17mm or 19mm brake master cyl?

[johnvan8]

Hi all,
I'm about to change my pedal cluster so I thought I might as well put in a new master cylinder. My question:
What difference(s) will I see or feel going from 17mm stock to 19mm?
Thanks

[Rouser]

The 19mm upgrade is recommended when replacing the original 17mm MC.

The 914's were crippled with the 17mm MC because Porsche knew it would out-brake the 911.

You'll notice a somewhat higher pedal pressure, but better overall braking (once you get the whole system properly bleed; what a chore).

[bowlsby]

There is a well written article on this site that explains that the MC is sized to get the right BALANCE for the braking system.

Bigger is not better on this item. Only consider upsizing, if you increase the brake calipers and hence the internal volume needs of the hydraulic brake system.

http://www.pelicanparts.com/techarticles/914_BMW_calipers/brake_calc2.htm

[Jim T]

Yep, Bowlsby is right. That article is very nice, too. Its summarizing quote it correct:

"To obtain the greatest force between the brake pads and the rotors you will need the largest caliper pistons you can get coupled with the smallest diameter master cylinder bore that will move the required amount of brake fluid without excessive brake pedal travel."

I've rallied against the 19mm MC on stock 914-4s for more years than I can count now. Normally it falls upon deaf ears, people just simply can't believe that bigger is not always better. And people also see that a 911 has a 19mm, and a 914 a 17mm, and they immediately think the 914 is smaller because it is a "cheaper" car.

The 914 is not a super braker as it is. You need to get as much pressure on the brake pads as possible. The 19 mm accomplishes the EXACT opposite, it requires more pressure on the pedal to obtain the same amount of pressure on the brake pads. Not good. (a 19mm will reduce pedal travel a little, at the expense of requiring MORE pedal pressure to get the same stopping, not a good trade off, in my book).

The 17mm was put on the 914 not because it is cheaper than a 19mm (they would cost the same). It was put on there because it is the optimal size for the stock braking system.

[Rouser]

If that's the case, than the Tech Article "Upgrading to a 19MM MC" would conflict.

If you're not going to the wild-side of 320i/911 front brakes, the 19MM MC is a worth-while investment to upgraded braking. The 17MM is marginal compared to the 19MM.

[bowlsby]

Think about it.

With no other changes to a braking system except enlarging the volume of the MC, the bigger MC will move more fluid. Which means less pedal travel. Which means less leverage and more effort to push the pedal to achieve the same effect as the smaller MC.

Enlarge the MC only to push more fluid volume if you enlarge the internal hydralic volume. Its a balanced ratio for the best effect.

If beefier brakes only required a larger MC, why wouldn;t we all just install 30mm or 40mm MC? (because the pedal wouldn't move at all).

[Jim T]

Just because there is an article titled "Upgrading" to a 19MM doesn't mean that the laws of physics and fluid dynamics have been repealed.

If your braking system is stock (stock calipers and lines), putting a 19mm on is a DOWNGRADE. You will have to press HARDER to get the SAME amount of stopping power. That is just a scientific fact that cannot be disputed.

[Rouser]

I believe it all boils down to what kind of braking performance the 914 owner is satisfied with.

Originally, my car was still equipped (by the PO) with the complete stock system (17MM MC, rubber lines, stock rotors). The system was properly maintained and bled, but I just wasn't pleased with the pedal's travel for the given performance.

After installing a 19MM MC and DOT S.S. brake lines, I was more satisfied with the pedal/braking performance. I still run the proportioning/regulator valve (not too keen on "T"-ing it off), and I agree the pedal engages sooner with slightly higher effort.

But when I say, "STOP NOW"; I'm content with the results. Later, I might consider the 320i option; but for now, very not bad.

[johnvan8]

Thanks guys, you've all been super helpful.
I see both sides of the argument and understand the pros and cons. I guess it'll be a personal choice based on what I think I prefer.
This board is great!

[Joe Bob]

Ummmmm, the 17mm is mostly MORE expensive than the 19mm......so, that may be a factor as well.

Just my $9.14 worth of advice.

[Jim T]

I'm glad to hear the 17mm costs more. Will make it more desireable to a lot of people, because, like "bigger = better", "more $$ = better".

People can feel good knowing they spent the extra money for the rare, expensive 17mm MC, rather than settling for the cheap, common 19mm.

[bowlsby]

OOOOOOOO...Jim T you're brilliant....

[bowlsby]

Originally posted by bowlsby:
ooweeeoooweeooweee...Jim T you're brilliant....

[pbanders]

I've posted this a number of times, but once more. I have a near stock brake system on my 914. The only differences are MetalMaster pads and ATE blue fluid. I didn't bleed the system - my mechanic here in Phoenix did it, and he is extremely good about bleeding out the proportioning valve completely. I run the car on the track for driving schools and I also do autox. I've never had any problem with lack of braking power, fading, or control. For 99% of driving, the stock system is just fine. And about 99% of the problems with the system are due to inadequate bleeding of the proportioning valve. BTW, my benchmark for braking is my '98 M3, which according to R&T stops from 60 mph in something like 112 feet. While my 914 probably can't equal that, it does pretty well.

Brad Anders