You could guesstimate, assuming stock components.
Or you could use one of these;
http://store.katechengines.com/whist...ster-p174.aspx
They use it to tech "limited" engines in circle track racing, haven't seen or heard of one being used anytime recently in dirt track racing, although it is mentioned in some dated rules, asphalt/road course racing may be a different story, they may be used currently.