75mm TB (split from For Sale thread)

[sharkey]

ford 4.6l throttle body. its 65mm. brad (treez24) is running one on his hybrid with 3500 manifold, but his stand alone uses a fast idle valve. im not sure about what you would do with the iac motor, ive seen adaptors to run a gm iac on a ford throttle body.

[PCGUY112887]

The question still stands, if you tossed a 75mm TB on a stock 3500 opening, aside from a very touchy throttle feel would it work? (with a MAF system in particular).

I totally understand what you mean, and I love your 65mm TB... but I just can't afford it and I don't know what else I could turn to for a cheap TB that would be 65mm or greater.

[SappySE107]

Everything was ported for the 70mm on a 3500plenum, but you can only open it so far, which is part of the whole problem I am talking about. If 70 was good, we would be making them or I would sell the adapters for them. Since I know its not a good thing to do, I won't sell that setup. I can't stop anyone and if the boosted people think they can use a 75 then I would like to see it done. I would also love to see a dyno comparison between the 65 and any larger.

[bszopi]

Good theory, but... The fact is, its letting in the air that the engine needs that much faster. What used to be 10% throttle is now more like 20% throttle. And you have 100% flow (the top end will restrict the flow of the TB) at around 70% throttle. So you have just cut your effective pedal travel (and thus pedal control) significantly.

You also have a large opening and then you have a ridge between the TB and plenum. This will mess up the flow of the air into the plenum significantly which can screw with the way the air flows into the plenum and down to the heads.

[PCGUY112887]

lets pit experience agaimnst experience. 70mm on a 3400 makes it near undrivable on snow and is extremely touchy on dry ground as well.

Is that with the upper intake ported out to 70mm as well?

My theory is just that... if the opening is only 65mm, only 65mm worth of air will get though.

[SappySE107]

lets pit experience agaimnst experience. 70mm on a 3400 makes it near undrivable on snow and is extremely touchy on dry ground as well.

[RebelGT]

Who would put a 75mm TB on a 3400?

on a 3400 no, on a boosted lQ1, sure.

[ryan.hess]

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

A larger throttle body isn't like putting a larger carb on an engine. When you put a larger carb on, the airflow through the carb slows to a trickle, which plays hell on the jets. Faster airflow through a carb means better metering and better atomization of the fuel.

A throttle body doesn't have fuel flowing through it. The only difference you see with a larger throttle body is the larger progression of throttle. I.e. if you open the throttle 10%, it might be like opening the throttle 20% before you swapped the larger throttle body. So it becomes more sensitive to your foot.

The hardcore racers get around this by using independent throttle bodies. This still alows the throttle feel of small throttle bodies, but they can get away with huge throttle areas for maximum WOT performance with no penalties.

[PCGUY112887]

Yeah but cmon lets find a way to make it work lol. I need something that will go onto a 3500 upper that doesn't cost $275.

[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

[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.

[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...

[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.

[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.

[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.