Jkay

ill-developed social skills.

Less hammer, more cordial. Following this formula, you might make it out of mom's basement one day.

ryoung

You can feel the effect of the vacuum assist when you start the car. With your foot lightly on the brake pedal, the brake pedal falls an additional amount once the engine's running and providing vacuum to the booster.

I've never taken a vacuum booster apart, but I'm having trouble imagining how it could provide something other than a fixed amount of assist once the brake pedal is depressed a certain amount. Pressure on the pedal pushes forward a rod connected to the pistons inside the master cylinder. Simultaneously, the rod closes a vacuum port to the right half of the booster unit and opens it to the atmosphere. The left half of the booster still contains full vacuum, and atmospheric pressure in the back half of the booster pushes the internal diaphram forward, which applies additional pressure to the master cylinder pistons. Releasing the brake pedal closes the opening of the right portion of the booster to atmosphere and opens it to intake manifold vacuum again.

<img src="http://static.howstuffworks.com/gif/power-brake3.jpg">

Another Drew

Where can I find one of these? Thanks!

Another Drew

1) The 0.50 cf Ferodo DS2500 is for operating temperature. Are the DS2500 track pads?. A cold pad would be slightly different.

2) Re: "the optimal brake bias is 2.2" The optimal brake bias isn't static. It changes constantly due to traction (road surface, tire temp, etc), brake pad temp, loaded trunk (an we know the S8 has a huge trunk , etc.

ryoung

1: The Ferodo DS2500 is suitable for street, autocross, or light track. The coefficient of friction is extremely flat. See my additional comments on pads <a href="https://forums.audiworld.com/a8/msgs/75190.phtml">Here</a>.
<img src="http://www.federal-mogul.com/images/ferodoracing/DS2500Graph.jpg">

2. Absolutely agree. -- With street tires, 2.2 would be about the ideal, but highest, brake bias and is the result of maximum load transfer to the front tires at maximum braking g's. Everything else (rain, rear seat passengers, etc) would result in a lower optimum bias. You can't go lower than 2.2, however, without risking the back end getting squirrelly under maximum braking conditions in the dry. My S8's original brake bias was 2.5.

nirad

yes, i realize you are getting the data from somwhere else. i have driven many of the cars on that list- Audi TT, , Audi A8L, Audi A4 3.0, Audi S4 B5, Saab 9-3, Toyota Camry, Range Rover, MB E320... And the B6 S4 brakes better than all of them, except maybe the TT.

Bauer

The RS6 a very good brake with execellent pedal feel.....but if pushed hard they will fall off pretty bad. They are no good for tracking for several reasons.

1. Piston are too small and concentrate the heat in a small area on the pads, causing them to crack.

2. Poor rotor design does not cool well enough and adds to the problem of the piston design.

I speak from experience when I say the Alcon's are better....I have tracked both, albeit on different cars but the Alcon's have never faded and I am always confident to late brake with them...the Brembo's on the other hand....I would not feel very comfortable after a few laps braking late with them.

ryoung

I don't know what else to say.

nirad

if you drive one, i'm sure you will agree that it can't be accurate

ryoung

70-0 mph in 174 ft. That's 0.94 g and equivalent to a 227 ft 80-0 stopping distance, still rather poor performance, but it's a heck of a lot better than the 0.84 g calculated from Road &amp; Track's 256 ft 80-0 mph stopping distance.

To be completely objective, I'll average the R&amp;T and C&amp;D tests, which is 241 ft resulting in a 0.89 g stop, and change the original post to reflect this result.

BTW: I was also extremely suspicious of the Road &amp; Track results for the S4, and I rechecked it about six times. In their 100 car road test summary (in the back of every issue), a Morgan is the only car listed with worse brakes.