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View Full Version : Hydraboost or 13 inch disc to upgrade braking



Bihili
11-12-2017, 11:52 AM
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
11-12-2017, 03:00 PM
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
11-12-2017, 05:29 PM
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
11-12-2017, 05:31 PM
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
11-12-2017, 06:48 PM
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
11-12-2017, 07:49 PM
I think I changed my mind on the order of the improvements. :eek: 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
11-13-2017, 05:52 AM
Change to the 13" front brakes. Like Laz said, you might have to change your master cylinder as a result.

Rick_L
11-13-2017, 06:13 AM
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
11-13-2017, 07:47 AM
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
11-13-2017, 08:36 AM
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
11-13-2017, 09:07 AM
Bitchin', yes I meant master cylinder. Will edit.

A pressure gauge will certainly help sort out the variables.

JT56
11-13-2017, 11:48 AM
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
11-13-2017, 07:11 PM
Well JT, looks like that's good input for bihili, since you have a boosted engine and similar front brakes.

JT56
11-14-2017, 06:15 AM
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
11-14-2017, 08:32 AM
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.

http://i686.photobucket.com/albums/vv226/bihili57/20171113_202837.jpg (http://s686.photobucket.com/user/bihili57/media/20171113_202837.jpg.html)

55 Rescue Dog
11-15-2017, 04:11 PM
I've found out that permanently mounted F/R brake pressure gauges to be extremely helpful on a hot rod. Of course on my 55 I will come up with a cleaner install than this one.7939

Bihili
11-21-2017, 06:41 AM
Update:
After installing a PSI gauge to the front caliper using the bleeder hole here are the stats.
With engine off my 125lb wife could only get 400 PSI.
I was barely able get 600 PSI with engine off.
With the engine running and 18 inches of vacuum from the electric vacuum pump I was able to get 750 PSI.
After I changed the vacuum pump to run continuously it is producing 20 inches of vacuum and was able to get 800 PSI.
Now before you criticize my vacuum pump just remember how many Hybrids and other vehicle on the road today use electric vacuum pumps.
This is a standard GM pump.
Other stats: CPP master cylinder 1 inch bore, CPP dual 8 inch diaphragm, CPP proportional valve, linkage 1 inch lower on pedal, rear drums on 12 bolt and 10 1/2 inch rotors.
The additional vacuum made a improvement so I will try the skid test and see if I can lock them up.

chevynut
11-21-2017, 07:20 AM
To me 800 PSI seems low. I'm curious how much force you're able to put on the pedal. More vacuum doesn't appear to help that much either.

FYI the spec for the front of a C4 is 1250 PSI at 100 lb on the pedal or 12.5 times pedal effort. I don't know what it is for other cars. The force at the piston depends on piston area.

I found this online:

"Typical brake line pressures during a stop range from less than 800psi under "normal" conditions, to as much as 2000psi in a maximum effort. "

What is your pedal ratio?

JT56
11-21-2017, 08:05 AM
If the system is all in working order...Cnut I think your on to something with the pedal ratio

Rick_L
11-21-2017, 09:13 AM
Thing is, the pedal ratio you can achieve without creating binding in the linkage is limited. On a 55-57, a flat mount booster or a manual setup is a setup that works well both with the geometry and with potential brake line pressure with the stock 6:1 ratio. An angled mount lends itself to a 4:1 ratio.

You also can work with master cylinder bore size to optimize this. More pedal ratio = more line pressure but more pedal travel. Smaller master cylinder bore = more line pressure and more pedal travel. This for a given caliper bore.

100 pounds pedal force is a lot. I think this is panic stop territory, not normal driving territory.

chevynut
11-21-2017, 09:21 AM
Thing is, the pedal ratio you can achieve without creating binding in the linkage is limited. On a 55-57, a flat mount booster or a manual setup is a setup that works well both with the geometry and with potential brake line pressure with the stock 6:1 ratio. An angled mount lends itself to a 4:1 ratio.

