I haven't done a compression check after the rebuild yet but I will this spring .I did not measure the cc's on head compression hence the chambers were untouched. heads not milled either. (920 heads) just a little cleanup on the ports and a gasket match . I run premium 93 octane fuel . you can here from the exhaust that the 273 does have some compression. My previous 273 with flat tops had compression no less than 9 . I'll be posting the info on my compression come spring time. Thanks for the interest .