Piston sizes I believe are 30mm.

I'll state again for the record that I am not an engineer.. physics was not my strong suit in school. So I'm learning all of this as I go.

I haven't had enough time in the car to determine how much I like the new brakes. I certainly haven't driven it on the track yet.. I've literally driven it like 30 miles.

I will say that during the bedding process it felt like they had more grip and really helped slow the car down faster. But, that was also in a straight line.. So I couldn't feel much in terms of brake bias.


Dave: Where did you read that WRX 4-pots suck? Have you tried them? I've read one review on this forum where the guy said he loved them. Where did you hear that they move the bias toward the rear?

Here's my math:
The stock BRZ calipers have 48mm pistons
if my high school geometry is correct, that would make for a radius of 24mm. Pi x R ^2 = 1810 x 2 pistons = 3620

The Z32 calipers have 40.45mm pistons
same math gets me 1282 x 4 pistons = 5127

So, in theory the Z32 caliper should inpute more clamping force right? So I'm not sure why that would result in moving the brake bias toward the rear?

Again, i'm not a physics expert..

One thing I noticed is that the Z32 pads are slightly smaller surface area than the BRZ stock pads. I don't know how much smaller though.
Does a larger pad surface area increase stopping ability? It seems logical that it would.. but I'm not sure about that.

Couldn't a proportioning valve be utilized to balance the bias out?

The pads sit on the extreme outside of the these 317mm discs. When I was test fitting them I was actually a bit worried they might "overhang".. where the top of the pads isn't actually touching anything.. That is not the case. The top of the pads is flush or just slightly under the edge of the disc face. I credit KNS Brakes for a good design here.

I should take a side-by-side pic of the pads so we can see size differences. I think the Z32 pads are slightly taller, but not as wide.. maybe a tad smaller in overall surface area. They seem to sweep the top 2/3rd of the LegGT rotor.

I read online that the further away the center of the swept area is, the better the braking power. Anyone else know more about this?

I believe pad choice can also impact brake bias as well.


Guys.. I hope I don't come across like I'm trying to "defend my choice" and save face or something like that. I'll share my thoughts on the performance of the brakes through my real world usage (when I get a chance). But I'll also submit to the knowledge of the forum members.. If *we* can do the math and figure out that brake bias is severely impacted or some other input comes forth that makes these turn out to be a bad modification.. then I'll happily admit that the case. So far though.. I like how they feel.

I appreciate your posts and this thread! Thanks!

I love to explore alternatives to high-dollar parts, and I'm just trying to gain as much info to determine if this is a workable alternative.

Part of me wants big, cool looking calipers too! I know a big improvement can be made with simply changing fluid, pads, and brake lines.

If I can find salvage yard parts at pennys on the $$ and it works, I'm all for it.

Piston area is smaller, which means less brake torque at a given pressure (which with no other changes is constant for our purposes). That means that given the same rear setup in both cases, bias will shift towards the back. I meant suck for this (and the WRX) application, not that they suck for any other reason (ie, high flex, crap seals, poor construction, et cetera).


It actually doesn't work like that for piston area on a slider. You measure only one side of both calipers. If you want to measure both, you'd just double the OEM ones (so 7,240 based on your math for the WRX calipers).


Based on the correction above, the piston area is actually smaller than stock, so it'll shift towards the rear if all else is equal. Until now all the comments I said about shifting towards the rear was directed at the WRX calipers as I didn't bother to lookup the Z32 ones. Given the better leverage though it could be a wash (leverage = further from the hub, no different then using a longer lever to apply torque to a bolt vs a shorter lever)



Swept area does have an effect combined with pressure and piston area as well as leverage. However, brake force by itself isn't the issue (OEM calipers have PLENTY - they can lock up R compounds so an increase is force is simply not needed). A potential change in brake bias can lengthen braking distances due to easier lockup (more ABS intervention). This assumes Subaru did the job correctly and optimized the bias as well as they could, which isn't a sure bet given the parts bin nature but clearly they tried (hence the LGT rears with the WRX fronts vs all LGT or all WRX, etc). Prop valve/abs pump/etc play a roll in that tuning as well so we should assume that it's at least pretty good from the factory.

Put it in the other reply, but just think of a breaker bar vs a normal ratchet. Same story.

