What to do with a 343?

Replacements are cast slightly shorter to make up for the overbore though. So a replacement 0.030" larger piston will still produce whatever stock compression was even though it's shorter. So for a stock rebuild -- no problem.

Dan, I know you're aware of this, mainly stating it for other readers who might not know. I'm sure many others already know this as well, but we get newbies all the time.

Quench doesn't make as much difference on a mid compression engine. The 4.0L runs 8.7:1 and the piston is 0.080" down in the cylinder and there is still the thickness of the head gasket. It won't ping on regular fuel as long as it doesn't have much more compression (no more than 9:1). With a 258 crank and rods combined with stock 4.0L piston it develops roughly 9.7:1, and strokers have problems unless they 1) reduce compression close to stock, usually by increasing dish size, or 2) deck the block and use custom pistons to keep the quench area down to around 0.040" -- which means a zero deck height and dish sized for the desired compression. With the later they can run 10:1 compression without detonation being a problem. 9:1 seems to be the limit without care to quench height, but of course other factors such as timing weigh in.

The 4.0L strokers I mentioned are still using stock EFI systems with no modification to timing. With an old school V-8 timing can be retarded and a bit more compression can be run, but that's a crutch. I found on my early stroker (which had 9.7:1/not quench compliant) I could drive carefully and run regular, but it would ping on any remotely "hard" pull at all. Mid grade was better, but it would ping a little under hard pull conditions even with premium. I had to pull the head later and dished the pistons to drop compression to around 8.7:1 (stock 4.0L). I was running (still am!) a Renix system with knock sensor. It was retarding the timing as much as it could practically all the time (I forget if the limit is 3 or 6 degrees retarded), so it was giving up some power. When I reduced compression the engine no longer detonated, but from the seat of the pants there was no loss in power. It might have even been a little spunkier at low speeds. Retarding timing or reducing compression are pretty much a trade-off, or close enough for the average engine anyway. If building a cruiser I'd keep compression in the 8-9:1 range and not worry much about quench. Ironically the 4.0L is said to have a quench design chamber! I guess the design is such that the piston can be 0.120" down and still work just fine (that's stock with a 0.040" compressed head gasket... 0.042" might be the factory gasket thickness when compressed, I forget the exact measurement). 

Yep, we're on the same page. But if the shorter piston drops it to 9:1 you will be okay and can run regular or mid grade gas with no problem. For a cruiser/occasional fun day on the strip car I'd recommend that route. If you're more interested in the strip than cruising, well, that won't do! I wouldn't drop it under 8.5:1 though, and would be happier with 9:1 as a minimum. 8:1 wouldn't cut it! Run as much as you can without problems for sure!


Edited by farna - Jul/09/2012 at 6:16pm

Agree deck clearance and quench play into detonation,

Edited by PHAT69AMX - Jul/09/2012 at 8:12pm

So wait a minute, I can put a 360 crank in my block and use the same rods and pistons I currently have, or is that only w/ 360 pistons?

Edited by kellysguy - Jul/09/2012 at 9:05pm