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SirRobin

Pre cross flow compression ratio, can someone check maths!

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Hello

Trying to get my engine back together

I have just measured the chambers at 30ml i.e 30cm3

The Pistons have a slight dish in the top approx 2.35cm3

Gasket equates to 7.7cm3 allowing for compression say 5cm3

 

Bore / stroke = 381.5cm3 (030" Pistons)

 

Am I right n saying the compression ratio will be 10.2:1 which is perfecto for my A2 cam

 

 

Thanks

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I make it 11.2...

 

Volume at TDC = 30+2.35+5 = 37.35

Volume at BDC = 37.35+381.5 = 418.85

 

Compression ratio = 418.85 / 37.35 = 11.21 ?

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I make it 11.2...

 

Volume at TDC = 30+2.35+5 = 37.35

Volume at BDC = 37.35+381.5 = 418.85

 

Compression ratio = 418.85 / 37.35 = 11.21 ?

 

Oopsey, Thanks, do you think 11.2 would be too much?

 

Terrt

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Oopsey, Thanks, do you think 11.2 would be too much?

 

I would wait until someone else confirms my calculation...

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I would wait until someone else confirms my calculation...

 

I think it's right as I have just checked it on one of those calculators. The problem is I am missing the distance from the top of the piston to the top of the cylinder. (I am waiting for my rings to arrive)

If that's normal about .035" then it will be 10:1

 

Thanks

Terry

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On lotus heads that are often skimmed too much you could use the various range of Cometic head gaskets these can be quite thick and lower the CR.

Cheers

Brendan

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Hello

Trying to get my engine back together

I have just measured the chambers at 30ml i.e 30cm3

The Pistons have a slight dish in the top approx 2.35cm3

Gasket equates to 7.7cm3 allowing for compression say 5cm3

 

Bore / stroke = 381.5cm3 (030" Pistons)

 

Am I right n saying the compression ratio will be 10.2:1 which is perfecto for my A2 cam

 

Thanks

 

So, swept volume is 381.5cm3.

Volume of chamber etc will be 30cm3+2.35cm3+5cm3+4.66cm3 (the .035" to top of cylinder).

Ideally, you need one more measurement/volume, the volume from top of piston down to the top of the first compression ring.

 

Anyways, the above should give (381.5+42.01)/42.01 = 10.08, i.e. a 10.08:1 CR

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Thanks Chaps , but I have just found I have a big problem!

 

(Please note the engine is a pre cross flow unit and not from my Lotus!)

 

Anyway I set the piston in temporarily (with the shells!) to measure the deck height and it is 3.3mm down. yes 3.3mm down which equates to 17.3cm3

Its my eBay engine coming back and biting me on the bum!

 

Whats the problem and what can I do? Currently Compression ratio is 7.5:1 :-(

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Thanks Chaps , but I have just found I have a big problem!

 

(Please note the engine is a pre cross flow unit and not from my Lotus!)

 

Anyway I set the piston in temporarily (with the shells!) to measure the deck height and it is 3.3mm down. yes 3.3mm down which equates to 17.3cm3

Its my eBay engine coming back and biting me on the bum!

 

Whats the problem and what can I do? Currently Compression ratio is 7.5:1 :-(

If you're correct (no reason to believe you're not) then it does look like you might need to have the block skimmed a bit

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