Have installed the Wilwood caliper/rotor kit on my 71 Coupe with power brakes and they work great. Now want to install the Wilwood master cylinder but they need to know the original bore size before recommending. What is the original Master Cylinder bore size for a 71 big block coupe with power brakes ?
Thanks, Tim

 

1-1/8"

 

They may need to know what other engine modifications you have made as well.
Some engine modifications like a high lift cam can alter vacuum which can effect the master cylinder bore size.

 

Same as my '72 small block.....

 

The amount of vacuum has nothing to do with MC piston & bore size. Bore size is a option difference for manual or power brakes. You must be thinking about the booster which may not have enough vacuum to support it.

 

All I know is that when I purchased my complete brake system from Wilwood, they told me to wait until my engine was assembled and installed.
Wilwood told me to run a vacuum test to determine what vacuum my engine was putting out before I ordered my master cylinder as it could make a difference as to which master cylinder bore size I order.
I'm just passing the information Wilwood gave me along to the OP.

 

Ok. That's a new one on me.

 

Me too.

MC bore is based on the fluid displacement requirements of the calipers. In the case of the D series parts those mimic the oe spec and thus the stock bore is retained.

MC size doesn't truly have an effect on brake capacity- it does determine pedal travel and how much effort you want to use to make the brakes work. Larger bores take more effort, smaller bores less. But the side effect is travel and feel: bigger are firmer and smaller are softer.

Anytime I sell a kit the first suggestion is to run the oe spec bore. Nearly every properly built kit, Wilwood or my own, are intended to pair with the stock MC. If you want to make a change to something else you'd be wise to evaluate the current situation first for example. Or if you move from power to manual that can necessitate a need for a smaller size as you need more pressure.

Vaccum use takes away from the assist being applied. If you lose vaccum you need to push harder as you've lost your helper. As noted; cams can have an effect on this or other means that dump vaccum too quickly. My persona recommendation to anyone building a new motor would be to start with the stock set up and determine if you lose it. You can guess on the needs sure, but it's adding one more element to the decision process you don't need. Then we get the guys who bad mouth product at times claiming the new brakes don't work correctly......no, you just deviated from how it was intended to be used.

 

My later conversation with Wilwood agreed with HeadsU.P. and Todd TCE. Since the Wilwood replacement kit is a close duplicate of the original, the correct M/C would be the same: 1 1/8".
Thanks for all the help, Tim

P.S. Todd TCE: I have had similar heated discussions with motorcycle owners when they would bad mouth or criticize a manufacturer's product. One that had been successfully installed by thousands of riders, but they thought was junk. I've always believed that most of the product installation complaints are a result of the installer, not the product. I always ask myself one question before straying from the manufacturer's recommendation : " Am I smarter than the engineer that designed this product " Usually, the answer is " NO "

 

Thanks, I know some stuff but generally rely on the manufactures and their tech support people to help me out.
The only part I did not order from Wilwood was the master cylinder because of what they told me.
I will call them back and ask again, so I can order the master cylinder, take it off my list of needed things and have everything I need for my brake system.
If I am told to wait again, I will get them to email me the technical information and post it here.
Greg

 

Be curious to what you're told. I can maybe get some reasoning behind waiting if you knew you'd have a different vacuum value and were trying to account for it ahead of time- a known loss, suggesting you counter that by going smaller so you can still push it manually maybe...but personally I'd start "same-same" and see what the result was.

 

The engineer might be smarter, but the minimum wage high school drop out that was hired to answer the phone at tech support only has a set of books in front of him!

Wilwood techs screwed me last year on a hydraulic clutch kit for a customers TKO600 install. I think what was sent had a 3/4 or something way too small a bore and it was like using a leg work out machine to press in the clutch in. I had to take it all out and get a bigger piston so it worked right with an easy pedal.