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Atlantic Racing Scene

Breath Work, It’s all about the Manifold Vacuum . . .

This is the first article on a series of diagnostic tests that can be done by any service technician to find difficult problems on today’s vehicles.     

Let’s begin with cylinder breathing. Most of us would refer to this as a misfire or a rough running issue. I want to take it back to fundamentals by checking the mechanical operation of each cylinder including the intake and exhaust. When you do a normal compression test, you are checking cylinder sealing, not cylinder breathing!. When you check the engine vacuum at the manifold, you are looking at the breathing of the entire engine. What you don’t see is specific cylinder breathing and that’s what we will cover this issue.

Testing the Breathing of an Individual Cylinder

The first step is to find the compression ratio for the engine before you start. This information is usually found in service information or in the owner’s manual. This is not the compression but the ratio. Example: 8.5:1 / 9:1 etc. Next, get a vacuum reading of the engine when it is warmed up and record it. You can use your scan tool and look for a MAP reading but be aware that some MAP readings are the difference between Baro and MAP. So, you may see 11 inches on your MAP reading. This is if BARO is 30 and MAP is 11 the difference is 19 inches, which is your vacuum reading.

2002 Toyota Highlander intake manifold illustration. Dual ACIS (Acoustic Control Induction System). Variable dual-plane intake manifold system is electronically controlled via vacuum to vary the intake manifold length and volume, to improve intake efficiency.

Following this, you will need a compression gauge. Install it into a spark plug hole and ground the wire. It’s best to do this when the engine is warmed up and in a well ventilated area. Also be aware you may set a check engine lamp for this test. Run the engine on idle and watch the compression gauge and record the number. Now, take all the recorded readings, so say the engine is running at 18 inches vacuum.

Atmospheric pressure is about 30 inches, so the difference (30 inches - 18 inches = 12 inches) is what the engine is sucking in. Twelve inches of mercury is equivalent to about 6 PSI of absolute air pressure ( If compressed at an 8-to-1 ratio(from your vehicle specs), you should get 6 x 8 = 48 PSI pressure if all the air makes it into the cylinder and then gets pushed out. So your idle reading on the running compression is about 50 PSI.

In fact, you can do running compression tests at various constant manifold vacuum readings by loading the engine momentarily. The running compression should roughly correspond to the manifold vacuum. For example, at 10 inches vacuum, the engine should be breathing in about 10 PSI air pressure. You should see a running compression reading of about 80 psi (at 8-to-1 compression ratio). If one cylinder reads low running compression compared to the rest it means that the air didn’t make it in. If one cylinder reads high, the air didn’t make it out (and the next pulse of air raised the pressure).

When using the above “equations” make sure you know the compression ratio, as this will give the accurate measurements needed. Next issue we will cover the volumetric efficiency for each cylinder and the test procedures for each method. If you would like to receive this document attached with a couple of case studies e-mail me at and we would be happy to send you a copy.




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