XTUNE GST N 3" cat back !!!

[Slippi84]

Backpressure is never a good thing, unless you are talking about a car that is tuned for backpressure and can't adjust itself for the increased VE and "double loading" with a carburetor.

Backpressure will make a turbo spool slower. The effects are double fold with backpressure behind the turbine outlet.

Scavenging and velocity are what you mean by backpressure. Neither of these are quite as important post turbo as say a n/a engine since the turbine smooths out the exhaust flow anyway. This is why turbo cars can get away with a larger diameter exhaust system.

You 4g63 guys should know this better than the 420a guys but take the bolt on turbine hosuing smaller in size vs the larger t3 or t4 style exhaust housings(both with the same wheel). The smaller bolt on hosuing creates more back pressure and fills up quicker then the t3 or t4 and spools the turbo faster. Is this not the same principle with back pressure in the exhaust.

 

[BigRand]

If I did my reading correctly, the Apexi N1, listed for the GST model isn't a full 3", its measured in mm, and isn't quite as big as a full 3" is in the metric system, its inbetween 2.75" & 3"

So the fact that xtune was modeling theirs after the Apexi N1 style, maybe true, they probably went with a full 3" but in copying apexi the very end of it got smaller.

Regardless you can't beat the price,

_________________

Regarding backpressure in a smaller turbine housing with the same wheel.

Both housings are made to flow, it all goes small to big. The smaller housing is obviously designed to create more velocity to spin the wheel, while saraficing the free flowing exhaust at higher rpms.....regardless the turbo snail designs all go from small to big to the downpipe which gets bigger, to the exhaust system which gets even bigger.

 

[Slippi84]

If I did my reading correctly, the Apexi N1, listed for the GST model isn't a full 3", its measured in mm, and isn't quite as big as a full 3" is in the metric system, its inbetween 2.75" & 3"

So the fact that xtune was modeling theirs after the Apexi N1 style, maybe true, they probably went with a full 3" but in copying apexi the very end of it got smaller.

Regardless you can't beat the price,

_________________

Regarding backpressure in a smaller turbine housing with the same wheel.

Both housings are made to flow, it all goes small to big. The smaller housing is obviously designed to create more velocity to spin the wheel, while saraficing the free flowing exhaust at higher rpms.....regardless the turbo snail designs all go from small to big to the downpipe which gets bigger, to the exhaust system which gets even bigger.

Right that's how the manifold design works but the question is isn't that same principal with back pressure with the smaller hosuing transferable to to the exhaust back pressure as they are both post turbo tech??

 

[Mr Peepers]

Right that's how the manifold design works but the question is isn't that same principal with back pressure with the smaller hosuing transferable to to the exhaust back pressure as they are both post turbo tech??

Great article:
http://www.tercelreference.com/tercel_info/turbo_exhaust_theory/turbo_exhaust_theory.html

 

[Slippi84]

I knwo I read that before and someone said it was bs but assuming it's true it says that "volume" is the key. Bigger the better. 3" for most of the exhaust and 2.5" for a very small section at the end of two connection points is better than 2.5" all the way. Not to mention I already stated my goal for right now is like 200-225whp which that says is cool for a regular 2.5" exhaust. This doesn't evn take into consideration the x factor....no cat

 

[Mr Peepers]

I knwo I read that before and someone said it was bs but assuming it's true it says that "volume" is the key. Bigger the better. 3" for most of the exhaust and 2.5" for a very small section at the end of two connection points is better than 2.5" all the way. Not to mention I already stated my goal for right now is like 200-225whp which that says is cool for a regular 2.5" exhaust. This doesn't evn take into consideration the x factor....no cat

I hate to keep arguing but I strongly disagree that 2.5" all the way is worse that 3" with multiple 2.5" neck downs throughout it. You can have a 8" diameter pipe with a 2.5" holes on either side and it will still only flow like a 2.5" straight pipe. But, there will be more turbulence from the transitions and the inertia will also be lost. Same idea applies to the 3" pipe with 2.5" neckdowns, but not as apparent.

Can you point out where it says volume is the key?

 

[Slippi84]

I hate to keep arguing but I strongly disagree that 2.5" all the way is worse that 3" with multiple 2.5" neck downs throughout it. You can have a 8" diameter pipe with a 2.5" holes on either side and it will still only flow like a 2.5" straight pipe. But, there will be more turbulence from the transitions and the inertia will also be lost. Same idea applies to the 3" pipe with 2.5" neckdowns, but not as apparent.

Can you point out where it says volume is the key?

The general rule of "larger is better" (to the point of diminishing returns) of turboback exhausts is valid

more volume = larger

Read this article. Import tuner did a dyno test and showed that a test pipe/no cat system made 4whp more on a NA car imagine the diffrence on a bosted car where less back pressure is even more important...important enough that we're fussing and debating on 3-2.75" necks.

