Advice for upgrading struts and brakes for 1973T

[PeteKz]

Yes, I know, there are a lot of threads about upgrading brakes, and I've read a bunch of them. However, new parts and options become available, and the advice changes over time. So let me ask for my specific project.

I have a 1973 911T, as described in my signature line. It has the original Boge struts, with Koni Sport inserts. I am not currently overstressing the original brakes on the street, but I did build and install a 3.2 engine, and I may track the car occasionally.

Being an early car, it has the 3" caliper mount spacing instead of the later 3.5" spacing. Thus, to change to SC or Carrera brakes, I have to get later struts with the 3.5" spacing. I've been watching for some SC or Carrera Boge struts. The Koni Sport inserts also fit in the later Boge struts, so I won't have to buy new shocks (I would like to avoid the slippery slope that leads to $3000 KW struts, etc.)

I also want to raise the spindles 19mm/3/4" to drop the front a bit without messing up the front steering geometry and roll center. When I went to the trouble of setting the ride height to even the lower of the factory specs, it almost looks like a "Safari" when compared to lowered look of some of your cars.

On my early car, the Boge strut tubes neck down a few mm at the spindle, so it's not easy to cut and raise the spindles on these early struts. I understand the SC and Carrerra Boge struts are the same diameter all the way down to the ball joint, so I can cut and raise the spindles easily. But, some of the "for sale" pictures I've seen appear to neck down at the spindle too. Which is correct?

Brake calipers and rotors: My thinking has been to get the front calipers and rotors from an SC or Carrera because they have more capacity than my early calipers, and should "bolt on." But, I understand that the Carrera rotors are thicker and the calipers wider than the SC. Thus, I would like to buy the complete strut, including calipers and rotors, ready to bolt in, to avoid possible incompatibilities.

I'm not particularly sensitive to price, but don't want to spend "stupid money" either.

Bill V and others, what's your best current advice?

[Bill Verburg]

Originally Posted by PeteKz

Yes, I know, there are a lot of threads about upgrading brakes, and I've read a bunch of them. However, new parts and options become available, and the advice changes over time. So let me ask for my specific project.

I have a 1973 911T, as described in my signature line. It has the original Boge struts, with Koni Sport inserts. I am not currently overstressing the original brakes on the street, but I did build and install a 3.2 engine, and I may track the car occasionally.

Being an early car, it has the 3" caliper mount spacing instead of the later 3.5" spacing. Thus, to change to SC or Carrera brakes, I have to get later struts with the 3.5" spacing. I've been watching for some SC or Carrera Boge struts. The Koni Sport inserts also fit in the later Boge struts, so I won't have to buy new shocks (I would like to avoid the slippery slope that leads to $3000 KW struts, etc.)

I also want to raise the spindles 19mm/3/4" to drop the front a bit without messing up the front steering geometry and roll center. When I went to the trouble of setting the ride height to even the lower of the factory specs, it almost looks like a "Safari" when compared to lowered look of some of your cars.

On my early car, the Boge strut tubes neck down a few mm at the spindle, so it's not easy to cut and raise the spindles on these early struts. I understand the SC and Carrerra Boge struts are the same diameter all the way down to the ball joint, so I can cut and raise the spindles easily. But, some of the "for sale" pictures I've seen appear to neck down at the spindle too. Which is correct?

Brake calipers and rotors: My thinking has been to get the front calipers and rotors from an SC or Carrera because they have more capacity than my early calipers, and should "bolt on." But, I understand that the Carrera rotors are thicker and the calipers wider than the SC. Thus, I would like to buy the complete strut, including calipers and rotors, ready to bolt in, to avoid possible incompatibilities.

I'm not particularly sensitive to price, but don't want to spend "stupid money" either.

Bill V and others, what's your best current advice?

I'd get Bilstein hd struts and sport rear shocks, have a digressive revalve done based on the chassis weight and t-bar specs. w/ 15" wheels 19mm is the max for raising the spindles, you can raise them more w/ 16 and even more w/ 17

while in there also add the steering rack spacers

the lighter the car the less brake is needed, depending on actual weight Carrera 3.2 front brakes(rotor and caliper) and SC ( rotor & caliper)rear would likely be adequate for the intended purpose. Do add 993 or Gt3 front A-arm cooling scoops and use good brake fluid like SRF for track days.

overkill for most but 930 brakes are still pretty much the hot ticket if money isn't particularly tight.

An interesting alternative is the Vonn shocks sold by Elephant racing, I know nothing about them but Elephant sure seems to like them.

[PeteKz]

Bill, thanks.

I already have the steering rack spacers installed. I want to retain the option to run 15" wheels, so I'll stick with a 19mm raised spindle. Are the SC and Carrera Boge strut tubes the same diameter all the way down? Bilstein strut tubes?

930 brakes are overkill for my purpose. Car weighs a bit over 2300 lbs. empty (no fuel).

brake fluid: Good and fresh before track events--of course.
Scoops: I'll see if I need them after a track day. If so, I'll add them, or scale back my speed at the end of the straight.

