I based my definition of "conservative" or "pushing the envelope" based on the recommendation of noted engine builders like Lingenfelter, RM and i can't remember who wrote the other book i have, but they all recommended a .040 quench. In fact Lingenfelter wrote the ideal combination is to run a .005 piston to deck and a .038 head gasket (.043 piston to head). Either Lingenfelter or RM said that at .040 the gap cannot support combustion and that benefits from further reduction were insignificant and that above .060, the benefits of quench were largely lost. I'll also add from experience that there can variances from cyl to cyl (crank throws, deck heights etc can vary). So my thinking was why go beyond those recommendations on a street engine. The risk doesn't seem worth the small reward to me, but it aint my junk. All that said, .030 probably wouldn't be a problem and maybe i'm just jealous cause my tightest cyl only has a .045 quench.

race on bothers.....................

 

So it would seem that we have the usual concensus of no concensus. Let me ask this. After the pistons are fitted and measurements have been taken, what is the minimum of clearance I would want beyond the highest reading when checking at the "rock" points?

 

On my 90K mile 350 with forged pistons the center dimension ranged from 24-26 depnding on cyl. At the edges the worst case was .020 tilted up and .028 pushed down. Another with a wide range was .023-.030.

On my poor old twisted 406, the piston to deck heights vary from .018 to .031.

I'll warn you that i measured these with a staight edge and a feeler guage....so your mileage may vary.

 

I've been following this discussion for a while and noticed that some are advocating the old Stroker McGurt philosophy of "if a little is good and more is better, then too much is just right."

Tighter quench values do help squish and mixture uniformity; however, there are limits. I've quoted John Lingenfelter and now I'll quote Smokey Yunick. On page 44 of "Power Secrets", Smokey sez that a safe figure for all-around performance is 0.040" while anything up to 0.045" is okay. He also sez that he feels 0.038" is the absolute minimum for a nominal 4" bore with a piston-to-wall clearance of 0.007".

I've worked with racers (guys who make their living running short ovals) who have used less clearance, but they were working with custom made pistons with swirl dimples and other exotic design characteristics trying to get the last ounce of performance out of their engines at low RPM's when pulling out of the corners. There is nothing stock about their engines from the block to the angle-milled heads so their quence values aren't going to work on a near-stock street engine period. YMMV, but remember you pays your money and takes your chances.

The real issue here is not quench values, but zero-decking a street block. This is monkey see, monkey do reaction to what many racers do without really thinking about the extra work required to ensure that parts aren't broken when the engine is pushed hard. If you insist on running zero deck clearance, then contact the manufacturer of of your chosen pistons and tell them what you plan to do. This may change their recommend piston-to-wall clearances to minimize piston rocking. More than likely, they may tell you to deepen the valve pockets on the pistons or specify a minimum valve clearance depending upon the cam and total valve lift you plan on using. If using a domed piston, you may have to trim the dome. Another problem with zero decking is compression ratios. Unless you plan on using dished pistons, your engine may ping on 91 octance street gas.

I've been "zero decking" race engines since 1973 (long before Chevy even considered recommending this approach to racers). (As late as 1978 the Chevy Power Book still recommended a negative deck height of between 0.015"-0.020" with a minimum piston-to-head clearance of 0.035".) I can guarantee you from personal experience that just building a "zero deck" engine with off the shelf parts without consulting the ring and piston manufacturers can result in broken parts. The amount of work assembling, trimming, and modifying (blueprinting) these parts is considerable, especially for a street engine. Go with John Lingenfelter's recommendation in an earlier post to use negative deck height of 0.005" and a 0.038" gasket for your quence values, and you'll be happy with the results!

Good luck with your project!

- Greg Smith, Perpetual Corvette Mechanic

 

Obviously, I am not able to communicate my point. I'll give this one more shot. I agree that .035" to .040" is considered the minimum piston to head clearance. I don't necessarily concede this, but to save further argument let's agree that it is .040". NOW consider the construction and the usage of the engine that the standard clearance of .040" was given. Do Lingenfelter, Reher-Morrison, Chevrolet Power, etc. publish books for Checker Auto Parts engine rebuilders? For the most part engines built from those pages will be race engines of one degree to another. The point that I am failing to covey, is that the "envelope" is elastic. Different engines built with different components for different usages, have different clearance requirements. I am not suggesting that under dynamic conditions that Corvette Kid NC's pistons actually get any closer to the head than any other. I am suggesting that with tighter piston to cylinder wall clearances, and lower rpms, that a tighter STATIC clearance will yield no greater of a DYNAMIC clearance. Keep repeating: flexible envelope, flexible envelope.


Was that a +.005"? Or a -.005"? Was the .043" their words or your assumption? I'd go with a +.005 deck clearance (piston .005" OUT of the hole) and a .038" gasket for a .033" piston to head clearance.


