Slow Car Fast
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January 22, 2010 - It's time to make sure I have room for some real brakes.
The car originally came with unboosted brakes, and long-time readers will remember that I rebuilt the master cylinder. That was back when I thought I'd be driving the car as a four-cylinder for a while. Ah, those were the days.
Purists, of course, claim the unassisted brakes are superior in almost every way. But those who are sensitive to subtle hints may have noticed that I'm not much of a purist. I have several cars with unassisted brakes (interestingly, they're all British or replicas thereof) but this car will have a fairly intense power/weight ratio, and I want power assist. Besides, the whole braking system has been replaced with Miata parts, and my rear discs take more pressure to operate than the previous drums.
The problem is twofold - first, there's not much room for a booster underhood. And secondly, the original pedals put the masters above the driver's feet - so there's simply no room for a booster at all in that setup. There are such things as remote boosters which are intriguing. A number of newer vans (and other packaging-challenged cars) also use a "hydroboost", which steals hydraulic pressure from the power steering pump for assist.
But there's an easier way. In 1974 or 75, MG added power assist to the MGB. And in order to do so, they used a very small brake booster, a master cylinder with an angled top to clear the hood, and a different pedal box that put the brake hydraulics in front of the pedal. And it's a simple bolt-in to the earlier cars - although most MG owners go the other way, installing the unassisted setup. I found a complete pedal box, booster and master on the MG Experience website and had it in my hands a week later.
The first trial installation showed that it's a perfect fit. The front outlet is fairly close to my shock tower mount, but a well-placed divot would take care of that. The rubber line that runs close to the booster is actually the vacuum line on the engine. It's just sitting there, I can easily route it with lots of room.
I need to do a bit of figuring with pedal ratios (looks like it's in the 4:1 range) and master cylinder sizes (0.75", apparently) and see what that does for my brake pedal travel. I may want a larger master, as the Miata has a similar (4.1:1) pedal ratio but runs a .875" master. I'll measure the MG one more carefully tomorrow. Luckily, mounting a Wilwood unit is fairly straightforward. So there may be no reason to put that dent in the sheetmetal anyhow.
First, I'll stick this setup in the bead blaster and make it all pretty!

I've been puzzling on the best way to package this for a while, looking at various booster combinations. But I'm really happy with this result. I think it's going to be very clean.
tags: brakes, hydraulics