I hate to beat a dead horse or start a debate such as I have seen in the TPI section but here is my question. I know that a stock TB will handle up to 500 hp based on air flow alone but what size do yo need to put 24lbs of boost through it to a 383 sbc? I am using an accel super ram base and SLP siamesed runners. It is 8.2:1 with twisted wedge heads and all forged interanls. ZZ4 hot cam w/ all rollers. T76 Q w/ .96 ar 1 3/4" Headers (304) into one 3" y to the turbo. I thik that I will need a 58mm TB and my buick friend (12.2@112 TTA mostly stock parts IE turbo) thinks that I will need only 52mm. Any thoughts? Thanks. John

 

I wouldn't go any smaller than a 58mm

 

first of all, its going to be one hell of a job getting 900hp out of the stock style TPI unit to begin with even with upgraded base and runners. I would go with something like the AS&M 1000cfm monoblade and maybe a mini ram or HSR. Even a victor EFI with an elbow and a LSX TB.

 

The 900 is just an estimation. Mike at Full Throttle tinks that 1000hp is what I can expect with this combo and Preston isgetting 917 out of a 305 (I know bigger bore shorter stroke than a stock 305) with a tune port style induction. So I just used this number a a high side base line. I do agree that a mini ram or even better a single plane with an elbow is best but I wanted to keep some semblance of a stock look.

 

The answer to the question would be every horse that you make you need 1.5 cfm of air

 

So my math seems to be fairly close. A 58mm T body usually is around 1100 - 1150 cfm and to make 900 hp I need 1350ish. Thanks for the info.

 

58mm seems awfully small! A stock 3500 (3.5l v6) gets a 65mm TB (225hp), and the stock LS6 is 75mm (400hp). Am I missing something?

In N/A applications it is easy to see if you need a new TB. Just take a vac measurement right behind the TB, and again at the back of the plenum. If you don't see more than a 2" difference at WOT, then you need a bigger TB. Boosted apps are a little tougher, but oversizing would only result in bogging with low/no boost (even with a partial throttle).

Here's some reading material...
http://www.cartechbooks.com/vstore/s...=3048&CATID=21

 

Did you typo that? The way *I* would guess if you need a bigger TB (coming from a carbed guy here, bear with me), would be to take a pressure (vacuum) measurement *before* the TB, then again after. Any *more* than a 1-2" pressure drop across the TB means you need to increase the size. This is at WOT of course.

The way i'm reading your suggestion is to measure behind the TB (after?), then again after, in the plenum. What difference would TB size make there? And you're looking for *more* than a 2" difference to be ok? Please tell me you typo'd in there, and I don't have to re-learn everything I know about this stuff

 

Sorry, you are correct (I'm a boob, need to proof read more often ). If you followed the link, it explains it better than I. You are trying to measure the restriction by seeing the difference across the TB (or throttle plate for that matter).

Would the same test work in a boosted situation? If so, a couple of guages hooked up while driving could work.

 

Yes, it certainly should. Just instead of a difference in pressure (in the vacuum scale) it'd be a difference in pressure in the boost scale. But same concept.
Two sensors, hooked up to a differential pressure gauge would work. That's what they do to measure when an air filter needs replacement in the oil industry..

 

Problem is, when boosted under a turbo there is lag time where the engine is actually running N/A. Finding a happy medium will take some testing, a TB large enought to carry the boost, but not bog on a WOT jab might be difficult. A street/strip car vs. an all out drag car would make a difference as well. Don't see where the OP specifies the purpose of this car.

 

TPI systems use a twin-plate throttle body. So he is talking about 58mmx2, which means you have a total surface area in the openings of 8.17 square inches... a 65mm single blade TB would only give you 5.13 square inches of area, and a 75mm would give you 6.84 square inches. A stock 48mm TB gives 5.60 square inches.

To the original poster, adapting a 90mm LS1 style throttle body might be a good option if you need that much flow..

 

Funny you should say switch to an LS1 setup. I am currently going to sell the 383, TPI stuff and go with a 6.0l .