I am aware that quench greater than .060" is of little to no value. I have never heard that quench closer than .040" was of no further benefit. I have alway understood that the tighter the quench, the greater it's effect. Much of the benefit of quench is in the force imparted to the air/fuel mixture, to cause turbulence in the rest of the combustion chamber. Even if nothing additional happens at a clearance of less than .040", a quench of less than .040" causes the .040" to be achieved a few degrees or minutes earlier in the crank rotation. If the actual tightness of the quench isn't a benefit, the duration of that quench, will be.



I believe we have a consensus that the spec is ".035" to .040", minimum." It is my contention that with your milder engine and application that you can "push" the spec (not the envelope, which is elastic) to your benefit. I'm not being successful in making that point to Ralph.


Without "pushing" anything, I would shoot for .040".

RACE ON!!!

[Edited] Only to correct "quote" fields I messed up originally. Not one letter of text has been touched.

 

Jim, i understand your point and i wasn't arguing if the viability of.030 in this case. The point that i was trying to make was that unless this is an applcation where every HP must be squeezed out, why venture outside a widely accpeted safe spec. Lingenfelter's book is a guide on building strong reliable street performance engines.

btw, what do you think reducing quench from .040 to .030 is worth...all else the same?

I give I knew I should have had my decks squared

 

There is no point in venturing outside of widely accepted safe specs. Forget I mentioned it. It is a totally pointless pursuit.

Absolutly nothing. All in all, a pretty stupid thought.

I appologise for deraileng the thread, with such foolishness.

RACE ON!!!

 

Jim......i was serious: what do you think it's worth? I dont think it was foolishness and i could be wrong. I thought this was a friendly discussion, and maybe i'm just too conservative

Have a nice weekend all.

 

Now I'm wondering whether you think I was discounting your opinion Jim, and/or if you got that impression from my second question because I maybe didn't state it clearly. It wasn't meant to be a repeat of my original question but looking at the actual needed clearance from another point of reference, that being measured from the point of maximum rock 90 degrees from the pin. Now I'm assuming that this clearance (quench) is generally measured at the pin and is to allow for the rock and such. So what I was asking the second time, let's say you take your reading from the piston with the absolute least clearance to the head measured at the rock point. Now what is the absolute minimum clearance one would want at that point? And then from there one could determine the average figure to shoot for across the board measuring height at the pin and truly minimizing quench safely instead of just using a generally agreed upon number such as .040" or whatever. In other words, I've understood what you've said from the beginning, a snugger piston has less rock, etc. and so things could concieveably be pushed farther than just using an arbitrary number. I'm just trying to determine a "worst case" figure from which to base it on.

 

I DID understand that THIS question was not a repeat of the original. My answer in post #25 was a serious response to the second question. I should have qualified it with the word, "maximum". In retrospect, however, I realize that ".040" maximum" is way too large. I am not familiar with the variation of the piston to deck clearance from the "at the pin" to the "90° from the pin", measuring positions with any given piston to cylinder wall clearance, let alone at varing clearances. I would never "shoot" for a piston to head clearance (at the pin) of more than .040", so a minimum, in the "rocked" position would be substancially less than .040".

RACE ON!!!

 

Who's questioning whose question where on which post here? Man some allways have to get the last word in no matter. Which question really is it - clearance, quench or squish? Can't tell anymore - to many posts from PHD's without they're paper. Is there a dyno expert in the house that has actually measured the effects of quench hieghts - and has that data. D. Vizard has a Masters in Mech Eng with a whole lot more dyno time than most of us says its possible to work with as low a .022" clearance - but myself with my budget for motors .035" is a good number. Its not like we're compressing gases here and c.r. or c.i, rotating parts strength, speed (rpm) would change anything? Or maybe more tensional stress at 7,000rpm in comparision to 5,500rpm needs the greater clearance. Or like quench hieght with 502 cubes at 14:1 may tolerate a higher clearance and still make good gas squish into chamber?? And hey if ur running 502 inches at 14:1 to 7,000rpm then a big clearance would be expected - not limited to .070". Big inches with hi c.r. shifts the equation into uncommon territory. PV=nRT has more to explain here than just saying .060" or .070".
To many possibilities/combinations to know them all. But hey, unless ur building a monster mtr to run at the nationals or hi-winder to run at Sebring then Smokey, G. Jenkins, the Power Manual tell us .035" to .050" will get the job done here without the rocket science guys. Well this is an unserious response to this delima of venturing into quench, squish and combustion chamber theory while out'a the laborotory.

Season's Greetings,
Cardo0

 

Well excuse the hell out of us for examining an idea and trying to learn something. Funny, by your own admission you didn't take time to fully understand the thread but that didn't stop you from taking the time to criticize it. Since you obviously know it all, please share.

 

He doesn't know diddley. He may have met diddley, but he doesn't know him. He posts a lot of gibberish. Whenever I see his name associated with a thread, I make a point of reading it. Often, it is hilarious. Like, "Which question really is it - clearance, quench or squish?". HUH??? Or what the heck is, "Its not like we're compressing gases here and c.r. or c.i, rotating parts strength, speed (rpm) would change anything?" WOW!!!Consider the source and go merrily about your business.

RACE ON!!!

