Why do petrol cars have vacuum pumps?

Vacuum can be insufficient to meet the needs of the car under certain circumstances, and the amount of vacuum capacity available may also not be enough under certain circumstances. If the car has an aggressive camshaft or has a very small engine, combined with vacuum driven accessories that "consume" lots of vacuum then a pump may be necessary. The engineers make that decision based upon a comprehensive analysis when designing the car.

JazzMan

JazzMan wrote in rec.autos.tech

An extreme case was a 63 rambler that I had. It had vacuum operated windshield wipers. Driving on a flat surface at a steady speed and they worked fine, going down hill could get them flying. Accelerating up a hill would almost stop them. Many cars also use vacuum assisted power brakes, and the concern might be that the vacum from the engine would not be enough to make that work.

You don't tend to accelerate up a hill and use brakes at the same time... Vacuum is at its greatest on a closed throttle - ideal situation for brakes.

Dave Plowman (News) wrote in rec.autos.tech

You're right. However, the brakes do take quite a bit of vacum, and usually there is a large canister for the vacum. Could be that the maker is supplementing that with the vacum pump.

I've seen a couple of applications where a vacuum pump is used as part of a cruise control set up.

John

- wrote in rec.autos.tech

modern

We are, of course, talking about a Citroen here. :-).

I've not read that suggested here - have you?

A vacuum pump can fail too.

Maybe it is just me, but I remember when power brakes were an option. In my experience, cars still stop when the brakes have no boost, you just have to push harder. The only reason I replaced the check valve in my '84 Dodge 600 was that if I got too used to braking with no assist, I would have problems if I drove a different vehicle.

When they were an option, brakes were of such a design that the amount of boost provided was small - usually drums. Modern ones multiply the effort by several times, and even the strongest person can't get near the same pressure.

I wouldn't know - this is a UK group. But they don't stick in my mind as being great cars from what little I've heard of them.

And how many do today? If it was so wonderful, they would still be in use.

One of my cars weighs 4100 pounds and has MANUAL front disk/rear drum brakes. I prefer its brake feel to almost any other car I've ever driven. Manual brake setups (as opposed to power brakes with the boost disconnected) have a different mechanical ratio between the pedal travel and the master cylinder piston travel, and a different ratio between the master cylinder piston area and the total wheel cylinder piston area.

Manual brakes require more pedal MOVEMENT (not necessarily more pressure at all). The extra movement allows finer driver control of braking effort, and they respond faster since there are inevitable delays (milliseconds) in the power booster as valves close and vacuum vents in order to modulate boost. But you do have to remember to lift your foot up higher before putting it on the brake pedal!

That's about like someone asking sarcastically if I thought Bentley (back when it was really Bentley) wasn't decent, and me saying "this is a US group, I wouldn't know."

Sheesh, Packard was one of the top 5 or so of all time. They're the ones that made many of the manufacturing changes that improved reliability of the Rolls-Royce Merlin during WWII, after all.

But will take for ever to stop from its top speed...

Heh heh - R-R engineers were tearing their hair out trying to find out why Packard manufactured versions of their V-12 couldn't make the required power output. Came down to poor tolerances.

Yours must be the Hollywood version...

Packard has come on greatly since merging with Bell and starting to make computer equipment instead of cars.

In the years since I've seen a couple instances of GE and P&W make some mistakes, but they sure tried to rectify them no matter what the cost. Never saw that happen with Rolls Royce. Their tech. reps blamed every operating circumstance and environmental condition they could drum up, but refused to point the finger in the_right_direction. Britsh arrogance and shitty engineering.

And the sun has most assuredly set on your empire, tough guy.

Garrett Fulton

My - you have got a chip on your shoulder. Wouldn't they let you play with their planes?

It's never risen on yours, wimp.

Of course, this is a car group, and we could talk about the shitty US products they export here. Right at the bottom of all reliability and satisfaction surveys.

Why would it take any longer than a power brake car with the same size rotors and pads? Je ne comprends pas.

nate

And so good you had to buy it...

Funnily, the *worst* electrical parts I've ever had on any UK made car were distributors by Delco.