320i brakes in front and now..., The rears feel undersized Options

[banderson]

I did the standard conversion to 320i front calipers and removed the proportioning valve last summer and now I think that I may have increased the stopping distance. The rears don’t feel like they are working hard enough. I overhauled the system with new rotors, rear caliper rebuild kits, DOT SS lines and street pads.

Because my front brake pistons are bigger, it takes less effort to lock the front wheels than it did before. Now the front wheels lock well before the rears come close.

I havn’t heard of anyone complaining about this before and Grassroots motorsports reported that they needed an adjustable valve to keep there rears from locking. This makes me wonder if my rears are working properly. They get hot, so I know they are doing something. The pedal is solid and they don’t bind.

I am considering changing to a high performance street pad, or putting different ones on the back, but I don’t want them to grab too much.

Your thoughts?

[nebreitling]

yup, probably 2 options:

1) hi-perf pads on the rears
2) stock 914 calipers on rear (maybe with aftermarket proportioning valve), someone else will have to chime in on the specifics of this.

or you could go back to stock. my 2 cents

good luck, n

[banderson]

I assume that you mean the stock fronts to the rears and give up the parking brake? I have considered that too.

[Joe Bob]

Even though you lose the ebrake, having the 914 front calipers on the rears is a good choice. You get rid of that STUPID venting of the pad thing and you get bigger pads.

I second the notion of the adjustable p-valve.....put in the cockpit between the seats.

[john rogers]

In the rear, make sure the venting clearance is about 1/2 of what the book calls for so the pads will grab faster. Also use a softer pad in the rear so they will grab a little harder. On our 1974 with BMW fronts only they all lock up about the same time. On my race car with BMWs front and rear, it is the same but there is a little more weight in the rear with the six.

[banderson]

John, do you have a recomendation for the pads?

[banderson]

Mike, they are a direct bolt up, arn't they?

[crash914]

brett, come by and look at my set up.

I have a tilton adjustable porportioning valve located at the old rear "T". I would recommend the dial in the cockpit.

I am running hawk hp+ in front, in the rear I am running porterfield r4s. plenty of stop...I know that I could out brake you at NHIS. had no power running pulling out of the corner though....

Your pedal should be HARD!!! If not, the rears might need to be adjusted. Pull up the hand brake and then stomp on the pedal...better?

Give me a call or stop by..I would be glad to show you what I have...on a side note, the engine is out, and cylinders are off.....ready for 103's........ (IMG:style_emoticons/default/laugh.gif)


see you later, herb

[campbellcj]

The 914-4 front calipers bolt up easily to the rear, but most people use a few spacing washers to center the caliper over the rotor a little better.

Frankly the loss of the e-brake is sort of a pain, at least for me as I live on a fairly steep hill and basically if the car backed up 3 feet out of the garage, it could easily roll into a neighbor's house. I keep wheel chocks handy at all times...the little folding ones fit in the glovebox.

Do you have equivalent pads front & rear now? How about tires? Also, is your suspension in good shape and aligned correctly? All of this stuff works as a system...weight transfer and traction are significant factors in lockup and brake "feel".

John's venting clearance tip is a good one. I used to always set it quite a bit tighter than "the book" spec.

[dcruiksh914]

If you move your old fronts to the back, you can always also add a 911 e brake set-up. I recently did a 5-bolt conversion with 911 brakes front and back. The 911 e-brake was a pretty easy add on. The only tricky part is getting a custom cable.

This might solve some of your problems.

BTW: where can we buy the adjustable proportioning valve? I want one but don't know where to find them.

[boxstr]

dcruiksh914 I would like to see pics if you have them of your 911 ebrake conversion.
The adjustable proprtioning valve should be avilable from Summit racing.
CCLINCAMP914

[campbellcj]

HEY...I found the answer over on the Pelican board!

A set of these 935 calipers will give you plenty of rear brake. (IMG:style_emoticons/default/aktion035.gif) That is a stock 911 (street) rear caliper next to it...

(IMG:http://forums.pelicanparts.com/uploads2/935vs911CALIPER1073339970.jpg)

[Brad Roberts]

What you guy's dont know is... GrassRoots is having issues with their choice of calipers in the front.. They cannot dial in the bias. If you have a Tee in place right now and they are not "locking up" << Never thought I would judge how my brakes work by "locking them up".. tighten the venting clearance like John suggested.

