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Using a Vacuum Gauge as a Tuning Tool


Well-Known Member
One of the more overlooked tools for engine diagnosis is a vacuum gauge. It can quickly and easily identify internal engine problems, but many people don't have a clue how to use one. And that is the purpose of this article, to clear up the mystery surrounding how to use a vacuum gauge to help you tune your ride.

One of the first steps in using a vacuum gauge on your vehicle is to have a pretty good idea of what vacuum levels you see when the engine is happy and running normally. A stock engine, when all sealed up, will show vacuum levels as high as 18-20 inches of vacuum, whereas a highly modified engine with a lot of camshaft can show much lower levels. When you are sure you've got your new engine sealed up and dialed in, stick a vacuum gauge on it and record what you're seeing as a stable reading.

An engine in good condition should show fairly consistent vacuum levels at all stable engine speeds. If you rev the engine 2,000 - 3,000 RPM over idle speed and release the throttle, you should see vacuum jump anywhere from around 2 - 5 inches over your normal numbers and then quickly snap back to normal.

If you notice a consistent drop of about 2 inches of vacuum from normal, you've likely got some slightly retarded ignition timing.

If you notice a consistent drop of about 5 - 10 inches of vacuum from normal, retarded valve timing is likely the culprit.

If you see a consistent drop of 10+ inches of vacuum from normal, it's time to find the leak in the intake tract. Carb base gasket, warped intake, cracked intake, manifold gasket leak or a bad vacuum hose are some of the probable causes.

If you see the vacuum level dropping steadily and rhythmically, then something is happening in a single cylinder. It could be a bad distributor cap, a bad plug wire or a bad plug. It could also be a valve that is not seating properly.

If you see a very intermittent and rapid drop that will just as intermittently recover, you've got a valve guide trying to grab a valve and keeping it from seating normally.

If you see the vacuum level rise 2 - 5 inches on rapid deceleration, but the gauge slowly returns to normal, start looking at the exhaust system for a possible restriction.

If the vacuum level doesn't rise much on rapid deceleration, then it's time to break out a leak-down tester to check piston ring seal.

If you see the vacuum levels continuing to fluctuate 2 - 3 inches either side of normal, you probably have enough worn valve guides admitting air to the port and giving the carburetor fits.

If the vacuum gauge seems to lazily change at idle and then get stable at a higher RPM, you've likely got a shot valve or a blown head gasket.

If you see the vacuum level continuing to vary at a steady RPM, it's time to start tuning. If you're running multiple carbs, they're not properly balanced. Ignition timing could be off or plug gaps may be set too far away from what makes your engine happy. Or you could have some bad valve settings.

If the vacuum level is fluttering back and forth at idle but flutters in a narrower range at a higher RPM, you likely have a leak in a manifold gasket.

If the vacuum level flutters back and forth at idle and gets worse at a higher RPM, start looking for a broken rocker arm, a bent pushrod or a broken valve spring. If everything appears to be in good shape, get some springs off and check them for load.

If the gauge makes slow and steady flutters, matched by a change in engine idle RPM, you are either looking for a very minor leak or your idle mixture settings are off. A quick tip here is to work back and forth on the mixture screws, adjusting them for the highest, steady vacuum level.

If you're running a Holley carburetor and notice vacuum levels fluttering, engine RPM hunting and seeking and a definite gasoline smell at the exhaust, the power valve in the carburetor is either blown or wrong. Once you get a good valve in the carb, a quick tip to determine the best power valve choice for your application is to note your lowest vacuum figure at idle and your lowest vacuum level at a steady cruise RPM on level ground. Those numbers should be very close to one another, but take the lowest number and select a power valve about one to one and a half points below it. For instance, if you record a low number of 10 inches, select an 8.5 power valve. That will prevent the valve from sneaking open on you during idle and steady cruise RPMs, but it will also position the valve to tip in quickly when you need it.

I hope this will help some of you, the next time you're trying to diagnose a problem or just tuning for maximum efficiency.
Excellent info I bookmarked this page. I have worked on a lot of engines but the holly power valve was new to me. knowing that could help a whole bunch. Thanks Mike.
Mike, thank you for an informative and long overdue article. My question is will an engine that is 500 cubic inches have the same vacuum reading at idle as an engine of 350 cubic inches (all things being equal, cam/, compression, etc. ) Shouldn't a larger engine pull more vacuum at idle than a smaller one?

Curious, John
Mike, thank you for an informative and long overdue article. My question is will an engine that is 500 cubic inches have the same vacuum reading at idle as an engine of 350 cubic inches (all things being equal, cam/, compression, etc. ) Shouldn't a larger engine pull more vacuum at idle than a smaller one?
Good question and the short answer is no.

