On the cars I have worked on, seems there are four bolts to bolt the power brake booster to the firewall, but only two to bolt the master cylinder to the brake booster. Now due to the function of the brake booster, it seems the force between the firewall/booster is less than the force between the booster/master cylinder. So this seems backwards, and there should be MORE bolts between the booster/master cylinder than the firewall/booster. So why the use of only two between the booster/master cylinder, as that is where there is higher forces at play (or why four for the firewall/booster as those are a pain to get to)?
The firewall to booster has to hold the overall weight and forces of both the booster and master cylinder and sometimes abs and or proportioning valves, where the booster to master cylinder does not have to support the weight of the booster… Plus the booster to master mount is normally thicker and stronger then the large area of the firewall with very little support, more bolts spread’s the load of the weight over a larger area…
Some others can probably explain it better in more engineering terms…
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Reads like a perfect description to me as to why 4 bolts vs 2.
I find it odd why someone would create a login just to ask such a question, but hey, Welcome to the CarTalk forum.
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Inquiring minds want to know.
It seems to me it’s usually not bolts securing the booster to the firewall, but studs 
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Seems like a pretty good question to me. No complaints. The booster is wider (bigger in the lateral direction), so lots of room, a bolt at each corner makes sense imo. Harder to place 4 bolts on the master cylinder, not as much room to work with. It’s probably true that the forces on the 4 bolts, for each one, force is less than on each of the 2 bolts, but still works ok as long as all the bolts are strong enough.
That’s how my Corolla’s booster is configured. Studs on booster, nuts on firewall side hold it fast. I’ve had to remove the nuts and move the booster out of the way on occasion, for example when replacing the clutch master cylinder. Referring to a stud as a bolt? Pretty similar, not something I’m losing any sleep over.
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Why odd, isn’t this the perfect place to ask such a question?
OK, studs, I stand corrected. Doesn’t really change the basis of the question.
A relative and I came to a different conclusion. Back before power brakes, MC just used 2 bolts/studs to connect to firewall, and that was the precedence for their design. The interface could be easily modified to use 4 bolts with the new Power Brakes, but why change that tried and true design?
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But did you understand the answer, or I guess better worded, did you get your answer??
The booster is heavy, the master cylider is now mounted 6 to 8 inches from the firewall and it is heavy. Hitting bumps puts a much larger force on the firewall with a booster and master than just a master by itself. The forces from bumps are greater than the force from your foot.
Jump off a step with a bowling ball held to your chest and then again with the ball held a foot out. Which one takes more effort upon landing?
Master cylinders are still 2-bolt
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People seem to be concentrating on semantics, not the question., except davesmopar (you) and George. But I’d say the forces during brake application are greater that those supporting the weight of the MC hung out, so those explanations are of little support, also the booster doesn’t weigh that much. By the way, I have a BS in engineering physics (with honors) with courses in statics and strength of materials, so I understand the forces of supporting the MC hung out a far distance. Also see my ‘explanation’ above about precedence in MC design.
I think not. Bumps are smoothed by the shocks. Also it’s the forces in the booster/MC interface, those are multiple by the booster, and greater than those applied by your foot. Your foot (even petite women) can supply a pretty high force, the foot supports you weight all day long, used to supplying 50 or more LBs, the booster/MC don’t even come close to that sort of weight.
The bumps explanation is correct. Hit a pot hole, that’s quite a shock to everything attached to the car.
Then maybe you can contact the manufacturers and explain why they are wrong.
I also thought it was a strange question .
Good. Then you should also be able to envision the stresses placed upon a fabrication made from sheet metal supporting that weight and the lever it presents. You should also have the background to understand that a shock has limitations for absorbing impacts. It has a limited slew rate that is not intended to minimize certain impact rise times or harmonics. Large, 1/2 sine wave impacts are one thing. Then there are repetitive sinusoidal vibrations that also need to be factored in. Over time, those small vibrations will work harden the material and it will eventually fracture. So having the proper mounting points, reinforcements and supporting structure are all carefully engineered using various mechanical engineering disciplines, modeling and simulation tools as well as empirical testing to confirm the design meets all the goals and requirements. But you already knew this given your background so I am curious why you brought it up? Maybe that particular degree and area of study isn’t as all encompassing as one might think…
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By the way I have a Masters degree in mechanical engineering, worked for the division of a car company that made brake systems and suspension systems for 27 years and was considered a subject matter expert in suspension design.
I can do the math for you if you wish but the result will be the same.
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what harmonics get past the shocks?
I often had to help my Mechanical Eng. house mate and his friends with their homework, so I think it was pretty encompassing
Please do. Show why the PB/MC interface only needs two bolts, and horizontal, not vertical that would be needed to stop bump shocks, that several of you point to!
Boy this is a tough crowd, no need to stick around here, sorry I even brought this up.
OK…so what is your explanation? You profess to know it all…
I gave one possibility above if you’d read. But I really don’t have an explanation, therefore the post/question I raised, I am the OP.
This question below refutes the above-
If it was close, I’d invest the time to explain but I can see there is a huge gap and I don’t have that kind of time or energy…
Decipher: I don’t know
Low frequency gets past the shocks, but low frequency is not the impact that would need reinforcement.
My question is not why there are 4 studs on the firewall, but more so why are there only 2 at the PB/MC interface.