AFR vs. octane vs. pinging

[gradon]

I should ask over on Naxja to see what idle timing people with obd2s are getting(if they have a scanner).

[lafrad]

I've tuned quite a few hemi's, and something to keep in mind when using piggyback tuners:

Timing is based off of RPM vs MAP.

So, when you use the PSC, MAP CLamp, Map adjuster... *anything* that screws with the MAP signal, you are encouraging the PCM to use the timing at the NEW value that it receives.

Where I see trouble with this: Going bigger on injectors! If you go to a "big" injector, and end up having to PULL FUEL OUT by reducing MAP voltage, the computer is going to think that MAP pressure is lower, and it WILL dial in more timing. Usually these computers have a seperate timing map for WOT vs Closed loop, so you then deal with the fact that you get elevated timing in closed loop, then when it switches over to the "WOT" timing it falls back to something more conservative and the problem goes away.

My suggestion would be, if you NEED bigger injectors, make sure you get an adjustable fuel pressure regulator. then, you can get your "WOT" curve "close" with the big injectors and fuel pressure ONLY, then go back and "clean up" the fueling with the MAP adjuster. This way, your WOT-area MAP values should remain pretty close to stock (for timing in WOT and closed loop), yet you still have extra injector capacity for the extra HP.

[SilverXJ]

Except they don't make an adjustable FPR for the 97+ internal fuel pumps... and I don't feel like converting to an external pump.

I'm letting the PCM deal with the closed loop operation, but adjusting the WOT fuel.

Good point on the timing.

[lafrad]

Well, then I would suggest that, where you are getting ping... use the Map adjuster to make it RICHER, and let the long term fuel trims get rid of the fuel. this way, you should get lower timing numbers, and after some time, it will get the fueling worked out on its own (gotta keep it with 20-30%, however, so don't go TOO big on your injectors....)

[gradon]

So I closed the rc9's gap to .030" the other day and pinging was barely audible in the 2.5K-3K closed loop area(where pcm keeps it at 14.7:1 while nearing peak torque). On Monday, I rented a Uhaul tow dolly and picked up a 95 2dr 4wd aw-4 sport w ~190K that had overheated and was running rough(prob head gasket) for $300(another craigslist score). On the way back, I had a major backfire that blew a hole in the Borla muffler(I had to gut it a while back due to the preforated S that had blown in half and was loose and vibrating--exhaust pays the price for the tuning learning curve) and the Jeep immediately lost power and died(had to recover two jeeps that day). Anyhow, I couldn't figure out why it wouldn't start--tried swapping cps, coil, pcm--every tenth try or so it would start. I unplugged and replugged all sensors and disco'd adj map and it ran for 5 minutes and then died . I let it sit for a day and then yesterday I did the same and now it is fine. It runs great and no more pinging(we'll see if the pcm keeps it this way). I will have to reco the adj map cause WOT was ~13.5:1(6.0V brings it down to 12.5 on mine). I promise I'll use the profits from this latest score to buy the Unichip. Anyhow, you can get away with an 8.5:1 dcr on 93 octane without pinging if you have a good quench(mine's only .010" better than stock--could be .017" if I swap HGs), the right injectors, the right spark plugs, and the stock pcm, but as you can see mine's on the edge. I recommend shooting for a dcr of 8.25:1 or less, so pick your cam and piston(dish size) wisely.