75mm TB (split from For Sale thread)

[SappySE107]

[Originally from the following thread... http://60degreev6.com/showthread.php?t=34483]


Who would put a 75mm TB on a 3400?

[ihatemygrandamg]

Damn I thought 65mm might be a bit overkill unless your boosted or cammed. 75mm??? Not for us 3400 guys, thats 19mm over stock lol

[SappySE107]

I refuse to sell an adapter to put a 70+mm TB on a 3400/3500. Its a bad idea and I won't help you guys get shitty throttle feel and WOT well before the plate is all the way open. No one listens to me it seems but whatever.

[patgizz]

I refuse to sell an adapter to put a 70+mm TB on a 3400/3500. Its a bad idea and I won't help you guys get shitty throttle feel and WOT well before the plate is all the way open. No one listens to me it seems but whatever.

hahaha.

[ihatemygrandamg]

I refuse to sell an adapter to put a 70+mm TB on a 3400/3500. Its a bad idea and I won't help you guys get shitty throttle feel and WOT well before the plate is all the way open. No one listens to me it seems but whatever.

I agree. No need for that much bigger.

[ryan.hess]

I would not recommend this throttle body if you're making less than 225hp. 75mm is used on a 250hp aurora from the factory, so a little bit less hp is probably okay.

[SappySE107]

You have to look at other factors though, like the intake manifold design. How are you going to possibly use a 75mm TB on a 65mm opening? Why have such a large hole feeding a much smaller hole. Stock 3500 is 65 and stock 3900 is 70mm. Stock 3400 is 56mm. I think GM figured out the TB sizes for the best power/powerband for their engines finally.

Id love to see more comparisons between TB sizes for boosted engines as well.

[ryan.hess]

When I did a back to back comparison of stock 75mm and an 80mm throttle body on my northstar, I gained 6hp and 15lb-ft of torque from 3000-6500. The whole torque curve just shifted up, it was unreal. I'd bet the 3900 would have similar gains if the intake could be opened up. The 3500 could probably gain some too, but you would be decreasing the throttle's dynamic range unless you had a high RPM screamer or something.

But we're getting off topic. 15 posts and 3 of them have to do with the original post. If you guys want to discuss throttle body sizes and pros/cons, post a new thread.

[SappySE107]

Your dyno comparison is for a 4.6l V8 DOHC engine that has a 75mm stock TB (and went to 78.6mm, not 80). I saw your throttlebodies on ebay. You think a 225 hp 3400 is going to be able to use a 75mm TB? ROFL!

Can't split threads or id move it so you could sell your 75mm TBs to people that don't check the pushrod general section see this information. Sorry.

[bszopi]

As Ben stated, it is not physically possbile to open up the plenum to fit a 75mm TB. I'm not sure what the 3500 is capable of, but I know the TB neck walls on a 3400 plenum start getting pretty thin when going to just 65mm. Put anything bigger on and the metal will be so thin, it'll probably rip the TB neck right off the end of the plenum!

The fact that the size is overkill is a mute point if its not physically possible to fit it on the plenum.

[CNCguy]

I know the TB neck walls on a 3400 plenum start getting pretty thin when going to just 65mm. Put anything bigger on and the metal will be so thin, it'll probably rip the TB neck right off the end of the plenum!

So true Brad.... I don't have a 3400UIM here to measure but I do know that there was barely any material left after a 65mm.

[ryan.hess]

Your dyno comparison is for a 4.6l V8 DOHC engine that has a 75mm stock TB (and went to 78.6mm, not 80). I saw your throttlebodies on ebay. You think a 225 hp 3400 is going to be able to use a 75mm TB? ROFL!

Actually the 96-99 engine's throttle bodies go to 79.4mm, and stock is 74.5mm. That's what was dyno'd... not that it matters any.

Like I said, I don't know anything about 3400's. Generally speaking you can go 50% more area and not notice it all that much.

So if the stock throttle body size is 56mm, then you can take it to 69mm and not take away much of the low throttle characteristics. It will certainly be more sensitive, but after a couple drives you'd forget you changed anything.

You may need to adjust the acceleration enrichment though to keep it from bogging. Depends on the setup, it's less of a factor for MAF engines.

[PCGUY112887]

I'd love it just becuase I can't afford the CNC'ed 65mmTB...

Not tryin to get into an arguement, but what's the difference between a 65mm TB and a 100mm TB if the opening is only 65mm? If your using an adapter, then it seems like TB size doesn't matter as much since you can still only flow so much through that opening behind the adapter.

But is there ANY way to use something this big without a tune? Seems like the car woulden't be able to keep it's idle or anything...

[prophiseer]

I'd think you could pull it off without a tune, as long as the MAF doesn't go out of range.

But, theoreticlly, slapping a 75mm TB up to a 60-65mm bore on the intake won't produce exceptional results, it's just too much of a step for the intake air to plow through. Although, putting some kind of spacer/adapter that is tapered from 75mm down to whatever (like a velocity stack) would really help, but for it to be effective it would need to be a slow taper (i.e. 75mm -> 65mm through a 5" bore vx. 75mm -> 65mm through a 2" bore), possibly even putting a "screw" thread (like in the LSx "vortex" TB spacers) would also help speed up the air and direct it into the UIM bore.

[Superdave]

lol, guys are using 75mm TB's on LS1's as an upgrade...

that would be like putting a 750 carb on a 305... haha