i have a full bolt on(minus runners,tb and base) 1988 l98 gta. i recently put a comp 08-305-8 cam in and want some kind of idle. it is really rich and i would like an idle of around 650-750rpm. i know i need to tune an eprom for it, but i am currently waiting for the programmer and software, and would like to try this while i wait, as i have a spare tb to experiment on. i wonder if i could do as i have on ls1's and big cammed carb motors in the past and drill a small hole, 3/16 or smaller and maybe it would slightly lean it out? any positive input is appriciated.

modifications are:
-stock short block and heads
-ported plenum and base
-self made air intake(3.5 inch)
-air foil
-afpr
-underdrive pulley
-a/c, smog delete
-msd ignition, wires, cap and rotor, accel module, and blaster ss coil
-edelbrock coated headers, no cat
-4 inch mufflex
-th350, 2800rpm stall

i relocated the maf down right behind the headlight. if i move it back it dosent seem to make a differance. could this be an issue? i also have an electric crankcase evacuation pump. with these modifications(and a factory lt4 cam w/ 1.6 rockers on the exhaust) i previously made 272whp and 378wtq. if i can make 290whp, i would be more than happy. but i am not sure i can do this with a stock tune. i am going to dyno it in the next week or so. again, any positive information is acceptable. i really dont want to hear about how you dont think this cam was right, or i need a better set of heads, or runners or tb. i just want to hear some thoughts on drilling a hole in the tb for the idle.

 

Why complicate things more if you can just pull out fuel from the spark curve?

You can adjust your idle speed with the minimum air adj. screw and via prom adjustments.

 

Don't drill holes man. Just adjust the minimum air screw.

 

if i only adjust the idle screw, then it will change throttle angle. have either of you ever drilled a hole to keep from uncovering the idle circiut slot on a carb? it works well. as it does on the ls1's. i would like to have an idle UNTIL i have the ability to tune.

 

I'm running a 218 228 @.050 cam and did not need to drill holes in the throttle body.

Whats wrong with changing the throttle blade angle? It achieves the same purpose as drilling through the throttle blade.

 

Needs to be smaller than that IMO.

Do a search, there are people that have done it... me being one of them. Start small, you can always put in a bigger hole but not a smaller one.

 

Until you have the ability to tune just do what's been said...adjust the blades, then readjust the TPS.

 

I did not notice that you had the 305 cam. Somehow I thought you had a smaller cam.

 

the car idles, just very rich. i would like a simple solution until i tune it. i am only asking if this has worked for others. i know i need to tune, i have already stated this myself.

 

An EFI throttle body is completely different then a carb. Drilling the hole will not make any difference in the AFR. If the blades can be opened with the adjustment screw AND the TPS can be properly adjusted, then all is good.

The hole is required when the blades need to be opened so far that the TPS can no longer be adjusted. With the hole the blades can then be closed down some and the TPS adjusted within spec (very important on a MAF 3rd gen).

RBob.

 

I drill two 1/8" holes in my 52mm throttle body after installing my LT4 hot cam, as I could not get the car to idle with the IAC disconnected. The throttle blades were open so far the TPS was at .80v. A speed density car does not have an adjustable TPS. After drilling the holes, I was able to close the throttle blades enough to get the TPS back to .60v. Also, surprisingly, I gained half an inch more vacuum and 3 KPa.

I'm tuning a car right now with a 224/230 cam, and it would not even start and idle unless you held the throttle blades open. I had to drill two 3/16" holes, and now it starts up and idles just fine.

 

Good point Kevin, but he has a MAF setup and should be able to get some adjustment.

Also, did you try a MAF TPS on your SD for adjustment purposes? On my '89, IIRC, I could set closed throttle from .3 to over 1 volt...pretty pronounced slot in the TPS.

 

This is true, but you can use the older part number TPS. The only difference is the elongated slots on the adjustable sensor instead of fixed holes on the SD cars. Otherwise they are interchangable. I'm using the adjustable one on my 90 SD IROC and it works fine. Also for some reason the adjustable one is half the price. The adjustable unit part number from GP Sorenson is 779-3538.

 

how about adjusting the afpr down some, i think that would help

 

i tried that. i pulled the fuel pressure down to 43, and it didnt help any, plus thats too low. i adjusted the tps to .49 lastnight, and it helps a little. im going to continue to mess with the tps and iac and see what i can do with it, before resorting to drilling.

 

I had to drill a few holes in my 70MM vortec TB. The iac was all the way in, and throttle blades out until the screw wouldn't turn anymore, and it was still choking.

I don't think you should adjust your throttle blades. Your ECM is gonna command your idle back down anyway. You can set MIN air, but then the ECM is going to try and use spark to lower the idle. The end result will be a surge, and you still won't be happy.

You need to calibrate your idle, fuel, and spark curve in the PROM. End of story.

-- Joe

 

We shall see today

 

Id only drill a hole in the TB blades if I couldnt get the desired min air to get the base idle of round 400-500 rpm (more if you have a cam). Other then that, use the computer to adjust things.

 

i ended up adjusting the tps to .61, replacing the iac and o2 sensor, setting the minimum idle as low as i could get it (about 700rpm), setting the timing at 6 degrees, and the fuel pressure at 45psi. now it seems to idle fine, but surges at times. the idle has cleaned up quite a bit, as it dosnt make my eyes hurt with it idleing, and that says alot considering the types of cars i have worked on. i think the culprate was the iac. i do not think it was working properly. after replacing it, it seemed to idle much better after adjustment. so i guess i am somewhat happy with it now, it no longer has the nasty dip it had at 2800rpm before the cam, back when it was making 271whp. it is pretty smooth up to 4800rpm. once the temp goes down into the low 80's i will dyno it and get the fuel pressure and timing set properly. i would like to see 285whp. i think ill want a bigger converter too, prob 3500.

 

Question:
Did you set the minimum air rate with the Set Timing Connector disconnected?

 

There is alot of confusion in this thread.

The guy has an '88 L98 with only bolt ons. If the IAC and O2 sensor are working correctly and he has no vacuum leaks, then all he needs to do is to adjust the idle by the minimum air procedure which is posted on this website, then adjust the TPS to around 0.6V. And set the idle timing at maybe 8* ( do not forget to disconnect the EST wire).

Forget about anything that your might have learned from carbs, as stated above by one of the best guys on this board, EFI cars are completely different,,, tuning is a completely different matter.