Can I decat my downpipe, ie replace it with a down pipe without a decat in, like a scoobysport one without actually changing the back box. I'm keen to get some more power but I dont really want all that noise from a new back box.

Will I still get 20-30hp more with just the downpipe or do I need the noisy backbox too?

Also how does the centre pipe affect this discussion?

I don't know of anyone who has tried what you suggest, but I can say:

With some downpipes, it will physically fit. Doing so will increase noise a bit anyway as the cat absorbs/blocks some of the exhaust noise.

The centre section has a cat at the front, which also acts as a sound muffler and also restricts power.

The backbox restricts flow a bit (probably less than a cat though), whilst removing some of the noise.

In very basic terms, each time you remove a restriction from the exhaust, you will get more power and more noise. A scoobysport backbox with an OEM centre/downpipe is not as noisy as a full scoobysport system, but gives much less power.

Having said that, some exhausts are much noisier than others - do a search here and you'll find a number of posts about that. From my limited experience, I would put the scoobysport in the medium noise category - eg Blitz will be louder, but several quieter. Quieter systems will often have a sports cat and a resonated centre section, as both of these will absorb noise without having too much effect on power.

That is very do-able, not sure you'll get 'that' much power, BUT you'll be amazed at how different the car drives, it should be a LOT smoother, less on-off!
The downpipe is the one bit on my car that made the best change, I would have considered doing it first if I'd realised.
Ron.

DO IT!!!!

I've run all sorts of combinations......

I ran a Scoobysport downpipe with a standard (STi) exhaust for a couple of years.

I'd be surprised if the downpipe ever gave those sort of power increases. What it does do though is get the turbo spinning earlier - it seemed to shift the whole response level down atleast 500rpm. It makes the car much more driveable.

IMHO - the down pipe is the best upgrade - I'd do it before the backbox every time !

BTW I'm now running Scoobysport downpipe, PE catted centre and Magnex back box.

Looks like i'll be getting me a scoobysport down pipe then :) wonder if I can pass MOT by leaving the centre section in!

It depends...... my STi failed which is why I've now got a PE catted centre - I just couldn't be arsed with swapping it about every year. I've heard that some UK's still pass if they're hot enough when tested.

I dont want to go through all that hassle, it sounds like a nightmare keep changing stuff to pass an MOT, would a decat downpipe and a better centre section pass?

I think the question I need to ask is whats the most powerful combination that will still enable a flawless MOT pass?

S55 HOT

How do you find your PE centre?
Any noticable power changes when it was fitted?
How about increased noise over your standard STi centre.
I have ordered SS DP & BB so far, but leaving OE centre to try to keep noise down, and pass MOT.

Jules

UHF - as far as I know Power Engineering are the only ones who do a catted center and therefore it is the most powerful one ;-)

Jules - The PE centre feels slightly freer and is only very slightly louder than standard.

S55Hot - Not true. There's quite a few people who do sports cat systems. PE do, but also Graham Goode, MRT, APS off the top of my head and I think there are others.

Re "most powerful" - I don't know, although IIRC someone did a back to back trial of many exhaust systems which may include several of these systems.

Edit - I believe the someone who did the tests was dalloyd. Haven't found the results yet, though.

[Edited by hades - 10/13/2002 9:17:25 PM]

Hades

S55 was refering to catted CENTRES, the ones you mention have catted DPs and this is likely to have some limits on performance IMVHO.

yes, indeed, what I wanna do is de cat down pipe, sports cat centre section and a moderate back box and get some nice power and still pass MOT cos of a decent centre pipe cat!

Jules - Fair point, I should read more carefully. The other sports cats I mentioned will not fit to a SS dp. If UHF has already got/ordered a SS downpipe, that is the best option.

Re: limiting performance - I'm not convinced. One other variable in the equation is that with SS dp and PE centre, (IIRC) you have a diameter restriction where the two parts meet as they run OEM compatable flanges. I know the APS maintains the larger pipe diameter by having their own centre sections (MRT too I think). Downside is that their cat is a little nearer to the turbo than the PE one, as you said. APS is apparently fine to 360bhp+. Whether it is more or less restrictive than PE, I don't know. Guess we need to find some dyno results to settle that one! ;)

I havent ordered anything yet, I thought I'd ask here first

So . . .It roughly boils down to you want MOT legal, not too loud, lots of power? I can see variations on two basic options

1. Decat dp/sports cat centre/fairly quiet backbox. I just found another sports cat centre - Milltek also appear to do one here .

2. Fit a full MRT or APS sports cat system - these are quieter than most systems and give good power gains.

The MRT/APS systems have a centre resonator as well as a sports cat - which makes them quieter than the PE system which just has a cat in the centre and no resonator. A resonator and free flowing backbox is (I'm told) a less restrictive way of reducing noise than no resonator and a more restrictive quieter backbox. Because of what I've said, I recently ordered an APS system, which I believe is right for me.

I would still love to see all the systems proved on a dyno to get a definitive answer about the power output. Without that everyone can express opinions, but we have no proof of who is right.

Does that help?

makes sense, thanks!

I like the sound of this APS system, who stocks them?

cheers,
dela

try here

They are aussie made, so take a couple of weeks to arrive - ordered mine last week, so not here yet :(

Phone them - Jamie is helpful, and has generally impressed people on here who have used him. If you tell him I pointed you at him, he might even give me a discount - how about it Jamie? ;) :D

Horses for courses I guess.

The problems with the integrated solution are
a) you have to buy it all at once
b) if you don't like it you can't tweak/experiment a bit

I noticed the most performance increase from the downpipe and practically none from the backbox - though the increase in sound makes it feel faster.

AFAIK I was the first person to actually test a backbox back to back (before & after) on a rolling road on the same day! The results are here.

Pete Croney didn't seem particularly keen on me doing it - he was there that day and was very interested in the results. The Power Engineering boys were very surprised - in something like 15 years of tuning, they'd never seen a backbox deliver any gains before let alone meaningful ones.

When I swapped my Scoobysport backbox back to the regular STi one, I noticed no performance degredation, but it was a lot quieter !

I'm thinking of going back to the STi one again as I find even the Magnex one quite loud and that's supposed to be quiet......