Piston volume

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Dec 17, 2019 | 02:59 PM

Electron447

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Intermediate

Joined: Dec 2019

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From: 616,Michigan

Piston volume

I am trying to determine what compression ration I have but I can’t seem to find out what kind of pistons I have( came in car already). Does anyone know how to calculate the + or - volume on piston tops ? I have a 4.155 bore, 4.125 stroke with stock LS7 heads ( 70cc combustion chamber ) I just don’t know what pistons I have and how to measure them so I can see what CR I have . Thanks

In advance

Dec 17, 2019 | 03:51 PM

Will_M1911

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Joined: Jul 2019

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How precise do you need to be? You can probably estimate pretty well the Clearance Volume at Top Dead Center using a pair of dial calipers.

Dec 17, 2019 | 03:58 PM

Will_M1911

Instructor

Joined: Jul 2019

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I'll take a stab at this. My numbers seem really high, but we can use this to get the conversation moving.

V_d = Displacement Volume
V_d = (pi/4)(b^2)(s) = 55.931in^3
where
b = Bore = 4.155 in
s = Stroke = 4.125 in

Compression Ratio = (V_d + V_c) / (V_c)
where
V_c = Clearance Volume = Combustion Chamber Volume = 70cc = 4.272in^3

Compression Ratio = 14.093:1

Again this seems really high so this is either a high performance motor or I'm bad a math.

Last edited by Will_M1911; Dec 17, 2019 at 03:59 PM.

Dec 17, 2019 | 05:47 PM

Electron447

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From: 616,Michigan

Quote:

Originally Posted by Will_M1911

How precise do you need to be? You can probably estimate pretty well the Clearance Volume at Top Dead Center using a pair of dial calipers.

the more precise ,the better for when I choose a new gasket thickness and for tuning concerns

Dec 17, 2019 | 05:50 PM

Electron447

Thread Starter

Intermediate

Joined: Dec 2019

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From: 616,Michigan

Quote:

Originally Posted by Will_M1911

14.093:1 doesn’t seem too out of territory. It is a high performance engine with the works,so I wouldn’t rule it out . I just want an idea for when this thing gets re tuned this season

Dec 20, 2019 | 12:46 PM

67Ranger

Racer

Joined: Apr 2009

Posts: 359

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From: San Antonio TX

This may or may not work with the engine in the car, but you should be able to seal off the top of the piston with a plexiglass plate and grease, then use a burette and water to determine the volume of the piston. If the piston is in the hole it gets trickier, but can still be done by sealing the piston walls, and doing more math, but could lead to getting some water down the piston walls and into the pan. It looks like there are plenty of videos online for "how to cc a piston dish".

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