Notes on brake "upgrades", What's worthwhile, what isn't. Options

[lapuwali]

A bit bored this AM, so I'm rambling a little...

The 19mm MC used with otherwise stock 914/4 brakes: this results in a reduction in braking force for a given pedal force (forcing you to push harder to stop), and the actual amount is 16%. Some people like the "firm" feeling this provides, but it doesn't mean "better braking".

The 19mm MC used with BMW 320i calipers OR Porsche M calipers (same size). This results in a 12% decrease in pedal effort for a given braking force. So, you will feel an improvement, but it's not a very big one. The downsides to this swap is the bigger calipers are heavier (unsprung mass, too), and of course you're doing a lot of work for a pretty small gain. You're also doing nothing to help brake fade. You're likely making it more likely, since you can now load a bit more heat into the brakes more easily. Happily, this also won't have a huge effect on front/rear brake bias, however, since the overall effect is so small.

A 17mm MC used with M calipers would produce quite a big change in pedal force v. braking force: 33%. However, this MAY cause a problem in that the 17mm MC is small enough that you couldn't move enough fluid with it before you run out of pedal travel to fully press the pads against the rotors. You'd certainly have to push it a long way to get the brakes to come on at all, and the brakes would likely feel touchy and mushy, but powerful. Rather like overboosted power brakes...

btw, "17mm" and "19mm" (and "42mm" or "48mm" for caliper piston diameters) aren't strictly correct. Even in metric countries, brakes are actually specified in 1/16ths of an inch, so 17mm is really 11/16, or 17.45mm, and 19mm is really 3/4 (12/16) 19.05mm. 42mm is really 26/16", or 41.275mm, and 48mm is really 47.625mm. Germans just list the sizes in the nearest metric size, whereas the Japanese perversely use both systems: 10mm for some parts, and 0.50" for others, for example.

[tat2dphreak]

better braking = rebuilt shea calipers, good bleeding with good fluid, good lines...

after that, 911 brake setup IMHO

I had BMW brakes, and they are no better than stock that works...


I like the 19mm M/C.. .feels stiffer... but I think with stock brakes, a 17mm is all that's needed...

[john rogers]

It appears your rambling is just a tad off, when in the second paragraph you mention you don't do anything for brake fade, but with M calipers and vented rotors the heat dissapation is increased quite a bit. In the 2004 Tecate race I suffered brake fade but after installing the 4 bolt alloy hubs, M calipers and vented rotors in 2005 I had no brake fade. I think I drove pretty much the same, maybe a little harder even.

[lapuwali]

QUOTE (john rogers @ Mar 30 2006, 12:20 PM)

It appears your rambling is just a tad off, when in the second paragraph you mention you don't do anything for brake fade, but with M calipers and vented rotors the heat dissapation is increased quite a bit. In the 2004 Tecate race I suffered brake fade but after installing the 4 bolt alloy hubs, M calipers and vented rotors in 2005 I had no brake fade. I think I drove pretty much the same, maybe a little harder even.

I didn't mentioned vented rotors, John. Just calipers. Plenty of people fit BMW calipers to the stock 914/4 rotors, and early M calipers, too. Fitting vented rotors requires a lot more work.

[rick 918-S]

First you need to decide whether you really need to up grade your brakes or simply rebuild the stock units. Porsche engineers designed the system that is one the the car. In hteory, with stock tires, wheels, and sprung weight, stock brakes in good tune should work excellent.

It has always been my opinion, that up-rated brakes have less to do with the amount of HP the car has, and much more with the changes in weight, wheels, tire diameter, contact patch, and balance.


stopping an 85 hp from 60 mph and stopping 500 hp from 60 mph, is no different if both cars weight the same and have the same tires/wheel and contact patch.

[lapuwali]

QUOTE (rick 918-S @ Mar 30 2006, 01:49 PM)

It has always been my opinion, that up-rated brakes have less to do with the amount of HP the car has, and much more with the changes in weight, wheels, tire diameter, contact patch, and balance. stopping an 85 hp from 60 mph and stopping 500 hp from 60 mph, is no different if both cars weight the same and have the same tires/wheel and contact patch.

