preignition from my 79~LARS HELP
06-16-2007, 12:04 PM
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preignition from my 79~LARS HELP
i have a 79 vette with a new 350 motor 340 hp about 1,000 miles on it and noticed on hot days after i drive it i get preignition from it. what causes this air/fuel settings or timing. and whats the best way to set my air/fuel for my edelbrock 600 cfm carb???
06-16-2007, 04:32 PM
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Biggest cause is timing and timing curve (how quick the curve comes in; where the total timing is set; and how the vacuum advance is set up). However, if the engine is running lean, it can also cause detonation.
Here is what I would do.
First, verify your compression ratio. If you're running over 10:1 compression, you're going to have to find some better octane fuel or do some serious timing retarding (loss of performance).
If compression is lower than 10:1, you should be able to make it run on premium pump gas. Do a TDC verification to make sure your harmonic balancer has the timing line in the right place.
Then, once verified, check timing. With a setup like that, you should have 36 degrees total timing, unless you have Vortec or fast0burn heads, in which case you need 28-32 degrees total timing. Initial timing should eb in the mid-teen range. Your total timing should not come in any quicker that about 2500-2800 rpm. You can slow this down to 3500 rpm without penalty to WOT performance.
Verify your vacuum advance control unit does not pull any more than 16 degrees of additional timing, and make sure it is correctly matched to the vacuum your engine is pulling (if it's too "light" it will suck the advance in too quickly).
If all aspects of the timing check out, richen the carb up a couple of sizes. If you still detonate, retard timing until it stops.
Here is what I would do.
First, verify your compression ratio. If you're running over 10:1 compression, you're going to have to find some better octane fuel or do some serious timing retarding (loss of performance).
If compression is lower than 10:1, you should be able to make it run on premium pump gas. Do a TDC verification to make sure your harmonic balancer has the timing line in the right place.
Then, once verified, check timing. With a setup like that, you should have 36 degrees total timing, unless you have Vortec or fast0burn heads, in which case you need 28-32 degrees total timing. Initial timing should eb in the mid-teen range. Your total timing should not come in any quicker that about 2500-2800 rpm. You can slow this down to 3500 rpm without penalty to WOT performance.
Verify your vacuum advance control unit does not pull any more than 16 degrees of additional timing, and make sure it is correctly matched to the vacuum your engine is pulling (if it's too "light" it will suck the advance in too quickly).
If all aspects of the timing check out, richen the carb up a couple of sizes. If you still detonate, retard timing until it stops.
06-16-2007, 10:29 PM
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my compression is 9:5 to 1 so can i run 87 octane fuel ????? also how do i check if my advance is matched to the vacume im pulling???
Last edited by tellcom2001; 06-16-2007 at 10:33 PM.
06-16-2007, 11:17 PM
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I'm unable to run anything less then 92 on 9.5:1 compression.
I've been having some issues as well, but I might make a change to aluminum heads to help offset part of the problem (and get some more get up and go).