Ragin' Bajan

This may not be the most frequent posting topic, but it's probably a contender :-) This is no doubt dealt with superbly in Adrian's upcoming book, but alas I can't wait....

I've a '91 964, and as all four rotors are worn and need replacing, I'm considering a brake upgrade, using my soon-to-expire PCA gift certificate. I attend about 7 or so track events per year and it's for this purpose I'm considering getting rid of all that pesky extra cash that is cloying my pocket. In the last three track events I've found the brakes not stopping me as quickly as I'd like. I like braking late and deep, and I'm currently right at the limit (and once over) of my car's braking capability. I'm running r-compound tires and it certainly feels like the brakes are the weak link in the system (well, after the driver of course). I'd like an extra margin of saftey for my current level of driving. It's either that, or I'm going to have to keep it a bit more sedate <img src="graemlins/crying.gif" border="0" alt="[crying]" /> .

Btw, I'm using Hawk Comp+ pads, which are apparently relatively gentle on rotors, but have eaten through mine in the past year. Getting more aggressive pads and swapping them out at the track is an option I guess, but not one I'm too eager to pursue--I'm also not sure of how much better they could perform than a brake upgrade.

I've done a bit of searching and asked a few questions here and there, but there are, unsurprisingly, different and sometimes conflicting answers.

So first, let me list my only limitation: money. What this means is, I've 2 sets of 16" rims and don't want chuck 'em and buy 8 new 17" rims & rubber, so any brake solution need to fit in 16" stock rims.

As far as I can tell, my options are something like:

Either 993 or 928 S4 calipers on the front like
<a href="http://www.proaxis.com/~tubbs/brakes.htm" target="_blank">here</a>.

Possibly also 964 turbo calipers/rotors, but they seem equivalent performance wise and more expensive than the 993/928S4 approach.

Upgrade the rear two piston calipers to four. Question: can I use the front calipers on the rear rotors? Do 928S4's fit? Would rear 993s be better?
Or, do not upgrade the rear calipers--is this advisable? I've seen several references to it being the first brake component of the '91 to upgrade, and other references (as above) where the rears were not upgraded.

If I can use the front calipers on the rear, do I need to add in an adjustable bias valve? Is this even advisable? I've been told by one mechanic (who might be doing the upgrade) that it's best to match the calipers eg 993's front and rear. Now, I know he also has a set of front+rear 993 calipers he wants to unload, so I'm not sure how much salt to apply to his opinion.
He said I'd always be playing with the bias valve, but this seems counter intuitive. However, I've been wrong before and would love knowledgable opinions. I'd also like to make sure the system is done properly--I don't want to do a half-assed job.

Another possibility I suppose is to fit some brake cooling ducts, but I've never heard good things about the 3rd party ones, and I don't think they would really help at <a href="http://www.na-motorsports.com/Tracks/Seattle.html" target="_blank">Pacific International Raceways</a> since there's lots of time for the brakes to cool off.

Also, assuming I can use 993/928S4s on front and 993/964f on rears (any other options?), can I get away with my current master brake cylinder?

Another, perhaps basic question: is it worth it? I've heard 1) "Yes! Yes! Yes!" and 2) "no, after improving my driving, I found it wasn't really justified".

Does anyone have any empirical data on what these brakes can actually do? Would a 993 give me, say, 10% reduction in stopping distance at 100 km/h?


Questions, questions questions!

Thanks for any light you guys can shed on the subject!

--Chris

Mr. C4

Chris,

As per my experience, you will find it difficult to "outbrake" standard size 964 crossdrilled/grooved rotors combined with a race/trackday pad (pagid, EBC, Hawk), stainless steel brakehoses, a good DOT 5 brake fluid and 4 piston rear calipers.

Secondly your front calipers will not be suitable for rear use and I believe 993 calipers require 17" wheels to clear the calipers.

After having driven the Nurburgring for 3 consecutive laps (1 lap = 20km) without any loss of brake efficiency, above setup get's my approval!

Ciao,

Johannes E.

Bill Gregory

Chris,

First, going to larger brakes won't stop you quicker. They'll give you more heat sink capacity, so your fluid won't boil as quickly. Once your wheels lock, there's no more braking capacity, no matter what calipers you have.

You should have already removed your brake backing plates, and definitely get a 4 piston caliper on the rears. Don't know if the fronts can just be plugged onto the rear, however.

You can fit 928 calipers on the front, with the proper adapters. They may require spacers with standard 16" wheels for clearance. You definitely should have the 4 piston calipers in the rear. With 928 calipers and 4 piston's in the rear, I'd either go with the Turbo 60 bar bias valve (highest bar valve offered by Porsche), a variable bias valve, or even no bias valve (ala cup cars). I know someone who runs the 60 bar valve with 928 fronts and 4 piston rears and is pleased with the balance. He uses the stock 964 master cylinder.

