Weak brakes with the 2002 brake booster

[pmansson]

One of my cars has a 3.5 L engine with the smaller brake booster, as the stock one won´t fit with the larger intake manifold of the 3,5 L engine.
I know that the 3,5L engine upgrade is very common in the US. How do you solve the brake issue ???

 

[x_atlas0]

I have this setup and I haven't noticed weak brakes. The only issue I've found is a very non-linear brake feel. They are almost on or off, with very little in-between, making smooth driving a little tricky. However, that could be because the brake lines were replaced with standard rubber lines, rather than steel.

 

[bmwmadman]

A simple engine swap to the 3.5 does not automaticly assume you bring over the EFI that comes with that engine. I have the old sytle intake with updated injectors and therefore still use the stock brake booster. No issues with the brakes..

 

[Honolulu]

"solve" brake issue

Use the hydraulic brakes that were likely used on the donor car from which the injected motor came.

They work just fine, but the "bomb" has limited life. Even the master on those cars, which sometimes leaks, can be rebuilt. Information to do so is on the web...

 

[Knut]

Brake booster/intake manifold

Saw one coupe with the large intake man. and the original booster. To make it work, he had simply shaved off the part of the manifold that came in conflict with the booster. The cutout was replaced with a plate nicely welded in. Seemed to work ok.

 

[Percy]

One of my cars has a 3.5 L engine with the smaller brake booster, as the stock one won´t fit with the larger intake manifold of the 3,5 L engine. I know that the 3,5L engine upgrade is very common in the US. How do you solve the brake issue ???

I am unclear as to what you mean by "weak" brakes. Are you describing higher than normal pedal effort? Or, are you describing brakes that do not stop the vehicle very well :?: A few more details like this might be helpful.

Although it is probably unrelated to any diagnosis, are you using a stock 02 booster, or is it a tii/2800CS booster? It is true that boosters, like most moving parts with diaphragms have a useful lifespan, but you have not indicated the history of this part. In any case, I would not be so quick to condemn the booster, before confirming so many other possibilities, including the usual suspects: old/contaminated brake fluid; poor friction materials; glazed pads; severely worn rotors; sticky, leaky or non functioning calipers; older flexible brake hoses, worn leaky master cylinder or even maladjusted brake pedal.

Consider that old brake fluid absorbs moisture and is thus susceptible to reduced boiling temperature resistance. This can lead to spongy brakes and maybe this has something to do with what you are describing or experiencing. The same may be true if you are using brake pads with a very hard compound requiring much heat/warm up before they become most effective.

Then again, maybe your booster does not provide enough braking effort. :wink:

A recent thread may be of some value: http://e9coupe.com/forum/viewtopic.php?t=4273

 

[pmansson]

New 2002 Tii booster. Higher than normal (E9) pedal effort. New brake fluid. Straight braking.
Can a different master cylinder improve (reduce the needed effort) braking? I have the E9 master cyl. mounted.

 

[x_atlas0]

New 2002 Tii booster. Higher than normal (E9) pedal effort. New brake fluid. Straight braking.
Can a different master cylinder improve (reduce the needed effort) braking? I have the E9 master cyl. mounted.

Do you have weak intake manifold vacuum? A larger master cylinder would, in effect, give you more braking power for the same force, so to get the same power, you would use less force, yes. I think the later 7/8-er master cylinders will swap in.

 

[Percy]

New 2002 Tii booster. Higher than normal (E9) pedal effort. New brake fluid. Straight braking. Can a different master cylinder improve (reduce the needed effort) braking? I have the E9 master cyl. mounted.

I would think your E9 Master cylinder is more than adequate for your purposes. I am also familiar with a couple of folks who used the smaller booster to clear their sidedrafts and they had no complaints. My experience with mixing and matching boosters is limited. And I understand there is a science to the manufacturer's original choice of such a large repository of vacuum, but - are you certain your present booster is working properly? If you haven't already done so, revisit the earlier post about testing the booster, and maybe you will discover the vacuum hose, the one way valve or even the booster itself, is/are not working properly rather than just being inadequate for the job.

Ten_Atlas may have been on to something regarding low manifold vacuum, you aren't by any chance running a high lift/high duration aftermarket cam with the super lopey idle - are you?

Please post an update so we can all profit from your experience!

 

[pmansson]

In fact I have the same situation on 2 cars: a CSL with triple Webers and the CSi with a 3,5L engine. Both cars have identical boosters and "problem". The CSi has a brand new brake booster.
The CSL has a 304 degree camshaft and the CSi a 284 degree cam.
I am not familiar enough to comment on the vacuum issue.

 

[Percy]

Manifold Vacuum

In fact I have the same situation on 2 cars: a CSL with triple Webers and the CSi with a 3,5L engine. Both cars have identical boosters and "problem". The CSi has a brand new brake booster. The CSL has a 304 degree camshaft and the CSi a 284 degree cam. I am not familiar enough to comment on the vacuum issue.

An E9's brake booster is wholly dependent upon the intake manifold for its vacuum source. (Heavy Equipment vehicles may use alternative sources such as external vacuum pumps). Most stock engines, with stock camshafts, produce 19-20 lbs/in2 of vacuum. A smooth idle and quick throttle response are indications of this and a healthy engine. When you use an aftermarket camshaft - especially with large overlap and higher valve lift, you trade off some of the low speed smoothness for more power production at higher speeds. (I am sure there are aftermarket camshafts with less overlap designed for greater low speed torque production as in the case of diesels). Hence, a reduced supply of manifold vacuum may be part of the so-called problem.

This is surmise on my part, but your CSL's cam may compromise your normal operating manifold vacuum pressure so that it supplies less vacuum to your smaller booster at low speeds. If you use the brakes quite a bit (city curves vs. autobahn), you will tend to more quickly exhaust the vacuum from that booster (per your complaint) and render the booster less effective than you would like. A larger volumed booster (remotely located) might be the only redress for that situation. The cam in your Csi does not seem that extreme and, absent some weird atmospheric phenomenon*, your braking effort should not be that much different from stock.

You seem to have covered most of your options, but I wonder if you couldn't resolve your concerns by a simple change of brake pads with a higher friction coefficient such as the carbon fiber variety. I know of another collector (Alfas) who complains of just the opposite situation as yours (too much braking effort) since he changed to carbon fiber pads (Hawk?) He claims his car's brakes are too sensitive and has considered driving without vacuum assist brakes. Coincidentally, the booster on his Alfa looks an awful lot like your 2800 booster!:idea:

Hth!

*Assuming you don't garage your cars at the top of Mt. Blanc or the bottom of Lucerne.