You aren't coming across like that. I'm not attacking your choice either. Just having a conversation on a mod.


Yes, pad choice can impact bias (I fine tune the race cars brakes with pad choice often).


Sucks that it's not using all of the friction surface.


If anyone has a correct year (10+?) LGT front caliper you should install that on one side with matching pad material and do some testing, easy way to see which is more effective in terms of force lol

I'm not sure I agree with this part..

And again, I'm no physics expert

Here's what I think I know.. Brakes work by having the master cylinder push brake fluid through the lines into the caliper to fill in behind the pistons. That hydrolic pressure pushes the pistons outward and that in turn pushes the pads into the rotor face. The friction created by the pad pushing against the rotor allows you to slow down.

Knowing this.. I don't think you should "double" the calculation for the OEM sliding calipers. They only fill with fluid on one side. The hydro pressure from the master cylinder is only filling in behind two pistons. That force is distributed to the other side as well.. but it's not double force.. it's the same amount of hydrolic fluid.

If anything, I'm worried they have too much front bias.

I'm planning to do the 2 pistons rears as well.. just need to come up with the extra $400 to finish it off. Doing so should help even out the brake bias (whether the Z calipers have more or less force.. it should help equal it out going to fixed on all 4 corners).

Any thoughts?

Equal and opposite reaction......


Do the rears fit? I can't recall 300z rear brake specs, haven't installed them for years it seems like. By fit I just mean wide enough to fit new pads and the rotor since you can make any caliper "fit" :)

Found these. The guy is selling brand new (front only) calipers, rotors, lines and pads for $750 (pics are of used ones).

More than I want to spend, but maybe someone might want to jump on these.

https://sfbay.craigslist.org/nby/pts/4547823750.html

Just checked, width is identical so rock on :) Diameter is close too so pad should fit nicely assuming a good bracket. Didn't check piston sizes or do any math :)

Those are not oem z brakes. Wilwood calipers, etc.

Ebay usually has a dozen listings for full sets of 4 calipers for 300 to 350.
Salvage yards are normally under 75 per front caliper. Rock auto or other online stores have them too. Hell.. i found mine on amazon of all places.

Great find @eikond! :)

I have some questions though, since my Google-fu is terrible right now:
*Aren't the stock front calipers 2-piston sliders with 42.8mm pistons?
*Your Z32 TT's are fixed calipers with 4x 40.4mm pistons?

If that's all true, then the piston areas are 2,877mm^2 and 5,128mm^2 respectively. That would be a massive front-bias movement. Sorry @Dave-ROR, your equal and opposite reaction idea isn't a perpetual machine that creates magi-double piston area in the formula. There's a fixed amount of piston area, and thus force, for any particular caliper, regardless of it's piston amount and orientation.

Now, what's happened here is since there's so much more front piston area, the master cylinder requires much more pedal travel to get the same line pressure. Chances are the car now has a longer and softer pedal. The pad mu is totally unknown and of course impacts brake torque significantly.

Using Legacy777's spreadsheet, the rotor diameter change is resulting in about a 5% front bias change.

That would all add up to a huge front bias increase. I don't feel like calculating it right now, plus without knowing the pad mu's it's fruitless.

If I were tackling this from scratch I'd want to find a front caliper with smaller pistons, but for $600, an inch more rotor diameter and 6mm more thickness, on top of EBD that everyone has seemed to forget about, this is probably a decent DIY solution even for light track use.

Bravo! :thumbup:

Thanks @Ryephile I appreciate you sharing some of your engineering genius with us.

I thought for sure I saw that the oem caliper had 48 mm pistons. I'm sure it still results in more front bias.. but I don't think as bad as you calculated at 42.8mm.

I need to find rear piston sizes and see how those change and how that affects overall bias

thanks again for your input

oh.. and you are correct.. the pedal does feel a bit longer. I was going to bleed them once more just in case I had a little air. But that makes sense.

You only use one side of the caliper for piston area.

The spreadsheet you linked to notes the following:

"The caliper piston area calculation for fixed calipers only utilizes the pistons from one side of the caliper."

