Thread: Engine Tuning Using a Vacuum Gauge

TUNING WITH A VACUUM GAUGE

You might be asking yourself, "What is a vacuum gauge, engine vacuum, and how can this article help me?" Good questions for someone that does not understand how their internal combustion engine functions. My hope is that this article will answer those questions and a few more.

Put simply, an internal combustion engine is nothing more than an air pump. This air pump just happens to burn a fuel that allows it to create torque and power. The faster you can get the air/fuel in and out of the engine the more torque you will make. The more volume of air/fuel you get through your engine, the more horsepower you will make. Think of torque as acceleration and horsepower as the engine's ability to sustain speed, or a top speed.

As the engine rotates, and the pistons move down each cylinder they create a vacuum signal. It is this vacuum signal that draws more air into the engine. In a carbureted application this will also draw more fuel into the engine. Modern computer-controlled fuel injected engines use sensors to measure engine vacuum and/or airflow which provide information to the computer to make complex calculations as to the fuel and timing curve the engine requires.

What is important to understand is that the higher the vacuum signal, the more efficient the engine is going to be. This counts for every given RPM and load level the engine sees. Using a medical analogy related to the human body, you could compare your engine's vacuum efficiency to that of your lungs. If your lungs are strong and efficient you are better suited to participate in activities of an athletic nature. Nimble on your feat, in better health, etc. Yet, if you're a chain smoker or overweight you are most likely going to be out of breath at even the slightest exertion of energy above your normal routine. With this example we are using a comparison to the efficiency of your vehicle's engine, and not necessarily that of a maximum horsepower or RPM potential. I will describe a racing scenario as the article continues, because aggressive cam profiles and internal clearances hamper efficiency to benefit other areas of performance.

Engine vacuum is affected by how the engine is built, its size, and intended use. Larger cams, carburetors, cylinder heads, basically anything to increase air volume will negatively affect air speed and the vacuum signal - at least at lower engine speeds.

Reading the Vacuum Gauge:

Many racers and engine tuners (I get caught myself sometimes) have opted for state-of-the-art diagnostic equipment but have forgotten one of the simplest, as well as most accurate tuning tools ... the vacuum gauge.

Of course if your engine is barely able to sustain 2" Hg vacuum at idle, it will be hard to tune using this method, but it is still useful in diagnostics of some problems. If you use a quality large face vacuum gauge, with some experience you can effectively tune your race car's fuel and timing systems. This is a starting point, we all know that dozens of passes and laps will more than likely be required to verify tuning adjustments and compensate for variations. We can also go too far using this method of tuning, something I call "artificial improvement" that we need to be aware of. I will provide more details a little later in this article.

• Vacuum gauge
• Screwdriver
• Vacuum "tee" for making a connection if you do not have an unused vacuum port
• Carburetor Spray

Optional Tools:

• Professional shops can make good use of a smoke machine for leak detection and other engine diagnostics. The Vacutec company (Model WV605 shown at right) makes various units to assist in finding vacuum and other leaks. I used to own a Vacutec unit when we were performing service work, a tool that saved my techs a considerable amount of time searching for pinhole leaks and used for other diagnostic processes. http://vacutec.com/aftermarket.html

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First, A Few Basics:

Connection of the gauge is made to a simple "manifold" vacuum source. "This must NOT be from a ported vacuum source (a source that pulls vacuum from above the throttle blades) which rises as RPM increases." In most cases, your manifold vacuum source will be a direct manifold fitting, or in some cases you can use the PCV port (larger port on the carb) on a typical carbureted application.

NOTE: Connection to EFI (fuel injected) applications is best done "Tee'd" into the source line. If you are not careful, disconnecting a particular line can affect the idle speed and interrupt information that the ECM needs to see. For this reason it is best to tee into your vacuum source for the gauge connection.

You must have the timing "Pre-Set," or at least do all ignition settings "Before" any carburetor adjustments.
After each adjustment is made, you MUST RESET your idle speed RPM setting. You do this to have a standard or baseline to compare against.
Small adjustments are best, and in fact "optimum" carb settings on the vacuum gauge (highest reading) is usually richer than it needs to be. In other words, after the highest reading is reached, the best setting (depending on engine) is to set mixture screws back "lean" approximately 1/16 to 1/4 turn.

