After plenty of research I ended up picking up a 3" catback for my turbo D16 project. But just today I was looking at my manifold, and the T3/T4 opening in it, which looked rather small. So I measured it up to find that the opening is 2" x 2.25" roughly, which is an area of 4.5". The area of my 3" exhaust is pi x 1.5^2 which is just over 7" area.

So am I missing something here, or is my whole exhaust system only going to flow as well as the most restricting point (i.e. the 4.5" area manifold)? By my calculations that works out to a pipe diameter of 2.4", so could I use a 2.5" downpipe and cat with no adverse affects? Or am I overlooking something here?

Thanks in advance for any help.

 

yeah but you're not bolting up your 3" exhaust directly to your exhaust manifold. Your turbo bolts onto the manifold (t3 flange to t3 flange), and then your downpipe bolts onto your hot-side turbo housing. Do not worry about the opening on the manifold, rather concentrate on the opening and transition into your 3" exhaust post turbo housing.

 

This is true, but exhaust flow rate is the exhaust velocity multiplied by the area of the opening. Meaning I could have a 5" exhuast all the way back from the turbo, but the exhaust is coming out of the engine, and it's only flow path is through that T3 flanged manifold, so the flow rate through the whole system can only be as fast as through that smallest opening, no?

 

not quite, you're forgetting that those exhaust gases from the engine must first pass through the turbo exhaust housing (and spin the actual turbo shaft), where there the exhaust flow rate is disrupted, and it picks back up when it exits the turbo housing and into your downpipe.

 

So are you saying the exhaust has higher or lower velocity leaving the turbo, as opposed to when it enters the turbo? Because it would seem to me that having to spin the turbine would reduce exhaust velocity. I just don't see how a bigger exhuast than your manifold outlet can help. The mass flow rate is restricted by the smallest point in the whole system, and that is the manifold outlet. But I also could be still missing some key point here, so if I'm wrong please show me how the bigger exhuast could help!

 

its been dyno proven, search if you'de like and you'll find out why. All i can tell you at this point is that the manifold t3 flange opening is not restricting exhaust flow and should not be considered as "the smallest point of the whole system" because the "system" actually starts post-turbine housing.

 

How much power are you looking to make? You can probably get away with a 2.5" exhaust just fine, but a 3" will show no adverse effect on power. It'll just be a bit louder depending on muffler size.

Anything regarding exhaust past the downpipe is basically a restriction though. In most cases, you don't need any more downpipe diameter than the size of the exhaust housing's exit diameter into the downpipe. The turbo's exhaust inlet isn't the restriction you are thinking it is though.

p.s. Expect pieces of cat to fly out and clog the muffler.

 

the idea behind having a "as big as possible" exhaust after the turbo is to reduse as much as possible the exhaust pressure to allow more flow through the turbo. think of the exhaust gasses as fluid, the t3 restriction is to incresse velocity to spin the turbo quicker and faster, the 3" exhaust is to decresse velocity and incresse volume. the more volume you move out the more efficient the velocity can turn the turbo.

 

Alright, thanks for the responses. I think I'm starting to understand the logic/theory behind exhaust sizing with a turbo. Actually this link seems to have some pretty good info:

http://www.tercelreference.com/terce...st_theory.html

As some of you have pointed out, the smaller opening at the manifold increases velocity through the turbine, and then the exhuast after the turbo should be as large as possible to reduce backpressure. So I will probably try to keep 3" all the way back.

Thanks again for the replies.

 

The reasoning behind all this talk is that I am using an EVO VIII turbo, and it's not easy to find pre-made flanges for it. I found a seller that makes a flange for the turbo outlet that has a 2.5" opening for the main flow, and a 2.5" opening for the wastegate.

So if I were to use this flange with a 3" downpipe welded to it, would that restrict the flow of the whole system as if it was 2.5", or would I still see reduced back pressure?

 

as far as i know you have about 1 foot of play in the size of the pipe before affecting performance. I would personally keep this transition as smooth as possible, i.e. pie cut the 3" pipe to down size it to 2.5" as closse as you can to the flange. this allowance has something to do with gas expansion, this thoery is not so clear to me but dyno # prove it right.

hope this helps, did extensive reserch myself before building my turbo D, its alot of stuff to absorbe. I would suggest to use the stock evo's turbo turn pipe if it works with your setup, good chances it wont.

 

Alright, thanks, I will try to make it a smoother transition and look more into this theory. And I don't have any of the stock EVO piping or anything, just the turbo with stock wastegate, so that's not really an option at this point. I think just using the flange with 2.5" outlets for both the main and wastegate flows will work best, and just have the two merge into one after around 10-12". I had been thinking about using a 'pocketed' flange that allowed for the stock wastegate to be used with a single 2.5" outlet, but I read that having the wastegate dump that close to the turbine exit was counteractive as well.

Overall I'm not looking to make crazy power, 250-300 WHP at the most, so the differences in performance between 2.5" and 3" are probably fairly negligable, but since I already have the 3" I'd like to try and understand the theory behind everything. Plus who knows, some day I might want to turn up the boost, and if I build the exhaust proper the first time that's one less thing I have to change/hack up.

 

try to get a hold of weirtech on here, hes got the best prices flanges anywhere, not to mention a great selection

(and the fact hes nearly local to you)

 

Its true that the opening in the manifold is smaller than the exhaust diameter, but you have to consider that the manifold is focusing the heat energy into a specific area (the turbine). Running small exhaust after the turbo based on the size of the manifold opening is only going to accentuate the resistance. Another thing to remember is that the exhaust gas is going to have less energy and move slower after the turbo since it has done work on the turbine. I hope this isn't too wordy and confusing to understand. I'm having a hard time wording this...