Engine displacement questions

[SLEEPYDUB]

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?

[digger]

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.

[SLEEPYDUB]

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?

[ForcedFirebird]

It's not going to be 8.8:1 with the added 10mm stroke.

[SLEEPYDUB]

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

Sent from my SAMSUNG-SM-G935A using Tapatalk

[nando]

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?

[ForcedFirebird]

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.

[digger]

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

Sent from my SAMSUNG-SM-G935A using Tapatalk

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

[SLEEPYDUB]

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

[digger]

Ask top end they will tell you

[SLEEPYDUB]

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?

[digger]

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

[SLEEPYDUB]

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?

[digger]

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