250cid L6 Turbo Piping Diameters

[TheSilverBuick]

I'm still quite a ways out from even purchasing a turbo, which will likely be a factor in answering this question, but figured I'd ask anyways.

What diameter would be recommended from the exhaust manifold or header flange to the turbo? Either single pipe or twin scroll (I think that is the word for it..)?

Currently I have entirely too large of a collector and exhaust pipe on the engine at 2.5", so I know I'll have to go smaller. I see on V8's it seems to be recommended around 2.25" pipe.

The current exhaust manifold has two 1-7/8" outlets, and I am likely to get an equal length header that could be made to have a collector that keeps the front three and rear three cylinder exhaust separated as well, so that can be a customized diameter as well.

An example of the header I'm talking about, though this example has slightly longer primary tubes coming off the to clear a triple webber intake than I would get. Its already made, the guy is just waiting for me to cave and purchase =P


Thoughts and experiences? An optimistic eventual goal would be 600HP at what ever boost amount the engine will hold, so if you have thoughts on an associated turbo too, through it out there


With that on an Air to Air intercooler, would 2.5" inlet/outlet diameters be sufficient or should I proceed directly to 3"? My gut says bigger is better and go with 3". I currently intend to use an aftermarket Ford 5.0L single blade throttle body, but I cannot recall at the moment the mm diameter it is. Guessing between 65mm and 75mm, which is ~2.5"- ~2.95" diameter. The throttle body could be upgraded in size in the future.

[DanStokes]

To get an idea you might look up the Argentinian 250 Chevy turbo manifolds. Those are a standard item there and are cast in regular ol' manifold steel. It would be a fair guess that they have been doing this long enough that they've scienced this out before ordering up castings. When I was running my Chevy I found a website for this and IIRC it was in English. I wonder if the casting would fit the Pontiac but I really have no guess there.

Anyhow - I'd guess that this will depend on the turbo's mounting flange.

As far as the intercooler plumbing - I'm going with 3" on the Mercedes but that's because the intercooler I'm looking at has 3" fittings. I don't think it can be TOO big unless it decreases residence time enough that the compressed air doesn't have time to lose it's heat load - but I don't think so. In fact, wouldn't bigger tubing slow down the flow and therefore increase residence time? (Thinkin' out loud here.....)

Dan

[Bob Holmes]

You need to determine the importance of the power-produced characteristics. If you want something that has little to no lag, you plan one way. If lag isn't a problem for you and you are looking for peak potential horsepower, you plan a different way.

[silver_bullet]

wouldn't the larger intercooler runners increase volume to a point where it would create lag? I believe that the Brazilian SPA turbo manifold would be a great idea, because less heat would be lost on the exhaust side of the spool, a tighter package under the hood, and the exit of the turbo is where back-pressure/flow restriction kills performance by keeping it from spooling quickly...Tom Lowe from Dysart Iowa has been doing inline Chevys for decades and may be able to help http://www.12bolt.com/

[greenjunk]

I imagine you can't turn an i-6 up that high, but if you're trying to make 600hp or 100hp per cylinder, I would recommend 1 5/8" primaries into 2 1/4" route them where you need to, then merge to 3" into a T4 plate. 3" fits perfect. Obviously 3" Downpipe. As far as charge side goes, a 2.5" pipe will more then handle the power you're looking to make. The most important thing you're missing is the turbo. You need to choose both the correct a/r on the hot side and the cold side. I researched for months before i picked a turbo, and its specs. Ultimately i picked slightly smaller cold sides and bigger hot sides, because i needed the tq to come in slow, at first but carry max cfm as quick as possible once it was up there (Land Speed Car).

Remember boost is a measure of restriction, so if you're running an OE mild ported head with a small cam it will take a lot of boost to make power, all you're going to do is make heat. There are so many factors here...

In response to your other thread about intercoolers, if this is going to be similar to your layout i would go water to air with the same side inlet and outlet, line the turbo up with the TB and silcone hose it. No boost leaks, simple and clean.