The other item that isn't really discussed in the links above (and causes a lot of balance issues with the Fiero) is in factory form the Fiero brake system is very unique and this stems from how it was put together. In very simplistic terms, the Fiero has a front drivetrain in the rear and this also applies to the rear brake calipers. Yes they added the parking brake feature, but didn't downsize the caliper piston. So the Fiero is about the only car in existence that uses the same caliper piston areas front/rear.
Most automotive enthusiasts know that the front does more braking, especially once the weight has been transferred to the front wheels, and the Fiero isn't significantly different in this regard. However, how this is accomplished is significantly different. In the links above they talk about largely fixed line pressure going from the master to the calipers and the smaller rear caliper area being the primary means to create the brake bias front/rear. With the fiero having the same caliper piston sizes from & rear, the Fiero creates the front/rear bias by significantly reducing the rear line pressure.
Roughly 7 years ago I built this brake test rig that allowed me to apply known forces to the brake pedal by regulating the air pressure.
Then I was able to measure brake line pressure at the front and rear calipers. It was primarily done to evaluate how the larger S10 booster changes the assist on the front and rear, but it clearly illustrates the reduced rear line pressure coming out of the stock fiero combination valve. For example with 30 psi input air on the cylinder (1.5 diameter if I am remembering correctly) the front line pressure is 1200 psi and the rear line pressure is 675 psi (44% less). Since the caliper piston areas are the same front and rear, the line pressure difference becomes the bias difference. So the bias at that specific pedal load is 64/36 front/rear.
Now the front/rear bias isn't constant as the proportioning valve (which nearly every car has, and it is included within the Fiero combination valve). At 10 psi air pressure, the front line pressure was 400 psi and the rear was 300 psi. So the bias at that pedal force was 57/43 front/rear. So this clearly illustrates that at low pedal input pressures, the rear contribute more to the braking, but as braking becomes more aggressive, and weight transfer unloads the rear brakes, the rear brake contribution becomes less (likely to avoid locking up the rear brakes -which is a bad thing, especially for a Fiero).
code:
Caliper Pressure Comparison Stock 88 Booster vs. Rodney S10 Prototype:
All Tests with Engine On, but there is data from two 88 Fieros - 3.4TDC and 2.5L
3.4TDC 2.5L Stock 3.4TDC 3.4TDC 2.5L 3.4TDC
Front Front 3.4TDC Front Rear Rear
Stock Stock vs. S10 Front Stock S10 Rear
Input Air Booster Booster 2.5L Booster Gain Booster Booster Gain
(psi) (psi) (psi) (psi) (%) (psi) (psi) (%)
5 135 n/a n/a <200 n/a n/a n/a n/a
10 320 400 -80 450 41% 300 300 0%
15 550 600 -50 700 27% 400 400 0%
20 675 800 -125 950 41% 500 550 10%
25 975 925 +50 1275 31% 600 675 13%
30 1150 1200 -50 1475 28% 675 775 15%
35 1200 1225 -25 1500 25% 700 775 11%
40 1200 1250 -50 1550 29% 700 775 11%
45 1275 1300 -25 1575 24% 725 850 17%
50 1325 1325 0 1600 21% 725 850 17%
55 1475 1375 +100 1675 14% 775 900 16%
60 1475 1400 +75 1700 15% 775 900 16%
To really illustrate the Fiero line pressure issue and why you don't want to just start throwing other calipers on, we need to do a couple of comparisons using clamp force at the caliper. This is what creates the stopping force and is line pressure X caliper piston area.
So let's stick to the 30 psi pedal input where the fiero has 1200 psi front line pressure and 675 psi in the rear.
88 Fiero with stock calipers:
Front: 48mm piston = 1.890" = 2.80 sq-in x 1200 psi = 3,364 lbs clamp force
Rear: 48mm piston = 1.890" = 2.80 sq-in x 675 psi = 1,890 lbs clamp force (bias = 64/36)
88 Fiero with C5 front and rear calipers:
Front: dual 40.5mm pistons = 2 x 1.594" = 3.99 sq-in x 1200 psi = 4,788 lbs clamp force
Rear: single 45mm piston = 1.77" = 2.46 sq-in x 675 psi = 1663 lbs clamp force (bias 74/26)
With the reduces weight on the front wheels of a fiero, having a 74% front brake bias is not going to improve your braking as the rears will be contributing less.
Now if there was equal line pressure going to the C5 calipers, which is the way most vehicles are setup, then we can just compare the relative caliper piston areas front and rear for the brake bias.
Front: 3.99 sq-in
Rear: 2.46 sq-in
So with equal line pressure front and rear, the C5 brake bias would be 62/38 front, which has slightly more rear braking that the stock Fiero bias of 64/36. The C5 runs a 12 3/4" front rotor paired with a 12" rear rotor. The distance of the caliper from the wheel center becomes a lever arm and can also be used to help dial in the brake bias. In this case, the Corvette front bias would increase slightly from the larger front rotor than rear and likely end up very close to the stock fiero brake bias is.
To get optimal braking performance, both front and rear tires must contribute as much to the braking performance as possible. To keep things safe, you do want the fronts to lock up before the rears, but lockup from the front shouldn't be that far from locking up all 4.
To get the full picture of the front/rear brake bias, to have to take line pressure, caliper piston areas, distance from the caliper to wheel center, diameter of the tires, and tire compound differences into consideration as they all contribute to the conversion rate of pedal force to braking force.
Brakes are very complicated with a lot of variables making contributions. The fewer parts you change, the easier it is to evaluate the impact to the braking system and the overall cost of the upgrade can be lower.
For example, you can see up to a 33% increase in braking w/o significantly changing the bias by keeping the stock booster, master, combo valve, and calipers and just moving the calipers further from the wheel center (front and rear) with a larger rotor upgrade. All this does is make the stock clamp force at the caliper much more effective (like using a cheater bar to break a bolt loose) without creating a lot of other issues that require additional expense to address.
[This message has been edited by fieroguru (edited 01-19-2019).]
