Downpipe question.

[Xeiros]

I've got a 2.5" HKS cat-back, and I'm in the process of buying a downpipe. More than likely I'll be picking up one of the ones off ebay. My question is, should I get one of the ones that is a 2.5" all the way (that necks down to 2.25 where the cat would be) or one that becomes 3" after the flex coupler and then necks down in the same spot?

Would having the 3" be better even if it's only that little bit there, or would the 2.5" one be more suitable (even though both neck down to 2.25").

 

[xveganxcowboyx]

Bigger is better than smaller.

 

[Xeiros]

Even when it's going back to something smaller? It would be like such..

2.5 ---Flex---3----2.25/2.5

vs

2.5 ---Flex---2.5---2.25/2.5

 

[silver eclipse]

I would say to see if you could find one that stay 3in and get a high flow cat thats also 3 in
-Chad

With the decrease in size it may cause allot of back pressure.

 

[Xeiros]

Ok, so going from a 3" hole to a 2.5" hole with no neckdown what so ever isn't going to cause problems? Exhast gas would be hitting a chunk of my gasket (rather than just the outer rims of it) as it's making it's way to the cat-back portion. Thats not going to cause some form of turbulance or something?

I'm very ignorant to exhaust flow, but wouldn't that be bad? Or at least worse than it necking down to 2.5 or something?

 

[silver eclipse]

I dont really understand what you mean.
What I was saying is that going from 3in to 2.25 for the cat is going to be restrictive and cause back pressure. See if you cant find a 3in high flow cat and have the pipe diameter stay the same.
-Chad

 

[greengoblin]

I've got a 2.5" HKS cat-back, and I'm in the process of buying a downpipe. More than likely I'll be picking up one of the ones off ebay. My question is, should I get one of the ones that is a 2.5" all the way (that necks down to 2.25 where the cat would be) or one that becomes 3" after the flex coupler and then necks down in the same spot?

Would having the 3" be better even if it's only that little bit there, or would the 2.5" one be more suitable (even though both neck down to 2.25").

You would be better off running one that is the same size as your Hks exhaust and getting a matching hiflow cat or test pipe unless you plan on replacing your exhaust too.

 

[Xeiros]

Alright, yeah, you're not following.

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I will be getting a downpipe like the one pictured. As you'll note, it's one that will replace the cat. As you'll also note, it necks down at the very end (where a CAT would hook up to the cat-back portion of one's exhaust). That necks down to 2.25" so it can be a direct bolt on.

I've currently got a 2.5" cat-back, and wondered if it would be better to get one thats 3" after the flex coupler, or 2.5" since either one is going to neck down to 2.25"

I wasn't sure if getting the 3" would cause more problems since it's getting a lot smaller than if it were just 2.5"

 

[greengoblin]

Alright, yeah, you're not following.

I will be getting a downpipe like the one pictured. As you'll note, it's one that will replace the cat. As you'll also note, it necks down at the very end (where a CAT would hook up to the cat-back portion of one's exhaust). That necks down to 2.25" so it can be a direct bolt on.

I've currently got a 2.5" cat-back, and wondered if it would be better to get one thats 3" after the flex coupler, or 2.5" since either one is going to neck down to 2.25"

I wasn't sure if getting the 3" would cause more problems since it's getting a lot smaller than if it were just 2.5"

Teh 2.5 would be better. The 3inch one would just allow the exhaust gas to expand and cool down some. That would just hurt you since it would reduce right back down to 2.5 for your catback. You should however have the 2.25 neck down cut off and rplaced with a 2.5inch pipe and flange.

 

[Xeiros]

Thank you, thats what I was wondering.

And yes, I did plan on having the neckdown cut off at some point. I'll likely throw it on for the time being, and have it cut of in a month or so when I've got some more money to spend.

 

[greengoblin]

I don't think a muffler shop would charge you more than 50 dollars to reflange it for you.
I'd do it at the same time but I understand if $ is tight.
Good luck and happy boosting.

 

[dsm-onster]

Forgive my lack of tact, but this is absolute BS:

Teh 2.5 would be better. The 3inch one would just allow the exhaust gas to expand and cool down some. That would just hurt you since it would reduce right back down to 2.5 for your catback. You should however have the 2.25 neck down cut off and rplaced with a 2.5inch pipe and flange.

Causing the exhaust gas to expand and cool down some is what you want more than anything in the entire arsenal of exhaust flow tricks, when running a turbo.

_________________

That would just hurt you since it would reduce right back down to 2.5 for your catback.

How would it reduce flow. How would it hurt?

_________________

You should however have the 2.25 neck down cut off and rplaced with a 2.5inch pipe and flange.

yes. This is getting bigger. That is what you want w/ turbos bigger is better there is no counter to this! As turbos operate on the function of differentiating pressures. This is a contradiction to your first 2 sentences.

 

[kmoore]

I would highly recommend RRE for their downpipes.

 

[greengoblin]

Forgive my lack of tact, but this is absolute BS:


Causing the exhaust gas to expand and cool down some is what you want more than anything in the entire arsenal of exhaust flow tricks, when running a turbo.

