350 stock cast short block, quench?
 

How tight can I safely make my quench height with a stock cast bottom end? ('77 350, 12cc dished pistons) I know around .040 is the best, but I'm not sure if cast will flex more under load. I shift at 5500, but would like to be safe a little beyond that.

I almost wish it should be bigger so I don't have to use a metal shim head gasket :crazy:

How wide can the gap be before it starts detonating and losing efficiency?

Thanks,
Chris

Re: 350 stock cast short block, quench? (LiveandLetDrive)
 

Okay first of all stock block are normaly 0.25" tall for deck hieght. That would be 0.025" above a flat top piston (or piston would be 0.025" below deck surface at TDC). So with a composite gasket compressed thickness of 0.039" it puts your piston 0.064" below the metal of head thats located over the piston (quench hieght). I'm no dyno expert but from most of what I've read good quench hieght is 0.040" or less. This is why most performance blocks are machined for zero deck hieght and a 0.039" composite gasket is used.
Now for less than 0.020" clearance (block machined < zero hieght), the valves can easily hit pistons. :nono:
A steel shim gasket for 0.015" compressed hieght should be just right for a stock block deck hieght - and add some badly needed compression too. A good quench hieght aids in combustion by forcing the mixture out into the chamber at high speed. This will allow for higher compression ratios with lower octanes and make more power too. As much as 15 more ponies. You just have to get that block and head surfaces cleanie-clean and FelPro says use some spary sealer (Permetex) too. I cleaned my block by hand and it took forever. Try a synthetic scrubber on a drill to speed this cleaning up.
Your on the right track Chris. cardo0 :seeya

Re: 350 stock cast short block, quench? (LiveandLetDrive)
 

My "Chevrolet Power" book from Chevrolet says, .035"-.040" minimum. I have heard of some, getting by with less. Your 5500 rpm limit will not over tax the clearance. Your cast parts are not a consideration in piston the head contact.

RACE ON!!!

Re: 350 stock cast short block, quench? (CFI-EFI)
 

I'm running .037 total quench on mine now with no problems. My rotating assembly is all forgerd, but I agree with CFI-EFI that it shouldn't matter. The less quecnh you have the better (without contact) to stave off detonation. .037 in mine now allows me to run 11.94:1 compressoin on 93 octane on the street, of course I have a large duration cam that bleeds off some compression at the lower rpms and the alum heads help to remove some heat also. On pump gas I have to run the carb a little rich and I can't run a very aggressive timing curve... 12 intial, 33 total with the ramp all in at about 3000 rpm is all I can get away with without detonation on 93 octane. The comp cam I'm running does have 5 degrees advance ground in and I was thinking of retarding it 4 degrees to get a faster ramp with pump gas, but I don't know if this will help or if it will even work to accomplish it?

A tighter quench= more compression and a lower threshold for detonation. You just have to make sure you have the rest of the engine components thought out to help stop it. The effect of a tighter quench in itself helps to stop it to a degree, it's just a balancing act.

Re: 350 stock cast short block, quench? (EDDIEJ82)
 

I am running the stock deck deck height with .039" gasket giving giving me 0.064" quench. If I was to go with a tighter quench I would be running into piston to valve tolerance problems with lift on my cam. Gm has been using this quench distance and more for years with no probelms.

I have 11:1 CR and was running 45 deg advance for a couple of days when I first put the motor in and had no detonation problems. You also have dished pistons which increase the quench distance even more.

