Hydraboost or 13 inch disc to upgrade braking

[Bihili]

I am not satisfied with the current 10 1/2 front disc and rear drums on 12 bolt rear end.
Because of the small supercharger I have an electric vacuum pump and additional tank which produces 18 inches of vacuum.
This is barely enough for the vacuum booster.
It feels like the brakes will not lock up, maybe because of the large tires, 245 & 295's

Which upgrade do I need?
Larger front disc or hydroboost?

[55 Rescue Dog]

The hydro will add more pressure with less effort, but it won't improve your brake stopping ability unfortunately. You should be able to lock them up if you push hard enough unless the pedal is to the floor. If so you need more brake grip. Cylinder sizes are another thing to look at.

[chevynut]

In your situation I'd do both. Install the hydroboost first and see if you're happy with the braking....I believe it will improve and you can get rid of the vacuum pump and booster. But beyond that I'd go with bigger front brakes if you can fit them inside your wheels.

[Rick_L]

How big is your booster and is it single or double diaphragm? What bore size on the master cylinder? Do you have a gauge on the vacuum reservoir? If so what is the typical vacuum reading?

[Bihili]

Duel 8 inch diaphragm with one inch bore master cylinder.
The vacuum pump kicks off at 18 inches of vacuum.

17 inch wheels on front would allow a 13 inch rotor.

[chevynut]

I think I changed my mind on the order of the improvements. With 17" wheels I think the 10.5" rotors probably not only LOOK a little wimpy, but they probably are small for your setup. For both looks and performance I think I'd do a rotor and caliper upgrade to at least 12" rotors first....11" isn't much of an upgrade, imo. That should help assuming everything else is working correctly. I don't know what calipers you're using now but if you go to multiple or larger pistons that take more fluid volume, you may end up with more pedal travel than you like with the 1" master cylinder, so you may have to increase the master cylinder bore size .....then you need a better booster. It's all kind of a balancing act and you have to optimize the setup.

[Bitchin'57]

Change to the 13" front brakes. Like Laz said, you might have to change your master cylinder as a result.

[Rick_L]

Many of the caliper upgrades are actually smaller bore size even with 4 or 6 pistons, so would require either no change or a smaller master cylinder. The Wilwood 6 piston is that way, as are C4-C6 Corvette calipers. You need to make sure the master cylinder matches the calipers. Remember that a smaller master cylinder will produce more pressure for a given push with the pedal - but the travel goes up too.

I think what you have to determine here is whether you have inadequate boost or just not enough brakes. Lots of cars seem to do fine with the metric brakes, but obviously bigger rotors with a caliper upgrade will be better.

[Bitchin'57]

Many of the caliper upgrades are actually smaller bore size even with 4 or 6 pistons, so would require either no change or a smaller master cylinder. The Wilwood 6 piston is that way, as are C4-C6 Corvette calipers. You need to make sure the master cylinder matches the calipers. Remember that a smaller caliper will produce more pressure for a given push with the pedal - but the travel goes up too.

I think what you have to determine here is whether you have inadequate boost or just not enough brakes. Lots of cars seem to do fine with the metric brakes, but obviously bigger rotors with a caliper upgrade will be better.

I think you meant a smaller master cylinder, right?

[chevynut]

Bill, do you have a brake pressure gauge you could install to see what kind of line pressure you're getting out of the MC? That might help answer some questions.

[Rick_L]

Bitchin', yes I meant master cylinder. Will edit.

A pressure gauge will certainly help sort out the variables.

[JT56]

My current setup on my car is the 10.5 metric on the fronts(I know old stuff), Hydratech HB, with 1" bore MC (2006 GTO), external pv and strange brakes (4 piston). I used to have vacuum pump(SSBC) on my car, 1 1/8 MC (CPP) along with 2004 cobra brakes on the rear. I never checked the with pressure gauge, but the car stopped pretty well. No complaints and did a lot of street cruising too. After mini tub and double bead locks I had to change the rear brakes so now the car stops on a dime and gives back change. My future plan is to upgrade the front brakes to something that looks a little better and lighter. However, no problem with the way it stops now. As everyone else has said, start with a pressure check to see what you have. If the budget calls for it...do both.

[Rick_L]

Well JT, looks like that's good input for bihili, since you have a boosted engine and similar front brakes.

[JT56]

I am not satisfied with the current 10 1/2 front disc and rear drums on 12 bolt rear end.
Because of the small supercharger I have an electric vacuum pump and additional tank which produces 18 inches of vacuum.
This is barely enough for the vacuum booster.
It feels like the brakes will not lock up, maybe because of the large tires, 245 & 295's

Which upgrade do I need?
Larger front disc or hydroboost?

The SSBC was setup for 24" of vacuum so a little more than the 18. I would still check brake pressure first. I have one of the reverse bleeders and it does help with getting them bleed. Also remember the skinnier the front tire is the less contact surface you have for braking. Looks like a race car and helps cut down rolling resistance, but not great for stopping a heavy car.

[Bihili]

I found a couple of PSI gauges. Now I need to figure out the adapters I need to connect to the front brake line at the caliper to test the pressure.
The first thing I want to test is disconnecting the vacuum pump regulator.
Just wire the vacuum pump to run continuously and see if it will increase the amount of vacuum and then maybe a test drive if I have more vacuum.