Installed a turbo 23mm master brake cyl , hard pedal ,Good or bad ?

[heliolps2]

I have a 76 chassi as some of you know , I added G50 and the boosted brakes system off a Carrera with the stock 19mm MC , and I have 993TT big reds and rotors and 965 rear calipers and 930 rotors ,

Over the years I have gotten use to a softer pedal , much like a my Mazda CX9 Gt.on the street it preformed good but at the track I would some time run out of pedal.
Over the weekend I install a 930 23.3 mm MC , now I have rock hard pedal and I really don't like it , is this Normal , it seems to take more effort to brake, almost feel like when the vacuum hose that goes to brake servo is disconnected but, almost,

Maybe I just need to get use to it, oh bye the way I'm useing stock Textar pads Front and rear,

Has anyone expeirenced same thing?

[Rodsrsr]

Yes its normal. I just did the same upgrade with Boxster brakes all around. From what I understand, The 19mm mc is too small for the 4-piston caliper applications and wont move enough fluid to operate them properly. I also think that you can actually modulate better with the stiffer pedal, but maybe others will chime in.

[McLovin]

Yes, it will take MORE effort to achieve the same braking force as compared to a smaller diameter MC.

This is contrary to what most people think. People think that a larger MC will give "more" braking power, but the opposite is true.

[heliolps2]

Thank you guys for chiming in ,
does it matter that the brake vacuum servo is a Carrera part not a Turbo part, if so what's the difference , Bill V seems to know a lot about brakes, but I have not seen a theards on brake vacuum servos .

[Bill Verburg]

Originally Posted by heliolps2

Thank you guys for chiming in ,
does it matter that the brake vacuum servo is a Carrera part not a Turbo part, if so what's the difference , Bill V seems to know a lot about brakes, but I have not seen a theards on brake vacuum servos .

it makes a small difference, the vac cans have been either 7" or 8" w/ boost ratios from 2.22 to 3.

The 19mm m/c was too small to move enough fluid w/ your calipers.

the # that describes the effectiveness of the m/c w/ a given caliper set is the slave/master ratio

your setup w/ 19mm is 58.286, w/ 23.8mm it is 37.342

the target # is always in the range from ~25 to ~50, the smaller ratio gives a higher harder pedal that requires more driver input but is desirable from a performance perspective because it is easier to modulate a pedal that doesn't move than to modulate one that does.

factory setups have been from 29 on RSRs to 43.171 on 964s

if you think yours is tough try a 964RS at 32.786 or a 993tt at 29.76 both boosted

but the prize goes to the '73 RSR at 24.259 unboosted

[spuggy]

Originally Posted by heliolps2

does it matter that the brake vacuum servo is a Carrera part not a Turbo part, if so what's the difference , Bill V seems to know a lot about brakes, but I have not seen a theards on brake vacuum servos .

930 booster is 8", 3.2 is 7", 930 boosters come in a couple of master/slave ratios, IIRC.

I'm pretty sure that's not the/a problem; that's what good brakes feel like.

I ran 3.2 brakes with a 19mm M/C & servo and it all felt fairly soggy to me; I like firm brakes.

After fitting a 930 booster & M/C with 996 brakes front/rear, it felt like there was a block of wood under the pedal for weeks. Even now it's hard.

Yes, you have to push hard(er) in comparison. Yes, your leg can get tired on the track. But there's more control - because the pedal isn't moving so much. As a bonus, heel'n'toe is much easier as the pedal doesn't have so much travel - so isn't in different places depending on how hard you're braking.

The only down side is that I can't keep anything under the seats anymore; it all ends up in the footwells...

[heliolps2]

Again guys for chiming in,
I have been useing SS Brake lines for awhile now , if I was to go back to stock brake lines would I get a softer pedal, I drive the car mostly on the street now, and maybe 2 or 3 DE's a year, Or I'm I just being a wuss ? I having a hard time with my current setup I could not imagen braking in a RSR, that's for that info Bil

[Bill Verburg]

Originally Posted by spuggy

930 booster is 8", 3.2 is 7", 930 boosters come in a couple of master/slave ratios, IIRC.

....)

