I just got done reading a buch of posts about quench. I see some people don't want to 0 deck the block and think that it is unecessary. I was on JE's website checking out their pistons and all the figures they had were for the piston sitting .020 in the bore. I looked in summit at the head gaskets and their thinest gastket at 4.530 bore is around .040. If that is compressed thikness then that already puts me at well over the .035-.040 quench height. Whats the deal should i get a taller piston, do i have to deck it. I don't really want to deck a brand new block

 

I was able to get .015 thick gaskets for my small block from Fel-Pro, maybe try there? BTW, what size motor has a 4.5" bore? Joe

 

Its all in an effort to increase turbulance in the combustion chamber...

Makes the flame travel faster. Kinda makes up for crappy flame travel by forcing it to one side...

 

I take it your working on a big block. Decking the block is a good thing to do, if you need it done. A machinist would be a better judge of this.

The ohter thing that needs to happen is to have the length of the rotating assembly measured and compared to the height of the block. Then determine the gasket thickness.

My pistons actually come out of the block .007" and I had to get slightly thicker gaskets. Check out Mr Gasket Ultra seals they have a range of thicknesses too. Cometic Gaskets are a little pricey but they have the widest range of gasket thicknesses.

 

Its a merlin 3 talldeck 540

 

That is what I do.

-Mark.

 

Mines in the 50-55 thou range and runs fine, stock motors run fine with over 60 thou. I for one think this qunech thing is a little over rated, not saying that there is not any benefit to it, just stating for all the trouble you have to go through ( with a high lift cam you really have to watch piston to valve clearance and I am pretty close the way it is now )
it was not worth it to me.

Like a machinist told me whan I was inquiring about it " Don't won't worry about it, you will never know th e difference "

 

I don't have first hand information yet, but the science behind it seems valid - tight quench (especially with high swirl chambers) causes higher combustion chamber turbulence and faster combustion. This lets you use less spark advance for the same power, avoiding detonation/knock issues. I don't think there will be much of a HP advantage except by allowing full advance with today's cheaper gas you won't be leaving HP behind by retarding the spark. All it took for me was finding the right head gasket, so it was easily worth it. Joe

 

Sure like I said I am not questioning the benefits, but I would have had to 0 deck the block, flycut the pistons or buy new ones but not sure if I could have easily obtained the 11:1 CR I wanted, decking I could have done before hand but measuring the piston to valve clearance and taking the pistons out and cutting them or replacing was just too much for me. I took no shortcuts when building the motor but I felt there was not enough gain for me to do this.

 

I spent some time in a previous life as a piston design engineer. The norm for production engines is around 0.060-0.080". We tried never to design so that the tolerance stack up went below 0.040". The reason for this is due to the accumulation of carbon deposits that can build to the point where the carbon fills the crevice in the squish area which has been known to cause noise problems. Much like the piston hitting the bottom of the head at every revolution. Sounds crazy but true. I've heard it!

....not that significant carbon deposits will ever build up in a Corvette motor to this degree before they are promptly BURNED away by the driver by STOMPING on it.

You will be fine at 0.060" for a driver.

 

I am in the process of putting together a 383. I had to to do some grinding on the block and aluminum oil pan. It seems I now have adequate clearance. When turning the crank the pistons are popping out of the bore about .002". I have felpro head gaskets and they measure about 0.042" uncompressed. Is this going to give me enough piston to head clearance? Or will I have to get thicker head gaskets?

 

The Merlin blocks are a little tall. I decked mine .028 on one side and .030 on the other. My SRP pistons are .009 out of the hole. There is another Forum member here with a Merlin 540. His block was just touched up...not really decked. His SRP flat tops (mine have small dome) and his pistons are .022 or so down in the hole. He ran regular gaskets at first, but with new head changes he's gone to thinner Cometics to get quench closer. There are a couple of 540 guys here using custom .015 Cometics.

You've already read my ideas on quench. I see no reason not to just get it right while you're at it. You're spending lots of money anyway. Make it tight! My total quench is .028 and I have no issues even turning 7500 rpm sometimes.

Will you feel it in your seat of the pants testing? No. Will it automatically allow you to run 5 more degrees timing? No. But it's just one of the many little things you do to a motor during design and building that adds a few HP. Find enough of those things and it's what makes one 540 put out 500 hp and the other one makes 700+. Add it all up, and you WILL feel it!


JIM


BTW- If you haven't got pistons yet, ML67 here on the Forum has a nice set of forged flat tops (4.500 bore) with very little miles on them you could get a great deal on!

 

JDRez42,
Jim is giving some good advice. You are already spending a "pretty penny" building a 540, you should go ahead and spend the little extra it should take to have the machinist zero deck it. I asked my machinist to zero deck my block, but I later discovered that he didn't and my pistons are .020+ down in the hole. I am now running Cometics to get reasonable quench, but one day I plan to have the deck zeroed. Another thing to consider is how much compression you will try and run. The more compression that you try and get away with on pump gas, the more important quench area becomes to help avoid detonation.

 

You are fine, even "good". Aluminum rod engines might need more clearance than that but I would feel fine at anywhere around .035" to .045" for good tight quench and maybe up to around .005" out of the hole or maybe a little more. A lot "depends" after that IMHO. Skirt length, piston to bore clearance, piston manufacturing process, max RPM and even then, it would be a SWAG at best. Experience counts at that point.

-Mark.

 

You need to think about what is going on here: you are measuring your deck clearances in a "static mode" at room tempratures. I would not have any problem with 0.040-0.035" clearances but would not run them an tighter. 0.040" is the number most engine builders I know shoot for when setting this dimension, any further tightening up of this figure is a game of diminishing returns as you risk contact between the piston and combustion chamber and the additional gain in power is not worth the risk involved unless you are building a "race motor" that has frequent tear-downs for inspection. I am surprised that the machinist you intend to use isn't aware of this and if he/she isn't knowledgeable, I would look for another shop to do my work.

 

I haven't talk to the machine shop as of yet. I purchased the block from a friend who had the block work done at the same machine shop I purchased the rotating assembly from. I still need a few odds and ends and will talk to him about the quench issue while there. I feel that there is adequate clearance but I was unsure if the head gasket would compress leaving me with much less clearance and the chance of the pistons hitting.

 

This takes care of any quench issues I have:



Thirst quench issues that is.

 

I agree completely. Reducing quench below 0.040" is a game of rapidly diminishing returns in performance (if any at all) with rapidly increasing risk of damage.

 

Yup, in my past life I've had cars towed in for "blown engines with rods knocking" simply due to this carbon buildup. For a street driven engine, I wouldn't make it too tight, even knowing that it's goin' in a Vette.

Steve

 

I agree going below 40 has diminishing returns. The closer you run leaves little room for mistake!!!! When going under 40 you need to take into account for rods,crank,pistons heat expansion.

Never went to tight with a quench unless the compression ratio, or head slection/material forced me to work that hard. I try hard to not look for trouble but sometimes it finds me anyway.