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Old 01-18-2010, 07:35 AM
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importance of piston to deck distance?

I am building a 383 sbc on the cheap and wonder how important piston to deck height is for good combustion. I want to use pistons for 6" rods on 5.7" rods with a 3.75 stroke crank. Works out to 10:1 compression with .09" deck to piston clearance (includes gasket space). I am shooting for lots of torque from 1500 to 3500rpm for a waterski boat. What do you think - workable combo with that deck/piston clearance? Thanks for input - I have not read about this approach before and wonder if there is a good reason why people don't do it.

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Old 01-18-2010, 07:45 AM
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That is your quench distance and .090" is not good, you either have some really thick head gaskets or something is off. Are you sure you added everything up correctly? The ideal is said to be .040"-.045" with up to .060" as acceptable but not optimum. Rather than try to explain it myself and do it poorly do a search for "Quench Distance" here, at Chevy Talk in the Performance Section and Team Chevelle. You'll get a lot of information and a much better explanation than I can give.
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Old 01-18-2010, 07:47 AM
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I would think .090 would be way to much. On a boat engine it's under constant load so you want a tight clearance to reduce possibility of detonation. Also on a boat you don't have situations where the piston is unloaded during moments like shifting a manual car unless you come out of the water or something bad happens like grounding. On my 489 BBC airboat I have the piston sticking .006" above the deck with a .038" composite gasket for .032"

My Porsche 350 has the flat tops .025 to .030 down the hole so I am going with .015 steel shim gasket. (Budget build unless I decide to put a set of pistons in)

Check out http://www.kb-silvolite.com/features.php Good read and I agree with their opinions.
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Old 01-18-2010, 07:51 AM
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Thanks airboat, I hadn't been thinking about detonation, just aiming for a workable static compression ration. Do you remember which article on that site were you referring to - looks like a long list?
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Old 01-18-2010, 09:42 AM
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Thanks Hippie. Because I already have the leftover parts, want to go cheeeaaappp and want 10:1 my project would end up with the big quench distance. With aluminum heads, open tbi computer control (has a knock sensor), 160 thermostat and lake water cooling I was dreaming that it might work. Thanks for pointing me to the "Quench Distance" search, probably will save me a lot of frustration!

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Originally Posted by Hippie
That is your quench distance and .090" is not good, you either have some really thick head gaskets or something is off. Are you sure you added everything up correctly? The ideal is said to be .040"-.045" with up to .060" as acceptable but not optimum. Rather than try to explain it myself and do it poorly do a search for "Quench Distance" here, at Chevy Talk in the Performance Section and Team Chevelle. You'll get a lot of information and a much better explanation than I can give.
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Old 01-18-2010, 11:11 AM
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Quote:
Originally Posted by 383skier
Thanks Hippie. Because I already have the leftover parts, want to go cheeeaaappp and want 10:1 my project would end up with the big quench distance. With aluminum heads, open tbi computer control (has a knock sensor), 160 thermostat and lake water cooling I was dreaming that it might work. Thanks for pointing me to the "Quench Distance" search, probably will save me a lot of frustration!
With tight quench and aluminum heads your combo might work depending on your cam. I know all about "leftover parts" LOL. Throw us your parts list, maybe we can help tweak things around for you.
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Old 01-18-2010, 11:34 AM
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I have some unidentifiable 2.02/1.60 aluminum heads that look very similar to fastburn design, but have never been able to find the casting number on any website. They were heavily ported by before I got them. Had the machine shop look them over and they saw no problems. 58cc chambers and I am guessing 200cc ports, . The pistons are kb 11CC dish for 350/6" rods. Bob weight is right at 1400g so will need to balance with the nodular iron external balance crank. I also have some LT1 aluminum heads that I can tap and route water to make work, they are 76cc with 1.94 valves. I would be able to go with a very thin head gasket and still keep compression target with a .06 quench distance that way. Don't know if those heads would be an improvement over the others with a larger quench - I have read that the LT1 heads are not special, but this is a low rpm engine. Thanks again for the input.

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Originally Posted by Hippie
With tight quench and aluminum heads your combo might work depending on your cam. I know all about "leftover parts" LOL. Throw us your parts list, maybe we can help tweak things around for you.
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Old 01-18-2010, 12:13 PM
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The 6" rod 350 pistons have a Pin height of 1.261. With a 3.75" stroke and a 5.7" rod I come up with the piston sticking 0.111" out of the top of the block.

Somebody double check me but I've run the numbers 4 times and that's not going to work no matter what you do. I'm quite sure KB pistons don't have enough material to whack almost an 1/8" off the top.

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Old 01-18-2010, 01:26 PM
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Well, I used the KB calculator and looks like there is no good way to make it work. Thanks for your thoughts!

Quote:
Originally Posted by Hippie
The 6" rod 350 pistons have a Pin height of 1.261. With a 3.75" stroke and a 5.7" rod I come up with the piston sticking 0.111" out of the top of the block.

Somebody double check me but I've run the numbers 4 times and that's not going to work no matter what you do. I'm quite sure KB pistons don't have enough material to whack almost an 1/8" off the top.
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Old 01-18-2010, 04:57 PM
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What am I doing wrong?

6.0 Rod
+1.74 Stroke (1/2 of 350 stroke)
=7.74

9.025 deck height
-7.74
=1.285 = 6" rod piston pin H @ zero deck

5.7 Rod
+1.875 Stroke (1/2 of 383 stroke)
=7.575
+1.285 Piston Pin
=8.850

9.025 deck
-8.850
=.175

I get .175 in the hole?

ssmonty
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Old 01-18-2010, 05:21 PM
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You're not... I was using the 6 inch rod in the 383 instead of the 5.7 , with the 1.261 Pin Height from the KB website I get 0.189" down the hole. I knew I was wrong somewhere but it still ain't gonna work.


Quote:
Originally Posted by ssmonty
What am I doing wrong?

6.0 Rod
+1.74 Stroke (1/2 of 350 stroke)
=7.74

9.025 deck height
-7.74
=1.285 = 6" rod piston pin H @ zero deck

5.7 Rod
+1.875 Stroke (1/2 of 383 stroke)
=7.575
+1.285 Piston Pin
=8.850

9.025 deck
-8.850
=.175

I get .175 in the hole?

ssmonty
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Old 01-18-2010, 06:07 PM
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.189 is what I keep getting, .9 is what I thought I remembered measuring when I mocked it up a while back. No matter, no way will it work. Guess it is time to pony up, maybe I can ebay some parts to take some of the pain out of a stroker kit

Quote:
Originally Posted by Hippie
You're not... I was using the 6 inch rod in the 383 instead of the 5.7 , with the 1.261 Pin Height from the KB website I get 0.189" down the hole. I knew I was wrong somewhere but it still ain't gonna work.
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Old 01-18-2010, 06:13 PM
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The compression height on my pistons in my 6" rod 350 are 1.260.
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Old 01-18-2010, 08:58 PM
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This stroke and piston (1.250-1.260" comp ht)will work with an 5.85" rod, comes up to 8.985" os so depending on the exact comp height and deck amount. Deck the block to 9.010ish and use a .015-.018" gasket for piston to head clearance of .040-.043".
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Old 01-19-2010, 06:58 AM
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Quote:
Originally Posted by ericnova72
This stroke and piston (1.250-1.260" comp ht)will work with an 5.85" rod, comes up to 8.985" os so depending on the exact comp height and deck amount. Deck the block to 9.010ish and use a .015-.018" gasket for piston to head clearance of .040-.043".
Well, ANYBODY could do it that way... IF they had thought of it.... Good thinking Eric.
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