Random Pre-Ignition

just curious on this... every now and again when i turn my car off i get a second of pre-ignition, it doesn't do this all the time... maybe once every 15 times i turn the car off. My specs are as follows

1978 4-bolt Chevy 350 bored .060
flat top pistions w/ 14cc valve reliefs
64cc cast iron heads 2.02/1.60
lunati voodoo 60102 cam .468/.489
performer RPM intake, holley 650 dualfeed
figured 10.77:1 compression
car typically runs about 190 degrees *200 in traffic*


runs fine on 93/94 octane and there is no detonation *pinging* while driving...

do you think this is due to my compression being to high for the octane fuel im running? im going to pull the plugs and see if they are carboned black... last time i changed them they were a nice tan color which *from what ive read* means its running perfect. I never have any extreme raw fuel smells or exhaust smells so the air/fuel mixture can't be far off. Id like to get this fixed befor i do any serious damage...

Any suggestions or added tips are greatly appreciated....

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Originally posted by HellCaminoKid
just curious on this... every now and again when i turn my car off i get a second of pre-ignition, it doesn't do this all the time... maybe once every 15 times i turn the car off. My specs are as follows

1978 4-bolt Chevy 350 bored .060
flat top pistions w/ 14cc valve reliefs
64cc cast iron heads 2.02/1.60
lunati voodoo 60102 cam .468/.489
performer RPM intake, holley 650 dualfeed
figured 10.77:1 compression
car typically runs about 190 degrees *200 in traffic*


runs fine on 93/94 octane and there is no detonation *pinging* while driving...

do you think this is due to my compression being to high for the octane fuel im running? im going to pull the plugs and see if they are carboned black... last time i changed them they were a nice tan color which *from what ive read* means its running perfect. I never have any extreme raw fuel smells or exhaust smells so the air/fuel mixture can't be far off. Id like to get this fixed befor i do any serious damage...

Any suggestions or added tips are greatly appreciated....

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The "dieseling" usually comes from the engine being a little bit warm for the octane. The high compression, mixed with the hot engine and low octane makes it partially ignite one last time before it cools down enough to kill completely. Sometimes a small bottle of octane boost will cure it. Having your idle as low as possible helps it stop as does backing off the timing if possible. Backing the timing off may let it run cooler.

You can put an electric solenoid-type throttle stop on the carb and stop the run-on. Lots of smog engines had them. Turn off the key and the throttle closes completely, it can't run then!
Or just let it idle for a few seconds to cool off the chambers. Shutting it off in gear works too, but not the best solution. They can kick back, blowing gas out the carb with the attendant fire hazard.

thanx fellas... ill try the octane boost first... i ran it once b4 and it seemed to improve performance... but the plugs were fresh then too... any brand better than the next??? i used the formulas 104+ race mix the last time... the car is a stick so shutting it off in gear doesn't exactly work... i have let the clutch out to stop the continued fire after shutting it off a few times tho ;)

oh, just installed some Delta Force 40 race series flowmasters on her too....OOH it sounds wicked now...

just for future questions similar to this one... if anyone has the same problem... i used the octane boost... now im problem free!!!.... Thanx guys.... love this board...

A rich idle mixture will cause dieseling. Also, check to make sure the choke is open when the engine is warm. I had a rod for my choke pop out which caused it to close. The engine dieseled badly when I tried to shut it down.