Dwell time at TDC

Discussion of the 331-354-392 HEMIs.

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NE57
Posts: 192
Joined: Fri Mar 27, 2009 12:54 pm

Dwell time at TDC

Post by NE57 »

Is there a relationship between piston dwell at TDC and detonation?

I'm wondering if a longer rod will have any effect(good or bad) on detonation, which might influence CR choice.
oldngood
Posts: 121
Joined: Thu Mar 13, 2008 5:14 pm

Re: Dwell time at TDC

Post by oldngood »

I can relate my personal experience: I've found the major factors with detonation are too much static compression ratio, too much timing, fuel mixture lean, or head gasket stack beyond .080"- all create detonation.

the long rod won't be as much a factor, because in making the rod longer, the pin height is moved higher so compression is the same. I've run engines with high rod ratios 1.77, and low rod rations 1.53, they both had the same octane requirements, but the short rod/long stroke motors do tend to ping a little more, mainly because they are bigger and create more heat- I found a 9.6 CR wedge motor with a long 6.625" rod, and 3.75" stroke, would still ping at WOT on hot days, even with 94 octane Sunoco. Eventually both long rod and short rod engines broke the ring lands on 2 pistons each. I had another engine destroy all the bearings from detonation with only 9.6 CR, and had to be rebuilt twice, until I wised up and dished the pistons, and cut the CR to 9:1 It has been running fine ever since. The little bit of power gained with marginally higher CR isn't worth the hassle and expense of fueling it with Cam 2 (98 octane) or airplane fuel, if you can still get either- or rebuilding it when it breaks. A 10.5 CR motor we had pinged badly on 98 octane Avgas from the airport

lowering the CR to 9 to 9.25CR range eliminates the problem entirely, and the car runs on 93 octane available at any gas station.

many hot rodders don't realize their engines are pinging with pump gas, because with loud mufflers they can't hear it. But if it's excessive and you teardown the engine, you can see it- the bearings will be curled and stick on the crank and be chewed up, and the top ring and piston lands sometimes broken. I've found this in the above engines mentioned. An engine will tolerate mild detonation for years, but eventually a few rod bearings will get destroyed and chewed up, and ring lands crack. I've taken apart many high mileage V-8's and often found one or 2 cylinders with detonation damage to pistons/rings/bearings- and the engines were still running.
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