Rough idle with big cam
 

I'm tuning a friends 6speed '95 LT1's idle. He's got a big cam but it's pretty bad. I increased idle rpm to 900rpm and that helped a bit. Has anyone experimented with idle timing for big cams? IAC adjustments? any other ideas or tips?

barometric pressure here is 86kpa
his map at idle is 53kpa
for reference, my stock engine is ~32kpa at idle

Also has a nice surge at low rpm under some load (like pulling away from a street light).

Not a big cam but for my 218/228 112 LSA Hot Cam. Cranked TPS stop setscrew a bit to get TPS=0% at .75volts, I have an A4 and upped the idle to 800 D/ 900 N/P as you did, also added 3 degrees to the "Closed TPS Spark Advance Vs RPM" table in the 400-1200 RPM ranges, then tweaked the main spark table on the 400-1200 ranges to match TPS closed settings so it transitioned out of 0 TPS smoothly. I stayed away from drilling holes in the TB but I hear it is sometimes necessary w/big cams if you can't get enough air past the TB plate w/o cranking the TPS stops past .75 volts. My idle MAP at 900 is now about 48-50 kpa that's as low as I could get it.

Set the idle timing for the 400-800-1200 rpm cells to 32 degrees sometimes 34 works better have to try it to see. Zero out the target idle over/under speed advance/retard tables for a couple hundred rpm around 0. I would also adjust the tb blades so the iac was in the 80-100 counts range.

As for the surging, you could change the o2 sensor switch points for low airflows to fatten it up and add some timing after looking over logs, modify where the blm cell boundaries are so that it can adjust itself. Usually what happens is the car will end up with lean missfires due to high load and low rpm and a pretty lean air fuel ratio.

Last thing is he will have to adjust his driving to keep the revs up and stay in gears longer. A cam of any decent size is gonna have issues under 1500rpm with any load.

I had to set my idle at 1000 rpm for it to stay running. Especially when it's cold.

So how do you know how big of a hole to drill in the throttle body? and where?
He has a 58mm TB.
His closed throttle position voltage is .63v, so just adjust the idle set screw till its like .75v?
It's a big blower cam, .580 or .590 total valve lift if I recall correctly


Also what are the IAC "counts"? are these like individual steps from a stepper motor or something? and what settings are idel for it?

You will need to either post a datamaster data file of the problems you mention or look at items such as timing since if you dont adjust timing on the new cam from the stock tune then it will run very bad at idle and just off idle.

Bigger cams need much more timing at idle and low RPM than stock tunes, ontop of this the MAP readings at idle are much higher than the stock cam so this adds even further reduced timing to the idle/low load areas.

I could not believe how bad mine ran with stockish timing from just a cam swap, increasing timing to around 34 degrees at 850rpm on up made huge difference and make sure the timing curves are smooth on takeoff or wherever your problems are, if you see a ~5 or more degree sudden change in timing right at the surge area then that is likely your problem. The smoother I made the timing curves (as seen in datamaster) the smoother the motor ran.

Aside from this, with my cam I had to finally run Open loop due to the 02's reading false rich and making the motor run lean under light accell, creating slight surging on accel from a stop, and increasingly worse bucking/surging around 30-45mph. I have a manual trans alum flywheel so this is very noticable.

In open loop all accel surging is gone and 30-45mph bucking is just about eliminated.

My cam is around 230 intake, 234 exhaust.

Some other tables to reduce at idle are "timing advance/retard" over under engine speed or something like that. A bumpy idle and stock values here make your idle timing jump all over the place.

Don't drill a hole in the throttle body totally not needed. Adjust the stop screw (do it with the car off, think cut and check).

What your tps voltage is, dosent really matter as long as its not too high or too low (out of range).

Counts are related to the position of the stepper motor in the idle air controller basicly it goes from 0-160 the farther toward 160 it is the more air is traveling through the idle air controller. Stock usually runs about 30 counts. I like to run more with a cammed car, the iac air passes through a series of small passages and feeds air directly to the ports. Getting the counts close to 100 usually helps quite a lot with distribution problems of the lt1 intake when exacerbated by a large cam.

I just noticed his IAC position in datamasters (at idle) is Zero.
Occasionally it'll bump up as high as 4.

In comparing my car (stock motor), it's more like 30.
Does this mean his IAC isnt making any adjustments and the throttle blades must be cracked open, even though the TPS reading is 0% (at .63v)?

Does setting the idle setscrew past .75v register 1% TPS or something?

Also he has an A/C compressor delete pulley. Should I zero out the "IAC Offset for A/C Anticipate Vs. A/C Pressure" table? (so it doesn't effect anything when his A/C request goes on)

The PCM looks at what the TPS voltage is at key on or when you start the car, one of the two and makes that 0%. Im sure it has to be within a certain range or it will throw a code and not reset to 0% any longer.

As long as your getting no codes and the TPS % is changing with every little bit of throttle opening then your ok. Sometimes you canont check this unless the motor is running since the PCM does some learning on that but not exacly sure when, I thought at key on or crank.

If his counts are zero it means the air is either all passing through the throttle body, or he has a vacuum leak (iac valve is closed completely). The solution is to close down the throttle body blades.

Stop worrying about the tps volts its inconsequential as long as its in range whatever the voltage is when the key is on the pcm assumes as zero. Setting it past 0.75 sometimes makes the car think its recieving input, that is out of the range the pcm deems things are acting 100% normal and may or may not cause some erratic behavior. It would be of more concern if you had an a4 to deal with.

I would zero the iac a/c pressure anticipation if its been eliminated. I would also zero the timing table for egr if thats been eliminated.

Awesome. :metal: Thanks guys.

I reset the IAC counts to 115 because thats where it was going after I closed his idle setscrew. I set Closed TPS timing to 34 degrees. Running great now at idle.

Still have a slight surging problem under cruising and slight accelration. I think i'm going to try and minimize the issue through adjusting the MAF calibration tables.

FYI, I have my idle at 42 degrees to get the best vacuum. This gives you a comparison. Further, I completely closed off my throttle blades and drilled out my TB to get my IAC to run around 40 or so when warm.

On big cams, cracking the TB will work but the best way to do it is to have ALL idle air running through the idle plenum NOT the main plenum. This evens out BLM's on Right vs. Left bank at idle. Mine are perfect at idle even with my VERY big cam.

whoa 42 at idle. what do your timing tables look like? you must be running a lot of timing after 1000rpm.