52 vs 58mm throttlebody

Hey guys, I'm finishing up my new motor and was on the fence between either a 52 or 58mm throttlebody.

Motor specs are as follows, forged 3.75" crank, forged 4.03 flat tops with 5cc reliefs, stock head castings fully ported with new seats, 2.02"/1.600" Manley race master valves, 56cc chambers, zero decked block, XFI 292, comp springs with 130# seat pressure, ported factory intake, 42# Svo injectors. Rings are gapped for upto a 250 shot of nitrous. I have no flow numbers on the heads.

Since i dont have huge heads, I am leaning toward a 52mm throttlebody body. I know LOTS of people run the 58 on milder set ups, but I feel it's overkill for me. I just wanted to see what some of you guys with 383's who have tried both sizes, and the veterans on this page had to say about it. Thanks

58mm. You have extra cubes to feed and air distribution is important. More air in the plenum will help with that monster cam.

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58 for sure

It looks like a twin 58mm throttle body flows 1000+ cfm. That just seems overkill to me, which is why I posted this. The 52 may not have the volume, but it will have much more air speed.

Having more than cfm is not going to hurt anything and if you are going to drop money on an aftermarket TB then you might as well go all the way. Forget air speed going into the plenum, you need enough volumetric flow (a function of speed and area). Air speed is most important at the port, not the plenum where ideal WOT pressure is ambient. Having a bigger throttle body than you need will only increase throttle response.

I had a stock TB on my 383 with AFR 210s and a smaller cam. The map would drop 9 kpa from 3k to 6k at WOT because it wasn't getting enough air. Swapping to a 58mm TB and 1LE elbow chopped the drop in half but I still think my used Moroso intake is holding me back.

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Some data from the engine dyno. 381ci, very good ported LT4 heads/intake, less cam (230/242 114 0.590/0.590 - cam was chosen with the goal of passing NJ emissions). Guys that put the engine together smirked at my 58mm TB, said it didn't need it. NA the engine made just under 500HP on the engine dyno. They showed me the results - At max HP, the 58mm TB was only 77% open.... opening it to 100% did not add any measureable air flow or HP. They felt that showed I didn't need the 58mm TB. If you simply look at open area, a 58mm at 77% is actually equal to a 52mm at 96%.

On the other hand, the 58mm TB didn't hurt anything. So it's a tossup. Just remember, because a TB is quoted to flow 1,000 CFM, doesn't mean the engine can consume 1,000 CFM. And, flow has to be accompanied by pressure drop, or it really doesn't mean anything.

But.... next step. Engine also consumes a 300-shot of dry nitrous, with the nitrous nozzles in front of the TB, mounted in the outlet of a factory ram air box (no MAF sensor). I still think at that point the 58mm was justified, and its still on the car.

Ok great info. I was under the impression the 52 would give me better throttle response. I suppose when using the nitrous, the 52 may not quite be enough.

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Originally Posted by Catmaigne

Having more than cfm is not going to hurt anything and if you are going to drop money on an aftermarket TB then you might as well go all the way. Forget air speed going into the plenum, you need enough volumetric flow (a function of speed and area). Air speed is most important at the port, not the plenum where ideal WOT pressure is ambient. Having a bigger throttle body than you need will only increase throttle response.

I had a stock TB on my 383 with AFR 210s and a smaller cam. The map would drop 9 kpa from 3k to 6k at WOT because it wasn't getting enough air. Swapping to a 58mm TB and 1LE elbow chopped the drop in half but I still think my used Moroso intake is holding me back.

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Moroso/k&n I noticed a difference for the better in throttle response switching to my ghetto pvc intake on my hotcam bolt on setup. Those intakes are funky.

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I researched this years ago for some hard data despite a lot of talk about the 58mm being worthless for most. Here's some actual data you can use for help.

http://www.camaroz28.com/forums/lt1-...ite-up-739747/

FWIW, I have a 58mm on my stock cube H/C setup, and throttle response and idle are great. It drives just like it did with the stock TB on it. Mine is a BBK that has the IAC passage closed.

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Originally Posted by nickp

Ok great info. I was under the impression the 52 would give me better throttle response. I suppose when using the nitrous, the 52 may not quite be enough.

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If you're going to keep your plate for the nitrous, it won't add any flow to the throttle body. But with the plate I can see an advantage with the larger throttle body. The wet plate is typically known for distribution problems. You don't want a high velocity air stream from the 52mm driving the fuel particles deeper into the plenum, starving #1 and #2.

When I started the stroker build I had a Callaway Honker intake, which is the same as the Moroso. The engine builders/tuners did not think the Callaway intake was going to handle the engine's air requirements very well. Gave me an excuse to buy the SLP ram air hood and airbox. To insure that was adequate we opened up the top of the box as much as we could, and eventually I cut the baffles out of the hood scoop.