Disc brake sizes?

I am working on a 1990 Sebring 5000 MX. It came with factory (CR) installed Mustang ll front suspension that included 9” front disc brakes. A 9” rear was also installed. A PO installed an 11” rear disc brake kit on the rear. (17” diameter wheels front and rear). I am by no stretch of the imagination an expert on anything, but I thought larger diameter brakes were always in the front???

I'm planning on installing a 2015+ Ford Mustang 2.3 liter Ecoboost engine and 6-speed manual transmission. This should provide plenty of power and also help reduce the somewhat typical heavy nose of this car. I've installed QA1 front coilovers rated at 375 pounds each. Thoughts on the brake set up?

Since the MII front end has been used for many years in the street rod market, there are many bolt on choices for larger brakes.  Wilwood and Baer both sell excellent brake kits in sizes up to 13" rotors.  With calipers from 4 to 6 pistons.

Since you have 17" wheels, you may be able to get away with 13" rotors, but 12" should fit easily.  The only downside to these big name companies is price.  They are very expensive compared to other kits that use more common parts from OEM manufacturers.

Here is one from MP Brakes

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There are other still cheaper conversions that use a variety of OEM calipers, usually GM metric.  Most of these use Ford Granada rotors.  The rotors will fit with no mods since the spindle dimensions of the MII and Granada are basically the same.  The main drawback is that the Granada rotors will add 1/2" width to each side of the car.  

If you want keep the stock width of the front end, you will need a kit that uses custom hubs and rotors, like the MP kit above.

Be sure to contact the manufacturer of the kit to find out if you current master cylinder will work. And that the width of the front end stays the same.

Here are several pages of various kits

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Thank you. What I’m hearing is that the front brakes should be larger (more stopping power) than the rear. Thus, I should get larger front rotors for my build. Ok, another budget revision. That's the way projects go, at least mine.

Not just you!

Most of these types of projects seem to take on a life of their own.  There always seems to be just one more thing to spend money on.

At any rate, the original MII discs were around 9" if I remember right.  You could probably get away with 11" brakes in the front to match the 11" rears.  Most OEM rear calipers have smaller pistons so going to better calipers in the front (with 11" rotors) would still give you some built-in mechanical front bias.  I would still put in an adjustable proportioning valve in the rear line.  This would allow you to adjust out any tendency for the rears to lock up first.  12" or 13" rotors would probably be better though, if you can get them under the rims.

Having the rears lock first can cause the car to spin out in a curve or on wet roads.  That is generally the reason for having more braking power on the front, when dealing with braking systems that are not anti-lock.  That, and the fact that weight transfers forward during braking.  However, the generally thrown out figure of 75% braking on the front doesn't necessarily apply to every car or truck.  Cars with a low center of gravity and weight distribution approaching 50/50 (like a Sebring) may not transfer that much to the front.

Hotrod, I very much appreciate learning. Thank you for the explanation and advice!! And, taking time to answer my questions. I appreciate this forum!