Using a Compression Gauge on a Small Engine

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A Compression Gauge is a useful tool, but sometimes it can prove misleading.

It’s great if you take a reference of when an engine’s in good shape for comparison purposes later in the engine’s life.

It’s also great for seeing if there’s a significant difference in cylinders in a multi-cylinder engine.

However, it isn’t necessarily great when you’re trying to determine the condition of an engine you’ve never tested before, especially a single cylinder engine, and in this particular case, a “small” single cylinder engine.

I’m going to use my Honda Z50 engine as an example. It turns out the adapter for the compression tester is a problem. In the picture below, you can see the compression gauge with the Schrader valve at the end. It’s a one-way valve, just like on a bicycle tube.

The adapter is used to screw into the spark-plug hole; I think a 10mm diameter thread. You can see the large air cavity in it.



This large cavity affects the chamber volume as shown in the picture below:



A little math now. Based on the swept volume of 49cc and the known compression ratio of 8.8, the chamber volume works out to 6.28 cc.

CR = (swept volume + chamber volume) / chamber volume)

Based on dimensions with the adapter screwed into the compression gauge hose, the adapter volume works out to 2.86 cc.

Using Boyle’s Law concerning pressure and volume (P1 * V1 = P2 * V2), one can work out what the gauge pressure should be knowing what was measured. In my test I got a reading of 90 psi, which I considered low. In fact, so low I thought it might not allow the engine to start, or maybe start, but run with low power, but that was not the case. It ran fine.

So, the pressure I should be seeing is:

P = 90 * (( 6.28 + 2.86) / 6.28 ) = 131 psi

That’s more like it. In fact, the example in my Clymer Overhaul manual says 130 psi is for a new engine, with the lowest reading being 95 psi for an engine with 8000 miles on it. So, it looks like everything is OK, but as I suspected, an automotive compression gauge that uses adapters will not give an accurate reading for a small engine.
 
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