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Old 01-30-2010, 11:49 AM
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The quest for quench

In conversation with a member on another forum the subject of quench came up and a discussion ensued, of course more research was required on my part, and I found this article . Anyone not familiar with what quench is please read the article I linked to. But reading the article and getting a general idea of what qunech is does not end my quest...

It seems apparent that quench, or squish, is yet another variable to consider or manipulate when designing an engine build, but it is NOT related to any one specific compression ratio as much as it does to ONE specific dimension, the space above the flat surface of the piston and the roof of the cylinder head, if I understand it correctly.

Though higher compression ratios are more prone to detonation than lower under corresponding circumstances, the ability to reach an optimal quench will bring us to higher compression ratios. Obviously though, to achieve the optimal quench range of .040 - .050 the compression ratio will be quite close to 10:1, as my calculations have proven, but sometimes higher or lower depending on the piston profile, volume, etc.

A few thoughts and questions:
The obviously easy way to set your quench is to zero deck your block/pistons and use a .040" head gasket. But it seems to me you can have the optimal quench setup as well with your pistons in the hole a bit too; for example: with a deck height of .035" (pistons in the hole .030) you can use a .015" compressed thickness head gasket and get a .045" quench - yes?

In the real world, even a zero decked block may not guarantee ALL pistons at zero deck height, rod and piston variations may push a piston above the deck or allow it to sit below, not accounting for rod stretch during operation at higher RPM. So the next question is, when choosing a head gasket thickness to tune your quench, do you go by the deepest deck clearance value, the shallowest or the average deck height?

Here is a real-world example, my SBC 357ci. Deck heights when assembled were as follows:

Cyl 1 - .028
Cyl 2 - .032
Cyl 3 - .025
Cyl 4 - .027
Cyl 5 - .030
Cyl 6 - .030
Cyl 7 - .024
Cyl 8 - .025

Average deck height .027625"

Using my average DH of .027625 the deck volume of all 8 cylinders is only .354ci, but is an average value accurate enough?

Using a standard .040" (compressed thickness) head gasket, my quench distance, using my average deck height, would be .067625, not within the quench sweet spot, and my compression ratio is 9.354 SCR and 8.531 DCR. But switching to a .015" head gasket would net me a .042625" quench distance, well within the desired range, and my compression would jump to 9.904 SCR and 9.026 DCR.

Using the example of my chevy engine, which is not entirely ping resistant, am I better off with a lower DCR or an optimal quench - if I were looking to optimize combustion and power production and eliminate the possibility of detonation?

I have two more engine build projects in the works, one for my Scamp and one for my buddy's Mustang, so mastering this concept is of valuable significance to me. Looking forward to input and discussion on the subject.

Last edited by octanejunkie; 01-30-2010 at 01:14 PM. Reason: fixed SCR/DCR typos and added link at top
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Old 01-30-2010, 12:10 PM
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Quote:
Originally Posted by octanejunkie
In conversation with a member on another forum the subject of quench came up and a discussion ensued, of course more research was required on my part, and I found this article. Anyone not familiar with what quench is please read the article I linked to. But reading the article and getting a general idea of what qunech is does not end my quest...

It seems apparent that quench, or squish, is yet another variable to consider or manipulate when designing an engine build, but it is NOT related to any one specific compression ratio as much as it does to ONE specific dimension, the space above the flat surface of the piston and the roof of the cylinder head, if I understand it correctly.

Though higher compression ratios are more prone to detonation than lower under corresponding circumstances, the ability to reach an optimal quench will bring us to higher compression ratios. Obviously though, to achieve the optimal quench range of .040 - .050 the compression ratio will be quite close to 10:1, as my calculations have proven, but sometimes higher or lower depending on the piston profile, volume, etc.

A few thoughts and questions:
The obviously easy way to set your quench is to zero deck your block/pistons and use a .040" head gasket. But it seems to me you can have the optimal quench setup as well with your pistons in the hole a bit too; for example: with a deck height of .035" (pistons in the hole .030) you can use a .015" compressed thickness head gasket and get a .045" quench - yes?

In the real world, even a zero decked block may not guarantee ALL pistons at zero deck height, rod and piston variations may push a piston above the deck or allow it to sit below, not accounting for rod stretch during operation at higher RPM. So the next question is, when choosing a head gasket thickness to tune your quench, do you go by the deepest deck clearance value, the shallowest or the average deck height?

Here is a real-world example, my SBC 357ci. Deck heights when assembled were as follows:

Cyl 1 - .028
Cyl 2 - .032
Cyl 3 - .025
Cyl 4 - .027
Cyl 5 - .030
Cyl 6 - .030
Cyl 7 - .024
Cyl 8 - .025

Average deck height .027625"

Using my average DH of .027625 the deck volume of all 8 cylinders is only .354ci, but is an average value accurate enough?

