Built an engine with negative deck height ? Where the piston crown is ABOVE the deck of the block? It would probably require the use of dished pistons to work right. In theory the combustion process would happen in the dish of the piston. Concentrating the power in the piston dish instead of power scattering across the crown/flat top of the piston and the combustion chamber. Think it would make more power OR the added cost of custom thickness head gaskets and/or custom pistons/rods would make the whole thang a 'wash'? This might go into the same category as the long rod theory !?! (some of the girl friends might enjoy this particular theory!) jus say'in. Rainy weather brings up a lot of these questions in my tiny brain............... Thx in advance 6sally6
A lot of race engine builders run em outa the block. most guys shoot for an .040 quench but some guys really push the limit depending on rod and piston combo's
A somewhat normal thing to do with drag race style Chrysler Hemi's. A change in piston pin location (piston "in the hole" or "out of the hole" was used to increase or decrease the compression ratio, depending on many things. Track location (Colorado vs. Calif), track condition, that is available traction. Ambient temperature and humidity, (again, horse power to be made with the current air density). Some street engines could be built the same for the same reasons as noted above. Mike
I know a guy who used to build a lot of Claimer sbc oval-track engines with the pistons out of the hole. I could see places where you could do it even on a street motor. Like if you were committed to using certain heads.
Sometimes what you believe is right is not for all applications. In other words, there's a hundred ways to skin a cat. But the cat, he don't like none of them. Nitro tuning talk. Lippy
I don't think my 401 had the piston above the deck. The pop up on the piston was, but not the piston.
On a small bore, some will go tighter than .040" quench clearance. I think we ran .030" on a OT race motor. We used to buy semi finished pistons, and I would do the final machining on the crown. Compression height, dome height, valve notches. We started with the official factory specs, but wanted more power. And we got it. Did one motor, seemed to make more power than usual. Hmmmmm. Strange. No idea why. On the next tear down, we noticed the ends of the piston domes had been touching the head. Didn't hurt anything else, turned over fine on the stand. Serious RPM does strange things. The domes got massaged just a little more in those spots. Never blew up a motor, wanted to keep it that way.
Yep, I try to do it with every engine I build that has quench area. Shoot for .040" most times, but my own stuff gets cheated a bit closer, like the .033" on my 8CM flathead in the Merc. It's worth the trouble.
It all depends on the engine, the heads, the desired quench, etc.. I've frequently used custom piston pin locations (compression heights) on some of my flathead build to achieve a .040 quench (without milling the heads). If you use a quench of less than .040, then you do need to validate how much piston "rock" you have (given bore size, piston material, piston-to-cylinder clearance, rod material, head style, head gasket bore dimensions, etc). For a purpose-built race engine one can tune the clearances to smaller numbers - though you need to consider all the details to do so.
Just a word of caution before you zero deck... Make sure your mains are all good and true before you take your measurements ... Because if you don't and you align hone it the pistons will be outa the hole if you want to or not.
The Jalopy Journal
