http://www.crankshaftcoalition.com/wiki/Quench

Haxby speak's to a issue that is a big question for myself im getting ready to plan a 383 and im getting caught up in piston to wall clearnce's...aka cold block and hot piston's//...Id really like to keep them quite tight to avoid rocking at high rpm.

'll start with MY thoughts on N/A applications and limit this discussion to small, and big block Chevy's. I often see numbers of .035-.045 thrown around, or it has to be less then .060, or .025 is the magic number, etc. The only problem with people regurgitating these numbers on the internet is that someone might follow it without understanding some simple variables. In an N/A deal I like 'em tight, within reason. You need to account for a certain amount of piston rock at TDC, a bit of tolerance stacking, rod stretch if you're turning some rpm, etc. If you have a large bore with a loose fitting piston, like a lot of BBC marine engines, and went for a .015-.030 squish/quench height and ran into even a little detonation at 6000rpm that piston will rock enough to possibly tag the head. Another issue is carbon build up. A really tight quench area usually accumulates carbon and it doesn't take long to build up enough that it starts contacting. Circle track guys like to set them up so the piston is literally leaving a faint witness mark on the head. The risk outweighs the rewards for that kind of set up in the type of engines we run.

Interesting, I've never seen a tight motor produce carbon in the contact area.

Chamber softening for NOS

I've seen some that were even more aggressive like a hemi chamber.

For my 10:1 NA build I went with the pistons zero decked and used a .040 head gasket. Run 93 octane and keep the A/F a little on the fat side as extra insurance. Spark plugs are cheap.

[QUOTE=Pliant;4288950]http://www.crankshaftcoalition.com/wiki/Quench

Haxby speak's to a issue that is a big question for myself im getting ready to plan a 383 and im getting caught up in piston to wall clearnce's...aka cold block and hot piston's//...Id really like to keep them quite tight to avoid rocking at high rpm.

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You may want to consider getting the piston skirts coated to control piston rock. I'm going to look into it for the same reason. A place called Line2Line coatings is who I am going to check out per a recommendation from KB pistons on my app.

BT

You also need to consider that the iron block and the aluminum pistons grow at different rates in relation to temperature as well. The piston manufacturer should be able to provide you with a clearance for marine use at your intended RPM.

My wife's Cadillac STS Northstar is so tight on quench, it has 11:1 compression, that if she drives it too easy, the carbon builds up and you can hear the piston slap. I take it out and drive it hard and it quiets right down. Which brings me to the question, exactly how rich is enough, because too much causes carbon build up and that causes detonation as well. I'm thinking about leaning up some of the low load profile to help prevent carbon build up.

When I attended school for general motors training, the north star was their baby at that time.

The air fuel ratio was programmed to go to 10:1 AFR, after 20 seconds of wide open throttle, to prevent piston melting . there was a whole lot of engineering into that engine for its time. I do not believe there was a north star that was 11:1 static. From what I recall they were around 10:1 range?

I was running low load fairly lean and ran into a slight knock after extended running at cruising speeds (3000-3500 rpm) due to heat build-up.