The classic differential pressure tester incorporates a pressure regulator, a precision orifice, an on-off valve, and two identical gauges (see Figure 1). It is connected to the cylinder via an adapter and standard air line fitting (not shown).
Line pressure from a compressor (Figure 2, blue) is regulated to 80 psi (green), an aviation standard. The orifice provides a pressure drop proportional to airflow through the orifice and the connected cylinder. At zero flow, the pressure downstream of the orifice (yellow) would be equal to regulated pressure. Given a small flow, the pressure drop would be small (high gauge indication), while a large flow—such as a leaky cylinder—would result in a large pressure drop (low gauge indication).
The gauge orifice for small engines has a specified diameter of 0.040 inch and an entrance angle between 59° and 60°. You will find references requiring a 0.060-inch orifice for use with a bore diameter larger than 5 inches. However, a manufacturer’s service information takes priority over any advisory material. Lycoming SI 1191A and Continental SB03-3 specify the 0.040-inch orifice for all engine models, including those with bores in excess of 5 inches.
The basic gauge set shown above is all you need for a Lycoming. Per SI 1191A, Lycoming cylinders should test within 5 psi of each other and 70/80 or higher, while readings below 65/80 or differences of 10 to 15 psi require a regular retest to determine wear rate and trend. Below 60/80 or more than a 15 psi spread requires investigation, and perhaps replacement.
Continental does not publish a minimum pressure. Instead, Continental SB03-3 requires a leak-down tester, which incorporates an additional master orifice and control valve, or the use of an accessory master orifice. The orifice is used to establish a Minimum Acceptable Pressure Leakage Limit prior to doing the leak-down test. The Leakage Limit is a field-determined minimum pressure value used in place of published minimum values. It is established by doing a leak-down through the calibrated master orifice prior to connecting to the cylinder (Figure 3). Mechanics report typical gauge values in the 40s.
If the cylinder pressure is higher than the previously recorded master orifice pressure, Continental considers the cylinder leak-down rate to be acceptable (Figure 4). However, mechanics should consult Table 1 in SB 03-3 for complete airworthiness information.
Although unacceptable to Continental, the use of a gauge set without a master orifice is quite common. The classic tool and traditional guidelines work fine for an owner who wishes to monitor trends for a specific engine using the same tester each time, in particular if the test values are well above 60/80. Remember, minimum values determined with a master orifice are in the 40s. If a Continental cylinder trends toward marginal, or if legality or warranty enters the picture, obtain the specified tool and use it to establish minimum acceptable pressure. If shopping for a new tester, spend a few extra dollars and buy one with the additional master orifice per the Continental SB. It is the cool tool for Continentals, and you will use it just like the classic model when checking a Lycoming cylinder.
An important comment – BE CAREFUL. Serious injury or death kind of careful! The prop MUST BE HELD to prevent movement during testing, lest it swings around and hits you. I speak from horrible experience, with a long time IA that lost control of the prop during a test. It crushed 7 inches of his skull. He survived, but was never the same. Be careful!
Thanks Dan for the great explanation and references. Prop control and ensuring you are on TDC are also learning curves.
Dan, thanks for the excellent explanation of the tools and their operation!
This might be a good point to add that the differential pressure test is very useful to determine where the leakage is. You can listen for escaping air to see if the intake or the exhaust valve or the breather (rings) are leaking. Exhaust valve leakage rarely gets better with use. Crank case pressure leaks often get better if they are due to sticking rings.
There is also the pressure tester used in the automotive field. It is a simple pressure gauge with a one way (bicycle) valve connected via the spark plug hole. The engine is turned over with the starter typically 10 compression strokes with the throttle open. All other plugs removed to make it easy on the starter. It then shows the total pressure each cylinder can achieve. The absolute value is not so important but the difference between each cylinder is. A fancier version of this test records the pressure gain from each cycle on a chart recorder.