(IMG:http://www.914world.com/bbs2/html/emoticons/agree.gif)

The primary reason, however, for upgrading brakes with large increases in power is that on a tight road/track, you'll be hitting 60mph in places where before you were only hitting 40mph, so you'll need to brake from the higher speed down to the same speed (assuming the weight really is the same, same cornering speeds) more often. In this case, you really need to upgrade the "fade resistance", so you need bigger and/or vented rotors more frequently than you need bigger calipers.

Of course, one rarely sees big increases in power in a 914 w/o big increases in weight, too. A V8 powered 914 is 20-30% heavier, at a minimum, than a 1.7 powered 914. The V8 is also going to shift the weight back, so you need more rear brake bias to fully take advantage of this. So, bigger calipers just on the front will not help this.

A turbo Subaru swap would probably weigh about the same as a stock 914, so you only have to handle the additional power, and could certainly get away with no brake upgrades at all, unless you intend to push the car hard on a tight course.

[jr91472]

(IMG:http://www.914world.com/bbs2/html/emoticons/popcorn[1].gif)

[rick 918-S]

QUOTE (lapuwali @ Mar 30 2006, 02:09 PM)

QUOTE (rick 918-S @ Mar 30 2006, 01:49 PM) It has always been my opinion, that up-rated brakes have less to do with the amount of HP the car has, and much more with the changes in weight, wheels, tire diameter, contact patch, and balance. stopping an 85 hp from 60 mph and stopping 500 hp from 60 mph, is no different if both cars weight the same and have the same tires/wheel and contact patch. (IMG:http://www.914world.com/bbs2/html/emoticons/agree.gif) The primary reason, however, for upgrading brakes with large increases in power is that on a tight road/track, you'll be hitting 60mph in places where before you were only hitting 40mph, so you'll need to brake from the higher speed down to the same speed (assuming the weight really is the same, same cornering speeds) more often. In this case, you really need to upgrade the "fade resistance", so you need bigger and/or vented rotors more frequently than you need bigger calipers. Of course, one rarely sees big increases in power in a 914 w/o big increases in weight, too. A V8 powered 914 is 20-30% heavier, at a minimum, than a 1.7 powered 914. The V8 is also going to shift the weight back, so you need more rear brake bias to fully take advantage of this. So, bigger calipers just on the front will not help this. A turbo Subaru swap would probably weigh about the same as a stock 914, so you only have to handle the additional power, and could certainly get away with no brake upgrades at all, unless you intend to push the car hard on a tight course.

(IMG:http://www.914world.com/bbs2/html/emoticons/agree.gif) Close... Here's my expanded thought on the subject. It still doesn't matter if your traveling 60 mph with an 85 hp car or if your traveling 60 mph with 500 hp. All things being the same the car will brake the same. When things change is when you add HP, stock tires and standard width wheels are no longer transmitting the added torque. When you move up in tires and wheels, the increase in weight, diameter, and contact patch, this decreases the brakes ability to stop the added mass. Think about adding a 4' pipe to your breaker bar.

[lapuwali]

I haven't done the math, but my gut feeling is the increased rotational inertia of heavier wheels/tires won't have a big effect on the brakes. There will be SOME, but I think the effect would be relatively small. I'll have to try to calculate the effect sometime.

[turboman808]

QUOTE (lapuwali @ Mar 30 2006, 08:17 PM)

I haven't done the math, but my gut feeling is the increased rotational inertia of heavier wheels/tires won't have a big effect on the brakes. There will be SOME, but I think the effect would be relatively small. I'll have to try to calculate the effect sometime.

WOW I wish I knew how to find the info but in a Grassroots motorsports magazine not to long ago they upgraded the brakes(on a neon) to some really beefy road race brakes and also upgraded to larger tires. I could have swore they lost 15 feet or so slowing from 100mph. But they did eliminate brake fade.

You would figure with the huge wheels and tires and huge brakes it would stop faster but it didn't. Think rotational inertia might have played a part in this.

[DanT]

That is why folks putting Dubs on their SUVs should be upgrading their brakes....huge increase in rotational forces.

