Right then,

A bit of a technical one, but I hope someone can help out.

I am about to install some Porsche 6 pots 335x32mm onto the front of our GC8 MY00 track car, then move our current Compbrake 4 pots 295x20 onto the rear... once I have designed and made all of the brackets etc!

Now by doing this i'm increasing the piston surface area by quite a bit over standard, so i'm thinking we need to upgrade the master cylinder to help things out a bit...?

Is the newage master cylinder larger to start off with, and does anyone have any info regarding cylinder bore sizes from 01 upwards...?

Cheers,
Kim.

 

Front 4 Pot caliper pistons will likely be to big to use on the rears.

STi 2 Pot Rear Brembos on the rear would work well with a bias valve fitted.

 

Are they front Porsche calipers pictured?

 

Yes they are the Porsche 6 pots that i'll be using...

We have the sti 2 pots on there now (red subaru type, not brembos), just thought with putting the 6's on the front (adding 2 pistons effectively) then adding 2 on the back kept things even...?

 

Doesn't work like that. Best to take a look at the piston sizes that AP do on their rear kits and compare it to yours.

 

Easy, just work out the areas its not hard maths.

I put Porsche Brembo's on my Integrale

 

AP use 4 pots on their rear kit (CP7615) with 41.3/38.1mm pistons generally, and that is exactly what our compbrake 4pot has...?

So i'm not following why you say it doesn't work like that. Surely if you increase the front axle braking capacity by 20% then increasing the rear axle to match makes perfect sense...?

Plus the impreza is already biased to the front a lot IMO... so some more rear axle braking couldn't hurt, especially on the track. As long as it's not pushed to far obviously.

 

You seem to be ignoring M/C size and pedal effort.

Yeah fit bigger pistons front and rear, but the pedal will hit the floor!

 

Ignoring it? Im sure that was the whole point of the question based around M/C size etc, I doubt it will hit the floor but there may be some more travel on the pedal, best to do some research I think there are some quite good explanations on the wilwood site describing master cylinder ratios etc and what works best with what. Of course the other way is try it and see how the pedal feels and how well the brakes work

 

Lots easier to do the maths.

There is a big difference going from 1" to 1/1/16" M/C of a new age car when using brembo's on an early car

 

Thanks Trevcoss... I'm not ignoring the M/C size at all, my initial questionwas should I be looking at a different M/C size..?

I have done some calculations for the bias using the piston/disc/pad sizes for all three setups, apollogies as they're in inches but here they are...



As you can see the proposed setup actually brings the car closer to the original Subaru bias setting, providing I have my maths correct that is!!

Also am I correct in saying that a bigger M/C size may increase pedal feel/hardness but will actually decrease the torque seen at the discs, and visa versa with a smaller M/C... softer pedal but more transmitted torque...?

 

Something up with the 4 pot one pot setup as I cant see the single pot calliper having a larger area than the two pot callipers

 

I thought that but apparantly it takes into account that a one pot value is a floating caliper, therefore the force is shared either side of the disc...?

I'm not totally convinced by it though... however a simple division of that value in half seems to make things look more like it??

 

Nah you need to make the area read half, halfing the dia reduces the area by more than half

 

Right then... we're getting somewhere now that someone who knows what they're on about is helping!! (Cheers Trevcoss, i'm clearly **** at maths!!)

Now it looks like our first upgrade retained similar bias values, whereas the proposed setup will give us proportionally more rear braking...?

The question is, if you increase M/C size it actually decreases the torque seen at the discs but would improve pedal feel...? I know this is all academic, and I will know more when the installation has been done!!

By the way Trevcoss, I'll be seeing you guys on saturday as we're bringing our engine down to you for a CDB 2.1 stroker rebuild...!! small world.

 

Interesting & Awesome.

 

So has anyone changed to a later master cylinder and noted the results...?

I'm assuming 172sport has done, what was the reason you did it and what was the difference...?

 

Pedal travel was long, not excessive but not too my liking.

Fitting a late one gave a nice hard pedal, more feel and consistancy.

Std was 1"

later model 1/1/16"

 

I would go for your plan of putting the 4 pots on the back and see what the feel is like, it may be that you need a bias valve and or the bigger cyclider to get it all properly set up and feeling correct or it may be fine, I will probably be in tomorrow changing the exhaust on the waggon if im there give me a shout and we can have a chat about what may be best, we have lots of customers running 6 pots front 4 rear etc so can find out what if any mods they have done.

 

Thanks 172sport, that's good info... have just picked up a late M/C 1 1/16" off fleabay for £20 so will see how it all pans out and use that if neccessary.

Spot on Trevcoss, we should be down there for approx 11:30am ish so will ask for you and we can have a chat... top man!!

 

so for 4 pots its bes to use a 1 1/16",how can this be seen ? model of car etc ?

cheers

 

Early cars have 1"

std 4 pot cars have 1/1/16

I used one off a v7/8 sti

 

In that case I have a spare one available, as i've just checked and we already have the 1 1/16" fitted to our car!! (MY00 GC8)

Should have checked before I went and bought one...

Am still not convinced that you would definately need a larger m/c due to a brake upgrade or increased piston size, as the new 08- WRX with Brembos has larger piston areas front and rear than even my proposed Porsche 6 + 4 setup, and that is using a 15/16" m/c...?

Bear in mind that a larger m/c will actually output less torque to the rotors using the same pedal input pressure, although will decrease pedal travel as it shortens the stroke.

Therefore ideally you want to have the smallest m/c you can get away with, bigger is certainly not always better in this case!!