RE: Brake Master Cyl Dimensions

["Weiss, Dave " <Dave.Weiss@mkg.com>]

91 V8 5-spd has 1" bore on brake master (25.4 cm).  Rear rotors are 
vented, same as S4, and I think 91 200Q 3B.  I thought the S4 and 200Q 
sedans used a smaller dia. MC, 23.8 or maybe 22.2 cm?  Seems like the 
non-turbo type 44's were even smaller.

23.81 cm = 15/16".  Interesting that these are still SAE dimensions. 
(Can't say English any more, can we?)

All other things equal, bigger bore MC gives harder pedal feel (for same 
force applied by foot, it is distributed over a larger area, so 
hydraulic pressure is lower).  To get the same pressure in the brake 
lines, you need to push harder on the pedal with a larger-dia MC.  For 
1" ID vs. 15/16", you have to push 14% harder (area related to square of 
dia).

Of course, all things _aren't_ necessarily equal between different 
models.  Could be different levels of power assist, different caliper 
piston bores...  And of course there are aftermarket hoses that don't 
give as much as stock ones.  (Does the servo give linear response?)  
Decel valve might affect feel?

Bigger bore MC should move more volume of fluid per stroke length, too.  
 Are the "19/17" and "18/18" related to stroke, F/R, ...?

Steve Buchholz wrote:

>>  my BDTD is on my 86 5ktq.  There were two MC's that fitted my car (I
>> used
>>  them both on the same servo) pn 443  611 021 size 23,81X19/17 and pn
>> 447 611
>>  021 size 25,40X18/18.  The second one, the wider diameter is for the
>> wagon.
>>  I used the smaller diameter one first with no noticeable braking
>>  characteristics.  Then I found the larger diameter one and decided
>> to change
>>  to it since that was what the book called for for the wagon.  Again,
>> I
>>  didn't find any braking differences between one and the other.

<SNIP> 
 
> ... well, I did look into the fiche last night and confirmed the 
> numbers that Lino posted.  In several places on the 5kq and 200q fiche 
> there are brake parts that are called out as being for the Wagon only 
> ... but in a few places it states that the parts were for "vehicles 
> with higher permissable total weight" ... I didn't happen to see a 
> "towing" package listed as an option, but as I suspected you should be 
> able to tell which parts the car should use by seeing if the rear 
> rotors are vented.

Dave Weiss
91 V8
98 CR-V