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Posted by: jhynesrockmtn Jan 22 2020, 09:17 AM

Hi, I am in the process of refreshing a 914-6 GT tribute car that was built for PCA solo events in CA and has been mostly in storage for several years. The engine is a 2.4 that was built by GD racing in San Bruno. The suspension and brakes are 3.2 Carerra. I pulled the calipers and they are at PMB. New soft lines are going in. I was somewhat surprised to find the stock 914 brake pressure regulator still in place. There is no after market proportioning valve.

The car will be used a few times a year for vintage wheel to wheel and then some PCA DE events. I have several years of experience racing vintage FV. As I got older and saw several buddies get involved in some nasty open wheel tire to tire incidents I decided I wanted fenders and more metal around me.

I am not the most aggressive driver in the world and this car will remain street legal. I also want it to remain as period correct as I can get. I don't want to go down the rabbit hole of cutting a ton of metal out of the car, etc.

Should I pull the regulator and put a T in and proportioning valve in the cockpit?

Should I leave as is and see how it performs? If I leave it I will send to PMB to have them rebuild it.



Cheers, Jerry

Posted by: stownsen914 Jan 22 2020, 10:30 AM

If you a google search, you'll find a lot of discussion on this topic. I have a similar question, as I just bought an old 914 SCCA racer that also still has the stock "proportioning" valve (it's really a spring loaded pressure limiting valve). I noticed that Eric recommends keeping the stock valve, and I see he rebuilds them. I'd think that it would be a good idea for a track car, to replace the stock valvue with an aftermarket one like a Tilton valve, but I'm not informed enough at this point to say if it's a good idea. Brake bias is highly dependent on which calipers you have and tire widths, and I'm not familiar with the Carrera caliper setup on a 914. Hopefully others will chime in.

I've also wondered if the Tilton style valves are really a good solution to limit rear brake pressure, since I believe those valves are more of a flow restricting limiting valve. I'd think that pressure would eventually rise in the rear brake circuit with one of those if you are on the brakes for a couple seconds or more, but maybe I don't really understand how they work. Curious if someone here knows better.

I can tell you that too much rear bias on a track car is very dangerous and to be avoided. I've driven a couple race cars with dual master cylinder setups, and have experienced rear lockup before the bias was dialed in. You can lose control of the car in an instant if you lock up a rear wheel.

Scott

Posted by: mepstein Jan 22 2020, 10:45 AM

Great looking car. Call up PMB and talk to Eric. He is very busy but still loves to talk about this stuff and is a wealth of knowledge. It's function is to keep the rear calipers from locking up before the fronts. It doesn't limit normal braking.

Posted by: brant Jan 22 2020, 12:53 PM

Adjustable aftermarket

Posted by: Charles Freeborn Jan 22 2020, 11:25 PM

Dedicated serious race car? Adjustable. But... that means you’re calculating ALL the grip factors (tires, pressures,temps, etc).
Casual racer? Have PMB go through it, set and forget. Go have fun.

Posted by: ThePaintedMan Jan 23 2020, 08:34 AM

I just switched over from the stock limiting valve to an adjustable. After 3 sessions, I finally turned it all the way to the back (at least I think I did) and I still don't feel like I'm getting enough rear pressure. Unfortunately I converted to the BMW calipers on the front a long time ago, which I think is part of the problem. But it did seem to be a stiffer pedal.

However, I don't think you necessarily *have* to eliminate the stock valve, if you don't mind giving up a little braking and pedal feel, especially if you aren't racing ten-tenths. But it's probably a good idea if you want to get the most out of the brakes.

Posted by: stownsen914 Jan 23 2020, 10:22 AM

The stock valve has a spring loaded mechanism that "absorbs" pressure instead of sending all of it to the rear brakes. I recall in my street 914 that if I stepped hard on the brakes when the engine was off, I could hear the valve actuating and could feel the extra brake pedal travel.

Posted by: yeahmag Jan 23 2020, 05:15 PM

The pressure bias is to account for varying conditions or non-stock brake bias ratios. They do (at least the Tilton) limit pressure, not flow. If you have too much rear bias you need one and the factory limiter won't help you as your bias ratios are already wrong, not just in a panic situation.

For example, I have 911 M calipers up front and switched from 914-4 rears to 914-4 fronts on the rear as I was originally too biased towards the front (with the stock rear brakes). We had done the calculations and knew I was on the harry edge of too much rear bias, so we put in a twist style bias regulator (wish I had done a lever style). It took a few runs to dial it in, but it's perfect now.