Also on the TCE brake bias calculator site:

"Note: multi piston calipers express their values using one HALF of the caliper body. This accounts for the floating aspect of the single piston caliper. A six pot caliper would be 1.625/1.125/1.125" for example. True clamping force would be double that but also double the single piston of a floating caliper taking into account the 'pull' of the outer pad to the rotor surface. Using total area (all six for example) would require you double that of the floating caliper also- thus the net result is the same whichever way you do it."

http://www.tceperformanceproducts.com/bias-calculator/

EDIT: The piston sizes everyone uses for BRZ are wrong. They're ~40.4 mm front and rear.

- Andy

I agree it's not exact, but you are still wrong because it's a lot closer to double than it is to zero. Ask any brake system engineer in the world about this, I know some that have worked at Brembo, designed calipers for NASCAR and other pro series. They will tell you the same, use one side of the fixed or double the sliding. @JRitt will tell you the same thing also.

All brake force calcs use one side, fixed or not, evidenced by your own link.

BTW, I did reference EBD indirectly to that point (referring to it as a modern ABS system since it's all the same system for this discussion), however a faster lockup STILL causes ABS to work more which reduces effectiveness (which is why threshold braking is still the best, you never lock/release/lock/release/repeat).

edit: I should have read Andy's post first ;)

Edit 2: Rye, I'm flying to DTW tomorrow to head to Gingerman, might be back in the area sunday night, then heading to Toledo for a few days to visit family. Adhoc FT86club bar meet? We can all discuss Z32 calipers lol

The plot thickens.

The adapters were made because of the price. The mount is the only difference and z32 brakes are much cheaper than the 4/2 pot set-up. Production volume was higher plus they are a lot older so more used ones around for cheap.

And I'm confused, are you talking about installing only the front brakes with the stock rears? If the 2 pots are small diameter how could it shift bias rear? I was under the impression swapping to WRX 4/2 pot set-up was very similar in function to stock, just gain a bit of feel but mostly the looks.


I have been researching the exact set-up and the piston size is the same for the z32 calipers as the WRX4/2 pot set-up. This goes for the front and rear from what I recall. I'll try to find my notes to confirm.

I want in on that.. though I'm not much of a bar fly.


I still don't get how the sliders can double braking force. I get the equal and opposite force theory.. but it seems like that would be pad against rotor and vice versa.

let's throw out a random example.. say you have a styrofoam ball. Let's say you have two arms (most of us do) and they have equal muscle strength and maybe you can exert 100lbs of force. If you put the styrofoam ball against the wall and pushed with one arm you be exerting 100 lbs of force and could compress the ball a certain amount. But if you put the ball between your two arms and pushed on each side you would have 200 lbs of force you would certainly be able to compress the ball more right?

Just seems like common sense that more total piston area would be able to push with more force.

In your example the wall doesn't move. In a sliding caliper, the force of the piston pulls the other side of the caliper, and thus it's pad, against the rotor.

I agree, but it looks like logic isn't right in this case.

The pressure from the two slider pistons are acting on the caliper to both push the pad and pull the slider. There's no pulley system to create (let alone double) torque. In a fixed caliper, all pistons are pushing against the rotor, not half of them. So far I haven't read an explanation that clearly explains this alleged "doubling" of force a slider caliper has.

If I'm wrong, assuming the pads are the same mu as stock, the total rear bias increases by 6.6%. Put a slightly more aggressive front pad and the bias will shoot forward.

But the act of pulling the other side towards it should take half of the force away?

I give up. Lol. Too late. Need sleep.

Thanks for the conversation today.. good stuff!

If using the same rears, and the same front leverage (this setup is not doing that though), the lower brake force WRX 4 pots (compared to our OEM calipers) would shift bias. Due to fancy ABS related tech (EBD specifically) that matter less than it use to, but can still engage ABS too frequently resulting is a loss of efficiency. Noticeable? Probably not without measuring equipment. The variable here is the bigger lever due to the mounting location being farther from the hub. It could offset that perfectly (or not) making this a perfectly stock bias larger heatsink upgrade. Or it could shift bias. I don't know the piston sizes on the rear of the car (OEM or 300ZX) so I'm not sure what the difference will be there.

