working out compression ratios

[jansens]

The standard Compression Ratios seem to be

1923-1933 4.8 (or 4.9) to 1

1933-1936 5.2 ? to 1

1936- 1939 6.0 (or 6.2?) to 1

I understand that Compression Pressures are between 17 and 20 times the Compression Ratio in a good engine giving 

4.8 to 1 = 80 to 96 psi

5.2 to 1 = 88 to 104

6.0 to 1 = 102 to 120    

I get around 100 psi from the standard early head and 110 psi from the 1936 head.

Cheers, Tony.

I didn't know that about the pressure to ratio figures, thanks. It works for my MGB certainly.

I put the spreadsheet up here (http://www.asciimation.co.nz/austin7/fil...lator.xlsx) to download. It's in .xlsx format. I don't have a modern enough version of Excel to test it but you can import it fine into Google sheets.

I made a slight mistake when doing my engine in that I equalised all the chamber volumes first (to 34cc) but then found the valve heads were different thicknesses so my ratios are a little different between cylinders. But we're talking a small amount (5.16, 5.13, 5.16, 5.15) I am not sure it matters? And given those volumes presumably change when the crank whips at high revs meaning the ratios will be changing anyway. It's one reason I did the spreadsheet, so I could plug in different values to see what difference it makes. I suspect the differences I have won't matter too much on a road car (it just upsets my OCD tendencies!).

I am sure the racing guys will have a much better idea how accurate and precise one needs to be with all this. I am not worried about power as much as smoothness. It's satisfying to build a smooth running engine for some reason.

If anyone wants the spreadsheet and can't download it PM me and I can email you a copy. Someone really should check it is sensible!

Simon