Ibanez540r

So I've been reading a ton of conflicting information all over the internet (apparently a hot topic). It's clear that a .040 quench or so is ideal for performance applications to reduce detonation. However, I have seen some opinions that state a dish piston (similar to a stock dish 4 valve relief), not a D dish with a pad, do not have the surface area to match the head to create or benefit the effects of a proper quench.

It makes sense, but is it true?

REELAV8R

To answer that question first know that the squish distance is the distance from the squish pad on the head and the top of the business part of the piston (not counting a raised edge for dish piston) regardless of dish or flat top.
Quench is the action of transferring heat to the head then the cooling system etc.

So if you have a dish piston that is not a D-dish and has no squish pad then the squish distance is the sum of the depth of the dish plus how close the piston approaches the head.

If the dish is .020" deep and the piston is .025" down the bore at TDC and the head gasket is .015" thick then your squish distance is the sum of these numbers. which is 060". If your target is .040" then you missed.
So to get a good squish distance and good quench a D-dish is the answer for a dish piston.
In addition, the action of squishing the charge adds turbulence to the air fuel mixture for a more even mixture which leads to a better burn.

What I can tell you from my own experience is the Squish distance is very effective at reducing detonation.
In my '77 I have .038" of squish 9.9:1 CR and run 87 octane. I have yet to hear any detonation.
On the stock motor I could run 91 and still get detonation on a hot day at WOT.

68post

Yes that is true.

A full dish is more prone to detonation, but seems to provide less emissions from a more complete burn, ( and perhaps a very minuscule power improvement).

Reelav8r is very correct also.

It's all in the compromises.

73, Dark Blue 454

Don't get too hung up on quench or piston design. There are millions of engines that run great with .050 or .060, and millions of performance engines that run dish pistons.

Worry more quality machine work, CR, cam, valve-train geometry, durability, and design (an engine that works with your car and goals).

Good luck!

Ibanez540r

Well.. here's where I'm at. I'm cleaning/freshening up a motor that has been in my garage for about 15 years. It was pretty fresh back then, but has been sitting uncovered. To be safe I picked up new bearings and rings, and tore it down to clean. I measured the piston to deck clearance at .055". Obviously someone miscalculated compression height and didn't deck the block.

I was originally going to put it back together and sell it on craigslist for a few bucks to go towards other things (Still likely), but due to several factors, I may use it temporarily just to get the car setup and quirks worked out, while funds are saved for the real build.

Compression aside, for the sake of quench discussion, I've heard the debate that .060 - .090 or so is the worst area you want to be and it highly encourages detonation. Once over .090, you are back to a safe zone because the whole effect is negated?

So, do you go with a .015 gasket @ .070 quench, or really open it up with say a .040+ gasket and get back to the supposed safe zone @ .095 or higher?

76strokervette

Mike,
I agree with the others that machine work and or parts will be needed.
If you want to go the cheap route and save your money for the new build I have the l48 out of mine on the garage floor.Everything is there minus the carburetor.If this seems like something you're interested in give me a call.FWIW you're making me contemplate pulling the body off the frame after seeing yours.
Dave

REELAV8R

I've not found any reliable source or data on really large squish bands vs a properly sized one. You got any links for that?

Ibanez540r

Reliable? Not so much, hence the topic here. I've been reading years and years worth of various forum discussions on the subject. Obviously the forums have both sides of the spectrum from Mr. Know-it-all with no experience to really intelligent long time engine builders. However, what I started to notice pop up on a regular basis were guys saying that the effect of a proper quench begins to really work against you around .060 up through .090 and some saying even .120. I think the explanation was that once the piston is so far down the cylinder (.090 or .120 +), the effect is completely diminished and the large area then becomes part of the chamber and burns similar to an open chamber design. Sounds logical. Real world? Who knows.

Then you add forced induction into the conversation and it really goes all over the place from no different then N/A, to the FI creates plenty of turbulence on its own to keep a good homogenous mixture, all the way to opening it up really far to slow the burn.

Then, you find the one of the most popular SBC crate motors, the ZZ4, running 10:1 compression @ 32 degrees timing on 92 octane with a piston .028 down the hole and a .051 head gasket resulting in a .079" "quench".

..I'm on information overload. I've read so much info on both sides of the spectrum, both with a great argument and rebuttle, that I don't know where I stand. Kind of the reason I brought up the discussion...

Ibanez540r

That's a GREAT idea! Then you can just let me use your new motor until your car is done! Don't worry I'll play gentle

76strokervette

I suppose you 'll want the stall converter and billet 2004r to go with it!
Sorry I don't have a flywheel to go with your tko.
I think the answer is in keeping the heat in the thickest cross sections of the piston and cylinder head.Look up the mazda skyactiv gas engine that runs 14 to 1 compression ratio on pump gas.

cardo0

Ok Ib i read the whole thread and it seems u want a single answer for several questions on quench height.

First i say that D. Vizard wrote any quench height doesnt become very effective until almost 9.0:1 static compression. Thats not my discovery so take it for what its worth.

Next the larger the squish area the more effective. My visual says the D-ring dish will give u more than the circle dish with vlv relief (stock cast pistons) as the circle type has significant squish area under the head chamber - doing nothing. U can see on the newer Hemi heads where Chysler made quench pads full circle on head and piston to put quench into a hemi head - works great from what ive read.

