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screamin_conure | OBD1 "Mystery timing"??? | ||
Senior Guru
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Gurus, Got a question regarding a strange phenomenon that I've recently discovered that may or may not apply to my '92 LT1/A4 coupe. I've seen a few posts on other forums, mostly the F-body sites, talking about "mystery timing" or what is essentially erroneous spark advance/retardation under acceleration that some OBD1-equipped vehicles have been prone to have. |
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Posted on: 2012/2/10 21:29
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Matatk | Re: OBD1 "Mystery timing"??? | ||
Webmaster
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I know very little about the tuning tables, I've never gotten too far involved in them.
Have you been able to locate the other table people claim the car switches to at 4000 rpms? I know that the bosch IIIs fire differently than the stock style injectors and a tune must amended to compensate for the differences. I wonder how much is related to that? Matthew |
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Posted on: 2012/2/10 23:15
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2002 EBM convertible, Magnusson supercharger, cam, headers, etc. 1989 Corvette...RIP |
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tjpreul | Re: OBD1 "Mystery timing"??? | ||
Elite Guru
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What octane do you use and how often do you fill up. They do have a preset tune for when wot. If you were getting a spark knock it may be a fuel issue.
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Posted on: 2012/2/11 3:00
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Vegas wasn't built on winners. '92 auto, MSD, Banski, Taylor wires, SS lines, C5 rims, 3.07 gears, white gauges, seats from a '96. Motor: Forged rotating assy, Probe pistons, CC valvetrain, AFR heads, Procharger P1SC |
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bogus | Re: OBD1 "Mystery timing"??? | ||
Grand Imperial Pooh-Bah
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Your injectors are not Lucas - they are Multec.
GM used Multecs in these things for years. I was talking to Jon at FIC recently. They found a problem with the Bosch injectors in reference to how the Multecs operate over their duty cycle range. In short, the Multecs are NOT linear (if I remember rightly). They have modified the Bosch III's to work in the same manner. Call Jon at FIC on Monday and talk to him about this. |
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Posted on: 2012/2/11 5:31
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The single biggest problem with communication is the illusion that it has taken place. - George Bernard Shaw Education is the best tool to overcome irrational fear. - me |
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Woodstock | Re: OBD1 "Mystery timing"??? | ||
Senior Guru
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Earlier L98 engines like 2° advance across the board a lot, not so much the LT1.
LT1 stock engines rarely show spark knock. I figured the spark table must be on the conservative side, but no gain could be achieved by advancing timing. The LT1 owner couldn't feel a difference, so we pulled it out again. Bosch injectors require some adjustments in PW correction vs. battery volts, because they have a lower resistance. I doubt anything can be done to the injector itself. |
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Posted on: 2012/2/12 14:29
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screamin_conure | Re: OBD1 "Mystery timing"??? | ||
Senior Guru
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Thanks for the replies guys.
@Matt - The tables for spark advance are simply called "Main Spark Advance Vs. RPM Vs. Load" and "Main Spark Advance Vs. RPM Vs. Load (EXT)". The first one covers the RPM range of 0 through 4,000 RPMs, and the (EXT) version covers the RPM range of 4,000 RPMs through 7,500. Not sure why the GM engineers broke that into two tables instead of keeping it all in one. Perhaps due to a limitation in the total bits and bytes those ECMs were capable of handling at one time, I don't know. Either way, the ECM makes a seamless transition between the two tables as the RPMs cross over. @TJ - I always run premium. Most of the stations in my area only offer 91 octane, but there is one that isn't very convenient for me that offers 93. I have to say (and I don't know if this is just a "placebo effect"), the car FEELS like it runs better on the 93 (i.e., smoother idle, more power, etc.). I've also been kicking around the idea of trying some 104+ Octane Boost in the spring too, but I've heard both good and bad about that stuff, and to me, it just seems like another way for a company to separate someone from their money. @Andy - Actually, the original Multecs were long gone when I bought the car, and these are the injectors that were in it: http://www.racetronix.biz/itemdesc.asp?ic=01D042x&eq=&Tp=. One of the previous owners must have ran into issues with the Multecs and canned them. I think Standard Motor Products now makes those injectors, so I guess I was incorrect in calling them "Lucas" injectors. It appears that they are closest in spec to the original Multecs that you're likely to find. @Woodstock - Yep, agree 100%. The LT1 timing tables are fairly conservative, and probably rightfully so given the temps that those engines usually run at. And I think you hit the nail on the head regarding the "Injector PW Correction Vs. Battery Voltage" table needing adjustment with the Bosch IIIs. I'm not really looking at this as an injector issue per se. It's just that I've modified my tune to accomodate a completely different style of injector from what the car was equipped with from the factory, and now I'm seeing an issue that is likely related to the tune, and not the injector. The only reason that I want to go back to my original injectors is for experimental purposes to see if I'm still getting any knock. They are closest in overall spec to the factory Multecs and should be 100% compatible with the factory tune, whereas with the Bosch IIIs, that's not necessarily the case. It would more or less be "going back to ground zero" to see if the problem still exists. I guess another thing I could try is to leave the Bosch IIIs in, and copy the timing tables from the factory tune into the tune that the tuner provided me, but leave the changes that he made for fueling (i.e., "Injector Flow Rate", "Volumetric Efficiency Vs. RPM Vs. MAP", etc.), burn that to a new chip and then datalog to see if I'm still getting knock and retard. |
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Posted on: 2012/2/12 15:06
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