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Proper Carb Adjusting Procedure:

With the above items taken into consideration, the first thing you do after connecting the gauge is to "lean out" one of the primary mixture screws (those on the front or primary venturis if using a 4-Barrel carburetor) until the gauge as well as the engine begins to shudder.

NOTE: With a properly jetted carburetor, turning either of the mixture screws all the way lean (or close), should kill the engine. If not, you're too rich! This may require re-jetting, or drilling a small hole into each of the primary throttle blades to add more idle air. Many of the newer performance carbs allow you to change idle air bleeds to address this.

If you know the jetting is close, or even if you are not quite sure, you may need to look down the primary venturis or remove the carburetor and see if the transfer slots are exposed at idle (click on image below for a larger view).

These are the small vertical slots at the lower portion of venturi, just above the high side of the throttle blades. If these are exposed at idle (below the throttle blade), additional fuel will be pulled into the engine causing an over-rich condition. To compensate you need to lower the idle screw position to cover the slots. Now, after closing the throttle blades you may find the engine needs more air. Modern carburetors often come with adjustable air bleeds, so you can exchange one that is too small for one that offers a larger orifice. Another way to solve this is by drilling a small hole in each of the primary throttle blades. The holes could be as small as 1/16" or as large as 1/8". This is often telling you that there are problems elsewhere!

You will now bring the idle mixture screw back towards rich while watching the vacuum gauge. I am intentionally NOT being specific as to the "in or out" movement (clockwise or counter-clockwise turning) of the mixture adjustment screws, for there are a few carburetors that are actually "In" for rich instead of the standard Holley and others that use "Out" for richening the mixture. As the gauge climbs, you will stop adjustment when the needle reaches its highest reading. Now perform the same procedure for the other primary mixture screw. You may have to repeat this process a few times to get optimum results, plus its worth the time an effort to ensure proper settings.

For carburetors with the 4-corner mixture screws (as you see in the image above), you have to spend a bit more time setting these. Setting the secondary mixture screws is best if you run the engine at a "steady-state" speed of say 2500 RPM to double-check your secondary mixture screw settings. Do this with temperament! It takes time to get used to what you are seeing as well as if it is actually helping. Each engine will behave differently.

What Does The Above Tell You?

Correctly reading the vacuum gauge can help you find errors or problems with:

• Incorrect Jetting
• Vacuum Leak
• Leaking Power Valve
• Misfires
• Leaking Component Diaphragms (distributor vacuum canister, EGR Valve)
• Internal engine problems (valve adjustment, burnt valves, head gasket condition)
• Clogged exhaust system (catalytic converters, failing mufflers)

When I was younger, I was taught to effectively tune an engine with just the vacuum gauge and some track (or driving) time. Not even a timing light (which can be scary)! Once the education and practical experience is there, it is a very effective technique. It takes some patience, but the knowledge of how your engine behaves to tuning changes is worth some safe experimentation. The typical screw up will be that you will set the engine with way too much timing (more timing increases vacuum). Correct vacuum adjustments will be less than the optimum high reading. This is why using a timing light is still mandatory. Otherwise, at what you think is the best setting will cause substantial pre-ignition.

IGNITION TIMING:

At the beginning of this article I mentioned that it is easy to go too far when using the vacuum gauge to set your timing. I alluded to this problem moments ago, so let me provide more detail. When you are reading the vacuum gauge and turning your distributor (advancing it), you are moving the firing point of each ignition cycle before TDC (Top Dead Center). At low-load idle RPM situations you will watch the vacuum gauge climb and climb. With a cheerful smile of accomplishment you may want to tighten the distributor down at this highest reading - DON'T! - unless you you simply want to test the results with a timing light. This timing value is going to be much too high!

Advancing the timing to the highest point on the vacuum gauge sounds good, but this is NOT the same as fuel curve adjustments. Use the vacuum gauge with your ignition system as a guide and for further testing using proper equipment. If you want to perform a few experiments to prove this fact, that is your choice.

As you move the firing point too far before TDC (advanced), you will finally reach a point where the engine starts to hiss, spit and cough, and it will probably not restart if you shut it off (even without the spit and cough), for there will be too much countering energy (for too much timing advance) for the starter to overcome. On almost all engine applications we want the ignition timing to send the spark to each cylinder "just before" the piston reaches TDC. If the spark is too soon it affects the speed of the piston, slowing it down and increasing the possibility of damage as combustion occurs while the piston is still on the upward exhaust stroke. If the spark is too late, and the piston is already headed down the cylinder on the power stroke, we have wasted fuel and efficiency, causing lost power, a rich condition, and excessive emissions. We are looking for that balance that provides enough time to ignite the air/fuel mixture and provide the most efficient explosion that pushes the piston down the cylinder. We do not want too much timing at the wrong RPM and engine load!