Third i never head of pistons 0.055" down the hole. But large quench distances beyond 0.060" dont hurt anything - just dont provide any turbulant effect. U have found for yourself the stock chevy engines are for the most part in that >0.060" height so done take faceless internet BS as fact unless u can verify it.

So u need to calculate your compression ratio with what u have and if <9.0 dont worry. It sounds like that "spare" engine has the pistons its going to get without changes. So pop the head and measure piston height along the axis of the piston pin where piston rock should not effect it - make sue u are at TDC (yes u need an accurate TDC with piston stop and degree wheel for this). Then measure the above piston volume along with the head chamber vol to nail an accurate static c.r.. Until u have that we are just blaberating "what if". And even for complete engine builds the final choice of pistons come down to a small few where u have to choose from and can calc the c.r. from before purchase.

I guess what im saying here is u need to take some "accurate" measurements before wasting time on internet searches. If u dont take the time for accurate measurements then the whole effort is pretty much a blaberation.

Ibanez540r

.055" results from a 1.54 compression height piston in a stock deck 9.025" block. That compression height is supposed to be used with a "0" deck resulting in a more standard .025".

Somewhat common mistake for someone who does not know how to take "accurate" measurements and build a short block properly . Like I said, it was a motor I obtained in the early 2000s that I pulled a head and the pan to confirm it was truly a pretty fresh build. It was bagged, but eventually came uncovered and dirty over the past 15 years or so.

What do my measurements have to do with the concept of piston to head clearance and the conflicting theories and effects of quench? I've taken extremely accurate measurements of the entire engine with a micrometer, deck bridge, and dial indicator. Also cc'd. I did not post them because I'm not looking for someone to apply the concept to my application, I'm capable of doing that.

I guess I'm looking for the same as everyone else.. reliable proven information. The only thing reliable is that .040" is beneficial. From my research, anything outside of that has not been tested and only hearsay from one to another. What are the differences at .060, .080, .100, .120, +? Are there any? (as mentioned, forced induction throws another curve ball)

It would be interesting to see someone scientifically research a motor on an engine dyno with the same tune, same load, etc. setup with a knock sensor and progressively increase head gasket size and observe what happens with detonation.

cardo0

Ok your confident your measurements are accurate and your not really asking for help. Well i believe u have answered your own question with the ZZ4 example. If u expect a series of lab results for this u would have to estimante the time and labor and equipment to do so and u would realize any information like that would be proprietary.

Sorry if my answer was of no use as u can read continuously on this forum or any forum were others are asking for accurate replies but provide only estimates of their parts and volumes. Its almost a disease with new enthusiests to use hearsay for calculating compression ratio and other critical parameters as finding the true TDC with a piston stop and degree wheel is too much effort. Or even verifing the timing tab TDC and will just use whatever bolts up to the crank and block for timing marks.

Again there may well be a lot of real data produced in expensive dyno rooms by trained technicians but unless its leaked will most likely become proprietary. Some authors do share thier experience and publish in books where u can read bits and pieces. My 2 cents is the quench distance >0.060 is not enough of a concern otherwise if it was hurting power it would become fairly common information by the common authors that publish. The asset to realize here is just optimizing the quech height. And D. Vizard also mentioned the ring type dish has no useable quench (or something to that effect and i dont recall which book he wrote that in).

REELAV8R

Ibanez I applaud your wanting to find out as much as possible about squish as it applies to your situation.
Many times you'll get responses about how such things don't matter and just move on. It could be about any aspect of the engine build and you may get responses like that.
IMO they are wrong. EVERY aspect of the engine build is important and worth investigating to gain what knowledge you can for the best results of your build in the end.
On my engine I chose to let Dart take care of the valve springs, valves and things like install height, seat and open pressures and distance from coil bind. It appears they did a poor job of that and now I am forced to deal with it after some damage has occurred.

So learn what ever you can BEFORE you have to go back and fix it, or find ways to deal with the problem later.

As far as squish goes the goal in an NA engine is to get the needed turbulence and homogenous mixture to effectively burn all the fuel in the combustion chamber. Getting the squish distance as small as possible gives the most quench and turbulence.
Detonation occurs when hot spots develop due to stagnant flow in the chamber. Stagnant flow burns much slower than turbulent flow. So more turbulence faster burn. Faster burn less time for hot spots to develop and turn into detonation.
Often times this can be observed when you get detonation at 2500 RPM that disappears at 4500 RPM. The burn is finally fast enough for detonation to not have time to set up.
The reasons for detonation are many and varied, my example is only one of them. So detonation can certainly occur at high RPMS or low RPM, cool days or hot days, under a heavy load or under a light load.
As near as I can figure .040" is as close as you can go and comfortably avoid hitting the head with the piston in most applications. With a 3.48" stroke and steel con rods, tight fitting hyper pistons, tight con rod bearing clearances and main bearing clearances a 6000 RPM limit, you could probably get away with .030". But it's a gamble.

Check this thread out, and others like it. Lots of good discussion about squish and quench.
http://speedtalk.com/forum/viewtopic...h+ratio#p31928
You may need to join to see the thread.

Bigger squish distances seem to apply more to emissions applications and for turbo, super charged and N2O engines.