Hole in Throttle Body Flap, Needed or Not Options

[914Sixer]

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.

[Superhawk996]

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.

[ClayPerrine]

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.