911 Brakes 3" vs 3.5", What am I missing and Options

[jesiv]

Ok so I am upgrading to a 911 suspension (Seperate Thread http://www.914world.com/bbs2/index.php?showtopic=87337 ). Then I hear brake caliper bolt spacing 3.5 vs 3.0. So (IMG:style_emoticons/default/WTF.gif) WTF is 3" vs 3.5" Am I making a mistake going with 3.0 vs 3.5? I really don’t understand the issues. Do I care?

Thanks for real world opinions!

Regards,

James

[McMark]

It depends on what you want to do with the car. What kind of motor you're putting in etc.

Basically, more stress (big motor, track time, etc) means more brakes.

3" = 'low' and 'medium' brakes
3.5" = 'medium' and 'high' brakes

[ArtechnikA]

3" = 'low' and 'medium' brakes
3.5" = 'medium' and 'high' brakes

To expand a bit, in case you were wondering where the 'M' and 'S' brake classifications came from, they are Ate (Itself an abbreviation for Alfred Teves) designations for the size of the caliper.

Mitte - middle/medium duty
Schwer - heavy duty

There are good brakes to be had in both sizes. a /4 on 'normal' street tires can lock 'em up repeatedly with stock calipers, which is -probably- all the brake you need.

The small caliper and unvented disk/hub assembly is quite light which contributes to the 914's nimble feel and good road-holding.

For more power and increased fade resistance (i.e. V8 or trackday) the vented rotor has its benefits. 911 'M' calipers are plentiful and cheap. It's what 914/6's came with up front.

Be aware that /6's also came with bigger pistons in the -rear- brakes, so if you go increasing front caliper size, you're tinkering with the front/back brake bias as well as approaching the hydraulic limits of the 17mm 914.4 master cylinder. This is all manageable, but it is additional stuff to think about.

You -can- put truck brakes on a 914 - after all, the 935 brakes were basically a standard heavy truck brake Ate was asked to make in aluminum rather than the truck's cast iron... But your car won't stop any faster.

And all this presumes you are going 4-lug, 'cause AFAIK, there are no 4-bolt 3.5" bolt spacing struts.

(BTW - that's a real 3.0" and 3.5" measurement, one of 4 or 5 English unit parts that are international standards. Another thread for those...)