I have been thinking about manual brakes for some time now. I am still in the research phase, and I won’t be pulling the trigger on this any time soon. I have several suspension related projects I need to get through first. Oh and money, I need money too. lol.
There was an Instagram post (now deleted) about converting to manual brakes using a well known conversion kit. There a couple of threads on Nasioc about deleting the brake booster as well. The feedback on manual brakes seems to be mixed. It is difficult to compare each setup as everyone is doing their own thing. And many people using the conversion kits, are mostly converting show/stance cars.
Note: For any one new here, I already deleted my ABS and I run an adjustable proportioning valve. I have STI Brembos up front and LGT vented rear brakes (same rotor as 06-07 WRX 2-pots with sliders).I don’t remember what pads.
Why Manual Brakes?
This is partially motivated by curiosity and the following:
Cleaner Engine Bay
This is not the best idea for converting to manual brakes. However, removing the vacuum booster and associated hoses will clean up the engine bay and simplify the braking system. This will also feed into some other mods I have planned (think chassis bracing and intake manifold swaps).
Less Weight
Removing the booster reduces weight. The location of the weight reduction isn’t perfect for weight distribution, as it is behind the front axles; nevertheless, it is in the engine bay. I looking at swapping to the composite intake manifold from a 2008-14 WRX as well to reduce more weight. Although, I prefer the look and symmetry of a center entrance intake manifold on Subarus.
Feel
I am a little worried that the pedal travel might be too long. Figuring this portion out will probably come down to trial and error. However, I do have a starting point. Read on.
Pressure vs Feel (Travel)
One common complaint I’ve seen from those who have converted over to manual brakes is that the pedal feel is rock solid, and the brakes are not responsive. As a result the car take significant effort to slow down. Obviously, this is not ideal.
A general rule of thumb I use for most fluids:
High Pressure (Smaller Cylinder) ~ High Velocity or Low Volume = More Travel
To decrease travel, you need to increase volume; however, will you lose the mechanical advantage of a high pressure system. This is less of an issue with power brakes, due to vacuum or electrical assist, and more or less comes down to personal preference. Most people prefer the feel of short travel brakes (aka, larger m/c bore).
This basic idea also applies to gear ratios. Short gears (equivalent to high pressure/more travel) rotate the engine at quicker speeds, whereas long gears (low pressure-less travel) spin the engine slower. More mechanical leverage comes at the cost of longer travel (revolutions per minute) or higher engine speeds. I’m sure there are some laws of physics that better describe this phenomenon, but I don’t know what they are.
For my manual brake setup, I will be looking at smaller master cylinders to compensate for the loss of power assist.
Pedal Ratio Change & Master Cylinder Sizing
One mod I don’t see many people make when converting to manual brakes is changing their pedal ratio. This is likely because this isn’t a terribly popular mod outside of drag or rally racing. I honestly didn’t know about this mod until fairly recently. It is also a pain to modify a brake pedal assembly for most enthusiasts who are used to bolt-on mods. However, I believe at least one kit now modifies the pedal ratio.
Since we are removing the power assist, we need to add some mechanical leverage to make up the difference. The stock pedal ratio for the WRX is around 4:1 to 4.5:1. According to Summit Racing, power assisted brakes are generally in the range of 5:1-4:1 and manual brakes usually in the 7:1 to 5:1 pedal ratio range.
Using Summit Racing’s example of 70lbs of pedal effort, gives us around 280 lbs at the master cylinder before power booster assist is applied. Power boosters add anywhere from 200 – 300 lbs of force. For a sporty car like the WRX, it should probably be on the lower end for better feel. Although Subaru also put a relatively slow rack in these cars (16.5:1), so the power assist could be higher.
Let’s assumes the worst case for our purposes and assume the booster applies an additional 300 lbs of force. That gives us a total of 580 lbs at the m/c with a 4:1 pedal ratio (615 lbs with a 4.5:1 ratio). To replicate this force with a manual brake setup with the same size master cylinder (m/c), we would need a pedal ratio of 8.3:1 (8.8:1 w/ a 4.5:1 pedal ratio) which is almost double!! Reducing the m/c to 7/8-inch effectively reduces the pedal ratio needed to 5.6:1 (6:1 w/ a 4.5:1 pedal ratio) to achieve the same level of force. Reducing the m/c size also increases the pedal throw in the above scenarios from 40% (33% @4.5:1).
The 7/8-inch master cylinder seems pretty common for most manual brake conversions and cars that come with manual brakes. Although, the long-term plan is to add 2018+ STI 6-pots up front and 2-pots in the rear. We may need to go up to a 15/16-inch m/c. For now, I am going to focus on the 7/8th m/c.
As mentioned earlier, a smaller bore will give us more leverage, but at the expense of travel. We want relatively short travel for the best feel, but I don’t want to Fred Flintstone the car at every stop light.
Next Steps
The EK Honda Civic uses a 7/8″ master cylinder. This will likely be my starting point. I will need to build an adapter plate and run new lines. I already have an adjustable proportioning valve from my ABS delete.
| Brake Component | Current Setup | Proposed Setup |
|---|---|---|
| Front Calipers | 2004-2018 STI Brembos (4-Pots) | 2018+ STI Brembo (6-Pots) |
| Rear Calipers | LGT 1-Pot Sliders | 2018+ STI Rear Brembos |
| Front Rotors | 326x30mm vented | 340x32mm vented |
| Rear Rotors | 290x18mm vented | 326x20mm vented |
| Master Cylinder | OEM WRX 1-1/16″ | Honda 7/8″ |
| ABS | Deleted | BMW or Toyota ABS |
| Booster | OEM WRX | Deleted |
| Proportioning Valve | Wilwood Adjustable | Wilwood Adjustable |
That’s it for now. If you converted to manual brakes, let me know how it worked out for you.
References
https://help.summitracing.com/knowledgebase/article/SR-05037/en-us
b.obsdian | b.orginal 
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