What does "compression height" mean? - Chevelle Tech
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  #1  
Old Feb 27th, 12, 9:50 PM
2BlueLS6's 2BlueLS6's is offline
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Default What does "compression height" mean?

In trying to figure some piston, crank, rod combos I saw a set of pistons that were described as 1.095 compression height. I would have assumed this was from the center of the wristpin to the top (deck not dome) of the piston, but when measuring a set of my LS7 454 pistons they measure approx. 1.095 from the TOP of the wristpin to the top of the piston. With one inch wrist pins this would mean that the ones I saw that were advertised as 1.095 would have to have the wristpin .500 higher in the piston than stock 454 and I can't figure out how that works out with any stroke/deck height/rod length combination I can come up with in my head. I've always ordered these componenents by intended stroke & deckheight before and let somebody else do the math, so I'm kinda' lost here. I'm trying to figure a short deck 572, with a 4.560 bore and 4.375 stroke and have no idea where I need to be with rods and compression height.
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  #2  
Old Feb 27th, 12, 10:58 PM
Mike Mike is offline
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Default Re: What does "compression height" mean?

If you have a block deck height of 9.8" ,you subtract 1/2 your stroke (2.1875") from it to get 7.6125 for your combined c/h and rod length.
Assuming a .400" over rod which is 6.535" ,that would leave you with a 1.0775" c/h.
If you used a shorter 6.385" (.250" over) your c/h would be 1.2275"
With a stock rod (6.135") your c/h would be 1.4775".
c/h is the distance from the center of the pin to the deck of the piston -not including the dome.
pic from Lunati

You said
Quote:
but when measuring a set of my LS7 454 pistons they measure approx. 1.095 from the TOP of the wristpin to the top of the piston
which is confusing.
If your piston has a .495" dome , that would be 1/2 the pin diameter (.495") so it would appear to have a 1.095" c/h.
That make sense?
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Last edited by Mike; Feb 27th, 12 at 11:26 PM.
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  #3  
Old Feb 28th, 12, 2:44 AM
Tom Mobley Tom Mobley is online now
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Default Re: What does "compression height" mean?

the pins are .990, so .990/2 = .495 FWIW

4.375/2 = 2.1875 = stroke radius.

6.385 rod so 2.1875 + 6.385 = 8.5725

9.800 - 8.5725 = 1.2275 = compression height, distance from center of wrist pin to deck of piston. Looks like Mikes math is pretty good. Plenty of room for a modern ring pack. I wouldn't even try to run the .400 longer rod in a short deck big stroke deal.

recent 540 was (4.25/2) stroke radius +6.385 rod + 1.27 CH = 9.780 decked the Big M block to 9.790, let it eat.
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Old Feb 28th, 12, 8:10 AM
2BlueLS6's 2BlueLS6's is offline
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Default Re: What does "compression height" mean?

Thanks guys, that's what I though and what I was looking for. I DID measure to the top of the piston as it relates to the deck of the block and NOT to the top of the dome, exactly as Tom's drawing shows.

I was almost sure that the advertised compression height was to center of wristpin, nothing else would be logical, but couldn't make the numbers work on a pair of pistons I saw advertised with any stroke combo I could come up with and any rod that was listed in Summit or on E Bay in either a 9.8 or a 10.2 deck height block.

Since it's clear now I guess I'll go back through my math and see if I made a mistake. I tried plugging in 2", 2.1875" and 2.25 strokes with 3 commonly listed rod lengths and it wouldn't work last night unless I made a mistake with the math.
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Old Feb 28th, 12, 8:23 AM
BillK BillK is offline
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Default Re: What does "compression height" mean?

OK,
Stupid question but are you sure the pistons you were looking at were not for a small block ? I dont see any 1.095 CH Big Block pistons. They might have been something custom ? Do you have a part number ?
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  #6  
Old Feb 28th, 12, 3:50 PM
2BlueLS6's 2BlueLS6's is offline
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Default Re: What does "compression height" mean?

Yes, I'm sure. The bore is 4.5. I think you're probably right, they were probably customs, special ordered for someone and not bought for whatever reason.

Thanks for the help guys.
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Old Mar 3rd, 12, 12:03 AM
2BlueLS6's 2BlueLS6's is offline
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Default Re: What does "compression height" mean?

I'm still wading around in the fog on this. I've done the math about 10 times and these pistons with a 4.375 stroke and 6.535 rods have a minus deck height of about .0175. I don't know how much to allow for rod stretch on H beam rods but I've already got a set of 6.535 Eagles new in the box and setting up with the pistons hangine out the top .0175 doesn't sound like a good idea, especially if the block has to be decked at all. Going back to the 6.385 rods makes the deck height about .132 and that doesn't sound logical either. I got bored with it before I started plugging every rod and stroke combination I could think of into a tall deck possibility, but basically I'm trying to build a short deck 572 using what I've already got (short deck bow tie from 502 & new 4.375 crank w/6.535 rods) The closeout on the 4.560 pistons seemed like a great option except for the compression height that seems to be throwing me a curve for this combination.
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Old Mar 3rd, 12, 9:19 AM
Tom Mobley Tom Mobley is online now
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Default Re: What does "compression height" mean?

your 1.095 is from the top of the wrist pin bore to the deck, add 1/2 the bore diameter to that, get comp height of 1.590.

Still can't work out any bore/stroke/piston that comes out to 9.800 using commonly known parts. stroke radius + rod length + comp height = 9.8
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Old Mar 3rd, 12, 9:54 AM
Tom Mobley Tom Mobley is online now
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Default Re: What does "compression height" mean?

2.1875 + 6.385 = 8.5725

9.8 - 8.5725 = 1.2275 which is the CH of the piston you need.

using the dial caliper method, 1.2275 - .495 = .7325 from the top of the pin bore to the deck

looks like most short deck 572 are 4.5 x 4.5
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Old Mar 3rd, 12, 10:04 AM
LevonH LevonH is offline
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Default Re: What does "compression height" mean?

Scroll down on this page. The sell a 572 short deck and you may be able to get the piston info from them???

http://www.ohiocrank.com/chev_bb_shortb.html

•Merlin III short deck block•4.625 x 4.375 big bore
•6.535 H beam rods w/ ARP 2000
•JE gas ported pistons w/ .043 rings
•The most cubes in a 9.8" block
•4340 steel crankshaft 0
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  #11  
Old Mar 3rd, 12, 10:55 AM
Tom Mobley Tom Mobley is online now
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Default Re: What does "compression height" mean?

they're using 9.8 block, 4.375 stroke, 6.535 rod, leaves a piston CH of 1.0775, pretty short.

still a few thou taller than a SBC 4" stroke x 6" rod deal, that leaves a CH of 1.000 on a 9" block
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Old Mar 3rd, 12, 6:15 PM
BillK BillK is offline
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Default Re: What does "compression height" mean?

Quote:
Originally Posted by 2BlueLS6's View Post
setting up with the pistons hangine out the top .0175 doesn't sound like a good idea,.
Have you actually measured the block that you have ?? Most of them are a lot taller than the "stock" deck height. You usually have to cut around .020" off them to get them to 9.800. So you might actually be around zero deck.
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