On my particular car I upgraded to Ms in front and 914-4 fronts in the rear.
I did this to get increased fade resistance for the track. (IMG:http://www.914world.com/bbs2/html/emoticons/biggrin.gif)
with my tire combo which will have a smaller overall diameter and increased braking efficiency I am looking at much greater overall performance. (IMG:http://www.914world.com/bbs2/html/emoticons/wink.gif) (IMG:http://www.914world.com/bbs2/html/emoticons/dry.gif)

[Trekkor]

I run rebuilt stock calipers with Porterfield r-4's race pads all around.
Stock MC, prop valve, SS soft lines.

These are fantastic. I've yet to find the limits on the track.

For the street I can't imagine any better.
Anyone who rides along is stunned (IMG:http://www.914world.com/bbs2/html/emoticons/shocked[1].gif)


KT

[racerx7]

If all your doing is a single brake test from 60mph to 0 then yes it does not matter
if your car has 500hp or 85hp.

If your upgrading your brakes for track events (not autox), there is a big difference
between a car that makes 500hp or 85hp. You carry so much more speed every
where and your on your brakes sooner because you got there faster. Less time
for your brakes to cool also.

[Andyrew]

Yup. I experienced this with my car at the wcc.

Before then I never got the brakes hot... But once I got them hot, they stuck like glue... and I KNOW I was on the brakes a lot more than most of the cars... when I was pushing the car.

Andrew

[Eric_Shea]

Cool thread.

Thought provoking stuff before my pragmatic view:

* Larger wheels and tires generally = larger contact patch - Talk amongst yourselves.
* Excellent new tire and pad compounds - Talk amongst yourselves.

My pragmatic view:

Stock 914 brakes were over-engineered for their day, just like any Porsche brake system. This mark is known for the best brakes in the business.

Today's tire compounds are leaps and bounds over what was available in the 70's... so are today's pad compounds.

If you have a 914 with stock fenders you're 'probably' good to go with stock brakes. If you are racing said 914 in a 'serious' road course environment you'll want to look into brake 'system' solutions. There's evidence of this within this thread (John Rogers).

If you're autocrossing you'll generally be good with stock brakes but it would do you well to get good pads. There's evidence of this within this thread (Trekkor).

BMW brakes are not an 'upgrade'. If you think they are, you had really shitty brakes on your car before you put them on. The exact same 914 with BMW brakes and stock brakes will stop in the same distance. Your tires (contact patch) will be the limiting factor. He/She who locks up his/her wheels faster stops last... sliding is not stopping. 914 brakes can lock the wheels with the right pads (even with most wrong pads).

If you've successfully increased your contact patch through the use of flared fenders and wider tires you could probably benefit from a larger brake system. If you haven't, you're probably just trying to be cool (IMG:http://www.914world.com/bbs2/html/emoticons/cool_shades.gif)

Flared 914's, with the exception of all but the largest wheels and tires can be successfully stopped on a regular/race basis with a set of A or S-Calipers. Anything larger is probably bragging rights. These cars raced for 24 hours back on June 13th and 14th of 1970 with 908 calipers (basically an S-Caliper with fat-ass pads). They were around 220hp, lightened a bit and had 7-8x15" wheels and race tires of the day. They won their class in that little race called "LeMans"! (IMG:http://www.914world.com/bbs2/html/emoticons/w00t.gif)

What do you plan to do with yours? (IMG:http://www.914world.com/bbs2/html/emoticons/biggrin.gif)

Talk amongst yourselves...

[jasons]

This thread is hitting close to home right now. I have a dilemma. I was planning on flaring my car and using some custom widened pedrinis I have, with Eric Sheas calipers. In fact Eric has already rebuilt my front calipers and the rears are packed in a box ready to go. I also have the FG flares (I planned to rivet, not bondo) and GT rockers.

Fast forward to now. I picked up this new 73 2.0 for a song loaded with an SC front end and 16x6 Fuchs. Plus a ton of other bonus parts.

Here is my dilemma:
A: go forward as planned flared with four lug widened Pedrinis (which are totally bitchin BTW).