_________________


How would it reduce flow. How would it hurt?

_________________



yes. This is getting bigger. That is what you want w/ turbos bigger is better there is no counter to this! As turbos operate on the function of differentiating pressures. This is a contradiction to your first 2 sentences.

When the exhaust gasses are hot, they flow better. As the gasses cool , bigger helps.

Since he is not going to replace his catback a three inch down pipe choked down to a 2.5 inch cat back system is going to do him no good. A bottle neck is never a good thing down stream period.

That is why I told him to replace the 2.25 neck down on the down pipe he is buying and bring it up to 2.5 so that his exhaust stays a linear diameter.

Your right your tact could use some work.

 

[greengoblin]

I would highly recommend RRE for their downpipes.

They make great downpipes and will most likely make you a 2.5 inch one to match your exhaust.
I'd give them a call.

 

[dsm-onster]

I'm cutting your posts up...please don't take that as a sign of disrespect. You've just been misinformed. This stuff can be found in Corky Bell's Maximum Boost. This book is great reading and I refer to it constantly.

When the exhaust gasses are hot, they flow better.

Given the same diameter, no. hot gasses are more "bouncy". Increase the air temp in a ballon and it gets larger. The air becomes less dense. There is less mass flow where there is more heat. This is why cold air intakes work great

As the gasses cool , bigger helps.

AS the air gets cooler and cooler, the density goes up and a smaller diameter can get away w/ exiting the same amount of mass flow.

Since he is not going to replace his catback a three inch down pipe choked down to a 2.5 inch cat back system is going to do him no good. A bottle neck is never a good thing down stream period.

I'm sorry but no. a big downpipe even w/ a stock catback has been shown to increase spool and flow. I'll never say go smaller. But if you have to choose where to upgrade do the downpipe first.

That is why I told him to replace the 2.25 neck down on the down pipe he is buying and bring it up to 2.5 so that his exhaust stays a linear diameter.

Linear is great but if it can't be done go bigger earlier where the gasses are hotter and there is less density. A pipe of hot air (downpipe) flows less than a pipe of cold air (catback). Intercoolers work wonders for this same reason.

Your right your tact could use some work.

This is why I apologized in advance... But, I also do not want misinformation in my favorite forum Xeiros, go w/ Corky Bell: get as much of the downpipe as big as you can get it. . .

 

[pboglio]

Yep, hot gases flow faster which increases pressure loss. Gains are gonna be higher nearer the turbo. Based on a quick CFD simulation I just did, my ported 2g 02 housing vs. a ported EVO III 02 housing is costing me over 1 psi of pressure loss using a mass flow rate of 36 lb/min and gas temps of 700*F. Thats more than my 3" downpipe and 3" catback combined.

So yeah, go 3" downpipe, gonna be a big gain over your 2 1/8" pipe.

 

[Xeiros]

Slight update. Got my downpipe today (which was supposed to be 2.5" all the way) but ended up being 3" from the flex coupler on. It does neck down, but it only necks down to 2.75" at the flange before hitting 2.5" for the cat back portion.

Is that area where the flanges meet (2.75/2.5) going to cause any exhaust problems?

 

[nightspeed87]

On a turbocharger car you dont want ANY backpressure since the turbo itself is backpressure. The bigger the better as long as there is ground clearance. If you could run 4in run it.

Direct passage from turbo magazine :

"Some self proclaimed motor gurus state that you should not run too large of an exhaust diameter on your car because negines need a certain amount of backpressure to run correctly. Although the statement about not running too large of a diameter is correct, the assumption of engines needing backpressure is not. This is one of the biggest misconceptions. You need the lowest backpressure possible to produce the maximum power by keeping pumping losses low. It is almost impossible to put too big of an exhaust aft of a turbocharger as a turbo depends alot on the pressure differential across its turbine to get power recovery efficiency as turbo chargers are also sensitive to backpressure. "

Basically the bigger the better.... I wouldnt even touch a 2.5 in anything when it comes to exhaust, only thing thats going to be 2.5 on my car is the intercooler piping.
I have a full custom 3in crush bent exhaust system ran back from the downpipe to the back of the car with no muffler. I know the crush bends can hurt my hp but shouldnt be enough to rub off hardly any significant power with the amount of air flow a e316g flows.
But if I did it all over ( or when i redo it all over ) it will be a 3.5 or 4 in system. Why... Because I can and have nothing to loose in doing it.

Now id like someone to say something stupid like " Oh but youll loose low end torque and scaveging.." Well yes if I were n/a i would...

 

[dsm-onster]

Slight update. Got my downpipe today (which was supposed to be 2.5" all the way) but ended up being 3" from the flex coupler on. It does neck down, but it only necks down to 2.75" at the flange before hitting 2.5" for the cat back portion.

Is that area where the flanges meet (2.75/2.5) going to cause any exhaust problems?

Well if you got at smooth transition like greengoblin said that would be best but dont' sweat it becaue bigger is better than smaller even if you have turbulance from a stepped transition such as you are describing.

I have a full custom 3in crush bent exhaust system ran back from the downpipe to the back of the car with no muffler. I know the crush bends can hurt my hp but shouldnt be enough to rub off hardly any significant power with the amount of air flow a e316g flows.