Your (not you specifically, the end user in general) goal of a brake upgrade is key to component selection. All of these kits/setups serve a purpose, I wouldn't have done this setup simply because over time it would cost more than what I do run (I've bought pads for my caliper and the Z32 at the same time and the Z32 pads were clearly more expensive.. not a little.. but a fair margin more based on my pricing which isn't always retail). My setup also has a larger weight savings. It's also more track focused and not as street friendly (noisier, etc).

I have nothing against this setup to be honest, I just like these discussions and like to see actual data, costs, etc and don't want people to rush out and do it just because it seems like a good price. And no, I'm not saying it's not, I'm saying we don't really have that data right now.

I'll let some brake engineers explain it, JRitt has someone on staff I'm sure. They can do a better job then I can.

Force via caliper *BODY* movement vs caliper *PISTON* movement is the difference. Given the same exact piston area, brake force is the exact same (everything else being equal).

The key benefits of fixed calipers are feel, modulation, and pad wear (less tapering as force is applied more evenly across the backing plates).

This is from my race car. Sliding calipers can exert PLENTY of force on the pad:
http://www.itrexpo.com/users/dave/ch...kes/brake6.jpg
http://www.itrexpo.com/users/dave/ch...kes/brake7.jpg

And so can the piston ;)
http://www.itrexpo.com/users/dave/ch...kes/brake3.jpg

That's my point, ignoring frictional losses of the slider mechanism. I obviously get the whole Newton's 2nd law thing, fixed calipers are doing that with line pressure instead of sliders, otherwise rotors would be pushed off the hubs. :bonk:

Nice picture. It looks like a fairly classic case of not enough mu. Did your bleeders [hell, anything] survive that much line pressure to bend the backing plates?

Z32 calipers:

1990 NA = 26mm aluminum
1990-92.5 TT = 30mm aluminum
1991-92.5 NA = 30mm aluminum
92.5+ = 30mm iron

I've had all the versions. The switch to aluminum was a heat/warranty issue, not flex. Not an issue with lower weight cars.

-alex

There's no spacer.

A comparison of all the Z32 calipers:

http://importnut.net/300zxbrakeswap.htm

-alex

Piston didn't. Didn't check anything else. That pad was a $20 advanced auto set when we ran out of real pads and wanted to finish the race.

The other pad pictured was a real race pad :) backing plate wasn't nearly as crappy lol

Ah. Now brake bias makes sense. I really know nothing about brake bias, clamping force, etc so I was trying to figure out how smaller pistons shifter force to the rear. Makes sense the electronics would attempt to compensate the change in force.

I'll try to look into finding the piston sizes for everything so the right people can figure this out. I probably could do the math if someone gave me a formula (hell, I'll try to look that up too)

right on. guess you can't believe everything you hear on zilvia.

I did some more research on caliper piston sizes.

From what I can tell, the BRZ has the same brake calipers as the 09 WRX.. Right?

http://i249.photobucket.com/albums/g...brakespecs.jpg

So based on this chart, the WRX has 42.8mm from pistons (x2) and a single 38.1mm rear piston.

The Z32 calipers have 40.45mm front pistons (x4) and 38.1 mm rear pistons (x2).

Simple math (40.45/42.8) says thats about a 5.5% reduction in clamping power. It's probably more like 8% if you use a proper goemetric equation for area of a circle. If the leverage increase by using the larger disc gets you 5%+ back.. then the brake bias is only changed by maybe 3%.. which is probably not noticable. I think this is what Dave has been concluding so far.


I will also get a measurement myself on my actual brakes since I have both at home.. but I have to wait until I actually get home to do it.. so I'll do that this weekend.


I'm also going to punch these figures and the bigger rotor diameter into the brake bias calculator (later today when I have some free time) and I'll see what it shows.

All USDM (and ROW "High Spec") have 2010+ or whatever Legacy GT rear brakes, only the fronts are WRX spec.

The Legacy777 spreadsheet that Ryephile linked to could probably be tweaked to get you the right numbers for the Z32 caliper + LGT rotor combo and compare to stock BRZ.