That's probably true, but maybe Bill's simplest fix would be to simply change the booster mounting angle and go to a higher pedal ratio. That's why I asked where it is now.


100 pounds pedal force is a lot. I think this is panic stop territory, not normal driving territory.

Yes it is quite a bit for one leg. :p But I think what Bill is trying to do is address those panic stops since he's trying to lock up the tires. At least that's what I understand. He said the MAXIMUM pressure he can get is 800 PSI. I'll bet he's applying a lot of force to the pedal to get that.

chevynut
11-21-2017, 09:36 AM
More info....the C4 spec I quoted of 1250 PSI with 100 pounds of pedal force is using a 3.5:1 pedal ratio and a 7/8" bore master cylinder with a 9.4" single diaphragm booster.

That means the force on the MC piston is 1087 pounds. The force on the pedal pushrod is 350 pounds. So the vacuum booster is providing over 3X boost.

NickP
11-21-2017, 04:34 PM
Thing is, the pedal ratio you can achieve without creating binding in the linkage is limited. On a 55-57, a flat mount booster or a manual setup is a setup that works well both with the geometry and with potential brake line pressure with the stock 6:1 ratio. An angled mount lends itself to a 4:1 ratio.

You also can work with master cylinder bore size to optimize this. More pedal ratio = more line pressure but more pedal travel. Smaller master cylinder bore = more line pressure and more pedal travel. This for a given caliper bore.

100 pounds pedal force is a lot. I think this is panic stop territory, not normal driving territory.

How does one validate "100 pounds of pedal force", calculation or with an instron?

chevynut
11-21-2017, 07:12 PM
How does one validate "100 pounds of pedal force", calculation or with an instron?

You could put some sort of scale between the pedal and your foot...like a bathroom scale or something similar that would fit and goes to 100 pounds. Or you could put a 2x4 on the pedal with a bathroom scale on top of that and sit in the seat and do a bench press. :p Maybe a hydraulic jack could be used pressing on the seat.

Rick_L
11-21-2017, 07:34 PM
You guys are right, it's not an easy deal to get real data on if you want to do it the hard way.

But you can get real data on the brake line pressure, and knowing all the other variables you can come up with something on a non-boosted system. This would take the boost out of the situation, which is a variable that's harder to nail down.

My logic says that for you to put 100 pounds on the pedal with your foot, you need to use all or most of your strength, and you have to apply a similar reaction force to apply this force. Sitting in a driving position, this means you have to pull on the steering wheel with that same 100 pounds of force because there's not much else to react to that 100 pounds from your foot. Yeah you can do it, but you probably can't sit in the shop and do it every time unless you're a big strong guy. The 125 pound wife can't.

Bihili
11-22-2017, 07:26 AM
Yeah you can do it, but you probably can't sit in the shop and do it every time unless you're a big strong guy. The 125 pound wife can't.

She doesn't drive it that much but the brakes are the reason why.
On a side note, my second 57 project (cruiser) is a cheaper build but has the 11 inch disc and hyrdoboost with the LS engine but it is 3 or 4 years from completion.
I could rob the hydroboost from it but CPP has a sale on their 13 inch disc brakes this weekend and I may go that route first.

Bihili
03-19-2018, 03:10 PM
Follow up:
Installed CPP 13 inch disc and their new caliper on the front. The rear twelve bolt drums were not changed.
Wow what a HUGE difference and now have good panic brake stopping. The other change is the vacuum pump regulator was removed so the pump runs continuously and produces 20 pounds of vacuum to the dual 8 inch booster.

55 Tony
03-20-2018, 06:24 AM
I'm way late here, but I got a tip from someone and found it to be true that the cheapest brake pads you can buy stop the best! The only downfall is they sure do dirty the front wheels fast. I have GM older style discs and I increased my pedal ratio. With the pedal as it is, if I try hard I can get the pedal to the floor, but the brakes lock up before getting that far. I'm comfortable driving it with no booster or hydro.