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Old 01-14-2019, 07:19 AM   #16
SLEEPYDUB
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So the piston is just the 8.8:1 style piston dome/dish profile (as opposed to actually being 8.8:1) and used on a 2.9L bottom end but the gasket is 0.080" and head has been skimmed a few times?

ballpark is around 10:1 give or take depending how much was skimmed
It is a JE 85mm 8:8:1 dished piston from TEP for use with 135mm rods, and using the M54 84mm stroke crank. The head gasket is a Cooper Cutring from VAC and says its a 0.080" thickness. Head has been skimmed a little bit, but they said its "well within spec" whatever thats supposed to mean.

10:1? That high?
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Old 01-14-2019, 01:42 PM   #17
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If it's an 8.8 cr piston then a slight amount off the head changes by a tenth or two so maybe 9:1. I thought it was an 8.8 style piston rather than 8.8 cr. within spec probably means the dimples are still there.
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Old 01-14-2019, 01:47 PM   #18
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If it's an 8.8 cr piston then a slight amount off the head changes by a tenth or two. I thought it was an 8.8 style piston rather than 8.8 cr
Its an 8:8:1 CR piston. With the increase in the headgasket thickness over stock (0.080 instead of 0.070), and the shaved head, that would keep me at 8:8:1 or maybe higher?
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Old 01-14-2019, 02:33 PM   #19
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It's not going to be 8.8:1 with the added 10mm stroke.
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Old 01-14-2019, 02:40 PM   #20
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It's not going to be 8.8:1 with the added 10mm stroke.
Does this mean it will be lower? Having a hard time wrapping my head around it. I thought when TEP said 8:8:1 piston it would literally be 8:8:1 + whatever thickness my gasket added

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Old 01-14-2019, 03:44 PM   #21
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It's not going to be 8.8:1 with the added 10mm stroke.
Wouldn't it be if they designed it for his crank & rod combo?
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Old 01-14-2019, 04:09 PM   #22
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Wouldn't it be if they designed it for his crank & rod combo?


I over read something earlier. I saw the 10:1 post and got confused.
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Old 01-15-2019, 01:00 AM   #23
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Does this mean it will be lower? Having a hard time wrapping my head around it. I thought when TEP said 8:8:1 piston it would literally be 8:8:1 + whatever thickness my gasket added

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8.8 includes an assumption about gasket thickness. The pistons will have a cc number either + or - value. It should say on the documentation provided. Usually on the Pretty JE box they come in, that's what you need to do the calcs exactly along with the combustion chamber cc

Since your head gasket is similar thickness to stock +.25 version the cr is unlikely to change much
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Old 01-15-2019, 06:36 AM   #24
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8.8 includes an assumption about gasket thickness. The pistons will have a cc number either + or - value. It should say on the documentation provided. Usually on the Pretty JE box they come in, that's what you need to do the calcs exactly along with the combustion chamber cc

Since your head gasket is similar thickness to stock +.25 version the cr is unlikely to change much
Yea I was searching for that documentation, but I think the machine shop kept all of it

Ill likely have the head skimmed again once it comes off of this engine, maybe that will be enough to get me closer to 9:1
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Old 01-15-2019, 11:14 PM   #25
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Ask top end they will tell you
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Old 01-16-2019, 02:50 PM   #26
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Here we go from TEP themselves.
"14cc of Neg Volume
with 206.3mm deck height and 30mm CD should be right at .030 out of the deck - a .070 OEM Gasket = .040 Squish and 8.8:1
If you use a .080 Gasket and do a slight Deck surface and a likely a few light surfaces of the head and you are right back to 8.8:1 +/- .1-.2"

That sound about right?
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Old 01-17-2019, 12:44 AM   #27
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Here we go from TEP themselves.
"14cc of Neg Volume
with 206.3mm deck height and 30mm CD should be right at .030 out of the deck - a .070 OEM Gasket = .040 Squish and 8.8:1
If you use a .080 Gasket and do a slight Deck surface and a likely a few light surfaces of the head and you are right back to 8.8:1 +/- .1-.2"

That sound about right?
yeah close enough a tenth here or there is worth SFA,

The only clarification i would make is 0.030 above deck and 0.070 gasket only means 0.040" squish with the correct shape piston dome e.g a perfect flat top piston has no squish with those dimensions. this is just an example
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Old 01-17-2019, 06:39 AM   #28
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yeah close enough a tenth here or there is worth SFA,
SFA? What is that an acronym for? Adding the extra 0.010" from the cutring head gasket, does that put the squish at 0.050"? That sounds like its less then ideal quench? Should be closer to 0.037-0.040?
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Old 01-17-2019, 01:13 PM   #29
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Sweet f all.

0.05 is probably max you'd want. With a 14cc dish you may not have squish area anyway as it depends if any areas ste raised to match the head. Depends on the piston
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