Using a standard .040" (compressed thickness) head gasket, my quench distance, using my average deck height, would be .067625, not within the quench sweet spot, and my compression ratio is 9.354 DCR and 8.531 SCR. But switching to a .015" head gasket would net me a .042625" quench distance, well within the desired range, and my compression would jump to 9.904 DCR and 9.026 SCR.

Using the example of my chevy engine, which is not entirely ping resistant, am I better off with a lower SCR or an optimal quench - if I were looking to optimize combustion and power production and eliminate the possibility of detonation?

I have two more engine build projects in the works, one for my Scamp and one for my buddy's Mustang, so mastering this concept is of valuable significance to me. Looking forward to input and discussion on the subject.
Without making a long post, simply put...you want both by the sound of it in your application. Also, need to correct DCR and SCR's. Static is higher than dynamic. Only way I could see that happening is tuned racing engine with over 100% volumetric efficiency taking full advantage of the any available ram effect while the intake valve is open.
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Old 01-30-2010, 12:16 PM
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Quote:
Originally Posted by octanejunkie
ONE specific dimension, the space above the flat surface of the piston and the roof of the cylinder head, if I understand it correctly.
The roof can be thought of as the deepest part of the chamber- it would be less ambiguous to say, "...and the DECK of the cylinder head", in your statement above.

Quote:
the ability to reach an optimal quench will bring us to higher compression ratios. Obviously though, to achieve the optimal quench range of .040 - .050 the compression ratio will be quite close to 10:1, as my calculations have proven, but sometimes higher or lower depending on the piston profile, volume, etc.
Quench is independent of CR- ANY CR can be achieved w/any quench measurement-within the parameters of the engine's internal parts- be it good or bad.

What I'm saying is, you can have an 'ideal' quench of 0.040" and have a CR of 6:1. Or 15:1. Just as you can have a quench of 0.125" w/a CR of 10:1.

Quote:
But it seems to me you can have the optimal quench setup as well with your pistons in the hole a bit too
Yes. As long as the piston flat to the deck of the head is in spec, it really doesn't matter too much how you get there- within the limits of available head gasket thickness. That said, most builders prefer to 'zero' deck the block and use a composition type head gasket instead of a steel shim gasket and having the quench "in the block".

Quote:
when choosing a head gasket thickness to tune your quench, do you go by the deepest deck clearance value, the shallowest or the average deck height?
You have to figure what's the least quench distance the engine combo will tolerate and keep all the quench figures at or above that figure. Usually, that means using the 'tallest' piston (#7 in your case) to figure from.

You would figure the tightest quench your combo can use- from #7- and let the rest fall where they will. Without re machining the rotating assemble, this is about the limit of what you can do, practically speaking.

I believe you've switched the DCR and SCR above.

Quote:
Looking forward to input and discussion on the subject.
I'm sure you won't be disappointed.
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Old 01-30-2010, 12:36 PM
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I would add that in the future you could use either a different crank grinder- or specify to this grinder- that he "index" the crank as it's machined.

This will all but eliminate the variations you're seeing from cylinder to cylinder and will make the quench much more even across the board.

You can still have variations but it will be due to the block machining and/or core shift. This can also be corrected if need be, to all or some extent.

Best to start w/a 'centered' block if possible. The bore thickness variations aren't affected so much (so I've been told), but IMO the cam to lifter bore relationship can be troublesome if the block's out far enough.
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Old 01-30-2010, 12:55 PM
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Corrected SCR/DCR typos - thanks for replies so far
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Old 01-30-2010, 01:15 PM
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Quote:
Originally Posted by Intense RT
Without making a long post, simply put...you want both by the sound of it in your application. Also, need to correct DCR and SCR's. Static is higher than dynamic. Only way I could see that happening is tuned racing engine with over 100% volumetric efficiency taking full advantage of the any available ram effect while the intake valve is open.
Yup, fixed typos and added article link - thanks
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Old 01-30-2010, 03:58 PM
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You can also get variations like you have from the rod resizing process if the machinist isn't careful to make sure that the rods are equalized in length. I would say you have both rod length variations and crank stroke variations going on judging by your numbers being so varied even on the same cylinder bank.

As being said, you need to figure from the tallest piston in your case. As an example if say you had pistons between .010" and .020" down and used the lowest piston, and used a .015" gasket, you would have a disaster - the tallest piston will crash into the head due to only .030" piston to head clearance(quench).
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