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# compression ratios

### Tech Articles

Discussion and questions related to the course Engine Building Fundamentals

Having issues with the maths on the compression ratios.

i must have watched the video about six times and continually come up with this number what am I getting wrong or how do I convert this to a ratio

4 cylinder sr20det

for one cylinder:

compression ratio 4.855059352

bore area (3.142 x diameter / 2) 6221.9455mm

cylinder swept volume (area of bore x stroke) 566.197cc

piston dome volume 87.171648cc

clearance volume 146.8711499cc

How can the dome volume exceed the head volume? Seems like that would be a tight fit!

I would agree, that definitely seems about an order of magnitude too great - it may technically be possible with a big deck clearance and/or gasket thickness, but...

Dan, how about you post the actuial bore diameter, the stroke, the deck clearance, the actual gasket installed thickness (or even just the unused), the actual measured head volume and the actual piston dome volume - either nominal from the piston manufacturer or, prefereably, the measured value. As it is, you're basically just throwing numbers around that have ludicrously impossible claims of accuracy.

I suspect they are reversed -- If the head volume was 87cc and the piston dome was 48cc, then the clearance volume would be 87.2+11.2 - 48.5 = 49.9cc and the compression ratio would be (566.2 + 49.9) / 49.9 = 12.34 : 1

That would make sense, and be in the expected range.

I'm still bemused by the degree of accuraccy claimed, though, for the various values. ;-)

Hopefully, the chap will update with corrected values.