I have a 58 MM TB from another application I am going to use with my setup. L98 with vortec heads and intake, Comp Cams 220/230 .510/.510 114 LSA, SLP runners, 1.6 RR's and a 125 hp NOS kit, MSD all the assorted bolt-ons. My question is should I open the upper plenum to match the throttle body? Yes, I know this throttle body is a little large but I have it and I am going to use it. There are many conflicting stories and I am sure you have heard them all. There is a slight mismatch with the TB, which would in theory let a little less airflow into the upper plenum. I sincerely appreciate any information or help. Thank you.

 

Well, where the front of the plenum is there is an aluminum weld/casting flash mark... (where the tb bolts up to the actual plenum 'tube'). If you take away too much material on the outside edges, you will weaken the aluminum and it will more than likely seperate on you. That is one thing you really have to watch out for when you are porting the plenum as well.

Though, to be honest, if I were you, I would cut out the space in between the two ports in the plenum front, then just widen the top of the two ports, because the bottom and sides can get pretty tricky.

 

I have already taken down the lip on the bottom of the plenum. I am referring to the top of it. There is about a 1/4 of an inch of material at the top of the blades.

 

https://www.thirdgen.org/techbb2/sho...highlight=58mm

search is your friend. BTW, thanks for bringing this up though, going to add it to the index threads at the top

 

Perhaps I should have made myself a bit more clear. I am wondering if I have to match the throttle body to the plenum? If I leave the mismatch won't it in fact reduce some of the airflow? If the throttle body is too large and creates too much airflow would it not benefit to leave the obstruction?

 

You don't have to do anything. But now you have pictures of how you would do it if you so choose, so now you can better decide if you want to go through all that. Yes that slight step could reduce airflow somewhat. At the least i'd round the edge over alittle bit, but beyond that, airflow at the surface is pretty slow so there shouldn't be any real destructive interefence or anything.

If you're looking for a before/after dyno of a 58mm TB on a stock plenum before and after port matching, i've never seen or heard of such a test. If you're just mining for random opinon on the board i apologize for confusing things with a scientific approach.

 

Thank you very much for the information. It is appreciated.