Downpipe Size?

[camboscams]

So i'm in the process of installing a VNT-15 on my 1.6 with a Giles pump. And im tring to decide what size to go with. I'm on the fence between just going for the 3 inch since i'm starting fresh or just go with a 2.5 inch. And i guess i need to decide on what size of exhaust system to go with as well, i've got 2.25 now and i don't have a problem going with something different.
Any input is welcomed

Cambo     

[theman53]

I am a firm believer in there is no such thing as underkill. I have 3" DP and 3" all the way back. I like it, but to say it is better than 2.5" I wouldn't know. VNT spooling faster it may not be as big of a deal as with a K24. Although 3" pipe will flow more air than 2.5" pipe.
If you are ever thinking of upgrading to more turbo it would be nice to have the 3" in place. That was my final thinking.

[DJPyro]

lmao Second thread I came into that you opened with "I believe there's no such thing as underkill"... But I agree, on a turbo app, my belief is that you should be 3" to the back end of the car too... 2.5" is acceptable for fairly stock levels IMO, but the best exhaust for a turbo is nothing, so as little back pressure as possible is best. It's not like an N/A app where exhaust tuning will have a profound effect, just get as much air out the tail side of the turbo as quickly as possible.

[camboscams]

True three inch would be really nice, it shouldn't be much more difficult to run then 2.5. NOw i have to decide to run a muffler or not, with the turbo i have now it is very offensive and quite loud. But i'm hoping that a vnt 15 will be quite enough to not run one. Any first hand accounts with mufflerless vnt15's? 

[theman53]

ROR2.0 has a vnt 15 without a muffler and says it isn't bad. He doesn't even have more than a downpipe IIRC. 3" is only 1/4" bigger on each side so it isn't that big of a deal. Also if you are going to do a muffler then the extra 1/2" might not back up the exhaust too much as compared to 2.5" with a muffler it should flow better

[vdubspeed]

yeah when you have a turbo...the bigger the better.

Instant spool and more flow. Win win.

[R.O.R-2.0]

ROR2.0 has a vnt 15 without a muffler and says it isn't bad. He doesn't even have more than a downpipe IIRC. 3" is only 1/4" bigger on each side so it isn't that big of a deal. Also if you are going to do a muffler then the extra 1/2" might not back up the exhaust too much as compared to 2.5" with a muffler it should flow better

yup, VNT15 with a short open downpipe.. all my friends think its bad ass.. turbo screams at idle.. and when you rod on it in down town, it sounds sweet when you got the sound echoing off all the buildings..

going down the road at 60 mph, you can still talk to your passenger, and hear some quiet playing music, but when you get on the boost, and the gauge climbs to 15psi or more, then it gets a LITTLE noisey, but still not bad..

VNTs are quiet turbos.. but they scream pretty loud, compared to a conventional turbo..

i have a small downpipe too, FWIW, its only 2" or so, but the outlet of the turbo isnt very big either..

[camboscams]

Very true on the turbo outlet size, but my thinking is that it will have more room for the expanding gasses to fill. and i guess the beauty of a of redoing my exhaust at the same time i could stop right where the muffler will be or have a joint right there and continue out the back, so if i get tired of it a can add one later.
I've looked all over youtube and all i can find are TDI's with the vnt 15 straight piped and they sound sweet but i don't know if there will be much difference being that its an IDI

[R.O.R-2.0]

Very true on the turbo outlet size, but my thinking is that it will have more room for the expanding gasses to fill. and i guess the beauty of a of redoing my exhaust at the same time i could stop right where the muffler will be or have a joint right there and continue out the back, so if i get tired of it a can add one later.
I've looked all over youtube and all i can find are TDI's with the vnt 15 straight piped and they sound sweet but i don't know if there will be much difference being that its an IDI

IDIs are louder because of the 5 or 6 points more compression..

vw idis are the loudest of loud, but sound very tame when equipped with a VNT..

if i were gonna do it again, theres no way i would ever muffle my VNT.. the muffler makes it SILENT.. any muffler.. just a little turbo scream, and thats it..

