Since I'm about to get a move on the front suspension, I'll be getting my front brakes soon, and probably all of the lines, but was wondering if I would need a proportioning valve. I'm getting Wilwood's 11" rotor, 4 piston calipers on the front and back, but the front pistons are 1.75" and the rears are 1.12". My question is if this difference is enough of a bias to not need a proportioning valve?

A while lot more info is needed than what you have provided and the answer would still be, maybe.

I would plumb one in, but I have 13 letters in my last name.

I used the sticky and the spreadsheet on the math, and got that the rear would be around 45% of the front, I'm assuming, if that math is near correct, I wouldn't need a proportioning valve, correct? I'm not worried about perfect bias, just not too much in the rear.

just plan for adding an adjustable proportioning valve. that way you can dial in whatever bias you need to achieve perfect braking. a lot will depend on what tires your running and how they lock up under braking conditions. too many variables to tell you yes or no, so the safe bet is put one adjustable in on the rear line only.

The OEM's spent a lot of time and money dialing in brake systems and figuring proportioning valve specs only to get it wrong (A bodies come to mind).

You're likely to get real close but find that the last little bit is off and an adjustable setup can let you dial it in right.

It all depends on how hard you are going to drive the car. Road race guys tweak the prop valve during the race as the fuel tank gets lighter. Most grocery getters have no idea what a prop valve is. Just depends on which end of the curve you are. When we did Tim's Valiant the prop valve was under the floor with the knob under the driver seat so Tim could adjust it while driving. That was a pretty hardcore setup though.

This being the handling forum I tend to think questions here are going to be more hard driving type than grocery getter type.

So I answer them with that thought in mind. Had this been the Q&A forum I probably would not have answered it the same.

My recommendation is to FIRST, look at the pinned thread about more brake math than you ever wanted to know. In a nutshell aftermarket companies throw together brake kits and for the most part don't spend any time or engineering dollars in making sure they will work together with what is on your car. Given the fact that the applications and usage can vary wildly, it's up to the end user to verify the brake kits are appropriate for their intended usage. An adjustable proportioning valve on limits maximum pressure, so if your fr/rr balance isn't right or close, you can have a very unsafe ill working brake system.