B: go forward NARROW with five lug fuchs, SC front end and dial the rear to match.

C: go forwad WIDE with five lug fuchs, SC front end and dial the rear to match.

I am leaning towards B for now, and C can always come later. I can't see forefeiting the SCbrakes and 5 lugs.

[lapuwali]

More rambling:

When "improving" brakes, you can really only do threethings: reduce the chance of fade, reduce pedal effort, change brake bias front/rear.

Reducing pedal effort can be done two ways: change the hydraulic ratio of master cylinder to calipers, or add a power servo. Adding a servo to a 914 is possible, but pretty difficult. Altering the ratios is certainly doable, and can make a difference. There's a sweet spot on ratios, which ranges from about 18:1 at the low end and 24:1 at the high end. Lower than 18:1 (master cylinder too big), and the pedal feels "wooden", there's very little pedal stroke, and you have to push hard enough that you end up losing the ability to modulate the pressure. Higher than 24:1 (master cylinder too small), and the brakes feel mushy and touchy. Pedal stroke is quite long. You also run the risk of running out of pedal stroke before the calipers are fully pressurised, since the small MC can't push as much fluid. These numbers are strictly empirical, and it really is a "feel" issue before it starts to become a safety issue. On a 914/4 with stock calipers, the front brakes are 22.3:1 with a 17mm MC, and 18.8:1 with a 19mm MC, so they're both in the sweet spot, but at opposite ends of it. 20:1 would likely be the most pleasing to most people, but there's no off the shelf combination of Porsche parts that will get you there. And yes, the front and rear brakes are considered separate systems, since the 914 uses a dual MC. The combination of the rear brakes will also affect feel. The ratio at the rear should be less than the ratio at the front, or you need an adjustable proportioning valve to reduce the pressure at the rear calipers, so they don't lock first.

Reducing fade is usually the most worthwhile goal in improving brakes, and certainly what you need to do if you're going to track the car. This cannot be accomplished by just swapping calipers. You must either change the pads (the material CAN make a difference), or enlarge the rotors, either with larger diameter rotors, or vented rotors (or both). The downside of rotor swaps is that they add unsprung mass AND rotational mass, the former making the suspension work harder, the latter making the brakes work harder. If you're a zillionare, you could go with ceramic or carbon composite rotors, of course. Using lighter aftermarket calipers can help offset the additional mass of the bigger rotors, though that won't help the rotational problem (which I need to characterize).

Improving brake bias is, I think, the overlooked upgrade for the 914. I wish I had more data, but I get the feeling that the 914/4, as stock, is actually underbraked at the back. This is a "safe" engineering decision, since preventing premature rear lockup is important in a street car to be driven by non-experts. However, I get the feeling some useful braking is left on the table by an overly aggressive front/rear proportioning. With the usual adjustable proportioning valves on the market, you can only get a useful change in brake bias by also fitting much larger rear calipers, since all these valves can do is reduce the braking to the rear. If you don't have enough braking at the back to begin with, you end up with a valve that's merely open (i.e., a Tee).

[Allan]

I'm currently have the parts to go with a '89 SC front end w/ "A" calipers, 19mm m/c, ss lines, stock rear calipers, w/no proportioning valve and Porterfield pads.

Hope it works...

[URY914]

here are a couple of my thoughts on the subject:

a. the BMW "upgrade" is more trouble than is it worth.

b. unless you live at the top of Mt. Hood, I don't thnk a street car only driven on the street will EVER have a problem with brake fade.

c. I have stock calipers on my car. I realize I'm stopping less car than everyone else, but even when I did track events they were fine.

d. Never heard of anyone having fade problems at an a/x.

I know most of what I just said has been said above but I didn't want to be leaft out. (IMG:http://www.914world.com/bbs2/html/emoticons/wink.gif)

[Eric_Shea]

Paul,

When I saw you name up on the list, that's exactly what I thought as the thread was loading... stock brakes for that car.

Helps prove the point and (IMG:http://www.914world.com/bbs2/html/emoticons/agree.gif) with all your points.