I finally bought a nice Sears inductive timing light with a neat timing advance offset dial on the back, so I can check timing advance, as well as base timing. Anyway, what I found was that my car was at around 2* ATDC with the ESC thing disconnected. I found that surprising. So I advanced it to about 3.5* BTDC. I run Mobil 89 and will be running 93 octane from either Mobil or Texaco on my next tank. My question is, should I try to go higher when I put in the 93? I do have an XP240 and I DO notice counts of knock but I assure you, there is absolutely nothing that is audible to the human ear. I think the knock sensor is really sensative and you can have detonation without even hearing it. With that in mind, I question the technique of some members of this forum who seem to enjoy advancing until you hear pinging then backing off 2*. I think if you had a scanner, you'd see all kinds of crazy knock counts on cruise, part throttle and WOT acceleration. I have very few counts at cruise, a little more at part throttle and a few at WOT. It has changed over 2* ATDC though, meaning more counts...so I guess I'm getting true detonation, according to my KS.

So what should I do? I was going to run some 93 and take it from there on the timing...as for SOTP differences, I noticed not much. It might be snappier off the line, it might accelerate with SLIGHTLY less throttle than before and it MIGHT just pull better from 4000-4700rpm (shift point). I have the stock cam (L98) and the timing spec is 0* BTDC, btw.

Thanks for reading.

 

I wouldn't run any additional timing. Set it at 0 and let the ECM do the work. If you really want to tweak the spark advance I suggest you get into PROM tuning.

Turning up the timing may cause knock, then the ECM will retard the timing...then you've essencially taken 5 steps back.

 

Hmm, that's a refreshing perspective, if radically different from the opinions of some others on this board.

I WAS getting some counts of knock even with the timing set at 2* ATDC, but I don't know if any of it was true detonation. Actually, before I messed with the timing, I did an interesting experiment. If I was driving along (or parked...doesn't matter) and put the car in neutral then booted the gas a few times to rev it up good and hard, the knock count on my XP240 would jump up like 20 counts or more, all at once. And this happened each time I revved it hard (pumped the gas twice, once to 2500, once more to about 4000). Is that supposed to happen? I still maintain that I cannot hear pinging whatsoever.

EDIT: I meant to ask...you say set it to zero and let the ECM do the work. What if I wanted to take advantage of 93 octane? Does the spark advance want to go high enough to take advantage of 93, just that the ESC is constantly retarding it with 87 octane? I would think 3.5* BTDC would be fairly ideal with 93 octane, but I won't speculate any more until I run some for a couple tanks and watch the scanner (and my performance). Thanks for the reply.

 

I agree with your last idea, Kev. The ECM does not advance until knock occurs. It advances a fixed amount unless knock occurs, then it retards. The higher octane should bring down the knock count. If it doesnt, perhaps something else is setting of the KS. As I've mentioned to you before, I am running about 10° BTDC and have noticed a good difference over stock. I dont know if Im getting knock readings but I always run Texaco 93 in it.