Big Cam guys, what size hole drilled in TB Blade...

[Loudmouth LS1]

Just curious cuz I just got my TB back from Nick Williams (he rebuilt it with his new stuff) and he said to drill the hole even bigger (I already had it drilled out, but can't remember the size) and close the butterfly some, he said it was open way too much and may be causing the not coming down under 1.6% after wot runs. Any help is much appreciated, thanks guys!

[TAQuickness]

That's a good question with no exact answer. The goal is to get get the IAC to about 40-80 steps at hot idle. Before drilling, I would strongly suggest you take advantage of the idle set screw, keeping track of how many turns you're putting on it, to dial in the idle where you need it. When it's all good, start with a 1/64" - 1/32" hole and start working the set screw back to it's original position. As the set screw goes further back, you will need to drill the hole larger, in very small steps, to compensate.

[Doc]

Take baby steps. I've seen too many go too far.

[Maggie]

With very large cams I like to use two small holes and go 1/64 to 1/32 on one hole at a time...1/32 on a big hole can go from 'not enough air' to 'way to much air' very quickly.
When that happens it's very hard to un-drill a hole ;)

[dfe1]

Drilling a hole in a throttle body plate is really a crutch that is a carry-over from the carburetor days. With a carb, you can get all the air you need by just turning the idle speed set screw. The problem with this, and an engine with a long duration cam, is that by the time you get the throttle plates open far enough to flow the required amount of air, fuel is flowing through the transition and/or main circuit. If you drill holes in the throttle plates, you can back off the idle speed screw, get the throttle plates back to their proper position (so fuel wasn't flowing through other circuits) and still have adequate air flow for reasonable idle quality.

On older EFI systems like the Tuned Ports, the TPS is adjustable, so all you have to do to increase idle air flow is crank the idle speed set screw in, get the air flow you need it, and reposition the TPS so its voltage output is consistent with what the system expects at idle (.53-.55 on a TPI system) and you're set. (Except for engines with EXTREMELY large cams. I have a 400 inch old style small block with a 256/256 at .050 cam on 106 lobe separation and it idles with no holes in the throttle plates.) Later systems, like those used with LT1 and LS engines have a non-adjustable TPS that zeroes itself electronically. With these systems, the lowest voltage generated by the TPS is equated with 0% TPS. Actual TPS physical position doesn't matter in this case because the electronics do the "adjustment". (You can prove this to yourself by holding the throttle open slightly and turning on the ignition. Look at the TPS voltage and indicated percentage of opening. Then release the throttle and watch-- a new voltage reading will now be indicated at 0% throttle.)

Obviously, there are limits as to how far you can open the throttle at idle and still be within the range that will be tolerated by the system. So where this long-winded explanation is leading is to the fact that drilling ever larger holes is a mistake because as Maggie points out, undrilling a hole is very difficult. Maggie's idea of using two small holes instead of a single larger one is also spot on. The important factor with regard to air flow is hole area, not diameter and the relationship between the two is not linear. By way of example, two 2-1/4" diameter pipes have approximately the same area as a single 3".

I'd work with both the idle speed set screw and throttle plate hole diameter and try to get the idle where you need it with the smallest hole. Depending on the size of the existing hole, you may want to drill a very small second hole, adjust the throttle screw and work from there.

Sorry for the long explanation, but I've just seen too many guys ruin throttle body plates and not solve their idle problems because they thought hole drilling was a magic "speed secret".

[Loudmouth LS1]

Yeah, what sucks is that I can't remember what size hole I had in the tb before (it was a lot bigger than stock though) and Nick Williams said I need to go bigger and close the throttle blade, that it was open much too much and could have been contributing to the not falling under 1.6% after wot cuz of how much air was flowing over the blade, i'm gonna drill out the original hole in there to around where it was by eye tomorrow and then drill a small second hole and play with it from there, thanks for all the advice guys, much appreciated!

[Loudmouth LS1]

Alrighty, so I got everything set today. I drilled the hole that was already in there out using a 13/64 drill bit and then put another hole in the top of the blade the same size, started the car with the throttle blade fully closed and it died right out, so I opened the blade 2 full turns with an allen key (angled part full 360 degrees twice) and she started right up and idled beautifully, I let her warm up and looked to see where the iac counts were, and they were at 0 when warm, so I tried closin the blade down to bring them up, got them to like in the 30's and then the car would die goin into gear and wouldn't idle in gear at all, so I put it back to 2 full swings after tryin a few other things that didn't work, reset the tps and she's beautiful, iac still reads 0 warm, but I guess that's just where my car likes it, at least it's not hangin at 1.6% and surging anymore after wot (went and tested like 3 times)!!!

[Beer99C5]

Just in case you need a new Throttle blade, Shaner has them for like $20. The Ported TB I got from a ZO6 had a hole drilled in it and I replaced it pretty easily with a new one Mark Shaner sent me.

http://www.s2performance.net/home2.html

Call him he is not an Email fan LOL