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Old 06-07-2005, 04:51 PM
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Quench - what would you do?

Hey guys, another quench question.

I'm pre-assembling my new engine right now. Pretty fun stuff.

Here's the question: what is the ideal quench and is it worth decking a block to get it there? I seem to remember 0.040" being ideal for quench, is that right?

Here's my combo, sbc 385 (bore 4.040, stroke 3.75). 72cc chamber dart iron heads. Mr gasket ultra seal head gasket 0.038" compressed, 4.130 bore. Flattop 2 valve relief pistons (assuming -4.0 cc?). I've measured the deck height to be 0.028".

So with the gasket and deck height the quench will be 0.066". Is this too much? With this deck height the compression calculates to 9.73. With zero deck height the compression calculates to 10.3. 94 octane gas is available around here so I figure it shouldn't be too hard to run with 10.3 compression. This is a street strip car and this motor should make an honest 400 hp.

Would it be worth it to have the block cut down 0.025" or so? I want to build this motor right, but I do have a limited budget so every penny must be spent wisely. What would you do?
Thanks,
dh
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Old 06-07-2005, 05:08 PM
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If you have the block in for machining, it's easy enough to have it decked for a reasonable price, go for it. Not only will you get a better quench number, but you'll ensure the deck surface is true. The machinist should recommend leaving the pistons approx .005" in the hole to account for any eccentricities as far as I know... a little on the safe side.

If you want to stick with the deck height you have, you could always go with a thinner head gasket.

As far as that motor goes... consider which camshaft you've selected, as the valve overlap will affect dynamic compression. A cam with a lot of overlap will bleed off some of that raw compression you're calculating.

Still... I would deck it.

$.02

tbw
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Old 06-07-2005, 05:10 PM
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It really is important to have to correct quench ( It ensures correct mixture in the combustion chamber-- It also helps keep off detonation).
.040 would be good to shoot for.
.066 is a little to much... if you don't want to get the block decked you can get a still shim head gasket... I got them from Fel Pro they are .015 thick use cooper spray on both sides and make sure the surface of the deck is smooth and very clean. So far I have had no problem with mine and it worked great.

If not then yes get the deck taken down to what you need the best part about doing is that your deck will now be straight across (normally after awhile a deck surface get twisted. When it get decked it will be even and smooth which is great for getting things to seal and for the heads to sit correctly.

Chris

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Old 06-07-2005, 05:20 PM
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From the drivers seat you won't feel any difference, use what you got unless your racing and absolutely must have that 5 hp. Head deck thickness is marginal these days and the thicker you can leave it the better.
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Old 06-07-2005, 05:50 PM
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According to most good articles, .066 is not good. Shoot for .035 to .045 for final quench. We mocked up my sons 400 and had it decked perfectly to "0" deck height. My 350 I had them take off .020 and ended up .003 in the hole, both turned out fine. Both ended up right at .037 to .039.
My machinist that I use says that he can build a 9.1 "non quench" engine that will ping like crazy, but build a 10.5 with proper quench that won't. I believe him. Both our engines run great on 87 octane. Mine figured out to 9.4 with my pistons and head combo.

Mark
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Old 06-07-2005, 06:37 PM
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Quote:
Originally Posted by dh79
Hey guys, another quench question.

I'm pre-assembling my new engine right now. Pretty fun stuff.

Here's the question: what is the ideal quench and is it worth decking a block to get it there? I seem to remember 0.040" being ideal for quench, is that right?

Here's my combo, sbc 385 (bore 4.040, stroke 3.75). 72cc chamber dart iron heads. Mr gasket ultra seal head gasket 0.038" compressed, 4.130 bore. Flattop 2 valve relief pistons (assuming -4.0 cc?). I've measured the deck height to be 0.028".

So with the gasket and deck height the quench will be 0.066". Is this too much? With this deck height the compression calculates to 9.73. With zero deck height the compression calculates to 10.3. 94 octane gas is available around here so I figure it shouldn't be too hard to run with 10.3 compression. This is a street strip car and this motor should make an honest 400 hp.

Would it be worth it to have the block cut down 0.025" or so? I want to build this motor right, but I do have a limited budget so every penny must be spent wisely. What would you do?
Thanks,
dh
do what you can to get it down.....
or run high test fuel.....( cheaper to get it down..........
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Old 06-07-2005, 07:52 PM
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HERE is the link to the .015 head gaskets. .028 + .015 = 43 which is great...

I used THIS on both sides after smoothing off the deck surface. It worked great not one problem.

Chris
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Old 06-07-2005, 09:41 PM
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But then sometimes shim head gaskets don't seal well if the head & deck surfaces aren't almost perfectly flat, so you might oughta have them checked with a straight-edge just to be sure they don't need machining anyway. Chevy makes a shim gasket that's .026" thick and a couple of composition gaskets that are 0.028" if the .015" ends up too thin.
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Old 06-07-2005, 09:48 PM
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Quote:
Originally Posted by Jmark
My machinist that I use says that he can build a 9.1 "non quench" engine that will ping like crazy, but build a 10.5 with proper quench that won't. I believe him. Both our engines run great on 87 octane. Mine figured out to 9.4 with my pistons and head combo.

Mark
Mark,you and your machinist are right on the money. Quench is very important for efficiency and detonation resistance. Depending on intended RPM, fuel and components used, I always try to stay around .040 - .045 in high HP engines with steel rods. Mild to moderate street engines can get by with slightly less.
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Old 06-07-2005, 09:51 PM
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This is true. The surface must be smooth and flat. I forget the roughness the gaskets called for (they give you a sheet) if you search through my old post you can find that thread. The surface must also be straight which can be checked with a straight edge (which I did) hopefully your block will be ok if it's not then I wouldn't risk useing shim gaskets and just have the block milled down to what you need and re finished and call it a day.

Chris
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Old 06-07-2005, 10:37 PM
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I would get the quench down and run the slightly higher compression. You will have more detonation resistance with the tighter quench. It's not all about power. I like to have my blocks zero decked and use the gasket to get my quench.

Some of the high end race engines even have the pistons out of the hole. I have also seen some with quench so tight the piston just kisses the head during piston rock. While I would advise that on a street engine it is just to point out that quench is not something to take lightly from a HP perspective or efficient burn point of view.

.035-.045 is good for a steel rod, street engine.

Royce
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Old 06-08-2005, 02:08 AM
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Thanks for the replies guys; I really appreciate it.

I think I'll take it in to have it cut down to zero (or maybe around 0.005"). I don't know if the deck is completely straight right now. I might check that tomorrow. It might not be straight enough for steel shims.

Just for the sake of doing things right, I think I'll be happier knowing the quench is close to ideal.
dh
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Old 06-08-2005, 06:08 AM
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Sounds like a good idea. If possible I don't mind leaving a little on the deck the reason being that in the future I might want to resmooth the surface or any number of things and would like just a LITTLE left to play with. I agree that it is really better to use the correct gasket. The .041 gaskets the places like fel pro make work so well I have heard little problem with them. If you cna have the block done then go for that. In anycase try to get the quench right in the long run you will thank yourself when you can still run pump gas and not here one ping...

Chris
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Old 06-08-2005, 12:54 PM
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Cometic makes some good head gaskets .027 (and thinner too i think) look them up. i know a builder/racer in omaha that stocks them i think, sells and uses them for sure, he can help you out over the phone i'm sure, i picked his brain over the phone for over a hr 2Xs on Olds 403 Questions, easy to understand and talk to.........


this guy builds some stuff, pm me if you want his website & ph.#












Mustangsaly
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