Gti vs golf downpipe

[TwinDad]

As most of you guys know, I'm running game the stock downpipe with a gutted cat. I'm considering doing the same thing to a gti downpipe though since it's larger diameter piping. I've heard mixed opinions and respect them all. I still have the stock cat back. Will the 2-1/2" downpipe help things flow better to the catback, or will it not make a difference because the catback is still 2-1/4"?

 

[anjoVW]

As most of you guys know, I'm running game the stock downpipe with a gutted cat. I'm considering doing the same thing to a gti downpipe though since it's larger diameter piping. I've heard mixed opinions and respect them all. I still have the stock cat back. Will the 2-1/2" downpipe help things flow better to the catback, or will it not make a difference because the catback is still 2-1/4"?

My guess is that it wouldn't be worth it really considering that it's only a .25 inch difference, which also doesn't seem significant enough versus the 3 inch aftermarket DP's. If you want more flow just save up for an aftermarket DP.

Someone else please correct me if my statements are wrong.

 

[MeltedSolid]

He already has an aftermarket downpipe, just testing a different setup.

Looking at this problem from a fluid dynamics point of view, the thing that matters is mass flow. Right at the outlet of the turbo the exhaust is extremely hot and not very dense, and therefore needs to be flowing at a higher velocity, but after it gets past the cat and leaves the downpipe it's much colder and so doesn't need as large of piping to flow the same amount of mass at a given velocity. IE the colder the exhaust the smaller the needed pipe diameter.

As to whether or not 4 sqin is a big restriction compared to 4.9 sqin at our power level (or if the restriction's all in the cat) is a different debate, but the above is why the catback doesn't matter for power.

 

[TwinDad]

My guess is that it wouldn't be worth it really considering that it's only a .25 inch difference, which also doesn't seem significant enough versus the 3 inch aftermarket DP's. If you want more flow just save up for an aftermarket DP.

Someone else please correct me if my statements are wrong.

I have an aftermarket one already. I pushed my car to the limit on it and got bored, so I tried a piggie pipe to compare. So far it runs at least 90% of what the 3" one did on the is12. It has its losses above 5,500, but with the is12 I shift at 6,000. I'm going to is20 next and want to see how the larger pipe will work. You have to remember that even with a 3" downpipe, it's still running into your stock catback. As for size difference. The 1/4" extra diameter is almost 25% more volume. 3.14xradius squared. 3.97 square inches vs 4.9. The beauty of it is that when I open my hood, I can say it's stock

 

[m3lonbr3ad]

He already has an aftermarket downpipe, just testing a different setup.

Looking at this problem from a fluid dynamics point of view, the thing that matters is mass flow. Right at the outlet of the turbo the exhaust is extremely hot and not very dense, and therefore needs to be flowing at a higher velocity, but after it gets past the cat and leaves the downpipe it's much colder and so doesn't need as large of piping to flow the same amount of mass at a given velocity. IE the colder the exhaust the smaller the needed pipe diameter.

As to whether or not 4 sqin is a big restriction compared to 4.9 sqin at our power level (or if the restriction's all in the cat) is a different debate, but the above is why the catback doesn't matter for power.

Agreed. I'll shortly be in a similar situation (with the exception of the gutted cat) as TwinDad, but with 4mo on my Alltrack. I have a Golf R stock downpipe, a 65mm to 60mm od reducer and an IS20 sitting in the garage... I plan on leaving the exhaust stock.

 

[Ironshade]

I see zero performance gain tbh. Better off removing secondary cat. That’ll do 100x more then upping pipe size. Especially when exhaust is 2.25 as well. Either way with gti pipe you still restrict to 2.25

 

[Gawernator]

Dad, seems you'd be better off going 3" Becker downpipe like mine

 

[TwinDad]

Dad, seems you'd be better off going 3" Becker downpipe like mine

See above posts. I have a 3" downpipe already. Becker only makes it for the awd as well I believe. Mine I'd the eBay rev9

 

[Gawernator]

See above posts. I have a 3" downpipe already. Becker only makes it for the awd as well I believe. Mine I'd the eBay rev9

No they don't .. I have it on my car lol. Well now I'm confused on what downpipe you want

 

[MeltedSolid]

Make sure to let us know all of the details about the piggy pipe vs. the aftermarket downpipe with an IS20! I've decided that I need a downpipe, but I've also decided I prefer the sound of the stock downpipe. If it turns out good with a piggy pipe I may buy a used downpipe to make one and then do something with the catback, but otherwise I want to keep my catback conservative and get an aftermarket downpipe.

 

[breaking badly]

Looking at this problem from a fluid dynamics point of view, the thing that matters is mass flow.
Right at the outlet of the turbo the exhaust is extremely hot and not very dense, and
therefore needs to be flowing at a higher velocity, but after it gets past the cat and
leaves the downpipe it's much colder and so doesn't need as large of piping to flow
the same amount of mass at a given velocity. IE the colder the exhaust the smaller
the needed pipe diameter.

Very true and good point made!

 

[TwinDad]

No they don't .. I have it on my car lol. Well now I'm confused on what downpipe you want

Gotcha. I thought you were 4 motion. When I looked at the eBay pipes, the Becker ones looked good, but they only had the golf R pipes at the time. I got Rev 9 one for dirt cheap and put it on. It took a little tweaking and had to get my own adapter for it. I then didn't like the catless, so I cut it up and clamped in a high flow cat. It leaked and vibrates because I needed it welded in, but I could never get it lined up perfectly to where I wanted it to be. So I never bothered welding it. Guys always did piggie pipes on the older 2.7t Audis and I wondered if it would work on this car. Then I saw a guy on Facebook who did it on his S3 and dynoed very well. I then gave it a try. It works just as well as the aftermarket up to about 5,500. It falls off a little bit past that. A big part of that I think is because of the heat the little turbo is creating at the higher boost levels. I think the is20 will be fine up to 6,000+. I can. Get a gti downpipe for almost nothing from the guy I'm getting the is20 from. So I figured I can try it out.

 

[Gawernator]

Oink oink? I thought the Becker was really inexpensive, the Rev 9 one is cheapppp!

 

[toledospeed]

It would be great if someone could compare the stock, aftermarket, and piggy pipes on a dyno to really see how they perform at differing RPM's. In my mind, one of the issues with sticking with the stock DP is the god awful design/bends it has. Removing the primary cat will definitely help, but the inlet has a flattened curve to behave as a baffle to reduce sound. After that, the next bend coming out of that topmost chamber is also abysmal and limiting as you can see in the pics below. The aftermarket DP's are definitely going to be better at accelerating hot exhaust out of the turbo and on through the rest of the exhaust. If you muzzle the flow up top, velocity through whole catback will also suffer under WOT.

 

[TwinDad]

I think with the cat gutted, that first flattened curve isn't really an issue. I think the biggest issue is the reduction to stock size pipe and a bend right after that first cat. You do also have to remember that the exhaust from the turbo is coming out of an impeller that's less than 2". Gutting the cat helps the spool, it's just a matter of getting it out if that first chamber efficiently. My current set up really hasn't shown that much difference on the is12. I'm about 1 psi less at 6,000, but peak is the same. That extra 1/4" may help get that back. With the is20, I won't be generating as much heat. I don't think it will hold up 100% against a 3" at 27+ psi, but I think it will work just as well at 23