Smaller master cylinder bore?

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Smaller master cylinder bore?

Postby Yoshi » Mon Dec 07, 2015 6:07 pm

E30s are apparently 21mm bores. I see stuff for bigger bore sizes (23mm/25mm) but I'm looking for something smaller to make up for the lack of a booster now that I swapped over to manual brakes and killed the vac assist and the hydro assist. I really would just love a master cylinder that bolted in and went. Somehow I doubt that will happen. I see 5 lug people with M3 brakes but I'd like to get away with this for the last cash money.
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Re: Smaller master cylinder bore?

Postby M20_fever » Fri Dec 11, 2015 3:31 pm

So you just want a smaller master from another BMW? Look at the very early e30s with drum brakes. Generally drum brake cars have smaller masters than 4 wheel disc cars since the rear drum cylinders take far less fluid to actuate. Those would be a direct bolt on.

edit:

'82 318i has a 19mm master. http://www.realoem.com/bmw/enUS/showparts?id=1074-USA-05-1982-E30-BMW-318i&diagId=34_0221

'90 325i has the 22mm master. http://www.realoem.com/bmw/enUS/showparts?id=1113-USA-05-1990-E30-BMW-325i&diagId=34_0221
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Re: Smaller master cylinder bore?

Postby Yoshi » Fri Dec 11, 2015 5:08 pm

That's weird but makes sense. The masters are dual bore and have two different bores in them. The standard M20 E30 has a 22mm front 17mm rear bore. The 318 has a 19mm front and 20mm rear bore. So I'd gain pressure on the front but lose it in the back if I used the 318 one.

I wound up ordering a VW rabbit master that I will see if I can get to fit on.
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Re: Smaller master cylinder bore?

Postby M20_fever » Fri Dec 11, 2015 6:06 pm

With the brake system your building you really should be using a manual bias valve in the cockpit to tune the brakes for track conditions. On a hot day you'll want much different bias than a cool wet day, especially with fully manual brakes.
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Re: Smaller master cylinder bore?

Postby pr0craztinazn » Tue Dec 15, 2015 2:59 pm

M20_fever wrote:With the brake system your building you really should be using a manual bias valve in the cockpit to tune the brakes for track conditions. On a hot day you'll want much different bias than a cool wet day, especially with fully manual brakes.

This x1000. There are resources for calculating the bore sizing you'd need rather than experimentation until you find Goldilocks; have you done any math for the brake pressure you're looking for?
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Re: Smaller master cylinder bore?

Postby Yoshi » Tue Dec 15, 2015 3:49 pm

Fuck math. I got this master, gonna slap it on and see how it goes.
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Re: Smaller master cylinder bore?

Postby M20_fever » Tue Dec 15, 2015 5:39 pm

some things never change...
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Re: Smaller master cylinder bore?

Postby Yoshi » Tue Dec 15, 2015 7:09 pm

At this point, my whole braking setup is too complicated to accurately math out. As a result, I know what direction I need to go in and there is only 2 options for bore sizes. Bias makes sense but irrelevant for this portion of the exercise.
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