With every passing day the Mule provides opportunities beyond our original expectations. Since the 467ci engine belongs to HPP editor Tom DeMauro, it is at our beck and call. This affords HPP the ability to do things, such as our octane test, that would not be possible with a third party engine. (Try telling the owner of a nearly 500hp Poncho that you want to dyno it on regular grade gasoline and see how far you get!) So far we have tested three different grades of fuel, a similar amount of oil viscosities, and a mix of rocker arm ratios beyond the original dyno testing and tuning.

At this point, the Mule has well over 100 full-power, full-throttle runs, not counting other partial pulls. Moving forward we’re planning more testing.

With all of this stress on Project Pure Poncho, both the author and RaceKrafters thought it would be a good time to perform a cursory check-up of its health. No, we were not going to pull it apart, but instead perform an automotive version of a CT scan or X-ray. Compression and leakdown tests allow the health of the engine to be quantified without any more effort than removing the spark plugs. In addition, we introduce a procedure identified as a bubble test that many may not be familiar with.

Under the Microscope

The best way for any enthusiast to keep track of their engine’s health is to perform routine testing and document the results. This way, as time or use accumulates, you can accurately track the internal condition of the Pontiac engine, not only as a confirmation of all being well but as a trigger to locate a potential issue before it becomes a major problem. There is no better method for this than compression and leakdown tests. Since each has it own value and protocols, they will be discussed separately.

Compression Tests

When discussing a compression test, the question always comes up about how the results are indicative of the engine’s compression ratio. The blunt answer is they are not. It is apples and oranges.

The compression ratio is the difference in cylinder volume with the piston at top dead center (TDC) versus the volume with it at bottom dead center (BDC). For example, if the volume at BDC is 10-times greater than the volume at TDC, the engine has a 10:1 compression ratio (CR).

CR impacts an engine’s thermal efficiency ( “Eating Crow,” Sept. ’12), throttle response, and output for both torque and horsepower along with its octane appetite. Though it is well documented that the CR has a direct impact on thermal efficiency, the engine’s octane requirement can be lowered by design protocols, such as high rates of mixture motion, spark plug location to the bore center, and combustion chamber design. That is how so many newer engines can employ a 12:1 CR on 87-octane gasoline without experiencing abnormal combustion.

The compression test establishes the engine’s ability to pump air and build cylinder pressure in the bore. It checks the sealing of the cylinder under pressure and the piston rings’ ability to keep the compression gases working against the piston crown instead of escaping into the oil pan. It also is an indicator of the ability of the valve to seal against the seat and make an airtight bore.

Most are familiar with a traditional cranking compression test, but there is also a running compression test that is very important. We performed both on the Mule.

The readings are measured in pounds of pressure per square inch.

Leakdown Test

While the compression test is dynamic (the piston is moving), the leakdown has the piston static (not moving). Instead of the piston building pressure in the bore by moving up and down, the leakdown test applies shop (compressed) air to the cylinder while determining the amount that exits. The leakdown test is read as a percentage of total cylinder pressure inputted. It is a differential test.