8" or 9" brake booster

[Northern Goat]

I am curretnly running a Wilwood brake package on my 71 Firebird. 6 piston calipers front and 4 piston dynalites on the rear. I have a 7" dual diaphragm booster with a Wilwood 1 1/16" bore dual tandem master.

I have been screwing around try to get a better pedal feel and to get the car to stop the way I expected and think I have finally narrowed it down to the booster being too small.

Now the question becomes do I use a 8" or 9" dual diaphragm booster. The math seems to indicate that the area of an 8" will be slightly more than a 11" single diaphragm unit and a lot of internet research seems to suggest I use an 8" also; however the DSE master and booster package is a 9" unit and they seem to know their s*#t.

Any opinions on which way to go and why?

[Apogee]

What are your effective caliper piston areas? What is your engine vacuum level at idle?

Wilwood makes a few 6-piston calipers, but if we assume a billet Superlite with 4.04 square inches piston area (1.12 x 1.12 x 1.62) up front and either the 1.98 or 3.00 square inch Dynalite rears, I would probably run the dual-9" booster over a dual-8" with your current 1-1/16" bore master cylinder given the choice. First, I would have probably ran a 1" bore MC, so use the larger booster (~27%) to offset master cylinder piston area being ~12% larger a 1" bore MC. Next, a lot of us are running around with less than optimal vacuum levels, so the more diaphragm area, the better off you'll be in low(er) vacuum situations. If you ever think that your brakes are too sensitive, you can always reduce your pad CoF accordingly.

Tobin
KORE3

[Northern Goat]

Thanks for the response Tobin.

For the full story, I bought the master as part of a package and was sent the 1 1/16 booster (I had ordered a manual package and expected a 7/8 master but I figured it was too much hassle to exchange the master since I had a 7" booster sitting in my garage). My original problem was that the car barely stopped with the Wildwood master and being a novice I swapped out the master for a generic corvette 1" master and the brakes got better but were not great. Given the fact that my motor is an LQ9 with just a GM hot cam I did not think the vacuum was that bad. I was talking to a friend who is running 4 piston wilwoods on all corners with a 11" booster and stock camaro master I got to thinking about the booster and then did the math for to determine a 8" is close to the specs of an 11", but I also understand that a dual diaphragm booster is not as efficient as a single. Long story short I would like to use the Wilwood master since I paid so much for it but can live with the plain 1” I have on the car now if that is the best solution.