Preignition or detination?

cmpcpro · 09-09-2002, 04:48 PM

Hey all, I took a trip out to Chatsworth today (PAW) which is about a 15 minute freeway drive away, and when I got off the freeway, and punched it at the light, I hear a real bad knock. So, I let off the gas and drove it normally, and it was fine, after a few red lights, if I got on it again, it was gone. So, before I left Chatsworth (Actually went there to get bigger jets for my carb, which really helped!) I retarded the timing (Was probably around the 15deg area) and filled it up with 93 octane (Much cheaper in Chatsworth than Burbank!), and drove home. It didn't seem to knock at all on the freeway, even when I layed into it WOT. And when I got off the freeway it was fine too.

Because I had the stereo cranked the whole way there, I don't know if it was knocking at some point on the freeway, I don't think it did much as I didn't get into the 3k RPM's for very long, and it stayed in the high 2's. Could I have hurt anything? And what could it have been I was hearing? Detination or preignition? It seemed to only do it becasue I was on the gas for so long with no stop. As, with much higher timing, I can take a hill at WOT well past 4k and still be fine... Before I put the 93 octane in, I was runing 89.. always have. The motor is a lightly modified 1974 302, with *cries* 8.0.1 compression.

Thanks,

Chris

Jeff65 · 09-10-2002, 10:21 AM

Low speed detonation is usually heard as engine knock. Making the fuel-air ratio richer, increasing the octane of the gas used, and retarding the timing all contribute to eliminating knock.

Did you hurt the engine. Almost certainly not because if you had done so, the engine would now be blowing black smoke badly.

High speed detonation is a lot worse because it isn't as easily heard and if allowed to continue, will eventually break something in the engine, usually a piston. If you see a piston to which this is happened you'll see a hole burned in its top (completely through the metal). When the hole breaks through, black smoke ensues (blown engine).

If you can hear the knock, ensure you take action to eliminate it from being continuous. A small amount of pinging on a hill is not likely to damage the engine but if you hear it, let off on the gas and take it easy until the engine stops pinging. If its a nuisance, then retune the engine or use higher octane gas.

cmpcpro · 09-10-2002, 09:23 PM

Great! Thank you!

09-09-2002, 04:48 PM	#1
cmpcpro Registered Member Join Date: Sep 2002 Location: Burbank, CA Posts: 24	Preignition or detination? Hey all, I took a trip out to Chatsworth today (PAW) which is about a 15 minute freeway drive away, and when I got off the freeway, and punched it at the light, I hear a real bad knock. So, I let off the gas and drove it normally, and it was fine, after a few red lights, if I got on it again, it was gone. So, before I left Chatsworth (Actually went there to get bigger jets for my carb, which really helped!) I retarded the timing (Was probably around the 15deg area) and filled it up with 93 octane (Much cheaper in Chatsworth than Burbank!), and drove home. It didn't seem to knock at all on the freeway, even when I layed into it WOT. And when I got off the freeway it was fine too. Because I had the stereo cranked the whole way there, I don't know if it was knocking at some point on the freeway, I don't think it did much as I didn't get into the 3k RPM's for very long, and it stayed in the high 2's. Could I have hurt anything? And what could it have been I was hearing? Detination or preignition? It seemed to only do it becasue I was on the gas for so long with no stop. As, with much higher timing, I can take a hill at WOT well past 4k and still be fine... Before I put the 93 octane in, I was runing 89.. always have. The motor is a lightly modified 1974 302, with cries 8.0.1 compression. Thanks, Chris

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09-10-2002, 10:21 AM	#2
Jeff65 Registered Member Join Date: Jul 2002 Location: Oklahoma City Posts: 208	Low speed detonation is usually heard as engine knock. Making the fuel-air ratio richer, increasing the octane of the gas used, and retarding the timing all contribute to eliminating knock. Did you hurt the engine. Almost certainly not because if you had done so, the engine would now be blowing black smoke badly. High speed detonation is a lot worse because it isn't as easily heard and if allowed to continue, will eventually break something in the engine, usually a piston. If you see a piston to which this is happened you'll see a hole burned in its top (completely through the metal). When the hole breaks through, black smoke ensues (blown engine). If you can hear the knock, ensure you take action to eliminate it from being continuous. A small amount of pinging on a hill is not likely to damage the engine but if you hear it, let off on the gas and take it easy until the engine stops pinging. If its a nuisance, then retune the engine or use higher octane gas.

09-10-2002, 09:23 PM	#3
cmpcpro Registered Member Join Date: Sep 2002 Location: Burbank, CA Posts: 24	Great! Thank you!