Looking through the 1997 LT1 code, it seems that there is a provision for low octane gas. Changes all sorts of tables centered around knock retard. I have yet to determine just how the code detects low octane. I wonder why the earlier LT1s didn't have this code (?) May have been a change when OBDII came in.
-Tom
EE does have low octane compensation tables and logic as well. it's probably the same code.Originally Posted by Tom H
the logic is likely the same as most low octane compensation systems, if there are a lot of knock events within a certain timeframe, then it adds in more low octane compensation. the attack/decay is tunable.I have yet to determine just how the code detects low octane.
that isn't really going to help this guys ethanol situation since spark is the not the biggest concern. if you don't have consistent ethanol levels, the difference between 50% and 85% ethanol is massive with regards to AFR.
Thanks for explaining this a bit better.
In the end does software come up with a binary: good gas/bad gas or a number representing the gas quality?
I looked through the air/fuel ratio code and can see quite a few modifiers...that isn't really going to help this guys ethanol situation since spark is the not the biggest concern. if you don't have consistent ethanol levels, the difference between 50% and 85% ethanol is massive with regards to AFR.
- Over temperature
- Power enrich
- Cat protection
- Cranking
- Open loop
and so on. Seems like the stock PCM probably isn't flexible enough to handle the situation. In the later code the OBDII command mode $AE will let you directly control the air/fuel ratio. I am sure the ALDL has a similar mode to temporarily adjust things. Perhaps he could do a test and find out if by adjusting the air/fuel the car could be driven with both gasses. Perhaps do this before spending on new heads. Also wonder if a thicker head gasket might drop the compression enough to be a help.
-Tom
Tom, Thanks for the thought, but the car can be driven just fine right now. It is just not operation at optimum, and it obviously wouldn't be with a "compromise tune" designed to allow it to just "operate" on various mixes. The car drives fine on 93
octane I Just need to take about 5 degrees of timing out of it, and that costs quite a bit of power. I am sure it is still over the 400 RWHP mark, so in a 3000 lb package, its not boring. Going to a thicker head gasket would change the quench area for the worst, I am doubtful it would be enough, or do any good. It could easily change the detonation issue for the worse.
I can't see how a thicker head gasket would change the quench area... volume yes but area? I guess the thinner the quench the higher the velocity/better mixing/more power.
Not sure I know enough to post on this thread, so I will watch with interest.
-Tom
it actually sounds like a good tuning challenge to me. you might be surprised what you can achieve on 93 octane if you don't just approach the problem as just dropping 5 degrees of timing. there is a delicate balance between AFR and timing advance that you can play with to get a lot of power back, and spark is really complex, some running areas might not need much help at all. you might have a lot of fun with it.
you're confusing deck clearance with quench, that's all. lots of people mix those up because even a lot of machinists seem to use the wrong terms and it gets passed around.I can't see how a thicker head gasket would change the quench area... volume yes but area?
Could you expand your answer and clear this up for me?
Thanks,
-Tom
If I am incorrect of what steveo is talking about, please advise.
Basically quench is the distance from the flat parts of the head to the piston. Let's say your engine is assembled with the piston .010" down from the deck(deck clearance). If you use a standard .038" thick gasket, you will have .048" quench. Most hot rod people try for .035" - .040" quench. The desired quench can be done with deck machining or gasket thickness.
-Carl
Tom, The quench area is the tight area between the flat portion of the piston and the flat portion of the combustion chamber. Its purpose is to squeeze all of the mixture into the combustion area, so you want it as tight as possible without making contact. If you put a thicker head gasket in, then that area becomes less efficient, holds more mixture, and becomes more prone to detonation.
i guess you could view it as a number representing gas quality. it's just part of an overall closed loop spark control system. you can think of regular knock retard kinda like short term trim and octane comp as long term trim if you want
if you have poor quality fuel and your timing map is too aggressive for it, it'll go into a cycle of knock, decay, knock, decay....so you're still getting lots of knock events spread out over time. low octane compensation tries to lengthen that cycle so there's less actual knock (by removing more spark from more critical areas, and having a longer decay time)
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