compression ratio Options

[CoolAirVw]

Compression Ratio

Maybe someone has done this before but I searched and didn't find anything.

I decided to omit my head gaskets while my engine was apart but was worried that it might raise compression ratio (CR) too much. So I needed to make a comparison of compression ratio with head gaskets and without.

These are the tools I used.



In the picture is..

1. Straight edge (blue-point tool made for checking flat surfaces.
2. Plexiglas disk link type 4 store
3. Syringe with CC gradients
4. Digital Caliper
5. Feeler gauge
6. Short and long spacers that I made to hold down the cylinder (optional)

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First
You’ll need to measure Head Chamber Volume. Lay the head on a flat surface. The rockers/valve springs make it uneven so I used 2x4’s on the valve cover surface to make the head sit flat. Put the Plexiglas disk on the sealing surface of the head and measure the amount of fluid it takes to fill the chamber. I used ATF. My Head Chamber Volume was 60cc.




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Second
You’ll need to measure Piston Dish Volume, or the size of the valve pockets in the piston using the Plexiglas disk again. Mine measured 4cc.




If you have flat pistons skip this, your Piston Dish Volume is zero. If you have domed pistons you’ll need to fill a container with fluid and measure the amount the dome displaces and use a negative number as the Piston Dish Volume.



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Third

You’ll need to measure Deck Height. You’ll need to “mock up” the engine with at least one cylinder and piston. I reused the original cylinder shim for the mock up but you could just add the thickness of the shim to the deck height. If for some reason you were going to use head gaskets you would add their thickness to the deck height as well.

I tried two different ways to compare which was more accurate. Both ways were close enough that I think either way is acceptable. My deck height was .040 inch.

Deck Height with feeler gauge (easier)




Deck Height with Digital Caliper (Looks flashy with fancy tool but don’t forget to subtract out the thickness of your straight edge)




Now for the math...

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Calculate Volume of one cylinder = Displacement / 4

My engine example…

My 2.0 liter is actually 1971cc.
1971cc / 4 = 492.75cc
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Convert Deck Height (inch) to Deck Volume (cc) with this formula

Bore (mm) x Bore (mm) x Deck Height (in.) x .01996 = Deck Volume (cc)


My engine example…

94 mm x 94 mm x .040 inch x .01996 = 7.0546624 Deck Volume (cc) (round to 7cc)

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Calculate Combustion Chamber Volume

Head Volume + Piston Volume + Deck Volume = Combustion Chamber Volume

My engine example…

60cc + 4cc + 7cc = 71cc Combustion Chamber Volume

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Calculate Compression Ratio with this formula

(Combustion Chamber Volume + one cylinder volume) / Combustion Chamber Volume =Compression Ratio

My engine example…

(71cc + 492.75cc) / 71cc = 7.9 Compression Ratio
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I think I can live with 7.9 but how does this compare with the compression ratio with the head gaskets?

Most numbers are the same except I’ll need to add the thickness of the head gasket to the Deck Height. My head gaskets were .030 inch making Deck Height .070 inch.

So…

Deck Volume
94 mm x 94 mm x .070 inch x .01996 = 12.3456592 Deck Volume (cc) (round to 12cc)

Combustion Chamber Volume
60cc + 4cc + 12cc = 76cc Combustion Chamber Volume

Compression Ratio
(76cc + 492.75cc) / 76cc = 7.4 Compression Ratio

I think I can live with the 7.9 CR. Maybe I can gain a little power without too much additional heat??

Maybe use this link if you don’t like math.
http://www.race-cars.net/calculators/compr...calculator.html


Thoughts…
1. Without too much work I could have shimmed the cylinders the thickness of the head gaskets and ended up with the same CR as before with no extra time spent on all this. But isn’t it better to know?
2. By omitting the head gasket I changed the rocker arm geometry. Didn’t realize this till after the motor was assembled. I’m going to go forward with it the way it is.
3. If I would have done number 1 then number 2 wouldn’t be an issue.
4. How does my Head Volume compare with what others have seen? Does this seem high. Does this mean my heads have never been cut? OOOHH! Untouched heads???


Comments welcome. If I missed anything or gave any misinformation please correct me.