anybody have a good method for calculating TB hole size to drill

[S2H]

just trying to determine how much I need to drill a TB...would like to get close on teh first shot as I dont really want to have to drive back again to do cold idle after we get it warm to figure out how much more of a hole we need...

we did 1 check..at full operating temp he's got 130 counts...we need hm down around 60 and at cold he's pegging the IAC at 310

it would be nice to know maybe a math formula to convert desired IAC to hole size to drill...
probably doesnt exist...but Its worth asking about

and I dont mind if it turns out to be a 6 line formula...
as long as it gives me a # that is area needed...then I can just do a area of circle back calculation to get what I need and drill a second hole...

Thanks if you are a genius and can help me out

[94SS]

Which cam is that? I remember seeing something like that and somebody had already messed with the TB stop. I have never understood why the only way to lower IAC steps is to drill or crack the throttle blades and if this is setup this way because this is a mechanical design of the throttle body from the factory. Seems to me the higher the Desired Airflow is the more stable idle is. Throttle bodies are not cheap enough to be drilling so I will stick with cracking the blades. Maybe it's me but I don't know why it's so hard to get an answer on how idle exactly works in the GM computer.

[Billiumss]

What I have found that works good is, for medium to large cams, I go 2-3 drill bit size bigger than stock size hole.

Just a FYI - My car has a 226 222 116lsa cam, STG 2 Heads, LT Headers, etc and I have the stock hole in my TB blade and my IAC counts are between 40-50 at warm idle (850 RPMs), this is after doing a RAF 2 times on my car.

[S2H]

What I have found that works good is, for medium to large cams, I go 2-3 drill bit size bigger than stock size hole.

Just a FYI - My car has a 226 222 116lsa cam, STG 2 Heads, LT Headers, etc and I have the stock hole in my TB blade and my IAC counts are between 40-50 at warm idle (850 RPMs), this is after doing a RAF 2 times on my car.

2-3 bits in 64th's or 32nd's or 16th's??

[Billiumss]

2-3 bits in 64th's or 32nd's or 16th's??

Sorry about that, in 1/64ths increases.

[S2H]

Which cam is that? I remember seeing something like that and somebody had already messed with the TB stop. I have never understood why the only way to lower IAC steps is to drill or crack the throttle blades and if this is setup this way because this is a mechanical design of the throttle body from the factory. Seems to me the higher the Desired Airflow is the more stable idle is. Throttle bodies are not cheap enough to be drilling so I will stick with cracking the blades. Maybe it's me but I don't know why it's so hard to get an answer on how idle exactly works in the GM computer.

on a carb its an idle screw....on a TB its a bump stop to keep the plate from geting stuck..people just call it an idle screw...
our vehicles reference the TPS and TB stuff and moving that screw can yield some undesired results and tuning issues..
I myself have only once ran into this as the TPS was wust too far and the computer wouldnt enter idle mode...
and it doesnt take much to get there....


drilling a hole is the most appropriate way to get more air into the vehicle for idle conditions

[Bill@HPTuners]

fwiw my tsp 231/237 112lsa cam in my stock cubes with an a4 ran the stock TB hole with just raf, set screw & idle adjustments just fine.

[jimmyblue]

Look at "where you are" counts and "where you want"
counts, each of these lies under a desired area number
which is mm2. The difference in these indices is the
difference in area you have to provide by up-drilling
the current hole or adding a new one. 1mm2=0.00155in2,
you can get to a drill diameter by the arithmetic.

If you look at the TB casting from the back, I do believe
it's possible to drill and tap for a bleed bypass screw
from the IAC inlet port to the IAC "outlet" side. One hole
from the back hollow to the IAC inlet port; one across
the back hollow, outer wall and inner; outer wall then
tapped for an fine thread (like 10-32) adjuster screw
and that screw maybe given a conical point on the bench
grinder. Might need to take some care with the tap angle
to sink it true, but I think this is all straightforward. Not
that I need to do it, so I haven't.

[S2H]

Look at "where you are" counts and "where you want"
counts, each of these lies under a desired area number
which is mm2. The difference in these indices is the
difference in area you have to provide by up-drilling
the current hole or adding a new one. 1mm2=0.00155in2,
you can get to a drill diameter by the arithmetic.

If you look at the TB casting from the back, I do believe
it's possible to drill and tap for a bleed bypass screw
from the IAC inlet port to the IAC "outlet" side. One hole
from the back hollow to the IAC inlet port; one across
the back hollow, outer wall and inner; outer wall then
tapped for an fine thread (like 10-32) adjuster screw
and that screw maybe given a conical point on the bench
grinder. Might need to take some care with the tap angle
to sink it true, but I think this is all straightforward. Not
that I need to do it, so I haven't.

one issue...IAC's are not created equal...
so on one car it may be 30 on anoher car it may be 40...

[chrisj6]

ok heres a question for you guys....how big of a deal are the IACs? I mean my base airflow reads -.01 lb/min directly across the ETCs when warming up and when warmed up, in gear and park. My IACs are between 60-70. Is it good enough there or should they be lower and do I need to drill the TB to get them there?

Chris

[WS6FirebirdTA00]

what is the effective area value at 130 steps and what is it in the range you want to hit at idle? I would dietermine the difference in the effecitve areas and use that to add area to the hole.

Example:

130 steps= 48 mm^2 in effective area table

40 steps= 26 mm^2 in effective area table

you want to increase the hole size by 22 mm^2 and just add that to the area of the stock hole (1/8"?). 31.65 mm^2 is the stock hole size if its 1/8. So you want ~53 mm^2 hole. So what would mean you need a 21/64 drill bit. Although I have never done this, it makes sense to me. Let us knwo what you do.

[WS6FirebirdTA00]

ok so I skipped the posts and responded and saw jimmy said the same thing pretty much lol

[WS6FirebirdTA00]

ok heres a question for you guys....how big of a deal are the IACs? I mean my base airflow reads -.01 lb/min directly across the ETCs when warming up and when warmed up, in gear and park. My IACs are between 60-70. Is it good enough there or should they be lower and do I need to drill the TB to get them there?

Chris

The car will run fine as long as you dont make out the iac. dont look at lb/min you dont get good enough resolution. change it to g/sec

[WS6FirebirdTA00]

one issue...IAC's are not created equal...
so on one car it may be 30 on anoher car it may be 40...

If the effective area curve was accurate this would be no problem. I know on my car if I go by the effective area table as I explained it woudl come out perfect and I can just shift the table over to get it lined back up.