Vonn shocks: I've read about them, but would want feedback first from someone who runs them on the street.

[nickelplated5s]

Go over to the early 911s site. Search for the Alfa Brembo thread. Chris P and my user name pretty much lay it out. After finishing my Boxster upgrade here's my advice; Go with Tuthill(sic) or a 930 setup. Add the GT3 front, 3.5" struts are pretty much an offset cost wise if you can find them. I know, I bought a pair from Chuck at ER. GT3 will shave weight from the brake upgrade, mine added about 200 lb's. Whichever route you go it won't be cheap.

Everyone thinks you need to change the spindle for a lowered ride height. You do not need too. Find the throw length shock you want and shim inside the strut. I'm 22" ground to fender in the front. My hip says I should get a detachable steering wheel...

[Bill Verburg]

Originally Posted by PeteKz

Bill, thanks.

I already have the steering rack spacers installed. I want to retain the option to run 15" wheels, so I'll stick with a 19mm raised spindle. Are the SC and Carrera Boge strut tubes the same diameter all the way down? Bilstein strut tubes?

930 brakes are overkill for my purpose. Car weighs a bit over 2300 lbs. empty (no fuel).

brake fluid: Good and fresh before track events--of course.
Scoops: I'll see if I need them after a track day. If so, I'll add them, or scale back my speed at the end of the straight.

Vonn shocks: I've read about them, but would want feedback first from someone who runs them on the street.

bone stock '72S brake stress index is 462, this is for a 2365# car, 190hp and S/m calipers.

for comparison a '73 RSR is 267

the RSR is 485# lighter and has essentially 930 brakes

i run 288 and 305 on mine but was very comfortable back w/ 320, 320 was 964 calipers on 930 rotors.

every one is different depending on technique and usage.

Unless you go to 3.5" mounts on the front there is no brake advantage from changing stock M/M to A/M or S/m or using the Alfa calipers on the stock rotors. All the bigger caliper pads provide is more endurance which is not really a big problem usually.

To get any meaningful brake improvement you need 3.5" front caliper mounts

adding the Carrera 3.2 fronts is only a small improvement hence the recommendation for additional cooling, back when my C3 was new w/ a brake index of 492 I would go through a set of A/M pads in a day of track use, adding cooling helped immeasurable as did using SRF.

to get the 3.2 you need a set of struts, I'm very partial to Bilstein for many reasons not the least of which is the ability to easily raise the spindles. I know little about Boge just that they are a second tier brand

[nickelplated5s]

BV knows more than I but, Pete, you're pretty close. PM me and I'll take you for a drive. I would still suggest the GT3 front and you won't need 3.5" struts.

[Bill Verburg]

Originally Posted by nickelplated5s

Go over to the early 911s site. Search for the Alfa Brembo thread. Chris P and my user name pretty much lay it out. After finishing my Boxster upgrade here's my advice; Go with Tuthill(sic) or a 930 setup. Add the GT3 front, 3.5" struts are pretty much an offset cost wise if you can find them. I know, I bought a pair from Chuck at ER. GT3 will shave weight from the brake upgrade, mine added about 200 lb's. Whichever route you go it won't be cheap.

Everyone thinks you need to change the spindle for a lowered ride height. You do not need too. Find the throw length shock you want and shim inside the strut. I'm 22" ground to fender in the front. My hip says I should get a detachable steering wheel...

A Boxster caliper on a 3.2 Carrera 24mm front rotor and a 3.2 caliper on a 3.2 24mm rotor, other than visuals provide, the same benefit, i.e. a small enlargement of the thermal envelope due to increased thermal mass and increased internal air flow which is the only real issue. The Boxster does increase brake torque by about 27% but that only exacerbates the heat issue by adding more heat to an already borderline situation and created a bias issue that needs to be addressed out back.

Similarly, the Alfa caliper on the SC 20mm front rotor provides no real benefit except somewhat longer pad life, there is no change in thermal capacity or brake torque.

The only real issue w/ 911 brakes has ever been thermal deficiency which can only be addressed by, weight loss, rotor mass increase, rotor internal flow increase, rotor external flow increase, brake fluid thermal capacity, pad thermal capacity etc

things that cause more brake related issues are weight gain, hp gain, increase of brake torque w/o commensurate increase in rotor thermal mass, decreased internal and extremal air flow, reduced radiation distance, old water logged brake fluid, thin pads etc

[PeteKz]

Bill, others, thanks.

Here comes the slippery slope...

nickelplated5s: I'll PM you now.

[PeteKz]

Bill, a fine point that I think you may have misstated: increasing the torque on the brakes, say by fitting Boxster brakes, does not increase heat load. The heat into the brakes is directly related to the change in momentum of the car. In other words, the mass of the car times its change velocity^2 is converted into heat. Other factors being equal, higher torque brakes only decrease pedal effort.

