Pieman

Thinking of putting SS downpipe on MY00 PPP with ITG, but re-fitting original (catted) mid-section – emissions bla bla bla...
Will I lose all the benefit of the downpipe (faster spool-up etc.) by still having some restriction in the mid-section, or will I still get most of the gains?
What do you reckon?

Adam M

No you wont lose out. You wont gain as much but if you think that each of the restrictions decreases performance then removing one will restore that loss.

Most important one to restotre is the downpipe as

1. it is the most restrictive cat.
2. the turbulence it generates is not good for scavenging which is even worse so close to the turbo.
3. the standard downpipe completely blocks the wastegate which which is not good when flooring it. Even if you retained the cat, unblocking the wastegate will yield a substantial gain.

Pieman

Adam,

Thanks for the info. From what you're saying I guess I could also consider the GGR catted downpipe but with my de-cat centre. That downpipe doesn't block the wastegate, but the car will still pass MoT/emissions tests?

Adam M

It may well do. The downpipe is much hotter than the centre section so it heats up quicker and makes cat more effective. the ggr cat will be a high flowing one so should be ok, it wont be as good a cat obviously.

I would rather have a full decat downpipe and retain a normal centre section, or get a high flow cat in the centre section and take car for a blast before mot'ing. Mine passed emissions with my centre hi flow cat. This combination is better for power and torque so aim for it if you can. Decat fully if you have the *****.

nom

Adam - what hi-flow centre have you got? I'm out looking for one at the moment.

Pieman

I was wondering – would a downpipe hi-flow cat (like GGR's), rather than a mid-section hi-flow cat, be less likely to cause overboost with a PPP'd car? Or is any overboost only likely to be cause by un-blanking the wasgate, not the lack of a cat?

Pieman

Can anyone help?

nom

Not sure if I can help (as in may be barking up the wrong tree here) but from some fairly tortuous reading of an excessively long & technical post somewhere, it seems as though overboost may be casued by the actual design of the downpipe, as in whether it's blocked, twin-dump or just a large tube. It appears that twin-dumps can give less accurate readings to the O2 sensor (as well as complications with turbulence etc.) which can mean the ECU gets things muddled/doesn't notice & overboost can occur.
However, all of this was 'theory applied to observation' at the time.
Flame-proof jacket/general hard exterior applied.

dowser

Overboost occurs because you're removing restrictions - a good thing for performance, but not something the stock boost control system can deal with that well (especially with PPP).

If you get overboost with PPP after fitting a decat downpipe, just refit the OE backbox...this puts enough restriction back into the system to tame things down again...but will also blunt the performance gains

Read John Banks' post re. his flow control valve for a proper fix, or start fiddling with restrictors.

Richard

Pieman

Cheers for the advice lads. I reckon I'll go for SS downpipe (not twin dump so possibly better boost control) and re-fit original catted mid-section.That way things should still 'happen a lot quicker'but I should be okay re. emissions and the middle cat might just help keep any overboost under control.
Here's hoping!

nom

Just thought I'd better reiterate (help help spweling) that on my drivel up there ^^^^^ somewhere, there's a great deal of "may"s and "can"s in there on purpose - the twin/single dump debate is raging, let me see if I can be clever here...
http://www.scoobynet.co.uk/bbs/threa...threadid=56686 & the results - such as they are - are mainly being based on a single car. I didn't intend to make it anti-twin-dump, just a point of view