A930Rocket

Cross posted to racing and 911 Turbo forums.

First: I bought some 928 S4 front brake calipers for my '87. They bolt on just fine, but I need a hard line that has a little longer male thread on at least one end than what I have now. The bottom of the hex portion bottoms out before the flare seats properly. What I need is one that measures at least 7/8" from the bottom of the hex to the end of the double flare.

Has anybody done this conversion? Is yes, how did you find/make/etc. a longer male fitting to go in?

Would one off a 928 S4 work?

Second: I moved the fronts to the rear. Now the hard line is on the bottom inside that is close to the control arm. How do I connect this up? Just run a longer hard line over or under the control are to the flex line mount?

I can have a set of braided lines made, but I feel more comfortable with a hard line, as I drive it on the street some.

Bill V. any advice here?

Thanks,
Jim

Bill Verburg

I've done it both ways, but w/ the big brembos a 100% braided line seems to work best. You can try to make a hard line w/ the proper length and ends but it's a PIA.

I've never put the front 930 calipers in the back, according to my figures the bias would be horrible and a pretty aggressive p/v would be needed. I know people do this but I have no personal experience w/ it. The LF will go to the RR etc. if you do this.

A930Rocket

Would you just leave the stock brakes on the rear? Thanks for your help.

Jim

Bill Verburg

With the 993/S4 front caliper on 304x32 rotors I like the 965 rear calipers on your stock 309x28 rotors. But the stock rear calipers left in place are useable if somewhat front biased. It might work for you depending on the rest of the car setup.

A930Rocket

Bill,

I'm a little challenged here by the brake information. I know how brakes work, but beyond that, I'm lost as to dialing in the bias, etc.

Can you explain what is going on here with my brakes?

I know I have bigger pads with bigger pistons up front. Bigger/thicker rotors would be better for heat absorbtion and disapation, but... I'm using the stock ones.

The front and rear calipers use the same pad, so now how do the smaller/bigger pistons in the rear calipers come into play here? I know with bigger pistons one may have to go with a bigger M?/C to compensate for fluid, but that's it. Can you explain the piston/caliper relationship here?

Thanks again,
Jim

Bill Verburg

correct, with the new 993/S4 calipers

correct again, you have gained little by changing to the 993/S4 front calipers. The larger pad area will distribute the heat somewhat better and reduce unit stress on the pad material, net gain is mostly in durability.
correct again in stock form, the 993/S4 pads are much larger

Simple hydraulics, a bigger slave piston(all else being equal) will provide a larger clamping force on the rotor. This is one of 3 primary factors affecting bias. The larger clamping force increases the net bias at that end of the car.
correct again, but the 930 m/c is adequate for the calipers in question. I use it on my 36/44 & 30/36 calipers.

The 3 primary variables in bias are relative rotor diameter f/r, relative slave piston area f/r and pad material f//r.[list=1][*]The bigger the front piston area relative to the rear the more front bias[*]The bigger the radius of the front rotor relative to the rear the more front bias [*]The more aggressive a front pad is at a given temp. the more front bias at that temp.[/list=1]


Since you aren't changing rotors from stock, the hydraulic bias change from the calipers and fine tuning via pads is the only concern.

The 930 front calipers have 38/38 pistons which when used at the rear w/ 36/44 993/S4 pistons has a hyd. bias ratio of ~ 1.1 as opposed to stock ~1.6. This means the rears are too effective and must be toned down via a fairly aggressive p/v

If the 993/S4 is used w/ stock 30/30 rears the hyd bias is ~1.8 this is an aggressive front setting, you will likelly want to remove the stock p/v for this set up

When the 993/S4 is used with the 30/34 965 rear calipers the bias is a neutral(for a 911/930) ~1.57, just a tad more rear than stock, retain the stock p/v.

Stock you car has a very mild 55 bar p/v, the effect of any p/v on the rear is to move bias to the front only under hard braking.

A930Rocket

OK, I'm getting it now.

So the fronts located on the rears are too much brake. Not good due to early brake lock over the fronts.

Using the stocks rears is better, but the fronts have a little more braking power. By removing the p/v, it will help to even this up.

If I was to replace the p/v with an adjustable type, would I place it in the same location as the stock one?

I've been using PFC 90 compound pads front and rear for the track with good results. What would you advise with the S4 fronts and stock rears? Stay with the 90's in the rear and a softer pad up front to help even out the brake bias more?

On an earlier post you said: "But the stock rear calipers left in place are useable if somewhat front biased. It might work for you depending on the rest of the car setup. "

Below is a list of my suspension mods. Would my set up work w/o removing the p/v or is it a personal tuning issue.

18" BBS magnesium race wheels, Dunlop slicks, 23 and 30 mm torsion bars, 22 mm adj. front and rear sway bars, urethane bushings thru out, adj. spring plates, strut tower brace, Koni yellow sports, Bilstein HD front struts, Camber Plates, full Safety Devices rollcage, corner weighted, GT Racing air dam with brake ducting.

Lots of questions. Thanks for your help!

Jim

Bill Verburg

Depends on whetheryou want cockpit adjustability or not.

Stay w/ the same f/r until a reason to change becomes evident.

Depends on the car , driver taste/technique. Your car is heavy in the back(relatively) and stiffly sprung(relatively) so more rear bias will be desireable to a point. Each car and driver and track requires a litle bit different setup. The tires are an additional factor to consider. I use a bit bigger pistons than most in the back because of weight and larger than usual rear tire relative to the front tire.