Mazda Rotary Evolution - Part 2

Racing Beat co-founder and chief engineer Jim Mederer has earned his belt buckle the old-fashioned way, by tuning rotary-powered Mazdas to perfection for almost 40 years and surviving a 225-mph rollover at Bonneville. Search "Mazda RX-7 crash at 255 mph!!!" on YouTube for the Discovery Channel's recap of the crash.

Racing Beat co-founder and chief engineer Jim Mederer has earned his belt buckle the old-f

Few people outside of Mazda's RE research department know more about rotary-powered Mazdas than Jim Mederer, co-founder and chief engineer at Racing Beat. Founded in 1971 by Mederer and Ryusuke Oku, Racing Beat has been racing rotary-powered Mazdas ever since, having filled shelves with trophies and putting its name in the record books along the way. In 1978 the company's first-generation RX-7 set a Bonneville top-speed record of 184 mph. In 1983, its IMSA RX-7 won the GTO class at the 24 Hours of Daytona. In 1986, its FC RX-7 set a land-speed record at Bonneville of 238 mph. And after a dramatic 225-mph rollover on the salt flats the year before, in 1995 Racing Beat's FD RX-7 bumped that record up to 242 mph. The company has worked closely with Mazda on a number of projects and even developed a single-rotor engine for NASA. Simply put, Racing Beat has learned a thing or two about rotary engines by putting them to the ultimate test in some of the most grueling motorsports events in the world.

If you read Part 1 of this story in last month's issue, then you already have a good sense of where the Wankel rotary engine design came from and evolved into. It's all well and good to understand the history and mechanics of the rotary, but what if you actually want to own a rotary-powered Mazda? What should you be looking for? To answer these questions, Racing Beat's Jim Mederer has very generously shared his thoughts so you can go RX shopping with confidence.

Rotary Shopping According to Jim, "Mazda has been improving the rotary constantly over the years. Therefore, it's logical that the latest models are the best - the most reliable, the most powerful - and they are. One exception to this pattern is the fact that in 1993 Mazda eliminated the engine-driven fan and uses an electric fan in its place. These fans are far less powerful than the mechanically driven fans they replaced. Along with this change, Mazda has packaged its engines very tightly, causing high temperatures to accumulate in the engine compartments. This has caused severe stress on engines that were otherwise very well designed, and as a result the reliability of the RX-7 and RX-8 has been less than it could have been. These problems have shown up most in hot weather and are difficult to fix since they are related to the shapes and arrangements in the engine compartment. It's a fact that rotaries are more sensitive to thermal stress than most reciprocating engines and will not tolerate high temps in either the water or the oil without undesirable results."

Jim went on to say, "The best way to check the health of a rotary is to do a compression check. These checks are subject to some interpretation. The starter-cranking rpm is an important question, along with the engine temperature, rotor compression ratio and the type of compression tester used. Rotaries typically crank around 300-330 rpm. If they turn above or below this number, the test result will be affected. Engines often have moderately different compression pressure hot versus cold, but this varies with the type of engine. You can use a conventional dial compression tester, but it is preferable to use a 'recording' tester that will take and display the pressure from each of the three sides of the rotor separately for comparison. Although the actual peak pressure recorded is important, it's equally true that you should have minimal fluctuations from one flank of the rotor to another as well as between rotors. The highest numbers I have ever seen are around 140 psi. Healthy late-model engines are usually around 110-130 psi, earlier engines 10 psi less. Engines below 90 psi may be hard to start, though they may still make normal power. The key is to watch for evenness between the pulses and between the rotors."

By David Pratte

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