QuestionWe are racing in a hobby stock class and must pull 14" of vaccum at the end of the race. We were insufficient the other night need to bring it up some. Will things such as timing effect this. And how much difference between pulling the vaccum off the carb rather than the manifold. What is the easiest way to raise the vaccum once we ensured there were no leaks?
AnswerHi Shawn:
I answered this question recently for someone else. Yes, ignition timing will affect your vacuum, so will camshaft timing. First you should do a leak down check to make sure your valves are all sealing corretly, a burned valve will destroy vacuum. Obvious remedy is a valve job. Have it done at a race shop and pay the extra.
If you retard your cam timing by 5-6 or 7 degrees you should see an improvement in vacuum. Depending on compression ratio, I run about 34 degrees of ignition timing at 2500 RPM, all advance in.
You might try a slightly heavier engine oil too- --if that helps it may be time to think about replacing the rings. I recommend synthetic always, and stay away from the one you see advertised the most- - you know- - the "M" brand. I use Amzoil, others use Royal Purple and Lucas, VAlvoline and Castrol Syntech are also good. Most of us use the 20-50 racing oils.
If you do decide to replace the rings, use nothing other than Total Seal - - - they WILL improve your compression and vacuum. You will also see a change in your torque and power curves, It might even help you off the corners.
I don't think it makes much difference whether you take your reading from the manifold of the base of the carb. Make SURE that at idle, your throttle plates are fully closed. If they aren't, the carb needs to be rebuilt and track tuned by someone that knows what he's doing.
The reason they check your vacuum is to make sure you aren't using a cheater camshaft. If you are using too much duration, or too much lift, there will be no vacuum at idle.
Good luck, let me know what works best
Dan Liddy
Sarasota, Florida
