Topic: Power Brake Booster Vacuum

[Building 3]

I have looked in the Ford Shop Manual and I can't find the answer so I thought I would ask here.  Does anyone know the engine vacuum number (or range) that is needed for the power brake booster to operate properly? This is for a 1966 289 2V automatic trans. Thanks.

[Brian Conway]

Don't know the exact ' range ' but usually somewhere around 18 Hg for stock applications.  Most vac. gauges have a green zone for this test.  Brian

[Texas Swede]

A friend of mine bought and installed a conversion kit to get power disc brakes instead of the manual drum brakes his 1967 C-code car was originally equipped with.
After installation he wasn't happy with the power brakes. Told him to check his vacuum at idle and it was 12 inches of Hg. Told him he needed more and after a fine tuning he got the vacuum up to 16 and the brakes worked fine.
Texas Swede

[suskeenwiske]

In a perfect world 14.7in/Hg, but having to allow for mechanical variations and efficiency the simple answer is 16 - 21in/Hg. This is also important for your Automatic Transmission to shift correctly. Your 289, assuming it's mechanically healthy, basically stock and correctly tuned, should produce around 18 - 20in/Hg at approximately 600 rpm.

Ray

[WT8095]

In a perfect world 14.7in/Hg...

That's a very precise number. Where did you find that?

[Building 3]

Thanks for the input.  That is great news. I should be Ok since my 25,000 mile original engine is at 21.

[suskeenwiske]

That's a very precise number. Where did you find that?

It's difficult to explain without getting into all sorts of physics and mathematical equations. In a nutshell, atmospheric pressure at sea level is 14.696 psi or 29.92in/Hg; usually referred to at Barometric Pressure. As an engines piston begins its intake stroke, the suction creates a low pressure area within the cylinder, intake manifold and carburetor. At idle, the throttle plate(s) are closed. This makes it more difficult for atmospheric pressure to enter the engine and is why engine vacuum is higher at idle; the throttle plates create a restriction. With the throttle wide open atmospheric pressure is not restricted and engine vacuum is very low.

When braking, you take your foot off of the gas. This closes the throttle plates and engine vacuum rises due to the restriction. The power booster uses, in combination, vacuum and atmospheric pressure to multiply the force applied to the brake pedal. The greater the vacuum, the greater the force is multiplied which requires less effort on your part to stop or slow the car.

In the booster is a spring that prevents the diaphragm from collapsing which effectively negates the atmospheric pressure; there's more to it but this is sufficient for the explanation. To restore the loss we need a vacuum that is at least inversely equal to atmospheric pressure but since all of the components that make up the booster as well as the check valve, tubing and other systems in the car that require vacuum and because none of them are mechanically perfect, 14.7in/Hg is not going to work very well if at all. The vacuum is measured at the intake manifold, by the time it makes it to the booster, dynamic vacuum losses due to friction and variations in atmospheric pressure at different altitudes, you no longer have 14.7in/Hg. As a result, in general, at least 16in/Hg at the manifold is needed though I believe that 18in/Hg is the benchmark that engineers strive for.

This loosely explains why the vacuum levels have to be within a certain range. Some may argue that this is not totally correct as there are systems that will work at lower vacuum levels but those figures do not apply to factory correct equipment and factory equipment is what I'm addressing.

Hope this helped.

Ray

[WT8095]

...To restore the loss we need a vacuum that is at least equal to atmospheric pressure...

Ken, I think you're confusing 14.7 psi atmospheric pressure with 14.7 inches of mercury. Please don't take this as a snarky response, I'm just going to show the math.

Atmospheric pressure (standard at sea level) is 14.7 psia (absolute), or 29.92 inches of mercury. A vacuum gauge reading of 14.7 lbs would be 0 psi absolute - a perfect vacuum. It is possible to approach a perfect vacuum with sophisticated pumps, but it cannot be surpassed.

When working with pneumatics, it's more convenient to work in differential pressure than absolute. It makes the math easier.

A few corrections: First, I used the diaphragm from a '67 booster before noticing the original question was about a 1966 vehicle. I also neglected to account for the brake pedal leverage when calculating "driver contribution" to the power system. And, in hindsight, 20 lbs seems a bit high, so I'm reducing that to 10 lbs. My apologies, I shouldn't have worked on this so late last night 

Let't start with the brake pedal. From photographs, I calculate the mechanical advantage of the brake pedal on a manual-brake equipped Mustang is approximately 6.5:1. I haven't measured how much pedal force is typically used when making a "normal" stop with manual brakes (and it's been many years since I've done that). But I'll estimate it at 50 lbs, to simplify the calculations a bit. That's also the value specified for pedal height check in the '68 shop manual (I don't have a '66 shop manual). Pressing on the brake pedal with 50 lbs of force therefore results in 50*6.5=325 lbs of force on the pushrod in the master cylinder. That's with an entirely manual system.

Now let's look at a power system. A brake booster for a '66 Mustang has a diaphragm diameter of 7 inches. That works out to about 38.5 square inches. To get 325 lbs of force, 325/38.5=8.4 psi is needed. 8.4 psi=17.1 in/Hg. That's if the booster is doing all the work. But since the driver is pressing down on the pedal, but with less force than for a manual system, the booster doesn't need to provide all the force. Let's say we want 325 lbs total force on the master cylinder pushrod with 10 lbs of pedal force. With 10 lbs pedal effort, and a mechanical ratio of 6.5:1 (using a 65-66 pedal - later years had less mechanical advantage for power brakes), the driver is contributing 65 lbs, so only 260 lbs is provided by the booster, which requires 260/38.5=6.8psi=13.8in/Hg.[/s]

The answer to the questions of how minimum engine vacuum thus comes down to the engineering design requirements - how much pushrod force for a given amount of pedal force? Given these numbers, and allowing for friction losses and the force of the diaphragm return spring in the booster, 14-16 in/Hg minimum seems reasonable. Higher vacuum will result in lighter pedal force. Lower vacuum will result in a heavier pedal. There's nothing in the physics that points to an "ideal" value of 14.7 in/Hg.

[suskeenwiske]

we need a vacuum that is at least inversely equal to atmospheric pressure

A correction of my own...I added inversely to the above, by no means am I confusing atmospheric pressure with vacuum.

I like and concur with your post, well written and concise. I'd like to point out to everyone however that, I was not saying that 14.7in/Hg is the ideal value; rather, this would be the minimum under perfect conditions to gain a mechanical advantage that is sufficient to enhance braking power that can also be felt by the driver at the brake pedal. Since perfect conditions do not exist and because original factory equipment was far from efficient, more than 14.7in/Hg is needed at the engine in order to achieve at least that number at the booster.

This brings me back to the original question of how much engine vacuum is needed for a 1966 C-Code. A daily driver engine, (that is not modified or high performance), which is mechanically healthy and properly tuned, will produce from 18 – 21in/Hg parked at idle or as the throttle plates close under braking conditions. Sixteen inches of mercury at the engine may be sufficient to achieve what is needed at the booster but it would be negligent for me to leave it at that without mentioning what a healthy factory's C-Code vacuum should be.

If the engine is running poorly due to mechanical issues, a poor state of tune or a vacuum leak, the power brakes may or will be affected as a result. The general rule is to have the engines vacuum at its peak so that not only the booster works efficiently but also for the automatic transmission, if equipped, to shift properly, for the distributors vacuum advance to maintain proper timing and for the PCV system to do its job.

Thanks

Ray