 

OK Cardo.......how about you tell us what you think the difference between .030 and .040 is in HP/TQ. No one else wanted to venture a guess, but you seem to know it all.

BTW, we were discussing quench.......so please try to keep up. And try to keep in mind who you're dealing with here. I might be a schmuck, but CFI and da Kid didn't just fall out of a tree.

 

Sorry, just not enough time to type everything.
But seriously u look for an easy answer to what would require a major dyno effort. Who's gonna have all the data from every dyno run where they compared the different deck to cyl head clearance let alone piston rock? And to what accuracy - every .01", .001"???

OK Cardo.......how about you tell us what you think the difference between .030 and .040 is in HP/TQ. No one else wanted to venture a guess, but you seem to know it all.

Wow, now quench height/clearance somehow makes more or less hp? And i thought it prevented detonation - heys thanks ralph. Now the decking/reduced clearance does increase c.r. which in turn increases power. But really the squish just reduces the unburned fuel pockets in the chamber. And u can see that c.r. change as a function of initial c.r. itself.
For ex. i recently crunched the numbers for a 383" stroker without (+.025") and after decking block (0 deck height). Well using F/T pistons at stock deck hieght c.r. was 13.3:1 but after decking to zero c.r. climbed to over 14.5:1 (1.2 point increase). Put in 15cc dished pistons for stock deck height c.r. = 11:1 then decking to zero only increases c.r. to 11.7:1 (by .7).
What i'm saying ralph'ie is that don't look for quench to increase power as much as it provides better combustion chamber swirl for better fuel burn - reducing detonation. This increase in swirl kind'a pays for its own way here in that it allows a higher c.r. to operate on the same octane fuel without detonation/piston knock. Look for the c.r. increase to increase power. How much? No, i don't know but there are desktop dyno programs that can answer that for u. D. Vizard says zero decking worth 8-10hp but again that could be from the c.r. increase as he dosen't specify. Smokey only says quench reduces detonation and helps to increase the c.r. in the same process.

BTW i didn't imply anyone fell out'a a tree - though i suspect one may have fallen on his head. And as long as i can keep him amused then i think we're still safe.

cardo0

 

One thing to remember while using Internet forums is that everyone has their own opinion, and the reader has no way of verifying information unless the individual is a) well known in the field of discussion for his/her expertise or, b) quotes readily verifiable sources. The later method is the more important of the two unless you're just looking for a quick answer to a common question. You can look up the information for yourself and make sure that the individual didn't misquote or misinterpret the source.

An example is on page 38 of David Vizard's "How To Build Max Performance Chevy Small Blocks" where David sez decking the block to 9.000" for a Fel-Pro 0.038" gasket or 9.010" for a steel shim gasket is worth 8-10 HP on a 350 c.i. engine. David goes on to say that some of his "more wealthy/adventuous engine-building friends have run quench clearances down to .022 (inches)". The lowest quench figure that I seen David quote is 0.024" in his Power Squeeze article. Mr. Vizard also goes on to say in the article that when using this quench value his pistons kissed the head at about 7,000 RPM. So if I just quote 0.024" is a good quench value from David Vizard, but don't give the reference for your own evluation, you may end up with a busted engine.

If I (or anyone else) say that I recommend a specific value, then you have the right to ask how I arrived at that value, if I've used it in my own engines, and what the results were. Otherwise, you don't know if it's fact or BS.

I don't recommend asking for minimum/maximum engine blueprinting values over the Internet. There is too much room for error or misunderstanding. The proper place to get this info is the manufactuer of the part you're using. Additionally, the responder doesn't know if you even know where to measure the part correctly or if you're using feeler gauges passed down from your Dad (who used them to set the valves in his Z28 before he met your mother at the drive-in).

These are just my opinions, so YMMV. BTW, if you can find a copy of David Vizard's June 2001 High Compression article he had posted on Motortec Online Magazine, it's worth the effort. (David's removed all his articles and closed down the site, but the article seems to have a life of its own and re-surfaces occassionally on the Internet. However, if you can't find it, don't worry, David re-cycles all of his material in new articles/books from time to time.)

- Greg Smith, Perpetual Corvette Mechanic

 

Someone combed through my post to find i may have been in error by .002". Though i tried to qualify my recommendation for using .035" myself. Guess someone completely overlooked my comments on c.r., c.i., rpm, and quality of parts as factors. Not like i posted some accepted safe spec science. Or was proposing pwr gains from quench hieght instead of detonation reduction.
This thread is no longer producing any usefull info and is now reduced to fault finding.

cardo0

 

 

What type of pistons were used in the buildup?

 

The section referenced in "How To Build Max Performance Chevy Small Blocks" only discussed recommended machining operations for block prep. However, if you buy the book, David does detail ten different engine combos as the last chapter of the book detailing specifically what was done to each engine as well as what parts were used and if they were modified. The book itself is a must read for anyone interested in performance engine building.

In the article "High Compression" David discussed essentially the same material he went over in the "Power Squeeze" article in much greater detail (four pages versus one page of material, graphs, and photos).

- Greg Smith, Perpetual Corvette Mechanic