If we have a cylinder with 50 cubic inches of swept volume, that volume is achieved by both bore size and stroke. With the block, crank and piston, we can sweep 50 cubic inches, but the combination will not create a vacuum, because there is no restriction. Now, if we add a cylinder head, with its corresponding intake and exhaust runners, we are now capable of creating a low pressure area in the runners. Top that off with an intake manifold and a carburetor and as we continue to add the restrictions, the cylinder's ability to create a low pressure area at the carb's venturi's is increased even more.

If we use a longer connecting rod in this engine, we can signal the venturis even harder, because we have a combination that will move the piston faster from 90° ATDC to BDC. That will create a stronger vacuum signal. We are also creating more cylinder pressure, as the piston will slow and dwell longer from 90° BTDC to 90° ATDC.

But before we get too carried away with how we can create stronger vacuum signals, we have to come back to another engine principle - volumetric efficiency. VE is the term we use to describe the amount of fuel/air in the cylinder in relation to that same cylinder being 100% full of air at atmospheric pressure. And VE cannot ever achieve 100% on a naturally aspirated engine, because we need that low pressure signal from the cylinder to create a pressure differential between the top of the carb and the bottom of the carb. Without that differential, the carb is lost and has no idea what it is meant to do.

If we wanted to produce a higher vacuum signal, we could install a smaller carb. Since the carb cannot flow as much air, it creates yet another restriction and increases vacuum. BUT, the smaller carb's air flow numbers also cannot carry as much fuel in the air stream, so we are losing chemical energy potential, which will lower thermal efficiency, which will lower mechanical efficiency, which will not put a smile on Joe Hotrodder's mug.

If we now increase the bore and stroke of our model engine to 100 cubic inches and make no other physical changes, we are going to have a stronger vacuum signal. So it would seem the answer to your question is yes. However, we have that problem of trying to carry enough fuel (with its chemical energy potential) into the cylinder to produce a maximum VE, because this is what increases thermal efficiency. And we need thermal efficiency to be as high as possible, because every engine has working losses created by all of its moving parts. And our thermal energy, after the working losses are deducted produces the mechanical energy that actually does the work we want the engine to do. If we've increased engine size without increasing available energy at the rear of the crank, we've wasted our time. So we have to increase carb size, manifold size, runner size, valve size, etc.

In order to maximize chemical energy potential (our fuel) to maximize volumetric efficiency (our cylinder fill) to maximize thermal efficiency (load exerted on the top of the piston at each power stroke) to maximize mechanical efficiency (power at the rear wheels), we have to make attending changes to a larger engine. And those changes will keep vacuum levels amazingly close to the levels recorded in a smaller engine.

And just to sweeten the pot, have you noticed we haven't even begun to look at the other side of our engine? We have to remember the exhaust side is every bit as critical as the intake side. If we are not efficiently removing combustion byproducts from the cylinder, we are diluting the intake charge, making the engine less efficient.

Obviously, we can increase our efficiency levels by using an external compressor to help force air into our engine, but now we are no longer dealing with vacuum, but with boost.

Now, here's a question for you to consider - If we check the compression on a stock 350 Chevrolet (static compression ratio ~8:1), a Sprint Cup 358 engine (static compression ratio = 12:1) and a 347 Competition Eliminator drag engine (static compression ratio +16:1), which engine will have the highest pressure figure?

Allow me to share a rather humorous story with you. Several months back, I was visiting another automotive discussion forum (which shall remain nameless) and found a discussion topic where a couple of individuals (who shall also remain nameless) were 'designing' intake manifolds for their engines. These 'design engineers' were employing some rather novel and creative design techniques. One individual had determined where he wanted his carb to end up and was constructing his intake with that particular height in mind. He had constructed a plenum and was using tubing to connect his plenum to the plates attached to the cylinder heads. Using his pictures to determine scale, I did some quick math to figure the approximate size of his plenum and could quickly see it was in the neighborhood of 60% too large for his naturally-aspirated engine. The tubing diameter he was using was close for a street-driven application, but was still 10% - 15% too large. For his particular engine combination and the RPM level where that engine wold typically make its best power, his manifold runners were 2.5" - 3" too long. In short, this individual was going to need a very sophisticated 4 valve per cylinder head and a properly tuned exhaust system to make things work. Otherwise, his intake manifold was going to do little more than hold his carburetor up off the floor. :hoist:

It is a constant and ongoing balancing act, trying to efficiently make power. And no matter how hard we try, we are always reminded ol' Mother Nature has some laws of physics that we are not allowed to break.
Awesome post Mike. Thank you!!!


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