Well I have the headers, cold air intake, slp siamese intake runners on and now thinking about jumping up in size on the throttle body. Looking for comments and suggestions on which size to jump to from the stock 48mm. Should I go to 52mm or 58mm ? I will port the plenum to match at that time. Will I see any difference at all between the 52mm and 58mm ? I know heads and cam is involved as well, but that's down the road.

 

Id say 58mm but even that seems small, dual bore?

 

Anything over 52 on a 350 is too big. You've done pretty much all you can with what you've got.

 

The runners in the lower manifold are too small. All the throttle body is going to do is keep the plenum full of air. If you install a vacuum gauge in the plenum and one before the tb and make a full throttle run, maybe that can help determine if the tb is a restriction.

 

I would not do anything but a 58mm especially on a TPI.....

 

How is 58mm too big for a 5.7l when a 5.3l has a 87mm.

 

You're talking about dual vs single.

 

How does a vacuum gauge work with atmospheric pressure before the throttle body manifold vacuum drops at wide open throttle what kind of test are you suggesting ?

 

Everyone's car is turbocharged. It's called atmospheric pressure. The only problem with big throttle bodies is low speed torque suffers. A progressive tb would be best.

 

Um ok I thought the op had tpi without boost

 

Not too big as such but of no benefit because as noted the TB is not the restriction point with a stock base.

Get better value out of spending $$ on something else

and FWIW
my L98 with 113 alum heads , ZZ4 cam , fully ported plenum and base , SLP big tubes , LT's and free flow exhaust ran exactly the same times with a stock 48mm TB and a 58mm TB.
( 30 runs ,same day , same track , same conditions )

 

Yes but when people ask questions about how to make thier engine breathe better they fail to realise that when the tb is opened air is forced in only by atmosheric pressure and nothing else. So to get the most power your plenum pressure needs to match atmosheric pressure otherwise the tb is a restriction. But then lowe speed torque will suffer actually making the car slower. So a progressive tb you can have low speed torque and max power. I mean the tiny little runners, tiny valves, cam, all need to be bigger for max effort but were focusing on throttle bodies here.

 

your missing the point of the op, The tpi base is the bottleneck and the tpi long runner design. A more suitable choice would be a hsr, super ram, t-ram sounds like a real mild 350. He never mention what cam he has or heads or compression ratio he is running or if its all stock and stock tune. There is no such thing as a progressive throttle body for tuned port. It sound like he has upgraded his exhaust but more information is needed from him to help guide him.

 

Yes I made that point in post #4. I've only had 2 tuned ports but have never disected them to verify their ****-poor design. I bet there aren't even radiused port entries. I figured there was no progressive tb for the tuned port available in the aftermarket but I'm sure there's one out there somewhere that could be adapted. But anyway I really doubt there are any gains to be had by a tb upgrade on a tuned port.

 

Go 58mm and don't think twice about it especially if you plan future mods!!

 

My point is the same as tunedperformanc, getting a 58mm throttle body will do zero for you if you're running the factory intake. Now, if you want it because of future plans, then by all means, but if you don't, then I wouldn't go over 52mm simply because you don't need it.

FTRspeedy, you are referring to the LS or Vortec 5.3L in the Silverado's? Those are entirely different than our old TPI motors, especially with their intakes.

 

Well thats all fair enough, i got it understood now.

 

5.3 can move more air than yours can

you dont need any more than a stock 48. Wasting your money