Is there actually a problem with using throttle bodies that are 'too big' for the engine?

Reason I ask is that I've recently decided to ditch the 32/36 carb on my Pinto and megasquirt it.

I already have some 750s but do I need to go with something smaller for the Pinto? I'd like to use the 750s as I have them but does the fact that these are useable for 'big power' engines mean they're not suitable for low end stuff like my 2.0 Pinto?
Peeps seem to use them (750s) with relatively standard Zetecs which aren't *massively* more powerful than Pinto.

What are the disadvantages of big TBs over sourcing something like some 600s?

Thanks,
James

[Edited on 5/9/06 by James]

Part throttle response is usaly a bit nasty on oversized carbs/throttle bodies. It only takes very little throttle opening to get significant airflow, meaning a potentialy jerky response. A suitable throttle linkage could minimise this however.

The 750 TB's are oversized for the engine which they came off (for low speeds/power anyway) which is primarily why they have secondary butterflies.

Surely the Zetec is more powerful because it is more efficient? What I mean is that it makes better use of air and fuel.

What I mean to say is that because the engine is less powerful it doesn't necessarily follow that the TBs won't be suitable?!

I was under the impressio that the choke sizes for TBs is not as important as with carbs. The carbs rely on high airflow and venturis to atomise the fuel, hence if chokes are too big, the air SPEED is insufficient to atomise fuel. With TBs, the atomisation is taken care of by the injectors, so it's less critical.
I'd stick with the 750s

Carbs are much more fussy as air velocity is crucial to their operation. However, going too big with TB's also has negative effects as I mentioned.

Also the zetec will be more powerfull mainly due to better volumetric efficiency (i.e. it sucks more air in, and therefore requires more fuel to go with it) rather than through any significant increase in thermal efficiency.

Agree with the coments above re twitchy off-idle response (as in my U2U). However, the cam for the throttle cable is designed to give finer control at closed throttle so this should help. You could make a new cam that was even more exaggerated. As said, the secondary butterflies were a Suzuki 'bodge' to help at low throttle settings but I don't think you could really make much use of them.

As I said in the U2U, my only problem so far is getting an idle below 1000rpm, at the moment it just dies as soon as you hit about 980 and this may be a symptom of how 'shut' the butterflies need to be with such a big body. Just speculating really though.

I've got a genuine 900rpm idle on my 750 bodies.

As already mentioned, they are very sensitive at low speeds. A gentle right foot is definitely needed around town.

Matt,
Have you balanced them using a set of 4 carb balancers. I borrowed some the other week and although mine weren’t far off, difference was astonishing. I had to back off the idle screw and could get a sub 700rpm idle, and that’s on non-stock cams. Fixing the ignition advance near the idle bins made the idle much smoother, stops hunting and allows a slower idle.
Just a thought.

Like MikeRJ says, on a seriously bumpy road it’s hard to keep your foot on low throttle without kangarooing the engine.

Stick with the 750s and MS and enjoy the learning curve =)

Paul.

If you can modify the linkage so the opening becomes more progessive the further you press the pedal the smoother it will be to drive.
Regds
Ian

see

http://www.team-integra.net/sections/articles/showArticle.asp?ArticleID=484

and

http://www.jenvey.co.uk/Tech_QA.htm