[Bill Verburg]

Originally Posted by PeteKz

Bill, a fine point that I think you may have misstated: increasing the torque on the brakes, say by fitting Boxster brakes, does not increase heat load. The heat into the brakes is directly related to the change in momentum of the car. In other words, the mass of the car times its change velocity^2 is converted into heat. Other factors being equal, higher torque brakes only decrease pedal effort.

I should have said "potential increase" of thermal load, of course the actual thermal load is directly proportional to the Δ V of the car but the Boxster have the potential to put more heat into the already stressed rotors, It all depends on on your control of line pressures, which by the way may also degraded by the larger caliper piston swept volume increase. That will depend on the m/c used.

What decreases brake effort other than a booster is t a change in slave to master ratio

stock '72 has a slave to master of 41.311 which is way on the wrong end of where you want to be, pop in a 20.6mm m/c and it goes to 35.328 which is on the good side of fair, use a 23.8 and it goes to 26.328 and it is well into the gone too far zone except for hardcore racers doing s20min sprints

Boxster f & r w/ 23.8 is 32.34 just about unboosted perfect, but w/ 19.05 it is a dangerous 50, a poor 43.17 w/ a 20.6mm

the higher the slave/master ratio the less control you have of line pressures due to the ergonomics of the human leg.

the f/r Boxster also moves bias a bit more front which you don't want, stock M/m, A/M or S/m is just about perfect for a fixed bias system. Though you could gainfully go a tad more rear.

[PeteKz]

I have the stock 19mm master cylinder. With the stock 1973 T brakes, it's a fairly stiff pedal effort. I can lock the wheels, but I really have to stand on it. So less pedal effort would be a good thing in my non-boosted brakes.

If I find the brake balance is too far off, I might consider a tandem MC setup with balance bar, then dial in the balance.

[Bill Verburg]

Originally Posted by PeteKz

I have the stock 19mm master cylinder. With the stock 1973 T brakes, it's a fairly stiff pedal effort. I can lock the wheels, but I really have to stand on it. So less pedal effort would be a good thing in my non-boosted brakes.

If I find the brake balance is too far off, I might consider a tandem MC setup with balance bar, then dial in the balance.

Have you driven a car w/ ~30 s/m ratio?

If you had you would see the difference.

Back in the day the factory Motorsports dept. offered a 20.5 replacement for the 19.05, Porsche # 901.355.012.04/ATE # 3.2120 - 3901.3. I haven't seen one in decades but the Corrado 22.2 also bolts right in to a non boosted car(original use 89-99 JETTA 93-99 GOLF 90-97 PASSAT w/o ABS) and gives a very nice 230.42 s/m ratio


most any one car get the wheels to lock that's not a useful metric

you don't need a balance bar w/ stock brakes, the the factory engineers are pretty good when marketing and legal keep keep their noses out of it.

as I aid the only ones that I know hat use the 23.8 are hardcore racers mostly w/ 2.7RS spec cars, you get exquisite control at the cost of a lot of leg.

[nickelplated5s]

https://www.early911sregistry.org/forums/showthread.php?171318-Alfa-Brembo-Calipers-what-hubs&highlight=alfa+brembo

I sort of hijacked the thread but it's pretty well laid out.

[ericwitte]

Originally Posted by PeteKz

Bill, thanks.

I already have the steering rack spacers installed. I want to retain the option to run 15" wheels, so I'll stick with a 19mm raised spindle. Are the SC and Carrera Boge strut tubes the same diameter all the way down? Bilstein strut tubes?

930 brakes are overkill for my purpose. Car weighs a bit over 2300 lbs. empty (no fuel).

brake fluid: Good and fresh before track events--of course.
Scoops: I'll see if I need them after a track day. If so, I'll add them, or scale back my speed at the end of the straight.

Vonn shocks: I've read about them, but would want feedback first from someone who runs them on the street.

I have the Von’s shcoks/struts -digressive valving. Matched for my 21/28 TBs in my 72 with a 3.4. I like them with the stock front rubber top hats, polybronze bushings all Around since 2005
I had the sport/hd combo prior to the Vons and the car was harsh. I love the way it drives now.


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[jac1976]

Originally Posted by ericwitte

I have the Von’s shcoks/struts -digressive valving. Matched for my 21/28 TBs in my 72 with a 3.4. I like them with the stock front rubber top hats, polybronze bushings all Around since 2005
I had the sport/hd combo prior to the Vons and the car was harsh. I love the way it drives now.


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Were your prior sport/hd valved for the TBs?

[PeteKz]

Eric, thanks for the feedback on the Vons.

Bill, others, I'm sure 930 brakes are the best you can get. It's also overkill for a 2330 lb primarily street car. That's not just the slippery slope--that's swan-diving off the cliff!

[ericwitte]

Originally Posted by jac1976

Were your prior sport/hd valved for the TBs?

They were not.


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