What bore size master Cylinder for C4 Corvette Brakes?

[Custer55]

I have a question on what would be the right bore size on the master cylinder for my C4 conversion. I will be using the stock Corvette calipers from a 90 Vette. The master cylinder I have now is for front disc and rear drum brakes, 1 1/8" bore with a 7" booster. All this came with the dropped spindle kit a bought from Auto City Glass. The spindles are McGoughys. (The brakes never seemed to stop the car as well as they should with this set up, better than the front drums but not by much). The stock 90 Vette master cylinder which also has a booster (ugly black Plastic) is only a 15/16" bore master cylinder. I'd like to use the 7" booster I have since it fits nicely in the space available but not sure what I should get for a master cylinder because of the small bore of the stock Corvette MC. I thought power brakes needed a bigger bore.
Any thoughts or help would be greatly appreciated.
Brian

[Rick_L]

The 15/16" bore is correct for the C4 brakes with a booster. The caliper bores are pretty small on a C4.

[chevynut]

There's more going on than just the master cylinder bore and the wheel cylinder bore. The pedal ratio also changed from year to year and I think it needs to be considered. I would shoot for about the same line pressure as the corvette with the same pedal force. The tri5 has a 6.42 to 1 pedal arc ratio which is significantly different. Here's the C4 specs.....

'85-'87

MC Bore .875"
Wheel cylinder bore 2.1" front 1.6" rear
Pedal arc ratio 3.5 to 1
Line pressure with 100 lb on pedal 1250 front 800 rear

'88-'91

MC Bore .87"
Wheel cylinder bore 1.5" (dual) front 1.6" rear
Pedal arc ratio 3.5 to 1
Line pressure with 100 lb on pedal 1250 front 750 rear

'92-'96

MC Bore .93"
Wheel cylinder bore 1.5" (dual) front 1.6" rear
Pedal arc ratio 4.0 to 1
Line pressure with 100 lb on pedal 1160 front 680 rear

So I would be careful with the MC bore because the pedal ratio is so high and you could get too much travel. I modified my pedal travel to get about 4.5 to 1 as I recall, and I'm using an ABS Power Brake electric booster/MC.

[Rick_L]

Good point on the pedal ratio. Pressure at the master cylinder outlet is a function of both master cylinder bore size and pedal ratio.

[Custer55]

So if I leave the pedal ratio as is what would be the correct Master Cylinder bore size? Would a 1" bore be ok? or should I go to a 1 1/8" bore?
Also would it work to just eliminate the booster all together and use a 7/8" bore Master Cylinder?
Thanks for the info,
Brian

[chevynut]

If you eliminated the booster here's what you'd get. The pedal ratio is 6.42:1 so with 100 pounds on the pedal you would get 642 pounds at the pushrod. A .875" bore is .601 square inches so you would have 1068 PSI line pressure to the front brakes. That's close to the C4 spec, but about 200 PSI below so I'm not sure how well it would work.

The other potential problem is you might have too much pedal travel, but I'm not positive about that either. With a late suspension you have 3.53 square inches of piston area at the caliper and the early one is 3.46 square inches. So for every .001" the caliper piston moves the master cylinder piston has to move about .0058" and the pedal has to move about .0375". The thing I'm not sure of is how much the caliper piston has to move to take up all the clearances and deflections when braking.

You can't fix an excess pedal travel issue with a booster, you have to change the pedal ratio. Either way, you have to displace the same amount of brake fluid. And if you drop the pedal ratio, your line pressure will drop and you will need a booster. Its a balancing act. ;)

[Custer55]

Great Info. I can see how the pedal ratio is a significant factor in master cylinder selection. I checked the Corvette master and pedal that I have and it is a .87 bore (It's a little bigger where the snap ring goes in so it looked like 15/16 measuring it at that point) and the pedal has the 3.5 to ratio. I don't know if I am figuring things right or not but here is what I come up with based on your info.

All this is based on a 6.42 to 1 pedal ratio and having 1250 lbs. of line pressure to the front brakes.
It would take 117 lbs. on the brake pedal with a 7/8 MC with no booster. So probably not a good option.

It would take about 55 lbs. on the pedal with a 7/8 MC with booster.

It would take about 72 lbs. on the pedal with a 1" MC with a booster.

It would take about 91 lbs. on the pedal with a 1 1/8" MC with a booster.

So based on this I think my best options would be the 1" or 1 1/8" MC with a booster. I am leaning towards the 1" MC as I could change the pedal ratio to 5.5 to 5 to 1 if I would need more pedal travel or better brake feel. I am leaning to the 1" also because I prefer to use a lighter touch on the brakes anyway.

Thanks again Chevynut for all the info.

Brian:)

[KStaggemeier]

There's more going on than just the master cylinder bore and the wheel cylinder bore. The pedal ratio also changed from year to year and I think it needs to be considered. I would shoot for about the same line pressure as the corvette with the same pedal force. The tri5 has a 6.42 to 1 pedal arc ratio which is significantly different. Here's the C4 specs.....

'85-'87

MC Bore .875"
Wheel cylinder bore 2.1" front 1.6" rear
Pedal arc ratio 3.5 to 1
Line pressure with 100 lb on pedal 1250 front 800 rear

'88-'91

MC Bore .87"
Wheel cylinder bore 1.5" (dual) front 1.6" rear
Pedal arc ratio 3.5 to 1
Line pressure with 100 lb on pedal 1250 front 750 rear

'92-'96

MC Bore .93"
Wheel cylinder bore 1.5" (dual) front 1.6" rear
Pedal arc ratio 4.0 to 1
Line pressure with 100 lb on pedal 1160 front 680 rear

So I would be careful with the MC bore because the pedal ratio is so high and you could get too much travel. I modified my pedal travel to get about 4.5 to 1 as I recall, and I'm using an ABS Power Brake electric booster/MC.

Follow on question: Are the pedal pressures you quote for 85-87 the same for 84? I've recently completed an 84 Corvette for the 24 hours of Lemons series and have way too much rear brake bias. Car has standard plumbing layout and new aftermarket master cylinder (discrete reservoirs) I ASSUME the front/rear piston ratio diameter is correct. I would like to get a baseline understanding before I install a proportioning valve. Thx.

[chevynut]

Follow on question: Are the pedal pressures you quote for 85-87 the same for 84?

I don't know and I don't my Corvette Specs book handy right now. I know the 84's had unique calipers that only fit that year. I don't know if the piston size is any different or what the master cylinder bore is.

[TrifiveRichard]

Am I missing something here concerning pedal travel? Why not use a pedal stop rather than change ratio (the location of the push rod). The Wilwood and hydroboost systems I’ve used say to limit rod travel to 1 inch, and a pedal stop works nicely. What am I missing?

[Rick_L]

What you're missing is that pedal ratio and bore diameter have similar effects. Pressure and volume are both affected. A stop only affects total volume.

[chevynut]

Am I missing something here concerning pedal travel? Why not use a pedal stop rather than change ratio (the location of the push rod). The Wilwood and hydroboost systems I’ve used say to limit rod travel to 1 inch, and a pedal stop works nicely. What am I missing?

With the same calipers, a smaller master cylinder bore requires more travel to push the same amount of fluid. It also give you more pressure. There is a tradeoff of pedal stroke, caliper bore size, and master cylinder bore size. A smaller master cylinder give you more pressure at the expense of more stroke required. That stroke can be reduced by lowering the pedal ratio. But then you need more pedal force to get the same pressure. It's a balancing act and I had to do a bunch of calculations to get my brake system to where I wanted it since I designed a bellcrank to move my master cylinder outboard. I hope it works as expected.