Maybe the rears were not working? I don't know. I did the nitrous thing back in 2011, then basically right after that I got too busy with other crap and parked the car for a few years. When I got back on it a few months ago the rears were badly locked up and the fronts were draging. I have since rebuilt the rears and put vw fox calipers on the front with brembo pads.

I just always assumed That I was surpassing the ability of the brakes. Now you have me wondering if I have a restriction somewhere in the system?

That is what I would suspect.
I was going to add this to my previous post, but too late:

Assuming a 2500 lb (1135kg) car @ 115 MPH (52m/s).
That is: E = ½*M*v^2 = 1534520 Joules or ~1.5 MegaJoules
1.5 MJ = ~1450 BTUs

So to come to a stop from 115 MPH, the brakes would have to dissipate 1450 BTUs essentially instantaneously.

1 BTU = the heat required to raise 1 lb of water by 1 deg F.
Specific heat of Mild steel is .122, so .122 BTUs will raise the temp of 1 pound of steel by 1 deg F.
914 front rotors weigh ~15 lbs each.
Assuming that the kinetic energy of the entire car is absorbed by JUST the front rotors, then we have:
1500 BTUs into 30 lbs of steel = 50 BTUs per lb.
50 BTUs into 1 lb of steel at .122 (BTU/(lb*degF)
50 / .122 = 409 deg F.
So If the 1500 lb car is travelling at 115 MPH, and comes to an abrupt stop using the front brakes ONLY, AND all of the heat goes into the rotors ONLY, then the rotors would increase in temp by ~400 deg F.
The temp of Steel becoming visibly “red hot” is 900 deg F.

In reality, the calipers, pads, and wheels get hot, and a significant amount of heat is dissipated right away via convection.
Therefore “rotors getting red hot by a single drag racing pass is ‘busted’”

Now of course if the heat could not dissipate between runs (and you did repeated runs back to back), then you could certainly overheat them.

Wow, that is good to know. I guess I need to figure out what went wrong here. So by your math I probably don't even need to fuss with vented rotor/billet hubs.

So at what point will these brakes not work? I have a Lm4 5.3 v8 and boxster six speed going in it. I plan on doing the nitrous thing again with the new motor. shooting for low 10's or high 9's with the next setup. Somewhere near 500hp and maybe 135-40mph in the 1/4 mile. If the stock brakes are good for that, I am done chasing the billet hub dragon.

Well if it was me, I would first do a couple of runs where I measure the temp of the rotors before and immediately following the run (immediately upon coming to a stop) using an IR temp sensor.
Then you would be able to verify the calculations to within a sensible variation and verify that the rears get hot almost as much as the fronts.

If you were having trouble stopping, double check your rear clearances and that the proportioning valve is actuating.

If everything seems to be in working order , then I would upgrade the pads to Porterfields or KFP Golds.

This is all great information. That's a good idea with the IR temp reader. I will do this. Thanks for your help.

The original intent of the 4 lug disc brake kit was NOT for plain jane daily driver vehicles.

It was for guys that tracked their cars and had multiple sets of wheels and tires. If you only have to buy one set of wheels to go to 5 lug not that big of deal.

If you already have lets say 2 or 3 or more sets of rims and tires that are 4 lug, the cost to replicate that setup in 5 lug can get pretty darn expensive.

The 5 lug billet hubs intent was for people that didn't want to swap out their struts for one reason or another.

One of the guys I race with says he wished he hadnt changed to 5 lug, because it costs so much more to replace the brakes with the 5 lug setup.
I bet he forgot that the wheel selection (width especially) is much more limited in the 4 lug version.
I have to have custom steel wheels to get up to 7 inches wide in a 4 lug.

Brakes should cost the same unless one wants to go with something fancier than stock 911 M (?) calipers.

bumping this thread as I'm contemplating my options for brakes (vented rotors??) on the car for occasional track use. Car will be close to stock weight, I weigh 175lb, and the engine will be 120hp.

I have brand new solid rotors with PMB 5lug conversion, rebuilt calipers, new pads, lines, proportioning valve, 19mm master cylinder.



based on your experience, I'm thinking my desire for vented rotors is probably not necessary?

thoughts?

I'm using my stock Boge strut bodies; if I want vented rotors using Porsche calipers and 5lug, I'm assuming the only option is to convert to 911 front struts?

I have heard many horror stories about vented rotors cracking on track cars.
There certainly are those that do it, but would it be a benefit for your application?

I would say if have have moderate HP (stock to ~150hp) then you might not need the extra heat dissipation. It also depends on how you drive.

true. do you use brake cooling ducting?

Vented rotors cracking? Drilled rotors-most certainly, but vented are pretty robust.

Yea, its a problem with drilled rotors. Vented are on most everything. I cant imagine how heavy the rotors would be on the cars with BIG brakes if they were solid. Carbon fiber excluded.

Sorry. Since he was talking about 4-lug upgrades and vented rotors, I thought 'drilled rotor'.

Yes if you have 911 style vented rotors, then they do not crack like stock (solid) drilled rotors.

Occasional track use will come down to driver skill and the tracks involved. Going back, I had a 2056/4 with 4 lug stock brakes. Like most track diving geniuses, I knew the only way to go faster was to improve the car (since the drover was already perfect ; ) ) .. So after 2 years of saving my pennies I upgraded to 911 front end (3.2 Carrera brakes) and never had an ounce of brake fade thereafter. For me, I was running 4-5 events (10-14 track days/year).

Yes, the 5 bolt 911 brakes are the solution, hence the factory using them on the 914/6.

The stock brakes are fine for a stock car and can likely handle up to 110-120hp. Track driving is about HEAT management. If you want to avoid the $$ to change to 911 brakes, do what others have done:

1) Quality lines
2) Quality fluid
3) Great brake pads
4) AIR - Get lots of air to the front brakes
5) Learn to drive better (brake less, carry more speed).