 

[Mr Peepers]

Larger as in diameter, not volume You could make the pipe twice as long and in turn have twice the volume, but it wouldn't flow any better. I may sound like an ass but I'm just trying to be clear

No offense

 

[Slippi84]

Larger as in diameter, not volume You could make the pipe twice as long and in turn have twice the volume, but it wouldn't flow any better. I may sound like an ass but I'm just trying to be clear

No offense

No you don't sound like an ass but I mean it sounds like were saying the same thing. You do understand no one is saying this exhaust is longer than most but that the 98.9% of the exhaust is 3" so I don't see how the volume couldn't be any better than a pipe that is 100% 2.5"

edit: And this is all pre muffler I haven't even brought up how HUGE this muffler is. hell the silencer for the thing was as big as the stock exhaust if not bigger.

 

[Mr Peepers]

Volume is a very broad term when it comes to exhaust, that's what I'm trying to get at.

You can have all the diameter you want but with 2.5" neckdowns between it, the flow will not change. A bottleneck is a bottleneck as stated before. The more transitions, and the more abrupt the transition, the more turbulence.

Edit: Lets just agree to disagree. Most likely you will never notice a difference on the butt dyno with the 2.5" neckdowns or not anyway, I just wanted to point something out.

 

[Slippi84]

Volume is a very broad term when it comes to exhaust, that's what I'm trying to get at.

You can have all the diameter you want but with 2.5" neckdowns between it, the flow will not change. A bottleneck is a bottleneck as stated before. The more transitions, and the more abrupt the transition, the more turbulence.

I am going to mic the neck down to see exactly how small it is but the neck downs are not abrupt in any way it smoothly transistions and transitions just as smotth back out and again this IS ONLY AT TWO LOCATIONS. Volume is not vague it's a forumla and very precise. The system either has more volume or it doesn't. You supplied the link I'm just going off what it said. the more volume the better to a point it said in so many words.

Well when I do start boosting and pushing the setup we'll see how it holds up. I'm not trying to justify my buy I would do it agian in a seocnd I just don't see how bottle kneck on a apexi is fine and by the way whoever said it's close to the turbine hosuing so it's ok that was obviously miss info as per the article it says if you are gonna have a kneck down do it as far away from the turbine hosuing as possoible, and yet the kneck downs in this system turn it into a bonified 2.5" catback???

 

[Mr Peepers]

Volume has more than one factor in its formula. That is what makes it vague when talking about exhaust systems.

 

[Slippi84]

Volume has more than one factor in its formula. That is what makes it vague when talking about exhaust systems.

I am gonna do some research and find the exact exhaust volume forumla plug the numbers in for both a straight 2.5" and this style at exactly the same length and see how diffrent the outputs are. Then we'll be able to see if the bottle neck theory is fact or myth. But it's def 1:30am and debating .25-.75" is not worth losing sleep.

 

[Mr Peepers]

We still aren't on the same page.

Of course there will be minimal difference. What I'm saying is that it's not volume which is being referred to but diameter. You can have two exhaust systems with the exact same volume yet completely different diameters and lengths. Finding the volume of the two will tell nothing about the neckdown theory.

 

[staticbrainwash]

It's turbulence of flow you have to worry about with this. It's like hollowing out a cat, the SAME principal applies here. Going big to small disrupts flow, plain and simple.

 

[Mr Peepers]

Well when I do start boosting and pushing the setup we'll see how it holds up. I'm not trying to justify my buy I would do it agian in a seocnd I just don't see how bottle kneck on a apexi is fine and by the way whoever said it's close to the turbine hosuing so it's ok that was obviously miss info as per the article it says if you are gonna have a kneck down do it as far away from the turbine hosuing as possoible, and yet the kneck downs in this system turn it into a bonified 2.5" catback???

There is a difference between necking down and a gradual taper after the turbine outlet. The turbine outlet on a 16g for example is 2.5" so having a pipe that is 2.5" that gradually tapers to 3" is ideal. The turbine housing is 2.5" anyway, so going straight to 3" and having a 1/4" step all the way around would hurt flow. Staticbrainwash was right.

I could very well be wrong when I said a full 2.5" would flow better than 3" with multiple 2.5" neck downs. My original point was that backpressure is never a good thing in itself, and that you want to avoid transitions whenever possible within reason.

 

[Slippi84]

We still aren't on the same page.

Of course there will be minimal difference. What I'm saying is that it's not volume which is being referred to but diameter. You can have two exhaust systems with the exact same volume yet completely different diameters and lengths. Finding the volume of the two will tell nothing about the neckdown theory.

