I just installed a cam and my idle is pretty rough. I would like to know how to set the proper closed throttle spark advance. I found this info on an LT1-Edit FAQ:

Closed Throttle Spark Advance

This is the amount of advance when the TPS reads zero. This will have a large effect on idle quality, and may need to be changed with larger cams/increased idle rpm. A vacuum gauge can be used to help determine the proper setting for this, as it will have a direct correlation.


According to Datamaster, I have 60kpa at 875rpm idle (stock cam had 32kpa @ 800rpm). Should the timing be advanced or retarded now that there's less vacuum?

 

You should probably be able to get better than 60, but you would advance the low end until MAP ceases to improve. With a big cam, you advance down low, a little at a time, and in a balanced fashion. While cell to cell there is automatic balance (interpolation), too much variance will make the idle rougher as it bounces from cell to cell in the MAP range, and as you decellerate, etc.

Most of your time will be spent working low end timing, if you want perfection, possibly a good deal of time. By comparison, a short quote as best I recall "it took us 200 chips to make the car run well, and 400 to make it perfect." I really wanted to spend more time down low because I felt there was amazing power to be found, but I kept running into pinging on 91 octane MTBE gas. If you can find those sweet spots though and have everything balanced just right (you may well be doing 1 cell at a time, burning, then testing), then you could have an even more driveable car than before.

So, while a pretty good overall dyno tune will take 1-2 hours, real perfection takes engineers plenty of time to perform I'm sure, for all kinds of varying situations/conditions, plus those guys are pros, so to duplicate the feat will take plenty of work for a real hot car through all the rev range.

Good luck

 

So what kind of timing are you guys running down low???????

 

The absolute timing others are running won't really help. Simply kick up the timing a degree or two at a time in the idle range....40-60 kPa and 800-1200 rpm.

I took a wild cammed 93 with DFI from a choppy 72 kPa idle to a smooth 50 kPa idle by adding about 4 degrees of timing in the idle range. Don't go all that way at once but try it one degree at a time.

perry

 

Unless someone has exactly the same cam as you, timing down low is going to be pretty individual, heads will play a role as well, so find someone with the same cam as you and look at thier timing. Try to use it as a baseline and then WORK with it from there.

 

This maybe my problem.
I have pinging along with part throttle stumble and idle stumble.
I have a chip from PCMforless.
When I pulled the chip out the idle stumble was all but gone but the car running rich and blowing smoke, soon as I put the chip back in the smoke cleared up.
What suggestions do you guys suggest.
I have hotcam, slp midlength headers, ported head 2.00/1.60 int/exh valves.

 

Your going to need to fix the timing on your own, or try to log and send the logs to Brian so he can make the chip for you.

 

Ok My car idles around 67-80KPA. So your telling me that if I keep adding timing in the 40-60range it will bring the idle down into that range. Sorry for the dumb questions

 

No such thing as a dumb question.

Yes you need to add timing.....I think.

The "I think" is due to the WAY high idle kPa you have. Make certain we aren't trying to tune around a vacuum leak.

But yes. Add timing a degree or two at a time in the base spark table for 800 and 1000 and 1200 rpm and around the 50 kPa value up to the 80 kPa value.

So if you are at 18 or 20 or 22 change all the values to 20, 22, and 24. And than relog and let us know where you are on idle now.

Hopefully that will pull your idle kPa down to 50-60 kPa.

Perry

 

With a cam like that, you should eventually see somewhere around 45-55 kPa is my guess. Anything close to 80 is ridiculous, that would be a huge cam, if tuned right.

Dont add to much timing at once, and dont leave your tables very unbalanced, as the car bounces from one MAP point to another, erratic timing will cause an even worse idle.

Good luck!