$100 brake upgrade (Page 6/43)
D B Cooper APR 05, 12:59 AM
The 18 in of merc reference...... yeah, I'm bored

given:
25.4 mm to the inch , 760 mm of merc is one atmosphere, 1 atmosphere is 14.7 psi (every square foot at sea level has a column of air weighing a shade over a ton sitting on it)
the conversion goes like :
(18 in Hg) x (25.4 mm / in) x (1atm / 760mm Hg) x (14.7 psi / atm) = 8.84 psi vaccuum.

doesn't sound like a lot, but 8 or 9 psi vacuum means almost 60% of the air molecules have been sucked out of a given volume. -14.7 psi (gauge) is a complete vaccuum, or 0 psi (absolute)

[This message has been edited by D B Cooper (edited 04-05-2006).]

Phil APR 05, 06:53 AM
The Fiero bracket has 4 riviets holding it to the booster can - when removed the bracket attaches to the 4 bolts on the new booster. Unfortunately the pushrod is attached to the booster by a clip on the inside of the booster so in order to remove the rod you would have to take the booster apart
Here is a shot of a stock Jimmy booster

The bracket just bolts onto the back side of the booster. The pushrod in the Fiero is the same diameter as the S10/jimmy only about 2" longer

fieroman87 APR 05, 12:41 PM
as far as idle vaccum for STOCK N/A cars goes, *MOST* produce between 18-22" Hg (30" being a "perfect" vaccum) how that relates to negative PSI/atmospheric, i'm not sure. but if atmo at sea level is 14.7psi and that becomes our referance of 0, then i can see -9psi being a viable number for STOCK N/A engines. the higher the number, the better the idle quality, which is what manufactures gow for as far as driveability is concerned. *MOST* boosters need a MINIMUM of 14" Hg to opperate with satisfactory results, and *MOST* N/A engines produce < 24" Hg during decel (throtle closed). within the time you release foot from gas and apply brakes, the engine (depending on RPM) will have drawn enough volume of air (@ < 24" hg) to have emptied the booster (in this case the 9" Jimmy booster) at LEAST 10 times if not more (using 2.8L as reference for volume and giving the jimmy booster a volume of 2.6L). so if you're worried about refersh rate of the booster, you'll never notice it, EVER.

now if your engine dies and you need to brake, here's where the larger booster becomes an asset: depletion time goes up (this is a good thing). as volume of booster goes up, so does the number of available pedal pumps that are in reserve.

------------------
Arty: a.k.a. FestYboy or Fieroman87
'87 GT 5 spd. '88 Festiva 5 spd. '87 Scirocco 5 spd.
Yahoo messanger: AMAbonus
AIM: FestYboy88L

1fastcaddy APR 05, 03:53 PM
So if you get a bigger bore MC, you would want the bigger booster? I think I know whats on my list of to-do's.
Quickster APR 05, 05:55 PM
Thanks Phil etc. but by a, "Soft Pedal" I do mean it takes more pressure to stop. I had all brake work performed by an expert mechanix and he couldn't figure it out! So.... bottom line, is this S-10 Booster exchange what I need to make my "Big Bore" M/C work? (Fiero Store Big-Bore M/C)
kcfiero85 APR 05, 07:07 PM
Quickstar,,

why did you up size your master cylinder? did you change calipers..

a larger Master will yield lower line pressure for the same pedal force.

What Master cylinder did you use... ( Some are Diagnal Split others are Front Rear, I believe the fiero is front rear split but could be mistaken)
if Split is worng it could cause a low pressure issue in the front brakes.

The best way to make up for this is to increase the mechanical pedal ratio...... ( but not the easiest )

If you have a mushy feel,,, a larger booster may make it even mushier (not sure if thats a word).

Boosters are designed with a input vs output slope..... which varies for each vehicle application. This slope can be chosen by the OEM somewhat independent of size.

If all things are equal internally to the booster
All the larger size allows you to do is, you can hold the gain ratio slope longer before you run out of boost ( therefore reaching a higer total boost prior to runout),, but typically drivers dont reach the upper edges of this during typical braking.

If Phil has noticed an improvement with this booster
Im guessing that this booster has a higher gain ratio... which would make the brakes feel more "snatchy"
usually larger boosters can get away with highr gain ratio since they have more volume to work with( but its not always the case)

If I have time at work I will see if I can dig up perfromance plots of each booster....
kcfiero85 APR 05, 07:16 PM
sorry didnt notice that you said fiero store big bore,,, so the split should be ok....

if you didnt go with larger calipers than you should not upsize the Master cylinder.

- Larger caliper pistions genterate more torque at the wheel with lower pressure, (require more volume) due to the larger surface area of the pistons.

- Stock Calipers (smaller) need the pressure of a smaller master cylinder to generate adequate Torque at the wheel.

flames4me APR 05, 08:51 PM
for the problem you are arguing about is easily solved, people with big cams, blowers, etc have been using them for years. there called an electric vacume pump... something like this:

http://www.jegs.com/webapp/wcs/stores/servlet/CategoryDisplay?lang=-1&catalogId=10002&storeId=10001&categoryId=22593

------------------
1986 Silver 5 speed Fiero 3.4 DOHC
Bored .30,Fully balanced and blueprinted
13.9@99mph as it is on the street

355/380hp sbc, 4 bolt main
spec stage 3, and many other extras.
87 GT 5-speed Getrag, power everything.

jstricker APR 05, 09:06 PM
I can't answer for Quickstar, but we have the Blazer M/C on our Finale.

I was very well aware of what would happen when I put it on. We autocross the car and I don't care for the (what I think is) the long stroke of the brake pedal on the stock Fiero system. Yes, the pressure requirements on the pedal from your foot go up, but because of that you can modulate your brakes better on an autocross course (or I should say that *I* can). It also shortens up the stroke of the pedal quite a bit.

I changed mine for two very specific reasons. I WANTED higher brake pedal force and I WANTED a shorter throw on the pedal. That's why I did it. If someone doesn't know, or want, both of those two things, they should leave the stock setup alone.

John Stricker

quote
Originally posted by kcfiero85:

sorry didnt notice that you said fiero store big bore,,, so the split should be ok....

if you didnt go with larger calipers than you should not upsize the Master cylinder.

- Larger caliper pistions genterate more torque at the wheel with lower pressure, (require more volume) due to the larger surface area of the pistons.

- Stock Calipers (smaller) need the pressure of a smaller master cylinder to generate adequate Torque at the wheel.
theogre APR 05, 09:07 PM
Phil,

I get it... I couldn't see parts of the other pictures very well.

It's been ages since I had a booster from anything apart. Stinks about the clip being burried too deep. The booster can be taken apart but I would not recomend people try that. You need special tools and odds are if you tried reusing parts the thing wouldn't last long if it worked at all when you got it back together. I don't think anyone sells booster parts to the public or even to shops much anymore. It's one of those things that isn't terribly complicated but you have to know exactly what you are doing or you get a big lump. (Hopefully it gets bricked before someone tries to drive with it.)