Erratic Vaccum Reading

It all depends on the design. If your builder likes tight quench and the heads are good, and the static compression ratio is high enough, you need much less. Typically mine like less than 32, some as low as 28 total. If the quench is loser than .035", and dished pistons are used, you will have more tendency to detonate. But really, every engine and location is diferent. It's like a carb. If it's tuned very well, you will need to adjust for seasonal changes. Your timing will be diffeerent from a guy in Denver, or Utah, or me in CT. Best thing I can tell you is take it to the track and see what yours likes.