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Discussion Starter #1
Putting together a new motor with a 3.75" stroke. I can custom order, or buy shelf pistons for a 3.8" stroke which would put them in the hole .025". That'd give me .075" quench. Is there anything wrong with that if my compression is where I want it?
 

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Nothing wrong with that at all. Just be aware that a large quench distance is not the same as a softened chamber or parabolic piston dish as far as detonation goes. Not saying you need to be concerned about it since I don't know the details of the application. Hope this helps.
 

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Discussion Starter #3
Will the larger quench promote detonation?

LS3 390ci
10:1
63cc heads
flat top
E85
25psi max

Thanks
 

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No. Small quench is worse. The real cause or at least a contributing factor is parallel surfaces like piston top and quench pad in the chambers. It has to do with reflected pressure wave and/or very dense mixture causing a localized high pressure spike that is enough to set off the mixture in these areas which starts a chain reaction of sorts. Also the shallower the valve angle in the combustion chamber becomes very thin if you look at a cross section which promotes a very rapid combustion event. This is great for NA or low boost production engines as it is more efficient and less emissions but bad for big power production. Basically the closer you can get to a Gen2 Hemi chamber the better. This is the primary reason the hemi has been the king of boost for over 5 decades. That big, slow, curved chamber with even pressure distribution low swirl and true cross flow design is nearly perfect in every way when it comes to making big power with big boost.
 

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Discussion Starter #5
Awesome! Thank you for the detailed explanation. Using the shelf pistons will save me some time and $. ??
 

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There is some info out there that suggests quench clearance up to around .065/.070" is fine, but then there is a quench clearance of between .070" and .120" where combustion efficiency falls off and this may promote detonation,poor flame propagation etc. Once you get over the .120" quench clearance figure -then basically you don't have any quench.
 

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Discussion Starter #7
Oh. Well that’s not good because depending on head gasket choice, I’d be .075 or .085.
 

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How thin of a gasket are you using to get the .075 quench? Might be worth looking into some thinner gaskets but that could lead to O-rings in the deck/head too. Hope you can get it figured out and report what you decide on.
 

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Discussion Starter #10
How thin of a gasket are you using to get the .075 quench? Might be worth looking into some thinner gaskets but that could lead to O-rings in the deck/head too. Hope you can get it figured out and report what you decide on.
Ls9 (.051) or cut ring (.059) + .025 in the hole.
 

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There is some info out there that suggests quench clearance up to around .065/.070" is fine, but then there is a quench clearance of between .070" and .120" where combustion efficiency falls off and this may promote detonation,poor flame propagation etc. Once you get over the .120" quench clearance figure -then basically you don't have any quench.
I was directly involved with some of the very first softened chamber development with Alan Johnson - AJPE on his symmetrical port SBC heads used by an Engine Builder on his championship winning Top Sportsman race car. Thru a lot of testing, racing, teardowns and inspection of parts we determined that running a flat top piston in a conventional wedge chamber/quench pad arrangement with the piston in the hole as much as .150" gave little to no relief from detonation. The pistons were hard anodized so they were tougher than the raw billet head. You could easily see visible pitting on the quench pads and not the rest of the chamber so we could tell where the problem was coming from. It was literally knocking tiny chunks of aluminum off the head. This is what you see causing "peppering" on the porcelain of a spark plug. The prototype angled quench pad solved the problem and over time changed to the parabolic dish shape we see used a lot now on chamber softening. When the chamber is not an option to soften and the CR will allow I try to use a parabolic dish piston. I think as long as one or the other is curved/angled you pretty much achieve the same effect. Hope this helps
 
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