steve geden

Hi,
First post from me. I have a 1990 C2 which I bought last year. I have been using this forum alot and find it excellent.
Amongst, many other things that need doing, some for neccessity some for fun, my rear 2 pot calipers are lifting and need to be refurbed. I thought, rather than do that why not replace them with 4 pots, no plans to change the front. I've considered the options based on simplicity and cost:

1. 964 C4 or late C2, part No. 928 352 421/2 03. These cost £410 each from Porsche however (think I'll avoid the ebay lottery).
2. 993 C4, part No. 993 352 421/2 21. Cost £252 each from Porsche but are silver. Piston sizes are same as 964 - 30 and 28.
3. 993 C2, part No. 993 352 421/2 00. Same price as C4 I believe, in black, but pistons are bigger - 34 and 30.

I know there is a lot of info on here on this subject, particularly from Bill Verburg, I've tried to digest it all but can't find the answer to my question, which is:

- Can I use the 993 C2 rear calipers, wihout changing anything on the front?

The car is used for the road and eventually occasinal track, when I sort out a few other things. I'm aware of the need to replace the proportioning valve.
Thanks,
Steve

boxsey911

Steve, the 993 C4 rear calipers will work directly with your normal front calipers (as long as you change the proportioning valve as you say). The 993 C2 calipers will also bolt straight on but are normally used with a big black or big red upgrade to the front. You'll have too much rear bias if you stay with your normal fronts when using the 993 C2 rears.

p.s. I'm amazed the 993 C4 calipers are so much cheaper than the 964. Now you just need to decide if you're going to paint your fronts silver or the new rears black.

ilko

The 993 rears will give you too much rear brake bias. They have larger pistons than the 964 4-piston rears.

anto1150

Steve (Boxsey911),
how come you removed the "Carrera" script?

SORRY for OT.

sundog

I went with the new 993 C4 rears, and changed the bias valve. They work great. And I'm just running with the silver in back and black up front. It's not a show car.

slips2

Just last night I finished replacing my 1990 C2's rear 2-piston (40mm) calipers with 4-piston (34/30m) 993 C2 rear calipers:





The rear calipers replacement was one portion of an upgrade project which also included Bilstein HDs, H&R springs, brake lines, & pads on all four corners, plus front rotors:





Lastly, the upgrades were concluded with Meteor Gray Metallic 17" Cup IIs mounted with Michelin 225 & 255 PS2s:



Haven't replaced the stock proportioning valve, and may not. With everything else unchanged (brake rotor physical dimensions, brake circuit pressures, etc.), the applied hydraulic force and thereby brake torque produced by these calipers are a function of their piston hydraulic cross-sectional areas. By my calcs, the net rear brake torque produced by the 993 C2 4-piston (34/30mm) calipers is only 6% more than that of the 1990 C2 2-piston (40mm) calipers. I submit this 6% increase in rear brake torque with the 993 rear calipers is offset in my installation by the following upgrades:

1. the H&R springs lowered the car 35mm at all four corners resulting in a lower center of gravity, thereby producing less forward load/weight transfer during braking. More weight remaining on the rear tires can promote more tire torque which can accomodate increased brake torque,

2. the stiffer H&R springs further limit forward weight transfer during braking, with the same benefits as noted in Item #1, and

3. replacing the 225mm stock rear tires with 255mm widths increases the rear tires' torque limit.

Surely these upgrades in sum more than offset the 6% increased rear brake torque, yes?

AndrewLang

You mentioned this is going to see some occasional track time. Yes I'm sure some of the other upgrades you did might offset the need to change the bias valve. But if you're planning some track time in the future why not upgrade the front calipers as well down the line to the 993 C2 calipers? Better yet, take it out on the track how it is now and see if you like slightly more brake bias. Some people like to have the rear end rotate a bit while trail braking. Might not be great for a new driver but it can also be a bit more fun!

Bill Verburg

It's ok to use 993 30/34 rears w/ stock fronts on a 964 but you do want to set the car up for it.
the lower the better
the stiffer the better
you want an effective lsd, a GT or motorsports 40/65 is very desireable
you want to leave the stock p/v in place

964russ

I recently fitted 4 pot rears (964 c4 type) to my car.
To replace the tiny standard twins, along with the fronts bein re-furbed i cant fault the brakes on the old flash.
They need a a good shove of pedal but allways provide wot iam after, loads o stop!.

Good pads and fluid are the way forward, unless u run mega power, imo.

boxsey911

Don't worry Antonio, it's still there....that's an older photo in my most recent avatar. I'll replace it with an up to date one when I get a nice track shot this year

steve geden

Hi, thanks for the help, much appreciated.
I'm tempted to go for the C2's with the bigger pistons and keep the stock p/v in place. More questions though:
1. How much do you think the rear brake bias will affect daily driving before I deal with the lowering and stiffening?
2. If I change the front to 993 C2 calipers, what else would I have to do? Also would the p/v need to be changed?
Cheers,
Steve

Bill Verburg

The only time it will make a difference is in extremely hard or emergency braking. It wouldn't be any worse than stock 3.2 Carraera

keep the existing p/v

slips2

Our 964's brake bias (HBR) with the 993 C2 rear & stock front calipers is 1.41 by my calcs, compared to the stock 1990-91 C2 brake bias ratio of 1.50. As Bill mentions, no worse than the stock 3.2 Carrera which has a bias ratio of 1.31. Furthermore, unlike the 3.2 Carrera, the 964s "should" be protected from rear brake lock-up by the C2's ABS during rare emergency/hard braking on the street. However for the common-occurence hard/threshold braking on the track, and as additional safety on the street, accomodating the increased rear bias with car setup s/b preferrable.

Post pictures, it's required ...

airfan

slips2, I am also considering the 993 C2 rear caliper upgrade. What's your impression so far, an did you decide to keep the stock p/v?

christallon

I am having late model 964 4-pots installed at the shop right now. I am assuming I don't have to mess with the proportioning valve. Am I right?

Thanks