For brake setup, I have stock 4 piston calipers on each wheel, slotted rotors in front, all 4 rotors cryogenically treated (frozen rotors), titanium backing plates in front (between the pad and pistons), ducted air from the front fog lamps into the wheel wells, and Pagid Orange pads (I should swap them out between events, but don't). I did around 13 track days this year, and found my brakes good for around 2-3 days before bleeding them. I haven't warped any rotors (a problem some have), and while there is some wear, the rotors won't require replacement until at least through next year.

As a related aside, I used the Kumho Ecsta 700's this year for track tires, and after 13 track days, they have alot of tread left. Despite the recent Pano 18" Kumho test, these will start me off next year, I'd guess another 4-5 days out of them.

Ragin' Bajan

Hi Bill,
sorry, I forgot to mention: I don't think I'm locking up my wheels. I assume that if I did the ABS would kick in, but it's not hence I was thinking I need (well,would like) more braking power.

Another question: I'm afraid I don't completely understand the advantage of the rear caliper upgrade from two to four pistons. I've read somewhere that the reason the rear calipers were changed on the 92 had more to do with consolidating the C2 & C4 production parts, rather than a braking issue. On the C4 they were needed just so that if one of the braking circuits failed, the rear brakes would still work (C4s apparently had a different brake setup). That's what I've read, anyways.

In addition, whenever people have said you can upgrade your rears, no mention is made of changing the brake bias. From this I assume that the ratio of braking power between front and rear remain the same, which would support what I've read. What gives? Or was there not enough braking force in the rear for pre 92 models?

joey bagadonuts

[quote]Originally posted by Ragin' Bajan:
<strong>Getting more aggressive pads and swapping them out at the track is an option I guess, but not one I'm too eager to pursue--I'm also not sure of how much better they could perform than a brake upgrade.</strong><hr></blockquote>

Chris, why the reluctance to switch out pads? It's soooo easy! Probably takes less than five minutes per wheel. Honestly!

That said, you should really give true racing pads a go before pursuing the upgrade route. Pagid Orange or Hawk Blues will provide a noticeable difference and were designed for tough track work, unlike other pads. And as Bill mentioned, fresh fluid after a few events is critical.

Also here's my favorite source for brake data:
<a href="http://www.pelicanparts.com/MotorCity/wmv/brakes.htm" target="_blank">Bill Verburg's Website</a>

Bill Gregory

[quote]<strong>Another question: I'm afraid I don't completely understand the advantage of the rear caliper upgrade from two to four pistons.

In addition, whenever people have said you can upgrade your rears, no mention is made of changing the brake bias. </strong><hr></blockquote>

The 2 piston 90-91 C2 rear caliper uses smaller pads with 112 cm2 area vs the 92-94 C2 and all C4's which use the 4 piston calipers with 172 cm2 area pads. While I can't talk to what you've read, there's value to having a larger pad with dual pistons versus a smaller pad with single piston.

The brake bias valve on the 90-91 C2 was 45 bar. Going to the 92-94 C2 or all C4's the bias valve is 55 bar. If you need further balance, you can update to the Turbo 60 bar valve.

Dave R.

Bill,

Why would 4 pistons be better than 2?

And why would more pad area be good (lower heat loading per unit area on the pad?)

Ragin' Bajan

Joey: well, I think my only defense is ignorance. I take it you have an otherwise stock setup, and you're satisfied? Hmm, it certainly would be one of the cheapest of the solutions. How long do your rotors last with this approach?

Jack667

I get scared about the brake bias. This can lead to trouble. My advice is to leave your calipers as-is for now. First, get stainless brake lines, use Pagid Orange or other comparable pad, get Super Blue or comparable fluid, consider cross drilled rotors (they look great too), and consider the brake cooling kit. Flush fluid often.
This should resolve your issues, and when combined with additional seat time you may find that is all that you need.

Maybe you can use your coupon for the stainless lines...

joey bagadonuts

Yes, I have a stock setup and it works fine. I had some worn rotors which came with the car, so I replaced them a few months back. Not sure how long they'll last, but it sounds like a full year of DE events (20 track days) is typical. It doesn't sound like you're overcooking them, which is easy to do when braking deep, so I think race pads may be all you need.

As for swapping them out, it couldn't be easier. You take off the wheels, flip back the spring (needle nose vise grip is handy), pull out the old pads, push back the pistons and drop in the new pads. It's THAT simple.

Good luck.

