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What about 2.25"? It's not as critical on twin setups because the piping to the turbos are usually short. 2" would be better for most IMO. I'm easily making 1000ish crank on 2" piping off each collector on a single T4 setup.
Really not that hard to transition to a T4 with 2" piping...
Can make cone adapters pretty to step up or down the pipe size. (All these images are from 73Runabouts twin turbo mustang)
Last edited by Forcefed86; Jan 17, 2016 at 09:54 PM.
I guess my concern is more with spool than merging to the flange.
If there's an advantage to using the smaller 2 inch stuff, then so be it. But it's only going to be 2.5 from the outlet of the stock manifold to the flange of each turbo, which will only be a 4-6 inch run.
There have been a TON of threads about this if you search a little. I believe the ID of my OEM manifold outlets was closer 2.25 than 2.5.
What kind of power is the engine making NA roughly? RPM limit? Can't imagine you'll be over 450chp or revving very high on a "Stock LQ4". 2" piping flows plenty IMO. There is zero advantage to running over sized pipe pre-turbo. The turbo is usually the bottle neck, not the piping pre-turbo. The higher you keep the velocity of the exh gasses the better for “spool”. With a short run twin setup, it may not make a huge difference, but there will be a difference with the smaller pipe. Same goes for your cold side piping. Every little bit helps. Smaller piping is easier to package, cheaper, and lighter as well.
There have been a TON of threads about this if you search a little. I believe the ID of my OEM manifold outlets was closer 2.25 than 2.5.
What kind of power is the engine making NA roughly? RPM limit? Can't imagine you'll be over 450chp or revving very high on a "Stock LQ4". 2" piping flows plenty IMO. There is zero advantage to running over sized pipe pre-turbo. The turbo is usually the bottle neck, not the piping pre-turbo. The higher you keep the velocity of the exh gasses the better for “spool”. With a short run twin setup, it may not make a huge difference, but there will be a difference with the smaller pipe. Same goes for your cold side piping. Every little bit helps. Smaller piping is easier to package, cheaper, and lighter as well.
Exactly. I ordered the 2 inch stuff just now. Like you said, any little bit will help and 2 inch certainly won't be a restriction at this level.
There truly aren't that many threads that touch on this subject for twins. Cross over pipe size for big singles, however, you can read about until the cows come home.
My motor made 390whp/390wtq with small 1 5/8ths primaries. It was out of steam at about 6800, limiter is at 7.
I think you’ll be happy with it, let us know how it goes. Personally I wouldn’t go larger than 2” off each turbo for the cold side either. And 2.5” to the TB if you are running an IC. Saves a ton of room, 3” charge piping is a bear to work with IMO.
I think you’ll be happy with it, let us know how it goes. Personally I wouldn’t go larger than 2” off each turbo for the cold side either. And 2.5” to the TB if you are running an IC. Saves a ton of room, 3” charge piping is a bear to work with IMO.
The piping off the turbos should have good room, but I like the idea of running 2inch and then expanding to 2.5 at the cooler.
Coming out of the cooler is tight. It's a 3inch discharge. It would help a ton stepping down to 2.5 because I need to clear a rad under it, so any extra room will allow for a bigger rad.
The only issue I see is stepping down from 3 to 2.5 out of the cooler then from 2.5 to 4 for the tb. Might look a little wierd.
Bah, I’ll take performance and ease of install over looks any day. Probably easiest to weld directly to the TB and reduce as necessary and incorporate a good lip on the TB at that time. I always had issues with the coupler popping off the OEM TB’s anyway. Neck it down to 3” (or even 2.5) with an aluminum reducer. Then use a coupler as necessary.
Bah, I’ll take performance and ease of install over looks any day. Probably easiest to weld directly to the TB and reduce as necessary and incorporate a good lip on the TB at that time. I always had issues with the coupler popping off the OEM TB’s anyway. Neck it down to 3” (or even 2.5) with an aluminum reducer. Then use a coupler as necessary.
What TB are you using? The LS1 TB is 3.75”.
Yea it's ls1. Good thing you reminded me of that, lol. I'd be buying some extra stuff real soon ha
If I had the ability to weld aluminum, I’m welding a bead all the way around!
Many don’t have the ability to weld alum. at home. I filed mine down so I had a nice little lip and made room for to get a wide T-bolt clamp behind the bead. Then JB welded a lip on it. Held up ok for the last few years. No more couplers popping off.
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2 or 2.5 piping for twins? 