I wold not be concerned about this , this quench business is overrated as far as I am concerned. Just my 2 cents :D

Re: 350 stock cast short block, quench? (LiveandLetDrive)
 

The quench area can provide more efficient and detonation proof combustion. A close to contact quench, squeezes the mixture and causes it to to swirl. The swirl spreads the flame front all through the combustion chamber more quickly than the flame front, spreading on it's own. Without going into the details of the unburnt end gasses end pressures causing detonation, suffice it to say that the swirl promotes more thorough, and a quicker, more efficient combustion, while suppressing detonation or knock. One test of combustion efficiency is the ignition advance it takes to make maximum power. The greater the advance required, the less efficient the combustion. If you have to "light the fire" way early, to get it all burned, it's taking too long to combust. I wouldn't brag about an engine that "will take 45* advance". I WOULD brag if it made maximum power with 32*, total. The tighter the quench area, the greater effect. The effects of a quench area diminish as the distance increases. The effects of quench are pretty much eliminated when the piston to head distance is .060" or greater. With a dished piston, make sure the dished area isn't directly below the flat portion (quench area) of the head.

RACE ON!!!

Re: 350 stock cast short block, quench? (CFI-EFI)
 

I merely stated that I was 45 deg, it was actually a mistake on my part, I don't know where you see any bragging. I have read the theory and tech articles too and can recite them if nessesary. I do believe most of what is said what I am questioning here is how important it is. :D

Re: 350 stock cast short block, quench? (MotorHead)
 

I wasn't accusing you of bragging. Maybe I should have used a number different than 45*. Many times I have heard comments like, "My engine will take 4x* of advance", spouted with great pride, as though, as an engine builder, he actually accomplished something GOOD.

If you have read and can recite the applicable articles on the subject, than this info isn't for you. Others that haven't, should be cautioned on the fallacy of lowering the compression ratio with thicker gaskets. Of course, if the piston to head clearance is already in the .060" range, the "damage" is done.

I have experienced too many examples of the benefits of a close quench. In identical engines, including compression ratio, the quench engine will develop more power, using less gasoline, of an equal or lower octane rating, with less spark advance. And with the tighter quench, the compression ratio can be raised. I guess that is a personal judgment. But it DOES work.

RACE ON!!!

Re: 350 stock cast short block, quench? (CFI-EFI)
 

Thanks for the civil reply, if you have done comparisons of identical motors and have seen the benefits of tighter quench my hat is off to you. I have not seen any actual tests of identical motors myself perhaps you have some data you can share.

I would imagine it would be pretty simple to do, you could run a factory deck height motor on a dyno with a with 0.039" gasket and then a 0.020" gasket and compare the numbers.

The reason for my post was to help the originator of the thread. His question was basically "Do I have to worry about this quench because I would rather not have to use a steel shim gasket etc. " and I was trying to tell him not to worry about it and run the normal deck height and normal gasket and I added the part where my motor was running at 45 deg advance to try and give more credence to my answer. I have 2 engine builders near me who also seem to agree with this thinking and as one told me when I was building my 406 " Keep the standard deck height and flepro (0.039") gasket you won't know the difference." :D

Re: 350 stock cast short block, quench? (MotorHead)
 

Even with a .020" gasket, you aren't really pushing the limits with some, piston to deck heights, at -.025" to - 030". I just had the block on the engine I'm building, "zero decked", and I'm kicking myself for not going to a +.005" to +.008" deck height. Very possibly, you wouldn't. It IS my belief, however, that the tight quench is the reason for the "factory freak" that shows up every now and then, that runs better than any other similarly equiped car. The one in a millon with all the tolerances stacked in the most advantagous direction.

RACE ON!!!

Re: 350 stock cast short block, quench? (MotorHead)
 

Actualy these tests have been done at chevyhighperformance and with stacking the gaskets the problems with detonation got worse.

All I know is this is the first motor I built with this tight of a quench and yes it does make a diffrence IMHO. Ayntime you move the pistons closer to the heads you will get a noticable diffrence in the feel and the actual power output of any engine.

There are a number of way to do this... you can run pop up pistons where the head of the piston go into the combustion chamber of head...but the quench or the flat part of the piston where it meets the head will be the same. You can shave the heads ... this makes the combustion chamber part of the head smaller giving you more compression. or you can shave the block making the flat part of the piston closer to the head giving you more compression.

