Brake problems in an autocross car

[91Saleen#15]

My s52 powered e30 is exclusively for autocross. It has the standard e36 M3 5 lug conversion in the front with 318ti 5 lug conversion, e34 rear brakes.

The standard e30 master cylinder didn't seem to be adequate for the increased caliper area of the bigger brakes. I tried an 88 735i master cylinder, but it seemed to require too much pedal pressure. The unit came from junk yard, so I can't guarantee that it was in good working order, but it seemed to be ok.

I switched to an e30 M3 master and initial tests seem to indicate that it will work well enough. I've only tried it out on the street so far. The next race isn't until Sunday. The e30 M3 master is 23 mm as far as I know. There is a 23 on the casting.

My question is: is anyone here running similar brakes with a 25mm master cylinder? Could it be that my booster (stock 89 e30 booster in stock position) is weak? From other posts, I concluded that it should have worked.

[SmokeE30]

So you have an ix booster? Or standard e30 booster?

[91Saleen#15]

So you have an ix booster? Or standard e30 booster?

stock 325is booster

[evandael]

stock e30 is 22.2 (early models had even smaller 20.??)

stock e30 m3 is 23.8

stock 735/750 is 25.4


is this problem related to your master cylinder pushrod issue i read about in another thread?

[91Saleen#15]

stock e30 is 22.2 (early models had even smaller 20.??)

stock e30 m3 is 23.8

stock 735/750 is 25.4


is this problem related to your master cylinder pushrod issue i read about in another thread?

no... that was a temporary setback... I fixed that.

There is a difference in the 750iL that might explain why it doesn't work as well as the e30 m3 MC... The 750 MC has a 23 mm piston for the rear brakes whereas the stock e30 MC and the e30 M3 MC have only a 17 mm piston for the rear brakes. Percentage wise, this is a bigger difference than exists between the front pistons. I am thinking that the initial pressure applied to the brake pedal creates an earlier contact to the rear brakes than the front because of the relatively larger volume of fluid moving in the rear. This contact creates relatively weak braking from the rear and the subsequent need to apply additional force to engage the front brakes and achieve the expected braking level. The strong resistance of the rear piston to engage further fights the activation of the front brakes and gives the illusion that the brakes are "stiff".

[E30stan]

no... that was a temporary setback... I fixed that.

There is a difference in the 750iL that might explain why it doesn't work as well as the e30 m3 MC... The 750 MC has a 23 mm piston for the rear brakes whereas the stock e30 MC and the e30 M3 MC have only a 17 mm piston for the rear brakes. Percentage wise, this is a bigger difference than exists between the front pistons. I am thinking that the initial pressure applied to the brake pedal creates an earlier contact to the rear brakes than the front because of the relatively larger volume of fluid moving in the rear. This contact creates relatively weak braking from the rear and the subsequent need to apply additional force to engage the front brakes and achieve the expected braking level. The strong resistance of the rear piston to engage further fights the activation of the front brakes and gives the illusion that the brakes are "stiff".

Any updates on your brake situation?
Im dealing with a brake pedal that requires maybe 5 inches of travel before the brakes really bite down. I have e36 m3 brakes all around with the 750i 25mm BMC.

[91Saleen#15]

Any updates on your brake situation?
Im dealing with a brake pedal that requires maybe 5 inches of travel before the brakes really bite down. I have e36 m3 brakes all around with the 750i 25mm BMC.

Sounds like you have a brake bleeding problem. With that size master cylinder, you should be able engage the brakes with less travel.