I know that the stock throttle body size is 48mm and my question is will a 58mm throttle body fit in place and not require porting?

 

The largest you can go without porting is 52mm. For a 58 you need to enlarge the holes

 

You don't NEED to port the intake, the 58mm will bolt on. But to get the max performance you should port the intake to the same size as the TB. The stock intake bore is 52mm.

 

I would think the shelf created by putting a 58mm TB over the smaller opening in the manifold would cause enough restriction to make it flow less than a stocker.


A 58mm is pretty pointless till after you have had the intake off anyway, what I am saying is the only way you need a TB that big is if you have already been into the engine, if you have not yet then wait and when the manifold is off port it and then put on the TB if you really want it.

 

So, are you saying that there are no gains useing a 58mm TB even with board out openings only?

 

You need free flowing heads and cam before a 58 is necessary. you can go easily into the low 12's with a 52.

 

I found a site that rated the factory throttle body for 780ish cfm. To me that should provide most street driven small blocks with plenty of air. You would have to be in heads and cam (or adding more cubes) before your engine needs more than that. Its up to you if you want to spend your green on it, but were it me I'd want to put my money where the hp is at. Just my $.02

 

Flow ratings quoted without a corresponding pressure drop are meaningless. My dyno tests with a 58mm TB showed it supported 500HP on my 381 when only 77% open. Opening the blades farther did not increase power. I could have run the 381 with a 52mm TB at that power level.

 

Thanks for the official insite injun, I love to see actual tests that back up ideas.

Hmm, I was thinking that if there was less Hg in the manifold with a 58mm as to opposed to useing a 52mm, that alone would make it a better thing from the engines point of view whether you made more horses or not. To be honest though, I dont have a clue as to what is good Hg and what is bad.

 

The site says "All CFM values are corrected for airflow at 28 inches of water." I'm assuming this is what you mean when you refer to a pressure drop?

http://dtcc.cz28.com/flow/index.htm