98Z28CobraKiller

iTrader: (17)

I started talking bout this in another thread but it was not titled correctly and was not in the correct section.

My question is a simple one ( I think) for the experienced guys out there. I would like to know how the OPTIMAL timing at idle and under normal driving conditions can be determined without raising timing until you get knock? My 98 knock sensors are crap and I have had to desensitize them in order get rid of most of the (false) KR that I was detecting. Please don't speculate as to whether I had real or false knock as it was certainly false and stick to the question of determining OPTIMAL timing. I know that OPTIMAL WOT timing can be seen on a dyno based on HP/TRQ output but not idle and normal driving timing.

THanks!

fast98

iTrader: (16)

i would think you could find optimal idle timing by using a vacuum gauge and setting the timing at the highest vacuum reading............but i've never tried it and its just a thought

Patrick G

iTrader: (14)

Actually, if you set your idle timing to where it makes the most vacuum, you will have a very UNSTABLE idle. See, your pcm will use timing first to raise or lower your idle based on the desired rpm. The secondary measure is the IAC motor (or ETC). Most motors will make the most vacuum at idle with timing in the 32-36 degree range. Problem with setting it there is you give the pcm no additional timing to raise idle speed if it falls below the desired level.

This is why many tuners will set your closed TPS tables (idle timing tables) at 22 degrees. This will give you plenty of headroom (in spark advance) to raise rpm with additional timing. Working with your idle underspeed and overspeed timing tables will also help keep the idle stable.

Once your throttle goes beyond 0% TPS, you go from your closed TPS tables to your main spark tables. You will probably want your low-load timing tables in the 28-32 degree range up to 1200 rpm, then gradually going up to the high 30s to even the mid 40s at higher rpm. But since the table is a sliding scale, your timing will be reduced as your load increases. This will keep you out of detonation. The stock timing tables are actually a great place to start.

98Z28CobraKiller

iTrader: (17)

So the vaccuum gauge is not the way to do it at idle as you stated due to the headroom required for the idle correction. How about a more clear definition of how to set the timing though. IS there rime or reason or is it based on how it "feels"?

RednGold86Z

If you have EGT measurements, a cooler EGT typically means timing is closer to good (of course don't go into knock, though). If more timing doesn't make cooler EGT (i.e. it levels off), then you've probably reached optimum, and maybe surpassed it. Cylinder head temperature might help indicate too much timing, but you've got to be real familiar with the engine to get useful data from that.
Of course the best way is on an engine dyno. There, you just swing timing at a constant RPM and MAP, and see what minimum timing gives most torque (within 1% or so). There's a bell shaped curve of torque vs timing if it doesn't hit knock, and you'll want to be on the less timing side of the curve, near the peak torque.

Idle: just do what they've suggested above. Use enough to smooth out the engine, and use little enough to mabye raise the idle MAP 3-5 kPa over minimum MAP achievable. Blend it into the normal driving areas. Use a little more timing in the MAP areas higher than idle MAP, to help it compensate for applied loads a little better, but keep things gradual and well blended.

At very light load, in the negative torque region, use less timing. Peak the timing at about 50 kPa for smoother drivability (reduces the jerk when exiting fuel cutoff).

It's probably fine to base the part load stuff off of a stock table, but with a bigger cam, you'll probably need a little more timing in the low RPMs. WOT should be done on a dyno.

EdmontonSS

For part throttle, you can find a way to apply a steady load at various RPMs and tune for least amount of throttle opening at each point (or most vacuum, however you want to term it). You could go on a steady flat road 30 MPH in 1st, 2nd, 3rd gear, 50 MPH 1st, 2nd, 3rd, 4th, 5th and then do the same on a hill, steady speed on a long hill and repeat, this should give you a few points at various RPM's and airflows. This is your "constant RPM and MAP" without a dyno. If you can find more hills with steady grades, or go at higher speeds, all the better for more points recorded. Tune each point for minimum required throttle opening or most vacuum, then smooth and blend your timing table from there. WOT tuning is definitely best done on a dyno.... Or at top speed (set timing for most terminal velocity at WOT! Free dyno runs!)