Wrenching Tips: The Concept of Vacuum

This week’s Wrenching Tips is less about tips or tricks, but wrapping your head around a very useful concept for diagnosing engines, particularly badly running carbureted ones.

Your motor is an air pump. It dedicates half of its to pulling in or pushing out gasses. In between, it converts chemical energy to motion and heat. Upstream from this pump is a valve, commonly known as a throttle. Upstream of the throttle is open air (because we’re ignoring blowers, turbos and air filters for now), between the throttle and each cylinder is the intake manifold. The intake manifold is where the magic of vacuum happens.

Ignore your car for a second, and let’s get super-abstract with three components: a pump, a tube and a valve. The pump pulls air past the valve and through the tube. Any time you have flow through or past a restriction, you have a difference in pressure. Since the air upstream of the valve is at ambient pressure, we say the lower-pressure, we say the lower-pressure, downstream section is at vacuum.

Keep the restriction the same but increase the flow? More vacuum. Keep the flow the same, but increase the restriction? More vacuum. Turn off the pump or open the valve all the way, and the pressure difference (aka the vacuum), goes to zero.

Pause and let that sink in, as it’s about to get more complicated.

While a throttle is pretty much an idealized valve and your intake manifold pretty much a series of idealized tubes, your motor is far from an idealized pump. There are a number of differences between the “ideal” motor and the thing dripping oil in your driveway, but it’s precisely these differences that allow us to use vacuum as a “signal” for what’s going on in your engine.

Though, running at idle, your actual motor’s pretty close to that idealized pump model: the throttle’s fixed in a mostly-closed position, the motor’s at a constant speed, pulling a constant amount of air through. In the real world, that means the vacuum (negative pressure relative to outside air) inside your intake manifold should about 15 inHg, aka “inches of mercury”, aka roughly 7.3psi, aka 51kPa, aka .5 bar.

Assuming it’s running properly, that is. My vacuum gauge-equipped Wagoneer takes a few tries to get going:

A vacuum gauge will tell you if one cylinder’s misbehaving, as you’ll notice a tick in the needle at half of RPM. That tick is typically a valve issue on a culprit cylinder: intake not opening or closing properly, or exhaust staying open when it should be closed. Similarly, you can use a vacuum gauge while adjusting valves or timing: fiddle with either (even while it’s running) to maximize vacuum. The more vacuum, the healthier your pump is.

One big way your motor differs from the idealized pump is that it’s stuck doing a bunch of other work. Stuff like spinning a fan or, you know, moving your car. Basically, the more power your motor’s putting into moving your car, the less that’s available for pumping. Because of this, we can use vacuum as a signal for load on the engine or “available power remaining”. More load = less vacuum.

Again, let that sink in. It’s important.

Combining our two basic concepts (the simplified pump-tube-valve and load Vs vacuum), let’s think about what happens as you stomp on the throttle. The valve opens, but the pump’s still moving the same amount of air, so vacuum goes to zero. As the motor starts to make use of the decreased restriction and increased fuel, it starts pulling in more air. More airflow = more vacuum, so we see the needle on the gauge climb back up. Get off the throttle, and the valve snaps closed while the pump’s still churning. Massive vacuum spike. Watch and learn:

Carburetors use vacuum in combination with how open the throttle is to actuate various spring-loaded diaphragms or needle valves to meter fuel. Basically, they deliver more fuel when there’s less vacuum. Remember: less vacuum = more load, so that’s when the engine needs more fuel. EFI systems just use a vacuum sensor (aka manifold absolute pressure, aka MAP sensor) and feed it into a computer algorithm that decides how much fuel to deliver.

Let’s get to a few more specific applications of vacuum.

[Image srouces: Tunertools, Hotrodders.com]

First up: carburetor or throttle-body sizing. Either is the “valve” in our idealized model. At wide-open throttle (WOT) and max RPM, you’d like to be as close to that idealized valve as possible: no restriction. If you’re still pulling vacuum at full throttle and max RPM, your “valve” is holding you back. Obviously, the easiest way to fix this is to have the biggest-diameter example you can find, as large tubes offer less restriction. Unfortunately, that restriction is the source of our vacuum “signal”, which a carburetor uses to meter fuel. Slap too big a carb on your motor and you’ll have almost no midrange throttle response as the carb can’t tell the difference between part throttle and full throttle (since both have no vacuum). This tradeoff is the basis for the four-barrel carburetor: you idle and cruise around on two small barrels, with two additional barrels that open up based on a combination of throttle and vacuum.

So what to make of the often vexing vacuum leak? Well, first of all, go buy a vacuum gauge to see how much vacuum you’re running. They’re cheap, just do it.

Going back to our simple model, a vacuum leak is a tiny second valve, feeding the same tube. This is a problem, because your carburetor or EFI system is delivering fuel assuming that all the air’s going through the main valve. You’ve got extra air and not enough fuel, creating a lean condition. Too lean = hot, pinging motor. This is bad. Though, we can see that the effect of a vacuum leak will only be obvious at idle, when the size of the throttle “valve” and leak “valve” are almost the same size. As the throttle opens wide, the extra air from the vacuum leak is a roundoff error compared to what’s going through the throttle.

Lastily, power brakes use engine vacuum to pull on a big diaphragm and increase the force applied to the master cylinder. It makes sense to use vacuum for this purpose, as when you take your foot off the gas you snap the throttle closed with the engine still spinning relatively quickly. More restriction = more vacuum = more braking boost right when you want it. Alternately, it makes no sense to use vacuum to power windshield wipers, as they basically stop when you floor it, then speed up as you get off the gas.

That wraps it up for this week. Hopefully it didn’t suck too much.

DISCLOSURE: Those links to vacuum gauges on amazon earn us some money. We earn a bit more money if you actually buy something. We’re not necessarily endorsing Amazon as the best place to get something, but they’re typically pretty good. We’d rather make money pointing you to useful stuff than begging you to punch the monkey in the sidebar.

[Lead image: Terry Walters Performance Parts]

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