Vacuum advance vacuum from throttle

In cleaning my throttle bodies on my 1988 V12, I noticed the two A bank vacuum hookups at the top at the butterfly, even though the hookups are the same size, the ports inside the throttle body - one is a tiny hole and one is much larger. (these are for the throttle edge tapping)

My question is, which one should I use for my vacuum advance regulator? I’m assuming the larger hole will make it respond faster? Or will the larger one have a bit of vacuum at idle, since part of it is behind throttle butterfly? Or does it crap matter ?

Did you find any difference between the two vacuum taps? I’ve always been curious about which one might be better. Which one is bigger?

Neither on the left side are for ignition controls. They open the carbon canister purge valves in stages. The right side goes to the distributor vacuum controls as shown in your diagram.

Besides the size of the hole, there may be a difference in how the butterfly passes over the opening. The idea is that at idle the opening is outside the butterfly and hence sees no vacuum. As throttle opens, the butterfly passes over the opening so the opening ends up on the inside and seeing vacuum. One of these ports might see a slightly different transition than the other.

I think this only provides a signal to the vacuum regulator. The vacuum regulator then sends vacuum from the large port on the back end of the intake manifold to the vac advance unit. Hence, it probably shouldn’t matter much how large the hole is.

No, I didn’t notice any difference. I ended up hooking it to the front one, as that was originally it’s location when I got the car.

Your diagram shows the vacuum hook up for the distributor on the right bank. Nothing on the left bank. When you got your car it had a lot of issues. Why would you hook it up differently than the factory diagram? Left bank is for carbon canister purging at different throttle opening positions. That is why there are two ports on the left side. Both purge valves feed off of a common hose from the canister so which one goes to which valve does not matter on the LEFT. RIGHT side is for distributor control. On your original post, you mention the two ports on the A bank. A bank is the right side. If you have two ports, then the throttle bodies are on the wrong side of the engine. right should have a single one on the top and one on the bottom.

Mine has two top ports left AND right bank. We’ve been in agreement that the carbon canister uses both ports on left side. The question was which of the two to use on right side for vacuum advance. One had a smaller port than the other.

The right side is only supposed to have a single one. Looks like someone put a extra left side on the right. That being said, neither is proper for the vacuum advance although it might work. One opens sooner than the other. If you are not going to put the proper one on, then just hook one up and see how it drives and then switch. If you are happy with either, then you are set. The correct answer is another proper throttle body. Readily available and not expensive used. Also, the proper right side has a vacuum port on the bottom which is used in the distributor controls. The left side throttle body does not have this port.

It does not go straight to vacuum advance, it goes to vacuum advance regulator. I have it hooked up like diagram on my car sticker.
My right throttle looks original, maybe some cars needed a second port on right side for some smog device? Mines an 88, btw.

You are right. The hose from the port goes to a vacuum regulator which maintains a minimum of 11HG of vacuum to the distributor except for full load which the Dump valve lets the vacuum go. One end of the dump valve goes to the port on the bottom of the right throttle body just like your diagram shows. Do you have the lower port on your right side throttle body and is it hooked up to the dump valve?

Yep! And easy enough, I just cap rearward top port on right side. No need for new throttle body!