The brembos.

I'm looking into a set, but want to know what wheels I can run without using spacers at all.

 

^ I see, then the OE coupe 18" and 19" should work in that case.

 

I can't answer the piston sizes but I had a completely opposite experience with my brake swap. I replaced my stock fronts with a used set of Wilwood 6 pot calipers and stainless lines a little over 2 weeks ago. The pedal got noticeably firmer, so much so that I started to suspect that maybe the "Soft Brake Pedal" issue might be related to the OEM calipers?? I was actually going to update that thread this afternoon. I wanted to drive on the Wilwoods for at least 2 weeks before I posted to make sure it wasn't a temporary fix. I contemplated buying a Wilwood MC if it was necessary (they're pretty cheap), but so far it's not.

 

Well, I suspect my soft pedal issue is something else. I am waiting for the correct side rear caliper to be sent to me first before I update the thread with my results. Basically, my rear caliper slide pins are all seized up and the piston has to travel far between presses. Its pad knock back basically. I need the new caliper first to confirm. Im pretty confident replacing this caliper will fix the soft pedal issue.

But yes, I think you are right and the soft pedal issue is a caliper issue.

But, the reason I brought up changing the master cylinder is because I notice there are two different mcs depending on if you have the 03/04 (and 05x) brakes and the 05/06 brakes. Makes me wonder if there is a piston bore size change, or something like brake fittings or reseviour change or something. Of course, if nobody is reporting any change in feel, it's probably a non-issue. I'm just thinking about it as an engineer and my experience with mustang brakes in which the MC needs to be changed due to the large difference in the size of the pistons in calipers.

 

I followed Jeff's link and found this: https://g35driver.com/forums/3890415-post20.html. According to this, all 1st gen sedan and coupe have the same size of master cylinder bore, except for the Brembo equipped. I'll take it as a confirmation that the master cylinder doesn't have to be swapped when upgrading to 05/06 calipers. It feels nice to learn something new everyday.

 

I can't pin it on anything else. I wish someone else that has gone to a non-Brembo BBK would chime in to confirm/deny what I experienced. It would help us identify the issue if we had more feedback.

 

If that info is correct, then that would confirm the MC does not need to change. 1" bore for all non-brembo 1st gen coupe/sedans.

Since the brembos is a 1 1/16" bore MC, that means swapping Brembos on and keeping the 1.0" bore MC would result in a slightly longer stroke, and softer pedal.


Interesting thing i found out doing some searching...the AWD MC and RWD MC is different. The AWD MC is this LONG slender master while the RWD MC is short and stubby. Kinda interesting as to why there would be a difference

 

^I wonder if the valve body thingy (can't remember what it's called) splits pressure differently on the AWD

 

Did you check the 06 AWD MC? I bet it's the same as the 05 2WD since they changed the brakes. I bought a Stillen MC brace and it didn't fit my 05. It was designed for the 03-04.

I didn't find the bore size for my pistons but would assume they're smaller since there are 6 per caliper. Would that cause my pedal to be stiffer following your math?

 

I'll have to respond later on tonight when i'm home from work.

I did check the MC's for the 2006 model since i wanted to make sure all the brakes were the same. I'll do some more digging tonight as well

 

Tim, Total Piston area = (0.5xdiameter of bore) squared x 3.1416 x number of pistons.

 

That, plus the total of all 4 calipers, divided by the piston area of the master cylinder gives you a ratio. Also, the pedal ratio and the booster factor into brake feel, but those remain constant, so lets ignore those.

Since i can't find ANY info on the diameters of the pistons in the G35, i'll make some up for the point of making an example.

*FAKE NUMBERS*

03-04 front calipers....single piston 60mm
05-06 front calipers....dual piston 42.5mm
03-06 rear..........single 45mm piston

So, do some simple math, and figure out the total area of all the pistons in a single caliper.

03/04 front 60mm = 2827 mm^2
05/06 front 2x42.5mm = 2837mm^2
rear 45mm = 1590mm^2

So see what I did there? the single piston is larger, but the two pistons equal the same surface area. The benefit of two pistons is the pads can be larger because the area compressed is spread out over a larger area lenghtwise along the pad. So, with BOTH front calipers having the same surface area, there is no change in feel if you swap between the two.

Now, let's take an 05/06 and add up the total area of all the calipers

So 2827+2827+1590+1590 = 8834 mm^2 total caliper piston surface area.

Now we calculate the MC piston area. 1.0" is 25.4mm which equals 506mm^2 of surface area .

Now divide calipers by MC and you get a ratio of 17.4:1. That means, for your current pedal feel, the bore is sized 17.4:1 to the caliper area. It's almost like leverage if you want to think of it that way. As this ratio become larger, 18:1, 19:1, the pedal feel becomes softer and the stroke becomes longer, but your braking ability increases. If the ratio becomes smaller, the pedal feel becomes firmer and the stroke shorter, and the braking requires more force from your leg.

Now, lets slap some random Brembo caliper off a Porsche on there...lets say 4x38mm pistons. Pretty large caliper.

SO, 4x38mm = 4536mm^2 and you have two, so the total front caliper piston surface area would be 9072mm^2. Add in the rear brakes (assume we keep it stock) and you get 12,252mm^2....those are some big brakes. Using the same 1.0" MC, the ratio is now 24.2:1.....and you are probably pressing the pedal to the floor to stop and the brakes are soft as hell. SO if you actually tried to make a bracket and mount this caliper on there...your brakes would suck basically.

How to compensate? Make the master cylinder bore larger!

Lets say we use a 1 1/8" bore MC...or about 28.6mm in diameter. Calculate out the area and you get 642mm^2 of area.

Now, divide the caliper piston area of 12,252 by 642 and now you get 19:1. The number if getting lower, but not in the range that stock was.

How about a 1 3/16" bore MC?? That's an area of 716mm^2. Divide and you get a ratio of 17.1:1. So it's lower than the original 17.4:1 value, but only slightly. The feel would be virtually stock in terms of travel and pedal effort.

So increasing the bore size compensates for the larger piston area of the calipers. This is prob why brembos use the 1 1/16" MC bore with the standard brakes are 1.0" So if you swap OEM brembos on, you should change MC as well. Otherwise the feel will be completely different with the 1.0" bore MC vs the proper 1 1/16" bore. The smaller bore will increase the ratio, and make the brakes softer a tad.

Hopefully, this makes sense. I wish i had ACTUAL numbers of the piston diameters to do REAL math, as these numbers above are made up. But basically when you swap calipers from one car to another, there is the potential for pedal feel to change.

 

It could, but without knowing any of the actual diameters, it's tough to say.

If the total area of all the pistons on both front calipers, is smaller than the total area of the original OEM calipers...they yes, it would make the pedal firmer.

But without real numbers...it's just guessing.

I've been searching, but can't find ANY info at all on OEM piston sizes. I'll have an OEM rear caliper in hand tomorrow to measure, but if someone with an accurate means of measurement (in millimeters preferably) can measure the pistons, it would help greatly.

 

Nice info. For reference the rear piston dia is a wash. It's the same piston size. Just with a different mounting bracket to accomodate a larger rear rotor.

 

Agreed...i should have stated that and said to simply disregard it...but in the case of the OEM brembos, the rear setup is also changes so it must be taken into account