What I dont recommend is an adjustable prop valve. I dont understand what this will gain him ? (asking the people who said he should install one) He currently has the equivalant of a wide open prop valve with the Tee in place and he doesnt feel that the car has enough rear bias.

Oh.. do a search online for the Grassroots 914 project. They fought the brakes for several months and didnt say anything in the brake article.

Put stock calipers on it.

B

[Brad Roberts]

Winston Cup caliper and a 914 front caliper.

B

[nebreitling]

i meant to beef up the rears and THEN add a adjust-prop-valve if necessary. clearly, you don't need a a-p-v now.

'course, i'd just go back to stock...

[Brad Roberts]

I need to get a engineer to do thermal tests on a 914 rotor with stock calipers/pads and with the BMW setup on a stock rotor. Heat is the killer here and I personally believe the BMW caliper is too much for the stock rotor to handle proper heat dissipation. It may stop 2-3 times real nice with tires smoking... but how many times can you do this before the brakes fade from boiled brake fluid ?? I know how many times and how fast the stock ones fade. I'm willing to bet the BMW setup fades twice as fast.


b

[banderson]

Thanks for the tips.

Herb, did you change to the 320i calipers? When you get your new Jugs, I'll come check them out.

I did set the venting clearance to a tight .004". I'll check it again. If that doesn’t do it, and I don't think it will, I will swap the fronts to the rear (after I get an inspection sticker cus' in NH they check everything but emissions). It won't be too bad to swap them out every year for the inspection, which is my only concern.

Brad, I am a mechanical engineer.... what do you want to know about thermals on the rotor? I am sure that not everyone would agree with what I think, but I do think.

As I see it, and as has been said before, your kinetic energy becomes thermal energy. First it goes to the rotors and pads. Then it goes from the rotors to air and from the pads to the calipers, then to air. So, big calipers heat up more slowly because of their thermal mass, and transfer heat to the air better because of increased surface area. Overall, it’s the same amount of heat. Why would bigger pads cause the rotor to get hotter? Cooling fins would help. Why don’t calipers have cooling fins?

Now if the front/rear bias was off and the rears weren’t doing anything, that would be a different story. I am beginning to thing that BIAS means EVERYTHING, stopping distance and cool fronts! Good bye parking brake!

I would do controlled testing but its snowing out…

[Eric_Shea]

I would like to see pics if you have them of your 911 ebrake conversion

Craig, you can check out my 911 e-brake conversion here.

It's fairly simple. I'll be using 911 cables and a center mounted hand brake ala the rally GT's. I should be welding in that area soon. Rich's new 6 mount is in the mail.

Regarding this thread. IMHO, it's hard to beat a stock 914 brake set up if it's "set-up". A good rebuild on stock calipers with awesome pads should be more than enough to keep a 914 out braking most anything out there. Anything beyond that and I'd use a "system-wide" approach, meaning; upsizing/vented rotors etc. I'm planning for 220hp out of a 2.5 six. I chose S-Calipers on Koni struts up front and 911 rear M-calipers all with a "T" fitting and vented rotors. Worked well on 911's up to 260hp (76 Turbo) and beyond. Weight transfer is a bit different so I'm waiting to see if I'll need a adjustable bias control. I doubt it though...

That being said, I'd tighten up the rears. Maybe try a different pad back there (softer compound) and make certain you do a two-man bleed job. Have someone in the cockpit doing a 1,2,3,4 hard pump and hold. I realise there's no proportioning valve back there to worry about but I still think this method is a good one. Maybe Herb would help you (but then he might not be able to out brake you any more...) (IMG:style_emoticons/default/blink.gif) Release the rear pressure and repeat until you're sure they're blead clean. Herb's right... pedal should be H-A-R-D.

[JWest]

BTW: where can we buy the adjustable proportioning valve? I want one but don't know where to find them.

I'll have the proportioning valves that work with the original metric fittings (cheaper than the Tilton metric one) available in about two weeks (not saying the original poster needs one, just answering this specific question).

[DNHunt]

I currently have the 320i fronts and still have the stock proportioning valve in place and I like my brakes. I spent a lot of time playing with the bias. I feel the rears working but the fronts will lock before the back. Thats fine with me. I hate going backwards unless I'm in reverse.

I have no way of knowing if my setup is the very best it can be, I just feel like it's safe for me.

Dave