To 3" or not 3" ?

[GT3]

..”To 3 inch or not to go 3 inch, that is the question” I’m guessing that if Shakespeare were around today, he’d definitely be driving a Polo GTI, no doubt about it. ..and just like in Hamlet, he would have agonised over whether to upgrade his stock exhaust down pipe (DP) to either a 2.5” or 3”, to the point of despair.

Now, ..the majority of R&D and testing by the main exhaust manufacturers and developers have all essentially come to the same conclusion – any bigger than 2.5” on a Polo GTI and you begin to get negative results. However, there’s one case study that has challenged this consensus and perplexed many of us – Timbo’s 3” DP mod. On two separate dynos, his car showed a gain of 13 kw atw with just that mod alone. How could that be?

Anyway, I decided to rework (customise) my stock exhaust and find out the hard way. In summary, here’s what happened:

3” DP – felt amazing up top, absolutely no restriction all the way into red-line. Down low it felt sluggish. On the dyno, there was zero gain power wise when compared to my very 1st base dyno (chip/CDA). …also at 4000rpm, the boost just, well ..died!

2.5”/3” combo DP – this felt a lot better, and gave another 6 kw atw gain, and showed a much improved power curve, eg boost sustained to 5.5rpm. ..although, it still didn’t feel quite right and lacked in low/mid end.

2.5” DP – perfect, a well proportioned increase in power that was spread evenly across the powerband. Yet to dyno but I don’t even feel it’s necessary – it’s spot on!

If you’re contemplating a DP upgrade, my experience may be worth taking into consideration when making that choice.

[Timbo]

Now, ..the majority of R&D and testing by the main exhaust manufacturers and developers have all essentially come to the same conclusion – any bigger than 2.5” on a Polo GTI and you begin to get negative results.

Where's the evidence that supports that grand attribution?

Indeed, your post would be well served by including the relevant dyno charts; you have data and if you want to offer an opinion, it is equally important to offer the evidence substantiating it.

There's another variable at play here, and that is the chip tune. I would observe that it seems certain tunes do not seem to offer any significant benefit from the addition of the DP. I think in the case of APR, a further tune update is required to suit any exhaust mod. Can someone comment if this is so?

[jasn78]

Timbo the update to the APR flash isnt for improved performance it is to remove an error code if you are running the incorrect sort of cat.

[Flaps]

incorrect sort of cat.

Please explain?

[GT3]

Where's the evidence that supports that grand attribution?

In their R&D, both Milltek and APR (the only ones to have developed exhaust systems for the Polo GTI) both use 2.5". As they're experts in the field, they wouldn't have reached that decision without evidence to support it.

[GT3]

Please explain?

I'm not 100% certain, but my understanding is that with the increased flow (through cat) it sets the O2 sensors off, the exhaust re-code prevents it from doing that.

[VW GTI]

Please explain?

It's higher flow, therefore it doesn't heat up as quickly which means warning lights will go off because it thinks there is something wrong. It just suppresses those lights until it has time to be the correct temperature.

[xposdu]

In their R&D, both Milltek and APR (the only ones to have developed exhaust systems for the Polo GTI) both use 2.5". As they're experts in the field, they wouldn't have reached that decision without evidence to support it.

Remus also have an exhaust system as well, and from what ive heard its pretty good. But i will wait to see if anyone else puts it on first.

[Timbo]

In their R&D, both Milltek and APR (the only ones to have developed exhaust systems for the Polo GTI) both use 2.5". As they're experts in the field, they wouldn't have reached that decision without evidence to support it.

That's for whole systems, this is about DPs. And, as has been discussed before, there's very little that's wrong with the stock GTI system after the DP bends and enters the cat (after all, on the same logic, we have to assume VW is also an expert in the field )

It would be good to see some before- and after- dyno charts of the APR downpipe and cat...or have I missed them?

[GT3]

1.That's for whole systems, this is about DPs. 2. there's very little that's wrong with the stock GTI system after the DP bends and enters the cat

(1) Milltek and APR sell DPs separately as 2.5" (in addition to full systems). They would only sell bigger DPs if they were matched to bigger turbos (2) VW designed that specific section to be quiet, that's why they added the primary cat, which functions as a stand-in resonator. This was a counter-balance for having no resonator between the cat and rear muffler. Depending on your perspective, ie if you're chipped, then that section isn't ideal.