All three ways will increase compression but tightening the quench is the only way to do this without running the risk..or compounding detonation because of higher cylinder pressures.

It's one of those things like broccli, if you ever try it you might like it. My next motor will be a +5 deck height. with a .039 gasket for a total .034 quench. Yes, I think it makes that much of a diffrence.

Re: 350 stock cast short block, quench? (EDDIEJ82)
 

Very good discussion, like I said I was going to zero deck mine but I ended up with less than .080" piston to valve clearance as it sits now. Don't get me wrong I do believe it is better to have a tighter quench but like everything else it there is give and take, and you have to decide based on what kind of motor you are building whether you need to worry about it. The same engine builder said " It's just as good if you think it is going faster" :lol:

Re: 350 stock cast short block, quench? (MotorHead)
 

I don't buy into the "trick of the month". I firmly believe that SOTP results reported by most car owers is directly proportional the the cost and/or the difficulty of installation. I like your mechanic's quote. " It's just as good if you think it is going faster". However, there are certain things that work. Quench, is one of them.

I just had my new, previously, decked and squared, block, re-decked for $300.00, for .007". If I have to flycut the pistons for adaquate valve clearence, so be it. This is for a daily driver, street strip engine. *I* think it's THAT important.

RACE ON!!!

Re: 350 stock cast short block, quench? (CFI-EFI)
 

:beatdeadhorse: Okay this topic is gettting more envolved than what I thought Chris asked for/ needed. I see I need to add something here that I should have said before. The main clearance concern here is between the piston and the head - yes piston slap. So I dragged out the Vizard book and he says 0.022" is min clearance here - yes I miss quoted by 0.002". And Smokey Y said .038 to .040 is optimum while anything greater than 0.060 is missing the boat - no free hp (or improved combustion) ride for you. But using stock dished pistons with valve reliefs and a stock deck hieght, it would be hard for Chris to get it wrong on the short side here.
Vizard also points out that there are side or slanted dished pistons that are worth more power (15 hp maybe) than a conventional dished piston. I'm guessing the flat of the side dish of piston matches the flat metal of the head thats positoned over the piston for more total quench area.
Valve clearance for high lift cams is important too. And the only way I know to measure this is with modeling clay ontop of piston and slowly rotate the assembled motor by hand for valve impressions. You (or even *I*) would have to calculate for gasket compressed height too and a solid copper gasket could work for this (or else use a lot of gaskets). The first time I heard this clay trick I thought it was really just BS. But if anyone knows of a safer way please post as I never have needed to try this with the weeser motors I've built. :rofl:

Re: 350 stock cast short block, quench? (cardo0)
 

Yes! The original question has long been answered. This is a continuing discussion on the value or benefits of quench. Quench clearance IS piston to head clearance from the flat portion of the piston to the corresponding flat of the head. Deck height (top of piston to the top of block) plus compressed gasket thickness equals quench distance. "Piston slap" occurs between the skirt of the piston and the cylinder wall. Vizard says .022", piston to head is acceptable??? "Chevrolet Power" agrees with Smokey (.035"-.040"). The .060" loss of benefit has been discussed. I didn't see where you previously quoted Vizard or referenced either a .020" or .024" piston to head clearance. "D" cup pistons have the dish located away from under the flat portion of the head. There is nothing wrong with the "modeling clay" method. An alternative is rotate the engine with light "checking" valve springs installed, and pushing the valve to contact the piston with a dial indicator measuring the distance the valve will travel beyond the existing valve opening.

Just more discussion, NOT: :beatdeadhorse:

RACE ON!!!


[Modified by CFI-EFI, 9:01 AM 7/7/2004]

Re: 350 stock cast short block, quench? (CFI-EFI)
 

I agree it is a good discussion with valid points on both sides, that why this place is here.

While I agree there probably are some benefits of a tighter quench I guess I need to see some numbers. Any horsepower gain could be associated with a .5 or so increase in compression. If I don't have any detonation problems at 45 deg advance and 11:1 CR at 93 octane then I do not see why I would need to reduce the quench distance.

