New exhaust... lower boost

[clived]

Ok, so a few of you probably know It's a TT I'm a talking about here, but for the sake of the forum, we can pretend it's a BAM S3

MTM big turbo fitted just before christmas. 340ps, peak boost 25psi - very happy. However, at the time I had the kit fitted and the mapping done, I had Milltek sports cats with a GTT Sportlite cat back exhaust fitted - which is basically just a centre silencer and two straight through pipes branching off behind it. Very free flowing I'm sure, but just toooo loud (couldn't run at Combe recently - 104.1db static test ).

Yesterday I had the GTT swapped for the APR cat-back exhaust - I wanted something louder than a Milltek, but not as loud as the GTT. Sound is fine, but the bad news is that I'm 2-2.5psi down peak boost (so nearly 10%), peak VAG COM measued airflow is down 10% and the turbo really doesn't seem to be spooling up quite as quickly / early as it used to.

So - my question: Whilst obviously I can appreciate that the APR exhaust is slightly less free flowing that the GTT (it does, after all, have a backbox ) does that mean my boost is gone for good - or do we think if I were to get it re-map, with the new exhuast in situ, I should be able to get back to my previous peak boost and turbo spool up characteristics?

Cheers, Clive

[Scotty]

I believe you need some back pressure and perhaps it's too free flowing. I certainly not a tuning guru so I'll shut up and read the following posts with interest.

Sorry to hear your not all sorted as you wished.

[clived]

I think you're right ScoTTy. You best shut up

The APR exhaust must have MORE back pressure than the GTT, not less - the GTT had just a centre silencer, the APR has a centre silencer and a back box (and it's a lot quieter!) so I don't think I'm suffering from too little. In any case, as I understand it, the cats, and more specifically, the turbo, provide the bulk of any required back pressure.

Thanks for the wishes though - thought I stood more chance here than TT-F

[Scotty]

I'll go stum then.

Apart from to apologise for using " your" and not "you're". On TT-F someone would have corrected me by now!

[simple1]

Kin hairdressers..... who needs them.............now where did I hide my easter egg

[clived]

Attempt at humour Simple1? Been drinking? Our contribution or discussion not welcome? Or your wit just too esoteric for me perhaps?

Or maybe you're a shy exhuast guru and didn't feel able to give us the benefit of your knowledge and experience all at once Come on, you must have changed a few exhausts on all those Ur's - any intelligent and informed thoughts on this welcome.

[BOWS3]

So, 2-2.5psi down on 25psi. How much power do you think you've lost ? And where have you lost this ? Overboost or sustained ? If its down low on initial spool up surely it would just equate to a little less low down torque. Haven't seen the GTT exhaust. How does it split ? A y-piece or just a branch off ?

Chris.

[clived]

Chris, it's a Y peice.

Previously the car would hold 25psi from just over 3k to about 4k, but no longer - so on the overboost I guess. I think there are two effects. Reduced flow slowing the initial spool up of the turbo and then the "boost limiting effect". I'm more worried about how much torque I've lost than how much power - the car has certainly lost some of it's punch.

Cheers, Clive

[PhilT]

I'd recommed you get it on an RR, and considering you have the MTM map it makes sense to go to QST. It seems logical that increased back pressure could account for reduced boost. I'm no expert, but here are some questions which I'd be asking myself:

- What was the length of time between pre and post exhaust swap logs?
- Have you changed anything else?
- Could the sensor be faulty?
- What are the intake temps pre and post?

Can you plot the boost graph showing pre and post?

[clived]

Hi Phil - I've got a boost gauge, so the length of time for spotting the boost drop was as long as it took to change the exhaust And the logs for air flow were done about 1 1/2 weeks apart with nothing else changed inbetween. It is definately the exhaust that has caused it. I don't tend to log boost as I can see it on the gauge Which of course means I can't produce a pre and post graph.... although I could get the old exhaust put back on to create the graphs, it's a bit of a faff just to get the logs.

I guess I really just need to ask Kim (or anyone here with any mapping experience) what the probability is of getting this boost "back" from tweaking the map.

I'm also wondering how good a performer back pressure wise, the APR exhuast is compared to Forge or Milltek - and equally, how far aware either of those are from the GTT...?

[Nige_RS4]

I'm no expert, but I thought boost was on the inlet side and changing the exhaust would have little or no impact on this. Spool up would obviously be impacted by increasing the back pressure. I'm wondering if you have an O2 sensor on it's way out?? I would say the APR exhaust would be less restrictive than the Miltek. Apart from that I'm struggling to suggest anything apart from putting your old exhaust back on and see if your power is restored.

[clived]

Hi Nige. I haven't gone from Milltek to APR - I've gone from GTT to APR. The boost reduction happened *directly* on changing the exhaust and the car still runs "fine", just with less boost and less thrust, so the chance of an O2 sensor picking that exact moment to fail are, IMO, pretty remote. I think that exhaust restriction can effect boost in any case - all that air has to escape, right? Not enough flow, more heat, less boost?

[Nige_RS4]

Hi Nige. I haven't gone from Milltek to APR - I've gone from GTT to APR

Clived, you asked how the APR would compare with Miltek, Forge?? Ref. the O2 sensor, I'm was speculating that as they have obviously been working in the area of the 02 sensors (and it's associated cabling), that they *might* have been damaged by all the hammering that goes on when an exhaust is changed. BTW, was it just the cat-backs that were swapped out - you running high flow cats (or bypassed them completely)??

[VERN]

@ clived

HI mate

you have already self diagnosed your prob, it's a flow issue mate, for sure , you will be down on torque mate, this is what you feel on the "butt dyno"

just out of interest clive what d/pipe you running?, is it mtm? if so what size is it ?
regs
vern

[clived]

Nige - sorry had forgotton my own question I agree it's worth eliminating any issue with the O2 sensor - I'll do some logging and a visual check. I think it's really hard to guess which exhausts have the best flow characteristics. I think the APR has a real drone at certain revs, and *that* can't bode well for good flow, IMO.

Hi Vern . Arse. The real question then is what do I do? Been thinking about getting Blueflame to make me something customer - not as loud as GTT please, but just as free flowing

I think it's only the manifold that MTM supply - don't know about the downpipe - as far as I know it's the OEM kit, which runs into Milltek sports cats.