help need on a 6-71 blower

i have a 6-71 blower with a 30 tooth pulley and the crank pulley has 28 teeth. what is the pecentage of over or under drive on this setup?
thanks for any help.
trvguy

 

6.7% under if the 30 tooth is on top/28 on bottom or 7.1% overdriven if it is the other way.

If you have your basic 350 with a large bore 6-71 you are looking at about 10lbs of boost. Doable on the street if you mind your air/fuel, timing and compression ratio.

Good luck, -Abone.

 

From what I understand, rule of the thumb is about 3% per tooth.

I have a 671 on a small block chev at 9% underdriven and it sees about 12lbs boost, But, I do have a mild cam which means it could just be showing pressure "backing up" in the manifold but it still gives you an idea of where yours could be in comparison.

You really want a boost retard in your ignition, I use an MSD with an adjustable boost retard for the timing when in boost.

Also, another point, it as recommended to me that if I was going any less than my 9% underdrive, I would be better to use heads studs rather than head bolts.....so I stck with the 9%!!