Well. . . I'll try not to say anything stupid as you've suggested . Have you seen a cruch bent 90 degree bend on a 3 inch pipe? It crushes to a terribly small area at the apex!!! even a 45 degree angle suffers profusely from a crush bend at 3". Do you have 2 45 degree crush bends on your down pipe? or 1 90 degree crush bend. Necking down that small early on in the exhaust is detrimental to turbo flow.

EXCELLENT quotes from those who know best . But It doesn't seam that you're following their advice exactly. Nevertheless, the concept is there.

IMHO, mandrel bending is more important as you go larger on exhaust pipe diameter.

 

[islandigital]

IMHO, mandrel bending is more important as you go larger on exhaust pipe diameter.

In a performance application, Mandrel bending is allways important. No matter what sizethe pipe. I cant agree thats its only important in large tube diamater. Look at proffesional track motorcycles, they are running maybe 1.75" tubing, Much smaller in the smaller classes. Do you tink they would say it does not matter because the pipe is small?

I understand why you said that, Heck if anyone would be running a 2" pipe on one of these, then crush bend it anywayus because its already resticted?

 

[nightspeed87]

Well if you got at smooth transition like greengoblin said that would be best but dont' sweat it becaue bigger is better than smaller even if you have turbulance from a stepped transition such as you are describing.



Well. . . I'll try not to say anything stupid as you've suggested . Have you seen a cruch bent 90 degree bend on a 3 inch pipe? It crushes to a terribly small area at the apex!!! even a 45 degree angle suffers profusely from a crush bend at 3". Do you have 2 45 degree crush bends on your down pipe? or 1 90 degree crush bend. Necking down that small early on in the exhaust is detrimental to turbo flow.

EXCELLENT quotes from those who know best . But It doesn't seam that you're following their advice exactly. Nevertheless, the concept is there.

IMHO, mandrel bending is more important as you go larger on exhaust pipe diameter.

Lets see my downpipe has two crushbends the first one being about 45 deg, the 2nd hardly anything, but theres an addition of four more crush bends down the system after that which does suck, but I know the exhaust guy hooking me up and i stood there the whole time he did it and we tried to make it as un crushbent as possible so its probably not as bad as youd think. I got it done that way because I got it done from the dp back for 120 bucks and at the time it was open dp and wasnt trying to drive around like that for too long while waiting on some 400 dollar exhaust system when I simply dont have the money for that right now , therefore I had to do something on a budget and unfortunantly they told me the biggest crushbends they can do is 3 in . I dont see my exhaust as a perminant set up as i mentioned, because if I can get my hands on some 3.5 or 4in I will use it, and at what im doing with the 16g im not really that worried about it hurting hp too much, and if it does ol well because I dont need exactly the max hp this turbo can put out, hence im fwd and also considering I only weigh about 2500lbs itll be fine. Im not building a race car here, but supposeing if I ever upgrade beyond the 16g then thats when I will definitly upgrade the piping size.
If i ever did go 3in again then yes it will be mandrel, and I dont recommend crush bends for anybody because it can hurt up to 50 percent of the flow in a worse case scenerio.

 

[islandigital]

Causing the exhaust gas to expand and cool down some is what you want more than anything in the entire arsenal of exhaust flow tricks, when running a turbo.

not to be on a rant but dsm-onster hit a point that i wanted to say something about. the downpipe size question.

Exhaust gas velocity exiting is also crutial at the downpipe as well, I can aggree that bigger is better but not nessacarally cooler. Keeping the velocity up helps exit the gases.

Get a larger pipe to flow more, not to cool more.

 

[Xeiros]

Alright, got the downpipe on along with my HKS cat-back today. Almost sound's Evo-ish. The one odd thing I saw was that my boost levels dropped. I used to run at 15PSI and now it's dropped to around 11-12PSI. That shouldn't happen should it?

I just disconnected my negitive terminal and let it sit off for a minute to reset everything. I didn't have any boost leaks before this either. :\

Also, is that ground wire that was connected from part of the cat to the frame of the car important? I just clipped it and didn't bother reconnecting one. Does that matter?

 

[nightspeed87]

Alright, got the downpipe on along with my HKS cat-back today. Almost sound's Evo-ish. The one odd thing I saw was that my boost levels dropped. I used to run at 15PSI and now it's dropped to around 11-12PSI. That shouldn't happen should it?

I just disconnected my negitive terminal and let it sit off for a minute to reset everything. I didn't have any boost leaks before this either. :\

Also, is that ground wire that was connected from part of the cat to the frame of the car important? I just clipped it and didn't bother reconnecting one. Does that matter?

I take it that your on the t25. That turbo doesnt flow too much so increasing the engines VE ( intake, exhaust, cams, manifold, piping etc. ) will cause that turbo to struggle harder and harder to hold its boost in the higher rpm range.

When we ran my friends 2g on open dp with the t25 his usual 15psi would come on a lot sooner but at the cost of the boost falling on its face a lot sooner also. Exact thing thats happening to you... Upgrade to at least a big 16g and youll be straight.