[DJPyro]

Also worth noting that ROR's is gonna be a lot louder with a 10" downpipe than yours'll be if you route it to the back of the car. Further, a 3" is going to give you a slightly throatier sound than a smaller diameter (if that matters to you)

I'm running 2.25" turbo-back to an inverted megaphone that tapers up to 4" and back to a 3" outlet, it dropped the tone of my exhaust drastically and gave it a much burlier sound. I want to step up to 3" but with the rest of my engine being fairly stock, there's not a lot of need for it right now. I'll be re-doing it to 3" when I change turbos. I busted out some Gr. 10 physics, and did some rough math on the wavelengths for the sounds I wanted to keep and eliminate which gave me my final dimensions for my tail pipe. I'm sure it was inaccurate at best but it yeilded the exhaust note I was looking for.

I recently elimnated my cat, when the flange broke and my flex pipe crapped out on me due to beat motor mounts. By giving myself a clean section of piping from the turbo to the 2.25 exhaust, and eliminating the clogged to hell cat I noticed a drastic improvement in turbo spool, a huge change in exhaust tone (that video is pre-cat removal, need to get another one), and I'm *sure* my EGTs dropped (still need a gauge, tisk tisk)... Now my car sounds even barkier and a little more like a piece of farm equipment. It makes me happy.

[R.O.R-2.0]

Also worth noting that ROR's is gonna be a lot louder with a 10" downpipe than yours'll be if you route it to the back of the car. Further, a 3" is going to give you a slightly throatier sound than a smaller diameter (if that matters to you)

I'm running 2.25" turbo-back to an inverted megaphone that tapers up to 4" and back to a 3" outlet, it dropped the tone of my exhaust drastically and gave it a much burlier sound. I want to step up to 3" but with the rest of my engine being fairly stock, there's not a lot of need for it right now. I'll be re-doing it to 3" when I change turbos. I busted out some Gr. 10 physics, and did some rough math on the wavelengths for the sounds I wanted to keep and eliminate which gave me my final dimensions for my tail pipe. I'm sure it was inaccurate at best but it yeilded the exhaust note I was looking for.

I recently elimnated my cat, when the flange broke and my flex pipe crapped out on me due to beat motor mounts. By giving myself a clean section of piping from the turbo to the 2.25 exhaust, and eliminating the clogged to hell cat I noticed a drastic improvement in turbo spool, a huge change in exhaust tone (that video is pre-cat removal, need to get another one), and I'm *sure* my EGTs dropped (still need a gauge, tisk tisk)... Now my car sounds even barkier and a little more like a piece of farm equipment. It makes me happy.

the nice thing about my VNT, is that it doesnt sound like farm equipment, it just sounds sweet.. the VNT really mellows out the exhaust note.. i did have 2.5" straight pipe off the turbo once upon a time, and it sounded good.. i guess i just never got around to building an exhaust for it.. there is still a 1.5" (stock size exhaust) hanging under the rabbit.. lol. guess i left it there for when it goes back to n/a.

[camboscams]

2.5" is plenty big.  You won't see any gains going to 3" and it's harder to route.

As much as i want the snark factor of 3" exhaust i can only see headaches when i think about routing it. would a 3 inch downpipe be of any value to this project of just cost me a reducer?

I'm sure it was inaccurate at best but it yeilded the exhaust note I was looking for

That sounded pretty good! stock turbo i assume?

[CRSMP5]

i did 2.5 on my burnt toast project.. i could have done 3 or 4 no prob with room fo rthat.. but realistically.. the output of the turbo is less then 2.5... so anything more then that would be silly as that would still be the restriction..

so i suggest looking at your turbo.. and use that as the how big to make it.. free flowing mandrel bent pieces with straight pipes.. no muffler to kill power.. if your worried about that make up a fake one.. aka straight pipe inside a bigger pipe with some ends on it to look like it is a muffler.. and enjoy the power..

[RabbitJockey]

vnt-15 isn't that big of a turbo to constitute 3" imo.  but just a note, small amounts in the increase of an exhaust increases the flow by a lot, a 2.5 exhaust has an area of 4.9 inches squared, where a 3" exhaust has an area of 7 inches squared, nearly twice the flow.

[R.O.R-2.0]

vnt-15 isn't that big of a turbo to constitute 3" imo.  but just a note, small amounts in the increase of an exhaust increases the flow by a lot, a 2.5 exhaust has an area of 4.9 inches squared, where a 3" exhaust has an area of 7 inches squared, nearly twice the flow.

for a VNT15, i would say that 2" is good for a normal build, while 2.5 is probably as big as you would ever need with this turbo.. because it can only flow soo much air..