zoso_turbo

Looking for perspective on cross drilled vs slotted vs solid rotors for a 85 951 that will be used for 20% track and 80% street.

Also, looking forfeedback on various vendors for rotors and pads.

thx

ZT

RKD in OKC

Solid are standard rotors.

Cross drilled are the holey rotors you see. They should be cast not actually drilled for strength around the holes and are mainly for water egress. They do help to cool the rotors some but under track conditions should be inspected frequently for stress fractures around the holes.

Slotted have sprial grooves or slots cut in the rotor. They help cooling by venting the gasses and brake pad dust away from the pad/disc friction area.

The recommendation would be dependent on your driving style and climate. If you drive the car a lot in rainy or wet conditions the cross drilled rotors would be great for street/track. If on your track days you experience brake fade due to overheating of your brakes, slotted will do better than drilled and be less worrysome as you won't need to inspect for fractures around the holes.

RPG951S

Aftermarket cross-drilled rotors should be avoided less it's purely for cosmetic reasons. They will crack easily under moderate to hard use.

Since the factory did not offer cross-drilled rotors for your application(pre-S4; you'd need to upgrade to the 968-M030(S4) front brakes as a minimum, or more likely 993 BigReds) you should stick to solid rotors. I seriously don't think the factory rotors are 'cast'-- the casting mold would be rediculous. The factory rotors are carefully drilled along the rotory vanes, with very small chamfered(sp?) holes, to minimize stress cracks. Even then, the factory specifies that small cracks (that do not connect holes) on rotors are considered "acceptable".

I've never noticed any difference running solids vs slotted vs stock, performance wise. Nobody, ever, has produced a test showing that modify a stock rotor by 'removing' material will improve braking performance. You can lighten a rotor by removing material (which can improve suspension/acceleration performance), but your not going to improve braking performance by doing this!

I don't think any of the other 'modifications' to stock rotors are worthwhile, save your money and buy good quality solid rotors.
Spend the money on good quality race pads with high temperature fluids and you'll be fine.

aeronautica86

if this car is used primarily for the street, go with stock rotors - cross drilled rotors develop stress fractures around the holes and will eventually crack, and for the street, the extra cost of slotted rotors really can't be justified by a performance increase in street conditions

if the car is used primarily for the track I'd go with slotted rotors - no stress fractures from holes and extra gas dissipation capacity

Eyal 951

Solid is the way to go. The only reason I would ever consider cross drilled is for street, and thats just if I was worried about my rotors getting wet. The purppse of cross drilled rotors was reducing weight.

RKD in OKC

When I had my 88 TurboS I had a horrible braking habits and boiled my brake fluid frequently. I typically had to re-bleed my brakes after a track session. And that was using super blue. I went slotted rwhen I replaced the factory rotors. They often turned blue, and the temperature color changing paint said they were getting to 800°F, but I didn't have any more fade and boiling fluid with the slots. I had exactly the same issues with the 90 928 GT I owned. And the slotted rotors cured it's brake fade and boilling fuild. This is why I said it depends on your driving.

My 2001 BoxsterS's factory "drilled" rotors were cast. The chamfer in the holes was rough casting and not machined. The rotor face was machined and the bore of the holes were machined, but the chamfer was still rough casting. In the specs for the BoxsterS it stated the drilled rotors were for better braking in wet conditions. This is why I made the reference to lots of wet driving.

In the 4 years I had the BoxsterS I drove it very hard and in many events. The factory holey rotors didn't have one single visible crack or fracture around the holes when I sold it. The first year of that hard driving ownership I got the brakes hot enough the pads smoked after just couple of autocross laps. thankfully, after that first year I learned to carry more speed into the corners and quit smoking pads. The rotors did still get a bluish tint, but no pad smoking.

And yes, bigger brakes are almost always better at least for heat dissipation. And slotted and/or drilled rotors are much cheaper than a big red kit.

Oh and FYI, I drove few laps in a 350Z with the big 6 piston track setup. It was horrible. The brakes were very very difficult to modulate. It was either into the ABS or no braking. When the ABS kick on and off it felt like I was hitting bricks. I didn't like it.

buxton

I see that many of you don't recommend the cross drilled rotors and wonder why?. Aren't they designed to allow gases, heat, and brake dust to excape under hard braking? And why would cars such as the Carerra GT, 911 turbo, and the Z06 all come standard with cross drilled rotors? It would seem that the engineers who design these ultra high performance vehicles would have put on slotted or solid rotors if they were better. Don't most formula 1 cars also use cross drilled rotors?

f1rocks

There's probably a few thousand track/street test miles by the guys that have responded. I concur that based on how the question was phrased, solid or slotted are going to be better for the purpose requested. The drilled rotors are just too prone to cracking.

A Carrera GT/911 Turbo is running Ceramic Rotors. Like comparing Apples to Tomatoes. Those are "cast" holes too.


Formula one....Carbon Fibre impregnated Rotors (100% Made in Indiana by the way at a Honeywell owned facility) are all thrown away after one use so unless your planning on throwing new rotors on every week, I don't think that is a good comparison.

sweanders

Please, don't post incorrect information. There is no such thing as cast holes - they are all drilled.

This is a picture from Zimmerman, that makes drilled rotors for Porsche.

zerMATT951

I saw a quote by a GM engineer once when asked about the use of drilled rotors on the Z06 and Z51, he said that they only reason the Z06 had them was because other supercars like Porsche and Farrari did, not because they offered any performance advantage. He said that it was for marketing and appearance purposes only.

I'll see if I can't find that quote... it's gotta be online somewhere.

Seems like a very slight weight savings could be the only real benefit over slotted if you were looking for water & gas dispursion.

toddk911

CERAMIC!!

or maybe ceramic coated?

toddk911

And I have seen plenty of cross drilled/slotted rotors. i.e. they are drilled AND slotted.

f1rocks

If that is a question, they are 100% Ceramic. PCCB option...about $10K+

Made in the mother of all press units. Porsche and Ferrari have been using them for like 4 years now. They originally were supposed to be for the life of the car, I think that has been backed off a bit as the life expectancy is less than originally believed.

Anders,
I know Zimmerman cross drills but there are rotor suppliers who advertise cast holes. I'm not sure that the information provided is incorrect. I have seen sand cores for manufacturing cast in rotor holes.

toddk911

yes, I know they use full ceramic now, but I was asking about ceramic coating them. pros/cons.

toddk911

Based on parts I have had ceramic coated in the past, would be pretty cheap, but not sure how the coating would hold up.

I know there is ceramic coating of internal engine parts, and that would obvioulsy have to hold up to friction, stress, etc.