So I bought a big bore throttle body a while back and just got around to comparing it to the stock one. Stock body has a hole in it. What is the function of the hole? To keep the idle stable? Do I need to put one in big bore? Inquiring minds want to know.

To an old thread on the topic.

http://www.914world.com/bbs2/index.php?showtopic=249925

Comparing apples to oranges. Do not drill a hole.

Notice on the throttle with the hole. There is very little gap around the perimeter of the throttle butterfly. Very little air flow when the butterfly is closed.

Now look at the big bore. There is more of a gap around the perimeter of the butterfly that is basically a controlled leak that takes the place of the hole in the smaller throttle body. You should be able to measure the difference with feeler gauges if need be.

Don’t forget that the big bore also has a larger area around its perimeter for air to bleed through. No need for the hole. To use Weber 40 vs 44s as an example: the 10% difference (4mm) in throttle bore diameter results in a 21% increase in area of the throttle bore. The area of the bleed around the butterfly will grow by this same proportion. Not sure what bore diameters these are but the same sort of non linear growth of the air bleed will apply as the big bore diameter grows vs the smaller stock throttle body.

Just different designs.

Without a flow bench it would be hard to judge what, if anything would be gained or lost by drilling a hole in the big bore.

Most likely what is going to happen is the air bypass screw for the big bore will be set further out allowing more air to bypass than the smaller throttle body with the hole.

Translation of engineering speak...

You don't need the hole. The butterfly is bigger and the throttle stop holds it open farther.

I am not an engineer, but I do understand engineer speak. My father was an engineer and I grew up with it.

Love the translation

The "big bore" looks smaller than the stock one?

Glad I could help.

Just the way the picture looks.

What did the larger throttle body come off of? MM size?. Don't forget to enlarge the plenum opening to match the throttle body bore. Tail pipe expander works great.

Yep! If you don't enlarge the plenum you won't see the benefits. I used an exhaust expander and bumped it out to just SLIGHTLY oversized and then worked the taper back into the plenum with a small chasing hammer until I had a perfect fit between the parts. About a thousand little taps with the hammer, but only about 15 minutes of time. Used a super thin smear or RTV instead of the rubber gasket, as the gasket was now too small anyway.

Also, are you using custom EFI or just DJet/LJet?

When I went from the stock TB to the 53mm GoWesty TB I was able to feel a difference, but only after retuning the engine to take advantage of the new higher top end.

Zach

Using D-Jet on with MPS tuned by Jeff B. Engine is 2056 with Elgin cam with slightly more lift and Hoffman heads.

You do not need a larger throttle body all the way up to 2.4 on djet.
Tuned on a dyno
Fact

"The hole." Answer me this:
Can't the idle bypass screw that you use to adjust airflow at idle do everything this hole does, but with adjustability?

Of course it can. I guess it's about a similar baseline.

I've seen people tape over that hole to try and slow a fast idle. LOL

If it is a 1.7L D-Jet, you need a bigger throttle body. The 1.7L throttle body is a little bitty bugger.

If it is a 2.0L D-Jet, you don't necessarily need a bigger throttle body, but it can make it a bit more peppy.

i have a similar set up , 2.1 9.5compression, d-jet, and i installed this summer a 49.5mm throttle body, bored out stock '73 2.0 TB from Chris Foley, I can tell you it does make a difference from mid to high end for sure . With the modified cam, conter balanced crank and new heads with larger valves ported and polished, it all makes a difference so with these other changes, more air coming in helps on that top end where it felt before like it was not getting enough. Id like to do a before/after dyno but i dont have that ability. Seat of the pants evaluation says it does.