- Andy

Based on this link. http://legacygt.com/forums/showthrea...ize-18615.html

the rear piston for leg gt is also 38.1mm. So it should be same for brz and frs


I used this online bias calculator:
http://www.tceperformanceproducts.com/bias-calculator/

Stock:
front piston size (inches) = 1.685 and 1.685 (42.8mm)
front pad cf I left at .48 (street performance example)
front rotor diameter 11.575
front pad height (I left at 1.875.. not sure how to calc this so I left it at the prefilled value)
rear piston size = 1.49999 (38.1mm)
rear rotor diameter = 11.42
rear pad cf = .48 (same as above)
rear pad height = 1.25 (left at prefilled value)
Result = Front brake bias 0.697 or 69.7% to front.

Z32 brakes:
front piston size (inches) = 1.5925 and 1.5925 (40.45mm)
front pad cf I left at .48 (street performance example)
front rotor diameter 12.44
front pad height (I left at 1.875.. not sure how to calc this so I left it at the prefilled value)
rear piston size = 1.49999 (38.1mm)
rear rotor diameter = 11.42
rear pad cf = .48 (same as above)
rear pad height = 1.25 (left at prefilled value)
Result = Front brake bias 0.709 or 70.9% to front.

So the Z32 swap that I have on my car changes the front bias from 69.7% to 70.9% or a variance of 1.2%.

If my data is correct.. and it's probably not perfect yet as I don't have the pad height calcs.. Dave is correct that the WRX 4-pots brakes would move brake bias to the back. I did the calcs with the stock BRZ front rotor and the new pistons.. The bias came to 67.2. So doing WRX 4-pots moves braking rear-ward by 2.7%. But, when you add the bigger leg gt front rotor the added leverage moves the bias back to the front to a new bias is 70.9%. So you gain 1.2% front bias with this setup.

Is 1% change in brake bias noticeable in a void of nannies like EBD? Would EBD make up for it before even a trained professional noticed the difference?

I'll see if I can complete this analysis this weekend. I'll measure the caliper pistons myself so I have first-hand knowledge and I'll see if I can figure out this pad redial height topic as well.

Holy monkey! Didn't expect this one to turn into a physics symposium! :D

Dave-ROR is correct doubling the piston area on a slider caliper.

Hydraulic pressure x The effective area of the pistons on one side of the caliper= One sided linear mechanical force generated by the caliper

The caliper reacts to that one-sided mechanical force by turning it into a clamping force. Clamping force is the one-sided linear force x 2, regardless of whether or not it is a slider or fixed caliper.

The above is paraphrasing my friend James Walker Jr. He wrote an excellent book titled, "[ame="http://www.amazon.com/High-Performance-Brake-Systems-James-Walker/dp/1613250541/ref=sr_1_fkmr2_3?ie=UTF8&qid=1405689625&sr=8-3-fkmr2&keywords=high+performance+brake+upgrade+book "]High Performance Brake Systems: Design, Selection, and Installation.[/ame]" I'd encourage anyone interested in this topic to buy it, read it, and then read it again. He used to work for Bosch designing ABS systems and is a brilliant engineer.

I'll try to get some engineering time today to do the bias calculations. We're tied up working on our new 2015 WRX Big Brake Kit, but hopefully I can get some real numbers here next week.

There are a lot of ideas flying around here, but I'll try to break down some of the basics in an understandable/simple way. One of the problems is that there are a million terms/jargon involved, and I see people in this thread saying the same things in different manners. If you're confused, don't feel bad, it's confusing sh*t! :D I've been doing this every day for over 10 years and I still have to reread and rethink some of these concepts at times. Anyway...

As stated elsewhere in this thread, the primary goal of a big brake kit is to manage heat. Brakes don't stop the car, they turn the kinetic energy of a spinning disc into heat. They do this by generating friction between the pad and disc. Tires stop the car by generating friction with the road/track. If you can lock up your tires with your existing brakes, you have all the brake torque you need (people call this braking power, brake torque, brake force, brake output, leverage, etc.).

Heat Management

Big brake kits are all about heat. They manage the heat generated from the friction between the pad and disc and deal with it in a number of ways. Think of the heat as flowing through the system in a few ways:

• Conduction- The transfer of heat through one component physically touching another component. Disc heat flowing from the disc face (the part contacting the brake pads) into the internal disc vanes is a good example. Another would be the heat flowing from the disc face into the disc hat, then into your wheel hub and bearings.
• Convection- Transferring heat through fluid flow. In brakes, airflow can be considered the fluid. The air moving through the disc vanes absorbs heat and evacuates it on the outer edge of the disc.
• Radiation- Heat emits via waves into the air. Put your hand six inches from brake disc that just came off the track, and you'll feel massive waves of heat on your hand. As the temperature goes up, radiation increases and becomes the main way that heat is shed from the brakes.

