So, heres the specs:
bored 4.040 block
3.75 stroke
6 inch rods
mahle -16cc dished piston
54cc chamber head
.027 cometic gasket

comes out to 11.27:1 compression.

BUT: its a .032 quench. I am getting conflicting stories about what a safe measurement is... So, is that too tight for manley sportsmaster steel I beams and the mahle powerpack piston? I really don't want to start it up and have the piston beat the nice ported AI heads off my block. I know also that there will be some differences in everything when put together too, so what do you think? Will it be ok? It will be a daily driver, occasional track use, and it will prolly get used and abused a bit, just becuase why build it if your not going to USE it!

Opinions wanted!

Also, experience running this tight too would be appreciated!

Thanks
Cody

 

I have also done a lot of research on the subject. Most say a .035" quench is about the safe limit. I have also read on this site of people running a .028" quench. I had decided to go with a .035" quench as my block has not been decked yet. However after looking at my dynamic compression ratio I decided to back off to a tad to .038" quench as I will be running on 91 octane and this is a 1st gen motor. On DynoSim it was only a 1HP difference. What is your dynamic compression ratio?

 

There are some racers that run considerably less than .035" clearance but you don't hear them brag about it all the time (competitive advantage). The tighter you can run the quench without hitting, the better the engine will run and the less it will detonate. If you stop and think about it, the piston is virtually stopped by the time it reaches TDC so "beating the heads off the block" is not a likely outcome but you also don't want it touching if you can help it either. You are correct that part tolerances will affect the final deck height so you need to check every cylinder at TDC. Clearances (bearing and piston) will also affect what you can get away with but if you have a relatively tight engine that should not be much of a problem. If your worst case cylinder is .032" using actual measurements, you should be fine. I know of people running .025 in street engine with no ill effects.

 

ok good, cuz the machine shop is going to start on my block, and I am frantically measuring a thousand times, cuz I can only cut once! The machine shop is gonna mock it all up, and then shave the block according to where the parts end up, not the advertised numbers. So, then I guess .032 should be ok? I have no idea what the dynamic compression is, its a custom cam from Advanced Inductions.

 

Mine is tighter than that.

 

Hey, what are you running? zero deck with an impy gasket? Just curios. Most of the engine shops I have talked to recommended between .035-.040 for target quench. Basically a zero deck with a felpro 1074 gasket.

 

Im .010 out of the hole with the .039 Felpros. 76cc head, 7cc dome piston. Had to port the chambers to fit the domes

 

I think those heads probably make quite a bit of difference on what your quench can be, much more space above the piston in that area. With his stock heads, they are still flat for almost half of the bore. I defiantely think with a full chamber head, low quench is not an issue, but with a stock head, i think maybe it becomes more of a concern.

 

well, I email jeff creech, and he said leave the pistons .007 in the hole, that would give me .034 quench, which shouldn't be a problem. or .009 in the hole would give me .036 quench. But, would lower my compression to 11.16:1 I think I may go for .007 in the hole, what do you think?

 

The tighter you can run the quench, the higher compression you can tolerate, nice how that works out.

 

hmm, so should I shoot for .034? Is that ok and about in the middle for everything?

 

I saw about .004" between the highest and lowest piston with stock components. To do it RIGHT, you should have any line bore/hone work done, the cylinders bored and honed, and a skim cut taken off the decks to make them parallel with the crank. Get the block back and clean everything up and trial fit the rotating assembly without rings. You can probably eliminate some of the variation by matching the shortest rod with the tallest piston. Measure the worst case deck to piston with the piston rocked over in the bore for each cylinder. Decide what quench clearance you are comfortable with. Take the engine apart and have the decks machined to provide that clearance at the piston that is highest in the hole.

Is that a lot of work? Yes
Does it cost a lot more for machining? Yes
Are you more confident in what you have when you get done? You bet!

 

Yes that is some work, but it saves ALOT of money when you are meticulous and do it yourself. Measure at least twice, (and 2 different ways) keep a notebook with all measurements and label everything.

 

wow, I like the way you guys think! I too subscribe to the "measure twice, cut once" methodology. Well, looks like I need to just dig in then! Thanks alot for the help, and we will see what happens!

 

Thats exactly what I'm going to do with my build up. Assemble, measure and then deck the block. Going for .035" quench on my L98. With DynoSim there is only a few HP differnence in a couple thousenths in quench distance. Not enough to worry about versus being absolutely safe.