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.

[Timbo]

Neil, that doesn't really make sense. AFAIK, Milltek only makes a full system for the Pog (am not sure one has ever been delivered in Australia -- or a DP, if they do in fact do one) and the line about making a bigger pipe only for a bigger turbo is supported more by the desire of marketing people to segment the market (in the same way VAG does) than actual data relating the DP to the stock turbo.

I'm still scratching my head to understand how the primary cat works as a stand-in resonator when it's basically filled with catalyst.

Regardless, my stock exhaust was flow tested before I had a DP built. The biggest restriction in my stock system (which still includes the middle resonator) was the stock turbo to cat pipe. The exhaust tech then explained the maths to me about going to 3" rather than 2.5".

Why go to 3"? Because even a 2.5" will restrict flow at its bends. So, having 2.5" full system from the cat back will still suffer from the bend restriction prior to that, and thus reduces the benefit of the 2.5" system. Wa a a y back, Guy did his initial work on a custom Pog 2.5" exhaust, and found little to be gained. Has this changed?

As far as I am concerned, the proof of the 3" DP is on the table and I'm not sure why people argue against it, without putting forward alternative evidence

[Blitzen]

Is it just me, or do the storm clouds seem to be forming(tongue firmly in cheek)

[VW GTI]

As far as I am concerned, the proof of the 3" DP is on the table and I'm not sure why people argue against it, without putting forward alternative evidence

Have you got a link to your dyno graphs somewhere?

[GT3]

As far as I am concerned, the proof of the 3" DP is on the table and I'm not sure why people argue against it, without putting forward alternative evidence

Ok I guess I got it wrong, what would I know.

For those who think Tim's isolated anomaly is compelling evidence, then go ahead with a 3" DP, it's your choice. I've tried to do the right thing and caution against it.

[Timbo]

Have you got a link to your dyno graphs somewhere?

Here's the first time I posted them, and this is the second time.

For those who think Tim's isolated anomaly is compelling evidence, then go ahead with a 3" DP, it's your choice. I've tried to do the right thing and caution against it.

Caution against what?

Neil, I would simply observe -- even though you seem to think deleting posts changes history -- that you have been singularly negative towards the whole idea of a 3" DP since I first posted, challenging the results, the dyno etc, etc. For what reason I don't know ?:

Yes, it is up to people to make their own minds up, based on the evidence they are able to see. Equally, as I have repeatedly maintained, it is important to get this mod done by an experienced and well-qualified exhaust technician. I certainly would avoid any chop shop that also suggests removing any part of the catalytic converter