If we look at the different components in a brake system, you start to understand that all of the features are designed in some way to manage heat. Again, it starts with the friction that is generated at the interface between the pad and the disc. A larger disc mass provides a larger heat sink that can store more heat. This is accomplished by thicker disc walls and/or a larger disc diameter. More effective/efficient vane designs allow for greater cooling airflow, and allow more heat evacuation from the disc via convection. Superior vane design also speeds the flow of heat through the internals of the disc (convection). Greater disc surface area allows for more direct contact between the disc and the cooling air surrounding it (radiation). An aluminum disc hat slows the flow of heat from the disc face to the wheel hub.


Again, as the disc and pad generate friction, the energy of the spinning disc is converted to heat. The disc has to deal with that heat, but so do the pad and the caliper. That's why having proper race pads designed for high temperatures is critical for track use. The pads absorb heat and transfer it in similar ways to the discs. The pads radiate a massive amount of heat into the air around them. The pad backing plates are touching the pistons, so heat flows into the pistons. The pistons have brake fluid behind them, so the heat flows from the pistons directly into the fluid (why stainless steel pistons are a huge help, since they dramatically slow the heat flow). That's also why titanium pistons are used in high-end race calipers.

As you can see, all of the features of a properly designed big brake kit are designed to deal with heat. That's what big brakes are all about, and why they are so useful.

Brake Bias

A big brake kit that is properly designed to fit on a particular automotive platform should include careful consideration of the OEM brake components that it is replacing. When we design a front BBK for a production vehicle, what we are trying to do is copy/mimic/mirror the OEM brake torque on a given axle as closely as possible. We are NOT trying to increase brake torque, provide more "stopping power," etc. Remember, brakes don't stop the car, tires do. Brakes just transfer spinning energy to heat energy. If the OEM torque output on a given axle is properly replicated, the big brake kit can be seamlessly integrated without any ill-effects to front-to-rear brake bias, ABS, etc. If the torque output created by the new brake components changes much from the factory setup, it can cause all sorts of issues (assuming the OEM master cylinder is left alone).

So how do we mirror the OEM torque output on the front of the car? We carefully select the components for our kit while working towards the goal of superior heat management. The process goes like this:

The first consideration is the disc. How big of a disc do we need for this vehicle? How much does the car weigh? How much power does it have? How much power could it have if our customers slaps a big turbo on it? What speeds will it reach on the track? How much tire can you cram under the fenders? On what tracks will our customers be running? How long will the sessions be that they're running? How big are the wheels that this car runs (we obviously need to fit behind them)? How much weight can we shave off vs. the OEM equipment (you only want enough thermal mass to get the job done. Anything beyond that is dead weight to drag around)? How thick can the disc be while still fitting a good number of popular wheels? What type of face groove/slot pattern does it need given the usage environment?

All of the above questions point us towards a particular disc design. We look at things like disc diameter, total thermal mass, number of vanes, vane style, disc wall thickness, overall thickness, hat attachment style, etc. Again, what we're trying to do is provide a disc that will bring our intended customers all of the heat-management properties described above.

So we've chosen our ideal disc specifications. Next we have to figure out the OEM brake torque.

A LOT of effort by the car manufacturer goes into finding the proper brake bias for a given vehicle. This is where things get rather messy. Curb weight, drivetrain layout, center of gravity, wheelbase, static weight distribution, aerodynamic downforce, etc. all lead into a model of dynamic weight transfer under braking for a given vehicle. Once that is established, this data is combined with a bunch of data related to the tires, and a rather complex Anti-lock Brake System program is developed. I don't want to go too far afield on that topic, so lets assume the OEM engineers are doing a good job of finding the proper brake bias, and we want to copy it!