Bill Gregory

[quote]<strong> First, get stainless brake lines, use Pagid Orange or other comparable pad, get Super Blue or comparable fluid, consider cross drilled rotors (they look great too), and consider the brake cooling kit. Flush fluid often.
</strong><hr></blockquote>

Good list of brake considerations. Couple of comments:

FWIW, I'm not a proponent of stainless lines unless you're racing and plan to replace them on a regular basis. While stock rubber hoses may give away a bit of brake feel to the stainless lines, the rubber lines have a life of 10 years, give or take.

If you consider cross-drilled rotors, do not get ones that have been drilled. Only get ones that have the holes cast-in, as Porsche rotors are. Drilled rotors are prone to crack easily, which ruins the rotor. Rotors with cast-in holes will eventually also crack, but you'll get much longer life from them. I prefer slotted rotors on the front, as the heat sink value is much higher than rotors with holes.

Bill Verburg

As Bill G. stated bigger brakes won't stop the car with any greater authority than stock, they will let you do it for a longer period of time. Stickier tires are needed for greater deceleration #S

I didn't hear the usual complaints about boiled fluid(pedal to the floor) and glazed pads(hard pedal that doesn't do anything), which are the primary signals that bigger rotors (at least) are needed.


Currently your car has a hbr of 1.496(a tad rear biased), a 45 bar p/v(to alleviate the slight rear bias), 298x28 and 299x24 rotors

If you only change to the 4 piston rear calipers with no other changes your hbr goes to 1.72 which is good if the p/v is be removed.

The biggest caliper/rotor package that will fit under 16" wheels is the setup from a 993 which consists of 993/S4 caliper and 304x32mm rotor this caliper shoul be matched in the back w/ the 993 4 piston rear caliper used w/ your stock rotors. Note (1)the 993 rear 4 piston is externally identical to the 964 rear 4 piston but is very different hydraulicly(the important part)(2)the 993 front is hydraulicly similar to the S4 but has a different mounting configuration which can be altered to work at the front of a 964

The bigger rotors will provide a tad more mechanical bias to the front but it's not an issue, they will also increase thermal reserves(more track time w/o boiling fluid), the 993 calipers have ~31% more pad area which reduces the per/unit thermal load

I also agree that cooling ducts, good pads such as the Pagid Orange, fresh fluid and if drilled rotors only the factory ones other wise slotted should be used.

There were 2 different 964T setups, early ones used the 993/S4 caliper on 322mm rotors later used Big Reds on 322mm rotors ,neither will fit in 16" rims The difference in these calipers is only in their pad height, the Big Red being ~10mm taller giving ~20% larger pad area.

SteveW@stig

[quote]Originally posted by Bill Gregory:
<strong>

I prefer slotted rotors on the front, as the heat sink value is much higher than rotors with holes.</strong><hr></blockquote>

Where do you get your slotted rotors/disks from? I have only ever seen drilled???

Bill Gregory

[quote]<strong>Where do you get your slotted rotors/disks from? I have only ever seen drilled???</strong><hr></blockquote>

My rotors are from Diversified Cryogenics in Minnesota (www.frozenrotors.com). They freeze OEM or Porsche rotors, then cut the slots in them.

Ragin' Bajan

First of all, thanks for the informative answers guys! A much more reasonable, and by all accounts effective approach, will be to get more aggressive pads and swap them with my street pads for the event.

To date, I've seen three brake duct cooling kits (no doubt there are more). Two from Porsche, and one after market.

The <a href="http://home.attbi.com/~jbalthasar/cbf.htm" target="_blank">after market</a> one appears to be a zap strapped scoop that redirects air to the rotor. From it's appearance I'd be a little concerned that the scoops get knocked off during daily driving.

The first Porsche one are the well known cooling ducts which replace the fog lights and have a tube feed the forced air to the rotors. Nice clean solution. I'd have to have the headlights wired to become my new daylight running lights (no biggie). Only downside is cost. The unpainted scoops were quoted to me as $695 (Cdn), not including a $150 required connection kit. Plus painting.


However, I suspect they really mean these ones with the integrated lights:

Which claim to be for the 993.
So I may be able to get the plain ducts for 200-$300.

The third, and cheapest option, is basically two scoops of plastic as described in this <a href="http://www.pcasd.org/tech/tech_apr.shtml" target="_blank">San Diego PCA tech report</a>

Porsche part numbers 964.341.083.01 and 964.341.084.10. Apparently $11.38 US each! Of course Wiessach wants $63.00 each <img src="graemlins/cussing.gif" border="0" alt="[grrrrrrr]" /> . Assuming I could find them for the cheaper price somewhere else, they would worth trying out.

Anyone with a brake cooling kit they highly recommend?

Thanks!