Again if you are building a mild engine with dished pistons I think the added work involved in getting your quench to .040" far outweighs any benfit you will see, or think you see. With a 406 you have to start looking at custom gaskets ( I couldn't find anything less than 0.039" with the steam holes ) then finding the right pistons then worrying about piston to valve clearance ( flycutting the pistons ) etc.

If on the other hand you are building an all out race engine I think I would consider getting the quench down in the 0.040" range.

I think the piston slap he was referring to was piston to head which is another valid concern with a small distance above the piston.

Re: 350 stock cast short block, quench? (MotorHead)
 

First, I understand that such a change doesn't come cheap. I am not saying it has to be done. If you were to deck the block and even unshroud the valves by removing an equal number of cc's to keep the compression ratio the same, you WOULD notice the difference. Your 45* advance would no longer be a consideration. Your maximum power would come at 38* and probably much less, depending on the rest of the combination. I also understand the you are not "running" 45*, only that you set it there once and got away with it. With less advance, 38*, 34*, 32*?, the "fire" starts later and among other things, there is less time that there is combustion pressure acting upon the piston BEFORE TDC. You might be surprised what it can do for you. Of course the best time to implement such a plan is during the conception. If you're going to top the block, anyhow, to make sure it's flat, or deck to square, the only additional expense there is, an extra trial assembly or two. Once it's in the machine, it doesn't cost much at all to take a little more off the top. On a DYI, it's time not money.

Steam holes can be drilled in head gaskets at home. And there is nothing wrong with a .039" compressed gasket with a zero deck. I imagine that is what he was referring to. But THAT is not "piston slap". The piston hitting the head makes a knock. Trust me on that one. And THAT is the reason for the .035" to .040" minimum piston to head spec.

If a guy is just slapping together an engine for a daily driver, with little or no concern for maximizing the potential, a LOT of details can be overlooked.

RACE ON!!!

Re: 350 stock cast short block, quench? (CFI-EFI)
 

I must admit my last motor (355ci , 296RWHP) ran real good and it was around 0.040 quench with steel shim gasket. I was at 10:1 compression with iron Sportsman II heads and I had 230-235psi cranking pressure. I was running at 40deg advance on that one and ran 93 octane and I had never had any detonation problems. The tighter quench could have helped on this motor.

The ony reason I was looking for a thinner head gasket was because I was going to leave about 10 thou on the deck so I could clean it up the next time. :D

Re: 350 stock cast short block, quench? (MotorHead)
 

Moter head whats your secret, those are some extremly high timing #'s for the compressions you are talking about. I struggle to get mine to run without detonation at 33 degrees. What springs are you running in your dist and when does the ramp come in fully? If your running heavy springs to accomplish that much advance you would see a big diffrence in the bottom end if you backed out the timing and ran a lighter spring.

I too am scratching my head about this too, but I don't mind. I have a MSD Pro Billet distributor and the springs in it are somewhere in the middle of the set they give you. Advance is all in at 3000 RPM or less. It just occured to me that one possible reason is I have never run a vacuum advance can. I think with the vacuum can you can exceed 50deg advance at low RPMs, maybe that is where the probelm is.

In my last motor (355ci) I had a CC hydraulic 270H cam in it and I checked the compression and it was 230-235psi across the board, I thought it was the hydraulic lifters not opening the valves because they might not be pumped up. Shortly after I changed to a solid cam 282S and I checked the compression again and cam up with the same numbers. I ran that motor for a long time at 35degs and just before I pulled it I set it to 40deg and it ran fine, did I gain anything, not according to the dyno, but didn't really lose anything as far as I could tell. My current 406ci is running at 35deg all in at 3000RPM it is 11:1 CR and I run 93 octane with no problems I doubt I would have as high cranking compression though as the cam is tad bigger. :D