Mr. WU

I currently drive an Impreza 1.8l '93, FWD. I am looking into swapping the 1.8l engine for the 2.5 RS in a near future. I would like to have your suggestions regarding brake upgrades for this engine swap (I personally believe it is best to prep the car to increase its safety before adding more power to it. This is why I'm considering upgrading its brake system).

The brakes are currently stock, but I wish to know if it is feasible to upgrade the rear brakes to a size similar to the ones in the front (stock), then upgrade the front pads to something better without changing the rotors. The reason I am considering this limited setup is that I have 13" winter tires which I will be installing every winter. I wonder if this setup would be safe enough for the 2.5 liter's power. Of course, we have to keep in mind that after the engine swap, I will be eventually adding more mods such as CAI, headers, larger exhaust (2.5-3" max), thus increasing the power maybe by another 10-15% over the stock 2.5 RS's power output.

I look forward to your suggestions/comments. Thank you in advance.

Mr. WU

ImprezaRSDriver - Thank you for your reply!

I would like to know if this setup will allow me to use 13" winter tires though. I'm afraid the disks will betoo large and the calipers would not fit inside the rims.

romanom

My brake website: http://mysite.verizon.net/romano.michael/

1. For a front heavy car the absolute worst thing you can do is have the same sized brakes in the rear. You will be locking up the rear wheels constantly. Front rear sizing is a matter of weight distribution, suspension dynamics and center of gravity of the car.

2. The size of the brakes is not directly related to the power. What you need to ask yourself is; will I be making the car heavier and by how much and will I actually be braking from higher speeds. I am going to assume the answer to both is yes.

3. First you need the tires and the suspension to actually make the brake upgrades useful. More brake torque means very little if the tires can't translate that to stopping force. Also, if the dive underbraking is already severe more braking will make it worse and cause instability. This increased dive will also cause the front tires to become overwhelmed.

4. #3 also holds true for the rear wheels. When the nose dives weight comes off the rear tires decreasing the amount of brake torque needed to go into lock-up. This will cause instability. Now say you do upgrade the suspension then you will still need more rubber out rear to translate the increased brake torque into stopping force.


You can use the below equations to get you an idea of what size brakes you will need:

Equations:
(for ease and consistency try and use meters and kg)

NOTE: Equation 1 does not mean more weight you have more stopping force, it is just to calculate the stopping force required. As you can see in Equation 8, the larger the stopping force is the larger and more aggressive the brakes need to be.

1.
stopping force total = weight of car * longitudinal coefficient of friction of tires

2.
Front Force = (weight front + total weight) * tire friction * height of CG * (1/wheel base)

3.
Rear Force = (total weight - rear weight) * tire friction * height of CG * (1/wheel base)

4.
% front = Front Force/Stopping Force

5.
% Rear = Rear Force/Stopping Force

6.
Area of Master Cylinder * pedal ratio = M
Area of front or rear caliper piston= N
Mechanical Force Ratio= M/N


7.
mechanical force ratio front = mechanical force ratio * %front

mechanical force ratio rear = mechanical force ratio * %rear

8.
Stopping Force (front or rear) = pedal force * brake pad coefficient of friction * mechanical force ratio * (1/radius of the tire) * brake rotor effective radius
.....solve for the parameter you need.

-for pedal force use 667N or 68kg

To measure pedal ratio (C)

Mr. WU

ImprezaRSDriver - I checked for the WRX brakes. Only 16" and bigger rims will fit them.

go go go - Yes, I currently have drum brakes in the rear.

romanom - I just had a look at your website. Very interesting! It contains quite a lot of info. The reason why I was thinking of replacing the drums in the rear to disks which match the size of the ones I currently have in the front is because they are the only ones which will fit for 13" winter tires. I was aware of the danger of doing so which is to throw off the balance of the braking system (more braking in the rear). However, I was thinking of re-balancing this by increasing the braking power at the front with better pads. I was wondering if this setup would do the job.