Okay... I have read a bit about quench, but I need a little help here.

I have heard that the ideal quench is .040 inches, betweent he piston and the head at TDC... I have also heard that having the quench set as close to that as possible enables you to avoid detonation, Finally I heard a 9.0:1 compression engine with the wrong quench will detonate more than a 10.5:1 compression engine with the proper quench.

How true is all this? How vital is the quench? What happens if it's too small or big?

I also read something about not all heads being "Quench Heads" and therefore it doesn't matter with some heads... Are AFR's affected by Quench?

If my pistons are .020 in the hole at TDC...

would I better off running a thin shim gasket to hit the .040 quench, which will have my compression ratio up around 10.6:1
- or -
would I have more success running a thick .039 gasket and lowering the compression to 10.1:1, with a quench of .059.

Which one would avoid detonation better, or would it be the same deal either way?

 

I'll try to answer some of these questions.

The ideal quench in a perfect world is .001 or as close as you can get it so it doesn't hit the head. However, in the real world material elongates from heat and changing directions at high speeds. Also pistons rock in bores. So the tightest and still have some factor of safety is around 0.040" running a forged rotating assembly. Quench helps avoid detonation by thoroughly mixing the air fuel mixture before ignition by smashing it against the top of the head and making it push into the combustion chamber. I.E creating more turbulence adds to better mixing. An engine running a 9.0 CR vs a 10.5 CR is dependent on other factors, like combustion chamber size, iron or Al heads (added heat absorbtion allowing a higher CR). Its definitely a possible situation if the piston was like .080" in the hole plus a thick head gasket for the 9.0 cr and a cam with little overlap to make a high dynamic CR. There's just a lot of variables there. Quench heads have a section that is a flat over the bores that makes up the Quench area. I'm not sure about the historical figures on those, but maybe someone can help you there. AFR's are quench heads, they have the flat area for "squish" to take place.
In regards to your set up, a little more infor would be better. 10.6:1 on 92-93 pump gas is possible but should be accompianed by AL heads and the right camshaft. Also, you're shortblock don't know about the components. If they're stock I'd go with the second plan. I hope that helps some, maybe someone else can help me out.

 

Well I bought a rotating assembly... it was one of those kits off ebay... Originally it was supposed to have Speed Pro Forged Pistons, which has a compression ratio of 10.4:1 on 64cc heads. After the auction was over, the seller asked me what bore size I needed, well I needed .040, which come to find out if not a standard over bore and Speed Pro doesn't make forged pistons for .040 over 383's.

So I had 2 choices.. he could refund me $160.00 and send me Hyper SpeedPro Pistons, or I could spend another $100 and get the SRP Forged Pistons... I told him I thought I should get the SRP pistons at no extra charge, because the auction said Forged Pistons, available in .030, .040 or .060. He then sent me the Forged SRP pistons, come to find out they make 11:1 compression on 64cc heads and zero deck block.

So My setup is going to be as follows...

Eagle ESP 3.75" Crank
Eagle 5140 SIR I Beam Rods
SRP Forged Flat top Pistons, .040.
AFR 195cc Street Heads, 68cc Chambers
zz4 Camshaft, 208/221 duration, .506/.544 lift (1.6 rockers)

I don't have the short block together yet, so I don't know how far in the hole the pistons are going to be, I was just assuming somewhere around .020, I rally don't know what go guess, maybe you could give me a more realistic guess on that.

But if that's the case, with a shim gasket (.018) and .020 in the hole, my compression ratio with the 68cc chambers is going to be 10.6:1, which is a little high in my opinion for pump gas, however the quench will be small, helping with less detonation, correct?

However with a thicker composite gasket (.039) and .020 in the hole, my compression ratio with 68cc chambers would be around 10.1:1, much better in my opinion for street gas, but now my quench would be out to .059, leaving more room for detonation.

 

The idea behind running a tight quench area is not to avoid detonation. The whole purpose is to increase combustion efficiency. By increasing combustion efficiency, the engine doesn’t require as much advanced ignition timing. Pre-ignition can be avoided by running less advance. In other words, not running more than the engine needs.

A benefit of running a tighter quench area is the turbulence is increased. The mixture is homogenized more completely which helps reduce lean spots (i.e., hot spots). This reduces uneven spots within the combustion area.

The area is also small enough to reduce the chance of burning the air fuel mixture. This helps locate a majority of the combustion process into a confined area, increasing efficiency. It’s easier to pop a small balloon with 5 psi than it is a huge balloon with 5 psi.

Then there are so many other things that can affect how much compression someone can get away with. They’ve been talked about here many, many times. Things like cam timing, volumetric efficiency, ignition timing, cylinder head material, spark plug heat range, coolant temp, oil temp, air density, a/f ratio..... etc. etc.

But that’s the jest of it. Relying on only the quench area to allow you to run 10:1+ compression on pump gas with a daily driver is not the correct way. There are way too many variables involved other than how close the piston is to the head.

BTW, all wedge shaped combustion chambers have a quench area.

 

The picture is all becoming clearer to me now... I think I got the idea.

I'm not trying to rely on just the quench area, I am just trying to understand how it will help me acheive my goals...

The idea I'm getting from all this is the closer the better... is there a point where it doesn't matter anymore? Is .059 gonna be a good thing or a bad thing?

 

Also how would 10.6:1 on Aluminum heads react to 91 octane pump gas and moderate timing. I will be running a 170 stat with this setup, so will I be okay? Otherwise I'm going to opt for thick gaskets and take my compression ratio down to 10.1 if possible.

 

10.6:1 is just fine for aluminum heads on 91 octane. I run 10.5 on iron heads on 91 octane. As mentioned earlier in this post there are lots of different variables that affect detonation but 10.6 is for sure in the "safe" range. I only run about 25 degrees total timing with an AFR of 13.6:1 . I have tried to richen up the AFR and play with PE and timing but the engine does not rev as freely and feels soft. Times are slower too. Talk to people that build engines for a living and they wil tell you to give the engine what it wants. You can try to plan it all out on paper which is awsome but some things might suprise you when it comes time to tune.

 

Mystic,

Sounds like you're going for a nice 383. Good choice on the SRP pistons. Everything in your setup looks good. The only thing I want to comment on is your choice of cam. IMHO I think it is too small for your 383 especially the AFR heads. They can handle much more cam. Also it won't run very well with the 10.6 CR. You also have to think that with the added cubes a cam that might have worked well on a 350 won't be able to fill a 383 as well. What kind of power or goals do you have for this engine?
You could go with a zero deck block a 0.041" composition head gasket and order the 74cc version of the AFR's and have them milled to the compression you like?