Three things impact brake torque output on a given axle:

• Total piston area
• Disc diameter
• Coefficient of friction (mu) of brake pad

Holding the others constant, increasing any one of these will increase the brake torque on a given axle. Decreasing any one of these will decrease the brake torque on that axle. So if we go up in disc diameter while leaving the overall caliper piston area the same, we increase overall brake torque on that axle. The same would happen if we kept the disc diameter the same, but increased the overall piston area. If we leave the disc and piston area alone, but drop in some really high mu pads, we increase the brake torque on that axle. So playing with these three factors allows us to hone in on the proper solution that will mimic the factory brake torque.

For the time being, lets ignore the pad mu by holding it as a constant. We look at the total OEM piston area and the OEM disc diameter. How much leverage, stopping power, brake torque, or whatever you want to call it, is our front brake system generating? Once we know that, we insert the disc we've chosen into the mix, and then choose a caliper to mate properly with it. If we are taking the disc diameter from an OEM 290mm to our chosen 325mm, we need to compensate for that disc diameter increase by lowering the overall piston area to counteract that effective radius increase. To simplify, disc diameter up, caliper piston area down. One effectively cancels the other out.

If you put on a larger disc diameter, and you also put on a caliper with a larger overall piston area, you're increasing the overall brake torque on that axle. That can cause the problems I posted in my earlier response. With too much torque output on the front, the rear brakes aren't doing their share of the work. You want all four corners of the car simultaneously generating their maximum deceleration. If you don't have that, stopping distances will actually increase (further distance required to stop the car). Someone else mentioned it earlier in the thread, but an overall larger piston area also requires more brake fluid displacement, and can give you a long brake pedal as well with all else held equal.

Everything above holds true for the rear brakes as well, except messing with the amount of rear brake bias can become downright dangerous. In a system with too much rear brake, the car can get extremely squirrely under braking. If the rear wheels lock up before for the fronts, that is an inherently unstable condition. The car can swap ends into a spin as the weight shifts forward under braking and the rear wheels lock.

What about pads? Wouldn't putting a higher coefficient of friction (mu) pad in the front, but leaving the rear alone, increase the front brake torque. Yes it would. The magnitude of bias shift caused by pad mu is generally not as great as that caused by a significant increase in the disc diameter or piston area. If you go from a street pad with an average mu across its temperature range of .36 to a race pad with a .48 mu, it will have an impact, but not like increasing disc diameter two inches with all else held equal.

Since the relative brake torque changes due to pad mu are fairly small, experienced racers/drivers play with pad changes to mildly alter brake bias. For example, someone in a car that typically understeers heavily might run a slightly more aggressive rear pad in an attempt to get the car to rotate under trail braking into a turn. I see this with WRX STI guys for example.

Recap

Hopefully you can see that brake upgrades aren't just about finding parts that will bolt onto a given car. There is a LOT of thought that goes into choosing the proper components that won't upset the car's overall dynamics. Throwing a bunch of improperly chosen brake components on a car frequently does more harm than good.

I will say that in the majority of the situations where someone is pulling "bigger brakes" from a "bigger" platform (I'll call it the "Bigger is Better" Syndrome ;)), the result is an over-amplified front brake setup. Most of the time they go for a larger diameter disc, but the overall piston area being run on that disc is also considerably larger as well. Over the years I've had a ton of phone calls from customers that begin with, "So I found a great deal on a set of Brembo's on eBay...." Then that customer explains that while their brakes look really cool, they actually perform like poo on the racetrack, cause all sorts of weird ABS issues, etc. Many times they also spend a good chunk of money fabricating, gathering, and scraping the bits and pieces to tie it all together. That also doesn't include the amount of time involved. For many people the time commitment to do all of that isn't actually feasible, and they'd rather just buy a professionally engineered, complete solution that shows up in a box the next day.

All of that said, sometimes you get lucky! :thumbsup: There's absolutely nothing wrong with what the OP in this thread is doing if it is done right with some thought (which he's clearly trying to do). Sometimes the numbers all work out, disc diameter increases are offset by piston size changes, and everything closely approximates the OEM brake torque for a given axle. That is what the other folks in this thread are trying to find out.
Good level-headed discussion gents.:cheers: I'll try to have the numbers run soon.

Done that twice on my SpecE30 racecar. Once the piston goes far enough through the backing plate the brake fluid exits the caliper body and then you finally realize there is a problem when you have no brakes. :(

Awesome!

Thanks Jeff for clearing some things up!

- Andy