Ok because this is going around in circles I'm just gonna point out the obvious and call it a day. No matter what you say his system has less back pressure then a 2.5" full exhaust so it's better for a turbo car period. We can agree sure it would be better if it had straight 3" but aas I said ALREADY it's but a cut and weld away. Thanks for pointing out the obvious though.

 

[staticbrainwash]

Dude, we don't need the attitude here. We're just trying to help you understand why it's not such a good thing. Thats all.

 

[Slippi84]

I don't have a attitude I'm sorry if it comes across that way but not my intension. I simply posted about the exhaust and to show how it looks and how it sounds and so far how it performs. The points you guys are trying to make are understood but I just don't agree sorry. I understand that your trying to educate me but I have modded a FEW cars and understand the diffrence between therory and real life especialy with dsm's. My 2gnt is my first non boosted car in about 2 and a half years(87 toyota supra x2, 90 talon tsi awd) so I have seen atleast 3 exhaust systems and the effect they have on performance. That article as informative as it is is just theory. saying 300hp is pusing it for a 3" exhaust is rediculous. There is a 11.09 sec gs-t runnig on a 3" exhaust with a custom 2.5" dp makin 443whp. Trust me I understand the theory but it's just that theory. I understand you two like to be safe rather than sorry that's why one of you is running a built motor on a 14b setup and the other is aiming the same path except a 16g but i'm not that how I mod. Thanks for the imput though

 

[Slippi84]

To apease everyone I mic'd it again and it's 2.75 at the neck and 3" everywhere else I guess thats' why they list 2.75 in the auction but it's true 3" everywhere else.

 

[4UH8ERS]

Put it this way. You have a true 2.75" pipe all the way through and you have a 3" pipe that bottlenecks to 2.75" in two different sections. You push 400 cfm of exhaust gases into each pipe. The 2.75" all the way through pipe will obtain and sustain a higher velocity due to decreased diameter and lack of turbulence, while the 2.75" restriction will still only allow the same amount of volume to pass while the turbulence and enlarged diameter decreases the velocity. Sounds like the bottle neck would actually be worse than a 2.75" all the way through.

 

[Locke]

Put it this way. You have a true 2.75" pipe all the way through and you have a 3" pipe that bottlnecks to 2.75" in two different sections. You push 400 cfm of exhaust gases into each pipe. The 2.75" all the way through pipe will obtain and sustain a higher velocity due to decreased diameter and lack of turbulence, while the 2.75" restriction will still only allow the same amount of volume to pass while the turbulence and enlarged diameter decreases the velocity. Sounds like the bottle neck would actually be worse than a 2.75" all the way through.

I concur, good Sir.

I would upload the paint diagram I made to explain it, but ut keeps saying its to large of a file, don't know why.

If you saved it as a BMP, it will be uncompressed. Try saving it as a Jpeg.

 

[Mr Peepers]

Put it this way. You have a true 2.75" pipe all the way through and you have a 3" pipe that bottlnecks to 2.75" in two different sections. You push 400 cfm of exhaust gases into each pipe. The 2.75" all the way through pipe will obtain and sustain a higher velocity due to decreased diameter and lack of turbulence, while the 2.75" restriction will still only allow the same amount of volume to pass while the turbulence and enlarged diameter decreases the velocity. Sounds like the bottle neck would actually be worse than a 2.75" all the way through.

I would upload the paint diagram I made to explain it, but ut keeps saying its to large of a file, don't know why.

Exactly what I was trying to say, but you explained it much better

The assumption of 2.75" flowing better than 3" with multiple 2.75" neckdowns is still in question though. Hopefully someone with more background in fluid mechanics and thermodynamics can chime in

 

[Slippi84]

Put it this way. You have a true 2.75" pipe all the way through and you have a 3" pipe that bottlnecks to 2.75" in two different sections. You push 400 cfm of exhaust gases into each pipe. The 2.75" all the way through pipe will obtain and sustain a higher velocity due to decreased diameter and lack of turbulence, while the 2.75" restriction will still only allow the same amount of volume to pass while the turbulence and enlarged diameter decreases the velocity. Sounds like the bottle neck would actually be worse than a 2.75" all the way through.

I would upload the paint diagram I made to explain it, but ut keeps saying its to large of a file, don't know why.

Also agreed but velocity is not flow as paul has pointed out in another thread I just got done reading. Velocity shoudl not be mixed up with flow because while they are related they are in no way interchangable.

 

[4UH8ERS]

Also agreed but velocity is not flow as paul has pointed out in another thread I just got done reading. Velocity should not be mixed up with flow because while they are related they are in no way interchangable.

Who is trying to interchange either variable. We are merely explaining how each variable is effected by the other. Not that one is replacing the other.

btw: Paul, read your pm =)