That's not the slave/master ratio, the boost ratio is the force multiplier provided by the vacuum canister

the early cars('77) used a 7" vacuum canister w/ a boost ratio of 2.2 but the same canister when used on a 930 was rated at 1.8

all the SC/Carrera cars and '78 930s used 8" cans but again the boost ratio varied by application

details can be found here

the slave/master ratio is a function of the m/c and caliper piston areas

[uwanna]

If you used a Carrera booster and MC, you have a 20.64 MM and a 8" booster. The 19mm MC is used on a pre '77 car. Perhaps you could verify whether you have a 19mm or 20.64 MC, The number 20 is cast into the 20.64 MC. If you find you do indeed have a 19mm, perhaps you could try a 20.64 for a better compromise
By the way, all these specs are on Bill Verburg's "the brake GURU" great brake spec sheet!
Brakes

I have a Carrera booster and 20.64MC in my '80SC with larger calipers and it seems to modulate just fine. I also have a mint original late '80s 930 booster and 23mm MC set on the shelf that I have thought of installing. After your experience, I may reconsider!

[uwanna]

If you used a Carrera booster and MC, you have a 20.64 MM and a 8" booster. The 19mm MC is used on a pre '77 car. Perhaps you could verify whether you have a 19mm or 20.64 MC, The number 20 is cast into the 20.64 MC. If you find you do indeed have a 19mm, perhaps you could try a 20.64 for a better compromise
By the way, all these specs are on Bill Verburg's "the brake GURU" great brake spec sheet!
Brakes

I have a Carrera booster and 20.64MC in my '80SC with larger calipers and it seems to modulate just fine. I also have a mint original late '80s 930 booster and 23mm MC set on the shelf that I have thought of installing. After your experience, I may reconsider!

[uwanna]

Sorry for the double post

[heliolps2]

Should I change to stock brake lines?

[tabasco]

Edit : the answer is in Bill's post :-)

Bill,

Can you explain how you compute those ratios ?
What's the ratio for a stock 76 Carrera 3.0 ?

Thanks,

[Bill Verburg]

Originally Posted by tabasco

Edit : the answer is in Bill's post :-)

Bill,

Can you explain how you compute those ratios ?
What's the ratio for a stock 76 Carrera 3.0 ?

Thanks,

sure, just add up all the caliper piston areas and divide by the m/c piston area

for all 911 from '69-76 the slave/master ratio is 41.311 unboosted
for all 911 from '77-83 it's 35.192, boosted '75-77 930 is the same
for all 911 from '84-89 it's 38.196 boosted
for all 930 from '7-89 it's 33.077 boosted

[KC911]

Originally Posted by heliolps2

...Over the weekend I install a 930 23.3 mm MC , now I have rock hard pedal and I really don't like it ...

Maybe I just need to get use to it...

Give yourself some time to adjust imo. I've had the 930 setup on mine for years and I really like the feel of the pedal. I do like a stiffer suspension too, so YMMV. It IS a sports car however .

ps: I don't drive much in stop-and-go "city driving".

[heliolps2]

Sorry , I said 23.3 but it 23.8 right? Also I'm not clear on the booster, the Carrera one is an 8 in and is the same as the turbo or not , and if I change to the turbo booster how much total ratio will I have ?
How about the useing the stock brake lines ? Will give me a softer pedal ?

[KC911]

Originally Posted by heliolps2

How about the useing the stock brake lines ? Will give me a softer pedal ?

Nope...

[wayner]

Although slight, I have noticed that some break pads can make a bit of a difference. Some have felt distinctly "wooden" while others less so.

I don't have any 911 specific examples but aside from actual pedal pressure required, pads can give a different feel at least on some of my vehicles (particularly wooden feel if your pads have become glazed).

[JP911]

Are you sure that your booster is working properly? When working properly, the boosted 930 setup with good brake pads will throw you into the belts without too much pedal effort. A local racer actually switched to the unboosted setup to get better modulation.

[KC911]

Originally Posted by JP911

Are you sure that your booster is working properly? When working properly, the boosted 930 setup with good brake pads will throw you into the belts without too much pedal effort. ....

Good point! I absolutely love the "feel" of my brake pedal (I'm using Ferodo pads, and "feel" is totally subjective)...maybe something isn't working properly?