Posted by: jd74914 Jan 24 2020, 10:07 AM

The Tilton proportioning valves are proportional pressure regulators; not flow limiters (like a variable size orifice, etc.). The attached image shows a chart of a lever-style valve. Their action is quite similar to the OEM valve, just with an adjustable knee point.

Note: All valves will have some initial flow limiting effect, but you're likely not to notice too much when you're really on the brakes hard. Stopped and just pushing in the brake pedal, perhaps, as already noted in some above posts.

Posted by: ThePaintedMan Jan 24 2020, 03:40 PM

Dual MCs with a balance bar always seems to be the best bet. But way too much fab for guys like me

Posted by: jd74914 Jan 24 2020, 05:27 PM

It's interesting; there are quite a number of reasons to have both a bias bar and an adjustable proportioning valve.

Primary is the fact that you can adjust both portions of the pressure ratio curve. The bias bar is used to adjust front to rear pressure ratio so it's not a 1:1 ratio as shown in the Tilton curve I posted. This makes it so you can adjust for different piston diameters and pad lever arm length on the rotor. The proportioning valve comes in at higher brake pressures to further reduce rearward braking. This becomes really useful when you consider weight transfer as a function of deceleration rate.

For example: when you first starting braking, before you upset the car, the front and rear do a similar amount of braking (in a car with 50:50 weight distribution) so allowing closer to 1:1 performance front-to-rear is a benefit for scrubbing off speed. As you really hammer the pedal weight starts transferring forwards so maybe say 60-70% of your weight is on the front wheels. At that point the front now needs to proportionally be taking care of more braking so the adjustable knee in the curve as set by proportioning valve is important.

Hopefully that made sense...it's easier to show with some plots but I don't have any handy at the moment.

Posted by: stownsen914 Jan 24 2020, 09:09 PM

Thanks for that - makes sense.

Posted by: mlindner Jan 26 2020, 02:23 PM

Jhyn......, I agree, send to Eric to rebuild. FYI I ran with a "T" for 15 years, lots of auto-x and high speed DE at Road America, also street legal. Never had a problem with hard braking wet/dry, and still in the car as it develops into a 914-6 GT Tribute. Best, Mark

Posted by: jhynesrockmtn Jan 27 2020, 08:56 AM

Hi, thanks for your response. I'm a bit confused though. You say send it to Eric but then reference a T that is still in the car? Beautiful car by the way!

FWIW, I'm leaning toward having it rebuilt and re-installed for this year and see how the brakes perform and if I'm not happy, address it later. I've been trying to not make too many changes as the car was built by a pretty knowledgeable person and I don't have a ton of time to get it ready if I'm to make the first available test sessions and there are many other things to do.

Posted by: mlindner Jan 27 2020, 05:50 PM

Jhynes...sorry for the confusion. Eric is the expert hands down. I'm just saying the "T" worked well for me under the conditions up north. Best, Mark

Posted by: jhynesrockmtn Jan 30 2020, 08:35 AM

Excellent, thanks Mark!

Posted by: mlindner Jan 31 2020, 06:43 AM

Jerry, you have very good taste in car and color. Wish I was closer to see your build. Cheers, Mark

Posted by: jhynesrockmtn Feb 3 2020, 08:43 AM

Well, I take no credit for the build. I wish I was close enough to check yours out! It was done by a Porsche tech in the Bay area many years back. I'm just trying to get it back on the track. I got the brake calipers back from PMB and am hanging those with new soft lines. A new MC will be installed. I also have a fuel cell from Patrick Motorsports ready to install. The Weber 40's were in bad enough shape that they went to a local mechanic after I failed to get a plug and jet holder out due to them being so stuck in the carb body. The hope is to get this ready for an event in late April or at least early June.

Posted by: mlindner Feb 7 2020, 02:39 PM

jhy... thanks for sharing. Nice calipers. Are you going to install braided lines?

Posted by: campbellcj Feb 9 2020, 03:19 PM

I put an adjustable valve in my car but then for years with its current brake setup I always have run it wide open; at least one mechanic suggested I could just ditch it and go back to a plain tee.

Posted by: jhynesrockmtn Feb 10 2020, 08:05 AM

Yes on the braided lines. They were installed this weekend. Getting the old rears out was an interesting challenge :-) I had ordered some for a 914 I ended up selling this past year and had never installed them. I also have a new in the box ATE MC that I never installed and that is the next part of the project.