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The header collectors are 3", but you could use a 3" to 2.5" reducer to adapt the existing exhaust. The 2.5" is a decent size and the Street Pros flow very well, so with stock heads, intake, and camshaft it'll be enough (especially being mandrel-bent). Start getting heavily ported heads with better intake and bigger throttle body, then you would want to consider going bigger. (it will sound louder, and reduce ground clearance).
 

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FWIW...My old combo ran 11.30's through the full 2.5" flowmaster exhaust, ran the same with no mufflers, and again with open headers. (well, .15 better with open headers, which I think was due to less weight) so don't get hung up on 2.5" Vs 3" too much.
 

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Discussion Starter #43
The header collectors are 3", but you could use a 3" to 2.5" reducer to adapt the existing exhaust. The 2.5" is a decent size and the Street Pros flow very well, so with stock heads, intake, and camshaft it'll be enough (especially being mandrel-bent). Start getting heavily ported heads with better intake and bigger throttle body, then you would want to consider going bigger. (it will sound louder, and reduce ground clearance).
that's great thank you.. so then no issues with keeping my current midpipes. Regarding porting the heads, no experience in that, and not sure I want to have someone touch it over here unless it's something easy to do. May I ask if theres a specific way to do so ? like port sizing? I'm looking up videos as well.

as for the throttle body.. which size should i go for? I'm still waiting to verify my intake # if it's an Ls1 or 6 thank you
 

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that's great thank you.. so then no issues with keeping my current midpipes. Regarding porting the heads, no experience in that, and not sure I want to have someone touch it over here unless it's something easy to do. May I ask if theres a specific way to do so ? like port sizing? I'm looking up videos as well.

as for the throttle body.. which size should i go for? I'm still waiting to verify my intake # if it's an Ls1 or 6 thank you
Look up videos that focus on cleaning up the bowl area. That's where the gains are. Stay conservative on how much material you remove. It's easy to go overboard, and hogging out too much in the wrong areas can affect airspeed and charge motion. Flow is not everything. And without a flow bench to verify your heading in the right direction, you don't even have that as a guide. TSP, PCM of NC, TEA, and WCCH all offer reasonable porting services that include CNC porting that's consistent and with proven results. However you would need to send a set of heads in and wait for turn around. so for your situation that may not be a good option.

As far as throttle body, with a LS1 or LS6 you're pretty much limited to stock size, unless you were to send it to TPIS. They basically lop off the manifold's throttle body inlet and add a larger one to take a 90mm throttle body. Not sure how they manage to make it robust enough to withstand vibration, thermal cycling, and also keep properly sealed to the manifold but seems to hold up, and makes some decent gains. 90MM LS6 Manifold Modification Service – TPiS (TPI Specialties) Pricey though, and again requires shipping turn-around.
 

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The OPs goal can easily be met with cam swap and bolt ons. The 317s are fine for what he’s doing. LS6 intake is better then LS1 but if it’s going to cost $300- $400 or more it’s not worth it. Aftermarket are an option but again expensive for what little you gain. and stock TB is enough. Cam swap and correct tune alone will get you 50 rwhp or more.

Fooling with the heads by a guy that isn’t experienced is asking for problems and sending them out is going to cost at least $1000 plus shipping back and forth to the UA. Once a shop gets the heads they’ll want to do guides and valves which will add more cost. Better to buy some 243s.

1st need to know if he has LQ4 or LQ9?
 
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