Rotary Tuning
So you've found a rotary-powered Mazda you want to buy and have compression-tested the engine and it checks out perfectly. You plunk your cash down and take your new RX home, where the next step for go-fast geeks like us is to make it more reliable as well as more responsive and powerful.
According to Racing Beat's Jim Mederer, "Cooling is the most critical concern when using a rotary hard. Start by inspecting the radiator and oil cooler(s) for debris (sand and bugs). If the fins aren't clean and straight, either fix it or install a new radiator and/or cooler. Mazda has a very good fin design, but it gets plugged easily because it's so small. While you are there, inspect the seal of the shroud to the radiator. If it isn't sealed, fix it."
In terms of improving cooling beyond the stock system, according to Jim, "The only thing that we have found to help is to install a 'spoiler' lip of 1/16-inch-thick ABS plastic under the front crossmember, 3 inches high and extending as wide as possible across the underside of the car. It will hit the ground in cornering, but the low pressure it creates at the engine compartment air outlet is helpful. If you want to see an example, just look under the front of a C6 Corvette. Thicker radiators have limited value because they tend to restrict airflow even more than stock. And if you don't have an accurate temperature gauge, at least on the cooling system (and preferably on the oil inlet system too), install one. Stock gauges are inadequate. We recommend that you never exceed 195 degrees Fahrenheit water outlet temp and 205 degrees oil inlet temp at high power. At light power, the engine can tolerate somewhat higher temps, but don't push it. We aren't saying that the engine will fail if you exceed these temps, we are saying that higher temps will shorten the engine's life at an increasing rate."
According to Jim, FC and FD owners need to beware of fuel delivery problems."Fuel pumps, fuel filters and the wiring and relays that supply the pump with power - none of these age gracefully. The pump performance decreases, the filter becomes restricted and the wiring, which wasn't very large to begin with, accumulates resistance at all contact points, reducing voltage to the pump. A new stock pump and filter are good starting points, but the wiring problems are tougher. Start by measuring the voltage at the fuel pump with an accurate voltmeter. With the engine idling, the voltage at the pump must be no more than a few tenths of a volt less than battery voltage. If it is, you have two choices: find the locations where the voltage drops occur and correct them or run new, larger wiring to a new relay at the pump, controlled by the old pump wiring. This isn't fun, but it's necessary in many cases. To date, the RX-8 hasn't shown this problem, but you would be wise to check the pump voltage anyway."
In terms of other reliability issues to be mindful of, Jim recommends you don't spin the stock turbo (on turbo FCs and FDs) beyond 14 psi. If you do, there's a real chance of overspeeding the turbo, killing the turbo bearings in the process and sending parts of the compressor through the engine. Jim also recommends that for track days you install two step colder spark plugs gapped to 0.020-inch to avoid possible detonation at high power. Just keep in mind that although colder spark plugs that are clean do not reduce power, but they can foul fairly quickly, so try to use them only during competition or track days.
With the reliability side of your RX sorted out, you'll no doubt start looking for ways to increase engine output. According to Jim, "Once you have the engine temperatures and fuel delivery under control, the best way to increase power in a turbo RX is to install a low restriction exhaust system. The lower backpressure helps in two ways: it improves exhaust scavenging from the engine and it raises boost pressure. As long as you have good cooling and good fuel delivery, the increase in power can be impressive and fun. The addition of a low-restriction intake duct and filter can offer a bit more improvement. The same is true for normally aspirated engines, but the gains are much more modest."
Beyond the bolt-on options available from rotary specialists like Racing Beat, the sky is the limit if you have the budget to match. It's not hard to find examples of race-winning RX-7s and RX-8s, including the awesome Grand Am GT class Speedsource RX-8 that took the win at the Rolex 24 at Daytona back in January of this year.