with no disrespect to Andy Forrest, being in the mapping business he's career-bound to recommend remapping whenever credible. Having said that, I would seriously consider seeking his services should I make any more significant changes (such as replacing the more restrictive downpipe cat).

Candidate replacement rear cat pipe here:

http://www.scoobyworld.co.uk/catalog/produ...roducts_id=1116

Any recommendations for other suppliers or trusted fitters?

Thanks again folks

[arch]

"Archie, I appreciate your advice re mapping... I must admit I'm a little sceptical though. From all that I can gather, these ECUs have some degree of adaptability to minor changes in the system. And with no disrespect to Andy Forrest, being in the mapping business he's career-bound to recommend remapping whenever credible. Having said that, I would seriously consider seeking his services should I make any more significant changes (such as replacing the more restrictive downpipe cat)."

If I had a Syvecs ECU i would be happy that it would protect my engine for increased airflow through the engine with no changes to the fueling or ignition timing, standard ECU's well thats another story, but hey maybe thats just me

[mystery machine]

Fair point, Archie. I'll do a bit more homework first, maybe seek some advice from an expert or two... but probably not the type of expert who maps cars for a living

I'd be surprised if removing the second cat makes that much of a difference to airflow through the engine that it'd put the ECU out of normal operating range (assuming there are always other environmental factors that can affect airflow anyway), but yeah - pure speculation on my part...

Anyway - cheers again, it's all good stuff

Edited June 1, 2010 by Mystery Machine

[marko72]

I should point out that when I had my 2nd cat removed I also had my uppipe cat removed (MY02 wrx) so as per Hypertech it was remap time for me, plus I was looking for extra power as I had now fitted all my goodies ready for remap time. I can recall Dunc and Andy mentioning boost issues or something like that when cats are removed. But for me with the uppipe cat away it was a def remap. I see your point in just removing the 2nd cat and thats it, as Richie said it would prob be fine. With my 2nd cat removed there was more burble which was a bonus. Cheers.