I am about to convert my four wheel drum 71 chevelle with a mild 355 to disk brakes in the front. I am shopping for a master and booster. What are the pros / cons to various size boosters. Say 8" vs 11"? I know 11" is factory, but I found a good deal on a 8" complete. I have good vacumn pressure in this motor. Thanks, Steve

Bigger is better in this case. But remember, it's about AREA not diameter. A dual 8 has more surface area than a single 11. With the dual 8 you don't lose any boost power (compared to the single 11) to the pedal and you gain important clearance for big blocks and/or tall valve covers. A radical cam with low idle vaccum will kill boost power no matter what so every little bit helps.

Area of a Circle = pi (3.14) X radius squared
Single 11" = 3.14 X 5.5sq = 3.14 X 30.25 = 94.985 sq in
Dual 8" = 3.14 X 4sq = 3.14 X 16 = 50.24 X 2 (because it's dual)= 100.48 sq in

Also, a smaller m/c bore creates more pressure. The 1.125 m/c was designed for the single piston caliper on the stock brakes. If you go to the bigger late model brakes like a C5 swap, they give you the 1" or 15/16" m/c. My dual 8 kit from Pirate Jack has a 1" bore m/c.

If you're looking at Hydroboost, they reccomend the 1.125 m/c because the hydro creates way more boost than a vaccum booster. With a hydro+1.125 combo, you get the best of both. More pressure AND more fluid volume.

FYI; I got excellent customer service at Pirate Jack and their price was $20 cheaper than anyone else AND they are offering FREE shipping for the month of March.

See kids...there really IS a reason to learn Geometry...:D

Thanks for the info. So based on your information, a dual 8" would be a better choice over stock. Steve