Lets talk a little more about quench

ok so whats the widest i can run my quench and still be effective, is .040 the number? im running a set of 915's and it looks like im going with a forged 12cc dished piston at 0 deck

i just have to cc my heads to figure where im at

.045"

thanks

A lot depends on the mahining on your block. I typical block, decked with an older miller will typically be out of flat and parallel by up to .003", the bad ones as much as .020" off over the length of the deck. So when you say you're building with quench you have to take into account the accuracy of the parts and machining. Minimum safe distance for a forged piston/steel rod engine can vary from .015-.030" depending on the compression height, the bore size, and the rpm of peak power. Normal "safe" is no closer than .030. I target .035. To get that accurracy I have to index and correct the cranks' stroke in some cases, match and set rod lengths, align hone the mains, and have the block square decked. Only then do you have the package that is accurate to run tight. Tighter is better right up until they hit. Anything farther than .050 IMO is not worth aiming for as the cost of machining will be great for little or no benefit.

David Vizard writing on another person's Quench testing in 0.010 increments:

http://www.enginebuildermag.com/Article/2266/surfacing_equipment_can_increase_horsepower.aspx

sample quote

I knew that minimizing the quench area clearance was also good for normally aspirated engines as well, and had it in mind to do some tests that would establish just what it may be worth. These got put off time-after-time because it is a time consuming and relatively costly experiment for a one-man business to do.
Fortunately, while lecturing at the SuperFlow Advanced Engine Technology Conference in 1998 I met a gentleman who had actually done these tests.
As I remember it, the 350 SB Chevy test engine was in the 380-400 hp range and started with a CR of just 9.9:1. After baselining with the piston-to-head quench at .065; (.040; for the head gasket and .025; down the hole for the piston) the quench clearance was reduced in steps of .010; until the quench clearance was .035. When you are looking at potentially small changes in power data, scatter is a consideration. However, by averaging out the numbers the results appeared to consistently indicate that each .010; reduction in quench clearance was worth 6-7 hp or 5.5 to 6.5 ft.lbs. of torque.
The .030 change in quench clearance increased the CR from 9.9 to 10.6. So I made some relatively sophisticated calculations to establish what the increase in output from this factor alone would be.
The answers I came up with indicate that, at best, the increases seen were only 60 percent attributable to the increased CR.
Other issues of note were that the engine needed less total timing to make the power seen. Along with this, the ignition swings in the advance direction indicated it to be no more prone to detonate even with the higher compression.
......So how close can the quench clearance be run?
Good question.
A friend of mine who ran a machine and dyno shop until recently has, using good stiff race bottom end parts, run down to .022 before contact was seen. I have run a SB Chevy with stock crank and rods down to .025 where it showed that contact had just occurred. This was a motor that peaked at 5750 rpm. In practice I usually build a typical domestic V8 for a customer with .035 total quench clearance and that seems to be safe so long as the pistons are close fitting in the bores. For my own motors I typically go to .028 when using a known combination of good bottom end parts.

FWIW, a friend's LT1 stroker was less prone to detonation on pump gas after he replaced the old head gaskets which gave him .054" quench to gaskets that gave him just under .040, while increasing compression about .3 points....

Page 73 of Charles Fayette Taylor's book
"The Internal Combustion Engine in Theory & Practice"
has this set of graphs showing that in addition to Quench Clearance,
Quench Area is also important.

The 'Performance Number' on the left hand side is another name for Octane Rating of Gasoline, in this case the US Army-Navy substitute that they claimed to be superior to either ROM or MON, but where PN=100 is still what the chemical octane acts like when used as fuel.

thanks guys this is all very interesting and helpfull

so with good machining i can run it as close as about .030 but .035-.040 is probley the best just to be safe

i was hoping i could run it wider.. i wont know anything untill i actually CC my heads

Oh you lucky guys.... NHRA spec's don't allow we poor stock eliminator guys to play much in that realm.

Moper, you are a smart guy with a forum through Mopar Muscle, how about enlightening us and elaborate on swept volume, you know, the gasket thing, and the useless area thing, and the STALL thing when holes get too big....

Peace, Rev. Mopar

Class of 65 (32 time NHRA and IHRA National Record Holder)

Rev,
thanks, but you have me confused with another fellow who has a MUCH more satisfying job than I do...lol. plus, the post is about quench.. not static or dynamic ratios, the differences, or what else affects it...lol.

i was already planning on a .020 hyper flat top from KB and it looks like im going with the stealth heads after all because they are the only choice for a quench zero deck flat top engine other than edelbrocks. my 915's have way too small of a chamber

Good point about piston wall clearance and varying metal expansion ratios.

For factory engines in daily driver vehicles instead of race engines,
what about wear with mileage ?

Let's just assume that a new & well assembled engine
with attention paid to each cylinder individually
will hit at 0.025 at 5500 rpm.

What would be people's guesses
as to how much increased head to crown clearance is necessary
to avoid hitting at 5500 rpm
when the engine ages to, say
50,000 miles ?

100,000 miles?

200,000 miles ?

Related question:
does a head gasket compress a wee bit more with additional years and tighten effective quench ... or does it relax instead and increase quench clearance for the worse?

why was it going through gaskets, im assuming you didnt have it decked to begin with?

Correct, I checked it and found it to be within .0015 so I ran it. Tried the steel shims dry, w/copper coat, Hylomar. It probably would have been ok if I didn't abuse it so much but that's what I built it for

Here's where it stands right now. I dropped a piston in all 4 corners and it's perfect. Special thanks to Don's auto in Kenosha, EXCELLENT work.All I gotta do is clean/inspect and assemble.

what pistons are those, look to have a pretty big dish. how manny cc's?

They are actually Speed pro 2355's, were flat tops w/ 4 valve reliefs. I cut 'em for valve clearance and compression. If I remember correctly they are 14 cc's now.

wow i didnt know you could cut that much out of them. how big of a cam were you running to need to cut that big of a valve relief?

The crowns were over .300 at the reliefs so theres pleanty of room to cut. Minimum thickness according to speed pro is .180, I still have over .220. I'm running .579 lift with 2.14/1.88 valves so the radial clearance was really tight. Had to unshroud the cylinder walls to clear the exhaust valves also. If you build what you're talking about you won't be dissapointed. Just watch where you "cut corners"

Any good and reputable shop can and will automatically do this quality of work. Will you have to pay more? Probably so but in a package deal I can't see it running much more. You can run tight quench with loose pistons also. If they are a good shop they will use a dial indicator on a bridge and rock the piston and take the average in and out of the hole, go to opposite side (outboard or vice versa), and do the same. I had to do this on my build. Then you you surface the deck of the block to get it where you want. May have to do that a couple time, get it close, then make a final cleanup pass. It just takes attention to detail and pride in workmanship.

I am shooting for .040" and milling each piston top till I get it perfect (in addition to the deck milling) which is why I'm going w floating pins so I can mock it up as many times as I need to till it's right.