In this one area where the vacuum gauge can mislead us, because it shows a higher reading when the timing advance exceeds what is optimum for the engine at this particular engine speed (idle). What we must be aware of is that there is very little engine load at idle, in comparison to a moving vehicle. If you perform this test, you will likely find that an engine that needs 10-14° initial timing reads 30-40° or more if you used the vacuum gauge to set the timing! The low-load and slower engine speed makes the engine see an "artificial improvement." If you were to drive the vehicle (add load), you would see that the engine is not happy with this initial timing setting, especially as the mechanical (and sometimes vacuum) advance starts to kick in. Another negative to all of this extra timing is that the exhaust valve as not fully closed, and the engine is now pushing unburned fuel right into the exhaust, dramatically raising the emission levels. The engine does not have the RPM necessary at idle to counter these negative affects. This is why timing is added as RPM increases on most vehicles.

Remembering that our vehicle's ignition system advances the timing as RPM increases, up to a point where the engine is supposed to be its most efficient, we do not want too much timing too soon and vice verse. We now know from over a hundred and twenty five years of science and experience with gasoline engines that varying levels of advance are more important than just RPM based changes, and the programming maps (fuel and ignition curves) on modern computer-controlled and fuel injected applications take this into account, to gain both performance and efficiency by modifying systems through thousands of readings and instructions per second.

DISCLAIMER: Yes, I know that many racing and performance applications use a high static timing (locked out with no mechanical or other advance system). The racing/performance engine with its aggressive camshaft timing, overlap, high compression, high idle speed and other factors offer something a bit different. These engines will always be "dirty" or inefficient at idle, because they are purposely built to be their most efficient and powerful at very high RPMs. This is the opposite of a stock or mild performance daily driver that spends most of its time under 4,000 RPM.

The racing system's locked-out timing allows a very high static timing setting and uses controllers to reduce (retard) the timing during cranking (start-up). These controllers also start TAKING TIMING ADVANCE AWAY as RPM increases, upon transmission upshift, boost levels, nitrous activation, and more. Each of these events initiate a controlled circuit that retards the timing. On the highest performance drag racing applications the engine may launch at 32° advance, and cross the finish line at just a few or ZERO degrees advance!

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Detailed Vacuum gauge needle readings explained below:

	WHAT YOU SEE ON THE GAUGE	DESCRIPTION
	Steady Needle	Normal reading (usually 15-22" Hg. in stock engines) Race engines vary "a lot" and in most cases will be considerably less, and may not be steady at all due to the variety of factors that can contribute to an unsteady needle on a performance application.
	Intermittent Fluctuation at Idle	Ignition miss, sticking valves, lifter bleeding off (hydraulic), or just a BIG camshaft.
	Low, Though Steady Reading	Late timing, low compression, sticking throttle valve, carb or manifold vacuum leak (remember most race engines with a big cam and a tight centerline and high overlap will be naturally low ... you must decide your baseline vacuum reading).
	Drifting Needle	Improper carb setting or minor vacuum leak.
	Fluctuating Needle as RPM Increases	Ignition miss, blown head gasket, leaking valve, weak or broken valve spring
	Steady, but Needle Drops Regularly	Burnt valve or incorrect valve setting (too tight), The needle will fall when the bad valve operates.
	Gradual Drop at Idle	Clogged exhaust, excessive back pressure (in extreme cases the engine will regularly die at idle)
	Excessive Vibration that Steadies as RPM Increases	Worn valve guides

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In Closing:

Of course all this info sounds really good, but don't throw out your timing light, multimeter and other required tools. What the information above offers you is one more way to check for proper tune of your engine, and also a few tricks to do some preliminary testing of other components that typically require special tools and equipment. The vacuum gauge does not replace the timing light, multimeter, exhaust system back pressure tester, leak down tester and other tools, it only enhances them. No one likes to tear down an engine to fix something broken, or spend excess hard-earned money on a trained mechanic or tuner to find a problem.

Always remember to use the right tool for the job!