drill hole in throttle body?

[cals400ex]

i have a speed density tune on my 2000 Z28. i opened the set screw and in order to get it to idle, it needs to be opened up so that the TPS reads 2-3%. i assume that the hole needs to be drilled in the throttle body so i can adjust the set screw farther in to read 0%? also, i don't know why i am getting a maf dtc?

[cals400ex]

here is a scan of the system:

[12secSS]

Never drill out the stock hole on the throttle body, the IAC valve can compensate for the wildest of cams. Also set the TPS back to .47V at zero (0) % TPS. do not use this to hold the idle, it will trigger DTC while trying to bring the idle back into sync with the flashed parameters.

You need to log STIT, since LTITs are at zero. To dial in the idle you need the timing to be at a set timing value (or at least that is what I do), also ensuring the fans do not come on at all. There has been plenty of discussions on preferred methods of getting the idle just right, so a search will reveal a slew of information.

[S2H]

Never drill out the stock hole on the throttle body, the IAC valve can compensate for the wildest of cams. Also set the TPS back to .47V at zero (0) % TPS. do not use this to hold the idle, it will trigger DTC while trying to bring the idle back into sync with the flashed parameters.

You need to log STIT, since LTITs are at zero. To dial in the idle you need the timing to be at a set timing value (or at least that is what I do), also ensuring the fans do not come on at all. There has been plenty of discussions on preferred methods of getting the idle just right, so a search will reveal a slew of information.

you must not do any "Big" Cams

the stock IAC is only good to compensate up to a certain point...
when you get a big cam, then you need more than the TB set screw can compensate for at cold idle...
if its opening to 310 counts at any point when not fully cold.. then you need to get more air in there...
the TB set screw can be used up to a certain point, and then you need to zero out the TPS sensor by doing a TPS reset.
the typical max voltage is around .55
beyond that, you need to drill out a hole in the TB.

Ideally.. you shoot for 60~80 IAC Position at hot idle with the AC off and your fans commanded off in the scanner
once you get there..your cold idle will be within range

the IAC motor not keeping up becomes a problem especially on a bigger cubes motor that needs a lot more air to idle..its made to compensate for a 346...not for something with another 40~60 cubes more

also.. you can just log, IDLE DESIRED AIRFLOW
it is the end result of LTIT and STIT...
log Idle desired airflow vs Idle airflow table temps.. and you can just log it, and then do a copy and paste....
you will still need to do two passes or possibly more to fine tune it...but the first pass will get Idle Airflow close to the right values.

[DSteck]

I adjust with the set screw, then just twist the TPS sensor after ovaling the the holes out. It has worked perfect for me.

[12secSS]

Scott,

"Big cams" is a relative and vague term, to clear up I have not had to drill out the stock hole with cams up to mid 250 duration (@ .050" lift). You have your tuning methodology and I have mine, I respect everyones method as none is the end all method. Don't judge my comments without knowing my methods. Have a good day.

[Frost]

Scott,

"Big cams" is a relative and vague term, to clear up I have not had to drill out the stock hole with cams up to mid 250 duration (@ .050" lift). You have your tuning methodology and I have mine, I respect everyones method as none is the end all method. Don't judge my comments without knowing my methods. Have a good day.

...in for the same... I had a car with a 252/259 @ 0.050 in a stock cube motor just a few weeks ago and had no trouble getting IAC counts into line at 0% TPS by working with the blade screw. Looking at my files and the number of cars I've done (not mail order) vs the times I've had to drill, it's around 200:1.

[2xLS1]

Tell me what is the difference in letting more air in with the blade screw vs a bigger hole. Both are letting in more air. You are either letting more air in around the edges of the blage or through the middle. Why is one ok to do but the other is taboo? Why did the GM engineers put a hole in the blade to begin with? Why did they just not open the blade more with the screw? Because the purpose of the screw is to keep the blade from sticking in the bore, not to set bypassed air.

[DSteck]

The set screw is reversible... drilling is not. That's really the only justification I have for using the screw.

[2xLS1]

Holes can be made smaller. I've done it.

[Bill@HPTuners]

Holes can be made smaller. I've done it.

As have I.

Personally I'm of the "whatever works" side. I'm not going to crucify anyone either way.

I've always, personally, adjusted the idle airflow tables but I can see the other side of things working just as well.

[NicD]

The one nice thing about drilling a larger hole is that you get more "sweep" area of the blade in the bore of the TB which can aid in smoother off-idle transitions. I prefer that once I get to the point of where opening the blade reads positive throttle vs rotating the TPS. There is no right or wrong way, they both work well and have their place.

[edcmat-l1]

It's nice to get close by drilling, and then final adjustment with the set screw. Put me down for both LOL

[Ben Charles]

Want to bring this back for help.
I have a 150 IAC at warm temps (170+) with my car in sig. I checked TPS voltage before I ever begin and it was .57. I set the set screw on my TB to show 2% TPS on the scanner, I unpluged the TPS and IAC and reset it to 0. I then went to adjust the TPS by slotting the holes, now I cant get a Voltage reading over .24 by clocking the TPS, I decided to go the other way and I get the same.

Confused on what to do next? The scanner now read 0% TPS at idle and even when I touch the pedal to WOT.

[Ben Charles]

Wanted to say I had my mechanic friend come by and set it up for me. These things can be real tricky, apparently they can't just be placed on therre, but have to have some type of load put on them from the TB.