Thread: Big Block Predetonation

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I'm old (52) but new to this forum stuff.
I have just finished building a dream and it started. I had a 69 Camaro when growing up and have recently got another.
Anyway, a Question. I started the motor for the first time in the car. It was broken in on a dyno. The motor is a 434 Chevy Big Block. (396 - bored- with 454 crank) The motor runs real strong. I warmed it up to make sure there were not leaks and all was well. Oh happy day. Then I tried to shut it down. It would not. The predetonation was consistant until I put it in gear and feathered the clutch to load the motor and it turned off. I thought the timing to be too advanced so I broke out the light. I set the timing at 10 degrees advanced as it was up around 25 or more degrees. I thought all was well until I tried to shut it down again. Still detonating. There is a mild street cam in the motor and I'm trying to get the specs from the builder. Should I be experiencing this predetonation with a 12 degree setting on the motor? The factory setting is only 4 degrees for a 396/375. I would expect more from this motor. Can anyone help or offer some suggestions? Thanks

what rpm's are you cutting it off at and what is the temp. of the motor ? 12 degrees is a little high, back it up to around 8 and see.

What you're experiencing is neither preignition or detonation, but dieseling or run-on. There is a hot spot in the chambers, could be the edge of a head gasket, a sharp corner on a piston top or even the spark plug electrodes if you are running extended-tip plugs. Whatever is hot is acting as a glow plug and still allowing the motor to run because of the air-fuel mix that is getting past the throttle blades at the idle setting.

Install an electric solenoid on the throttle arm that only allows the blades to come to the idle position when the solenoid is energized with 12V from the key. Otherwise the blades are completely closed and the motor cannot run on because it has no air/fuel to combust.

Or you can do it the hard way, disassemble the motor and polish everything to within an inch of its life

jimwhittaker, get you timing, rpm's and carb. adj. properly before you start pulling your heads off. sounds like to me you may have the motor idled up a little high or the carb. may be a little rich. assuming the temp. is ok.

"jimwhittaker, get you timing, rpm's and carb. adj. properly before you start pulling your heads off"

Don't get excited Mike, I was just teasing him about disassembly of the motor and I'm sure he knows I was teasing.

"what rpm's are you cutting it off at and what is the temp. of the motor ? 12 degrees is a little high, back it up to around 8 and see."

Mike, it has been my experience that more advance, not less, will kill the motor quicker when it's dieseling.

does sound like deiseling. I'd start with some colder plugs. Pull the plugs and read them.

Man, all of you guys are great. I was not expecting 'a' reply so fast let alone 8.
I did exactly what most of you guys suggested. I had the timing set (checked) by a pro and it was dead on at 12 degrees. The mechanic adjusted the carb with a vacuum gauge and that is now right. The idle speed is set now at 800. It was 850. The 'dieseling' still exists. I had a thought about the cam. The motor is new, a comp cam was installed. I tried today to contact the installer to see if he degreed the cam for performance or just aligned the marks. With a Comp Cam, I found out the aligning the marks gives you a 6 degree advance when setting the timing mark at '0'. So if I'm thinking right, the 12 degrees I have it set at now, is actually 18 degrees. Would that make sense? If so, the gentleman that stated to cut the timing back to 8 for starters may be right. That would give me 14 degrees and will tell something. I may try to set it back to 6 degrees, which will give me 12. I have a friend that races and I know he has one of those real fancy timing lights. The ones that shows the advance and a lot of other info all digitally. I prefer simple but I want this car to run as good as it looks. Since you guys have tried to help, I'll keep all of you posted. I'm not sure how to do that yet as I'm very new to this type of help system. I'll figure it out and make sure you all know how I make out. Also, as soon as I can get some digital pics, I'll post them. I am pretty proud of the car since it took 30 some years of dreaming to get to this stage. Thanks to all for all of the suggestions. Most are not what I would have thought about. Sincerely, Jim

i have wanted to build a 396 -402 with a454 crank in it i have built a lot of big stroke stuff up to630 + but i think this 396 with 454 crank would be a great combo i allways like my 396 s i had .on timing on the crank i like 10 and 26 in the dist. and check to see if top dead cent is rigth with timming tab at O .like teck said i polish the pistons the the head chambers the block valley and the crank counter weights and the crank case and took a lot of crap for this but never lost a piston or head gasket

Originally posted by techinspector1
"jimwhittaker, get you timing, rpm's and carb. adj. properly before you start pulling your heads off"

Don't get excited Mike, I was just teasing him about disassembly of the motor and I'm sure he knows I was teasing.

"what rpm's are you cutting it off at and what is the temp. of the motor ? 12 degrees is a little high, back it up to around 8 and see."

Mike, it has been my experience that more advance, not less, will kill the motor quicker when it's dieseling.

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i agree with that, but 14 to 15 degrees sounds a little high for base timing to me. hes got 6 degrees at the cam, one thing for sure, he should have a bad ass low gear. i just wanted him to change it just to see how much dif. it made, good or bad and it didnt, i dont think it should be dieseling that bad no way.

"So if I'm thinking right, the 12 degrees I have it set at now, is actually 18 degrees. Would that make sense?"

You're confusing camshaft degrees with ignition timing degrees. The amount the cam is advanced or retarded in relation to the crank has to do with the valve timing (when the intake and exhaust valves open in relation to the crank), not with ignition timing (which is when the plug fires in relation to the position of the crankshaft).