1LESSZ28

I say yes but I know how people claim that .035 is ideal. I understand that this may be ideal for a street car but perhaps not for a race motor or a motor has been modified properly. My thought is to run .015 quench using a zero decked block with a .015 gasket with custom pistons to avoid any potential ptv. Assuming that ptv contact could be avoided via custom pistons would this be a good idea? in short is the thinking: "the less quench distance the better" provided it is safe to do so?

thoughts?

mdacton

.035 is the rule of thumb.

I would not go that tight...it will burn some stuff up. I don't know if you can even get a gasket that thin.

rskrause

Yes, a 0.015" piston to head distance would be very good...for the manufacturer of the parts you would have to replace when the pistons smacked the heads. It might or might be good if you could do it, but you can't. It is too close and there is likely to be interference. There are lots of motors running well with more than 0.035". Most BBC's run a 0.039" gasket with the piston about 20 in the hole. People do run with ~30 thou clearance. As to that being "better" than 35 thou, I do not know.

Rich

MachinistOne

That's way too close.

.025" absolute bare minimum on a race motor with steel rods, and the pistons may still kiss the heads.

96capricemgr

I think you are failing to consider material expansion from heat, stretch, and deflection.

AdioSS

that is EXACTLY why quench is needed
I would be willing to bet that it would be possible to run .015 quench, but maybe only in a 2000 rpm generator. NEVER in any regular engine
just use the B-body gasket

1racerdude

Didn't somebody say they run their best when the piston just barely kisses the head?????
Might cost a bunch to find out if that is .025 or .035 or .040 on your combo.
Unless ya have the rest of the engine set up to utilize the tight quench .050 is as good as .035. You are only talking a very small gain.

mdacton

Your right....but it cost alot of money to find the sweet spot.

Valves start to get expensive, gaskets etc. But its fun

1racerdude

Yea along with pistons, bent rods, beat out bearings and new heads.

95Blackhawk

Found a website somewhere where they did in fact determine the quench for the pistons to kiss the heads. It was 0.024 for an engine built for racing and revving to 7K. I would never go that tight.

1racerdude

That depends on a LOT of other factors.
Ya can't just say a blanket number .024 is minimum.

Damon

Hah! I built one real tight years back. And by real tight I mean .025". It never smacked the head, but it was not a high RPM motor and never came anywhere close to the 7000 RPM range like that motor did (sub-6000 street motor). It also had cast pistons in it with tight piston-to-wall clearance, so they couldn't rock much in their bores.

I think it's one of those things that you really don't want to spend the time and money to figure out where the real limits are. It would probably be an expensive learning curve.

Steve in Seattle

I belive Smokey said in one book that steel conrods will stretch and expand 0.010" at 6500 rpms and operating temp. Ovbiously higher temps/rpms would be worse. Plus you can add a little wobble for anouth 0.015" or so. I'd be VERY careful about dropping below the 0.030" level... even there is a very tight fit.

My 396 is at 0.038" for most cylinders (I imagine this is +/- 1 or 2 thou)... and she runs AWESOME.

I plan on pulling a plug and sending a scope in there to check out the piston tops just to make sure... but idle and some quick revs at cold temps was good when we started it up Saturday.


BTW... ever heard a 12.5:1 396 with open headers and a big cam turn over? I damn near wet my pants.



0.035" would be my lower limit for almost any engine... beyond that you're entering the high-cost and diminishing returns area. I believe smokey said the same thing... most gains are going down to 0.040".