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Editorial: Racing Tips - Good or Bad?

By Tracy Crook . Originally published in Rotary Aviation News, Issue 3, 1996

The first place many builders turn to for advice on how to prepare an auto engine for aircraft use is the car racing community. The racing world can be a valuable source of information and parts but just because a particular idea works there does not mean it’s the right thing for aircraft use. Remember that in the realm of racing, the perfectly prepared engine will explode just as it crosses the finish line.

During the next few issues I’ll go over a few racing recommendations applicable to the rotary and see how they may or may not apply to our use. One of the most frequently heard tips refers to the oil pump.

Racing Tip # 1

Use the higher capacity oil pump of the Turbo II 13B. (Mazda used the term “Turbo II” to refer to 87 - 91 turbocharged 13Bs)

Almost every rotary car racer I talked to made this recommendation. First let’s define just what higher capacity means. What it does not mean is higher pressure. Oil pressure in the 13B is controlled by the two pressure regulators located in the front housing and the rear side housing. These are the common spring loaded piston type which bleed off oil back to the pan if the pressure exceeds the set point. The rear regulator is responsible for setting the maximum oil pressure under normal operating conditions. The front regulator functions as a relief valve in the event that there is a restriction in the oil cooler or oil filter path which is upstream of the rear pressure regulator. Oil pumps are of the positive displacement type. If something restricted the oil flow (i.e. clogged oil filter) and there was no relief valve, the weakest part of the system would rupture.

As applied to oil pumps, the term high capacity refers to the volume of oil that is pumped for every revolution. The reason for using a higher capacity pump in the turbo 13B is to insure adequate oil delivery to bearings and rotors under high load, low rpm conditions such as hard acceleration or climbing a steep grade in high gear. The turbocharged engine is capable of much higher torque output at low rpm than the normally aspirated version. The down side is that under normal conditions, the excess capacity of the pump is bled off through the regulator which results in wasted power (used to turn the pump) and higher oil temperatures as a result of churning the oil.

As you may already have realized, high load at low rpm is a condition never seen in an airplane, even when turbocharged. I will not go so far as to recommend this, but if I were trying to maximize efficiency while building a turbocharged 13B for airplane use, I would replace the high volume pump with the standard version rather than the other way around.

The standard pump has more than enough capacity at any rpm you will use in flight. This was adequately proven to me after the blown O-ring problem I described in the conversion guide. While fixing this problem, I found that I had inadvertently created an oil system bypass path when I removed the stock oil injection metering pump drive gear. The drive gear was removed when I first assembled the engine. This essentially left open a 3/16 inch passage connected to the high pressure side of the oiling system. There were 98 hours of flight on the engine when I found this out. Instructions for blocking this passage are in the manual, but the point here is that the standard pump was easily able to keep up with the demand even with this large “leak” in the system.

On the subject of oil pressure, higher pressure regulators are available from racing sources and many builders assume that more is better here too. Even the racers agree that unless you plan on operating your engine in excess of 8000 rpm, this is not necessary. It only results in wasted power, higher oil temperatures and greater wear on the oil pump drive chain.

The main point here is that just because something works well for racing doesn’t mean it’s a good idea for aircraft use. Each idea needs to be analyzed separately. More on this in the next issue.