Rebuilt Tri-POwer

New motor in and runnig again.. Yes again.. Made the big mistake of not running a colder plug with a high lift cam. and over the 2000 miles of run time I had developed hot spots on piston surfaces which eventually led to a violent detonation.. yes booom. #1 rod twisted and broken, cam broken, timing chain gear broken and sent through the side of block. I was on it good to when it happened. Had motor out and at the builders on the next monday. 3 weeks later new shortblock ready to go. Took the time to have the headers ceramic coated. and built heat shields for the carbs. I was having major issues with fuel percolation withthe amount of heat that headers were kicking out which may have cause of fuel seapage into the combustion chamber..

A blown engine because of a stock spark plug? :!:
Perhaps there's more to it than just that.

Well it can happen? It was definatley part of the issue if not the main cause. We went over possible causes and as soon as I brought in the plugs both my ignition guy and builder both felt that it was detonation that caused the failure. When we took the engine apart the crank bearings were solid even the wrist pin was solid. The rod arm was toast. We believe that scoring marking and heavy build up on the piston surface caused the hot sports which may have lead to the premature firing in #1 cylinder. I know that with high end motors which this was they are tempermental. Bottom line is that these things happen and it did..

Bad luck...dang. Car still looks fantastic though!

what type of plugs (and thier numbers) are you running now, I've been tempted to get colder plugs myself.

Both my ignition guy and builder felt that the AP46 autolites were way to hot for my application. That is actually the stock plug go figure. I am now running a degree colder AP45's. In the summer I may even bump to AP44's 2 steps colder. I'm just more aware of these things now.

One Heat range ( especially the stock range ) should NOT of caused what you ended with , However The Ignition curve ( especially the total ) would do Big Damage , What was your Curve , Gas used and what was the Compression ratio on your combo ?

One Heat range ( especially the stock range ) should NOT of caused what you ended with

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x2
I start with 2 ranges colder with a mild motor. Car looks great

Hey kevinl1058

I believe FalconSedanDelivery has raised a concern that should be seriously considered.
Think about it.

If all you do is move one heat range down without confirming or eliminating the ignition curve and certainly your total advance could be flirting with another Excedrin mind-numbing migraine headache..

If you haven’t already don so, at least eliminate that possibility for no other reason than peace of mind..

Your build looks Grrrrreat…

What model carburetors did you use…
Take care and best of luck…

Running 9.5 comp ratio. Distributor was recurved 36 deg total advance. with timing set at 10 deg. 24 degrees curved into the distributor. Running all mechanical with petronix. I did flirt with more advance but backed it off way before the detonation happened. There was major carbon deposits on the piston surfaces, which lead to the hot spots which doesn't help.I am running super unleaded feul.

Sooo...
If you're running "colder" plugs, what are they?
We've got 2 of these engines that are relatively "hot", both getting ready to go into service, and would like to know what everyone else has found to be the best plug to avoid a similar problem.

Can anyone recommend a specific plug?

Our engines are both C8 casting 200's (one is .040 over, the other is .060 over), both are 2-barrel (one direct Holley on C9 head, one Weber with an adapter on an E0 head), both have pretty stiff cams (a 274 on 110-LC and a 276 on 110-LC), milled heads, decked blocks, balanced, bigger valves, headers, etc.
One is using Mike's DUI, the other will be a Mallory Unilite (for now).

Any input that can help avoid an issue like this is greatly appreciated!

kevinl1058":2bndh09v said:

Running 9.5 comp ratio. Distributor was recurved 36 deg total advance. with timing set at 10 deg. 24 degrees curved into the distributor. Running all mechanical with petronix. I did flirt with more advance but backed it off way before the detonation happened. There was major carbon deposits on the piston surfaces, which lead to the hot spots which doesn't help.I am running super unleaded feul.

Click to expand...

Ok first your numbers don't add up 10 + 24 is either 34 total , or if you actually mean 24 Dist =48 that plus 10 = 58 , also when is it all in ? ( the total )

Sweet looking falcon! It's awesome that you went with the six instead of a V8. Hope this one lasts a little longer.

Oops I have a total of 34 total advance in at 3000 rpm. need to check post before sending.. Since the the Webers didn't have an off idle source for the vacuum we recurved the distributor to get the most advance with mechanical the can only provided 7" of vacuum so just meant a more thumping idle instead vacuum advance at idle. I could go to 12 deg of advance but leaving it at 10 degrees advance right now is ok for performance. I'm just putting the spark plug issue out there because it is documented that running too hot of a plug could lead to major engine detonation damage on motors with major modifications, which my motor had. Just didn't want to see anyone have any mishaps like me. Things happen and things get fixed. My neighbor had a blown 56 chevy and his thrown rod was a 12K mistake so I just did my research to prevent any issues in the future.

Called my builder again today. there are two Autolite plugs that you can run AP45's one step colder than the 46, which is the hottest plug. or a 124 which is 2 ranges colder. The AR33 (3 ranges colder) he felt would be way too cold and I would be fuel soaking them way to often. He runs 45 in his performance motors. He was just stern that the 46 was way too hot of a plug within my compression and cam profile.

Too hot of a plug will just melt the electrode. Too much timing will hurt the pistions, valves or blow a head gasket. 32* total is enough on a six cylinder.