I hope everyone had a lovely holiday.As an update, if anyone cares, the MC is crossing the border into my grubby hands soon lol.
I hate to steal your thunder on this one, but it was taking too long and a friend had sent me a MC out of a wrecked 2019 Golf R he had no use for, and owed me a couple of favors so... It does appear that my theory was incorrect; see below, clearly the GOLF R MC is not a stepped QTMC.
One thing I would be curious about, I have seen were you (and others) have mentioned the MC piston diameter is supposed to be 23.8 mm for the Golf R however, when I measured this one it is actually 25.4 or 1”. I would assume yours should be the same but would be interested to know since yours is out of a different year/model.
As I said there must be another explanation, so I did a little more research and came across this interesting paper titled “Brake Pedal Feeling: Target Setting Techniques” you can download the PDF FOC at this link: Brake Pedal Feeling: Target Setting Techniques
After learning that we do not have a QTMC and seeing this paper; it is now obvious to me why these modifications cause the brake pedal to be higher and lot firmer with much better feel. It all has to do with the servo brake booster, I knew it provided pressure assist, but had no idea just how much!
This study tested a wide range of vehicles, on page 51 it provides an example of typical pressure curves for these systems. I had no idea the pedal force to brake system pressure ratio was so extreme! Understanding this now makes sense of what I have observed with these modifications.
From the curves shown above, we can see why the pedal drops and feels soft when the engine is started and the servo brake booster comes online. With a typical standing pedal force (approx. 20 daN or 50 lbs) with the engine off the brake system pressure is <100PSI, once the engine starts the booster ramps the pressure up to around 1000PSI. At that pressure the brake components (lines, calipers…) start to deflect causing the pedal to drop causing even higher pressure and even more deflection, hence the soft dropping pedal that seems to sink further as you press even harder trying to find that hard stopping point.
The graph shows booster assist curve as a function of pedal force however, pedal force equates to the input rod position, which is what dictates the amount of assist the booster provides. By changing the piston take up point the pedal/input rod stops short due to the pressure increasing much sooner with reduced take up. This drastically changes the booster assist curve relative to pedal force.
With the Macan/Q5 calipers increased piston retraction the same pedal force causes the pedal to go lower causing the booster assist curve to be even more aggressive. This is why the initial engagement point is lower, and with more assist makes for an even lower pedal with even softer feel. So lubing the seals or just adding the RPV’s gets the system very close to the stock caliper piston take up, and that provides pedal height and feel similar to the stock calipers.
However, by doing both (RPV & lube), that significantly reduces displacement take up. This means for a given pedal force the input rod is further back significantly reducing the amount of booster assist. This is why when the engine is started the pedal does not drop and feel soft as it does with near stock take up. Clearly the booster is providing some assist otherwise it would be very difficult to stop the car, but it is not bad at all. It just has considerably less assist compared to stock, so it does not cause any noticeable deflection in the brake system when you start the car. This is why the pedal does not seem to change when I start the car and provides a much firmer higher pedal with significantly improved feel throughout the curve.
You do have to use a lot more pedal force to get the same amount of breaking power however, for my preference it feels just about perfect, in fact I would lower it a bit more if I could. I do not like a low soft grabby overly boosted brake system that is difficult to modulate. These mods fix that, and now I understand how and why. Like I said for track use...
The blue curve represents Macan caliper with increased take up and the green represents reduced take up with lube & RPV added. Obviously I did not take actual measurements so this is for rough concept purposes only.
I imagine the StopTech ST40 with its smaller pistons has the same effect, perhaps even more so. So there you go, I hope this makes sense?