Brake Upgrade Different Sizes question

Wheelz

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Hey all, so I'm looking to upgrade the brakes on my 09 Bullitt, specifically trying to upgrade to 4 piston 14 inch rotors upfront from the 08/09 GT500 and the 13/14 GT500 rear brake upgrade to the 13.8 inch rotors in the rear. My question is will I see any negative issues/handling by having almost similar size rotors front and rear.
 

07 Boss

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I think it all depends on the ratio between pad to rotor contact areas. In theory if you increase stopping power in the rear you may increase braking distance. But I’m not sure how exactly our abs system works and it may not have any effect at all. I know I have a car I’ve converted from drum to disc in the rear and without a different proportioning valve the rears would lock up early under hard braking.
 

Wheelz

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Hmm, maybe it would be better to just upgrade the fronts to the 4 pistons and leave the rear the same size if it could increase the stopping distance..
 
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I have the 4 piston up front with the 13/14 rear brakes and I have not noticed any ill braking effects from this setup. To be fair, my car is a streetcar so I don’t push it to the very limit on public roads, but during spirited driving it hasn’t been a problem on my car.
 

007Stallion

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The stock rotors are 11.8”, so using the larger 13.8” rotors in the rear will increase braking torque due to the longer lever which will be increased by 1” (1/2 of the change in diameter) I would assume clamping force would remain relatively unchanged at the rear since the stock caliper is reused. A pad with a higher COF can increase clamping force however.

You will also have increased rotational mass, decreasing acceleration by a small but measurable amount. I doubt it would affect handling and probably nothing the butt dyno would detect.
 

DieHarder

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Hey all, so I'm looking to upgrade the brakes on my 09 Bullitt, specifically trying to upgrade to 4 piston 14 inch rotors upfront from the 08/09 GT500 and the 13/14 GT500 rear brake upgrade to the 13.8 inch rotors in the rear. My question is will I see any negative issues/handling by having almost similar size rotors front and rear.

Just upgrade the fronts for now and reassess. That's what I did and never found the need to do the rears. Unless you're going to track it upgrading the rear is probably not necessary.
 

bambam 06

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Hey all, so I'm looking to upgrade the brakes on my 09 Bullitt, specifically trying to upgrade to 4 piston 14 inch rotors upfront from the 08/09 GT500 and the 13/14 GT500 rear brake upgrade to the 13.8 inch rotors in the rear. My question is will I see any negative issues/handling by having almost similar size rotors front and rear.
I did the exact set up about 5 yrs ago (Front/Back) and had no problems whatsoever. The stopping power increased by about 20% and they work like normal OEM. You will need at least 18" wheels.
 
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AHaze

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Vorshlag extensively road raced a 2011 GT and used the 4 piston fronts with both rear rotor sizes at different points. I honestly recommend going through the build thread. It's long but IMHO, worth the time if you're even semi serious about making an S197 handle.
 

Wheelz

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Thanks for the input, i'm gonna compare the pricing on both set ups and make a choice from there. I will look into that Vorshlag thread, it looks hella informative
 

GriffX

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The biggest complaint I have with the brake is that I cannot push hard enough to get into ABS braking at 100 MPH.
A bigger breake booster would be nice to have.
 

Wheelz

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The biggest complaint I have with the brake is that I cannot push hard enough to get into ABS braking at 100 MPH.
A bigger breake booster would be nice to have.
Hmm I'm curious about that, why's it matter if you cant activate ABS?
 

GriffX

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Hmm I'm curious about that, why's it matter if you cant activate ABS?
That's a good question. Chicken legs? Despite I'm 6'2'' I'm more the marathon runner guy. To be fair it was with the OEM Akebono pads, after that emergency brake on the Autobahn, I changed them to more aggressive EBC Yellow pads (they brake much harder but they ruin the rotors, no recommendation from me).
The 07 doesn't have a brake assistant, so the ABS pump gets not activated from the brake pressure sensor alone, you must step on the pedal so hard that the brake pad friction is higher than the tire-road friction. When the tire stops the wheel sensor activates the ABS system.
I was not able to step hard enough on the pedal to overcome the tire-road friction at 100 MPH and the ABS got activated much later (from 70?).
 

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