- Saturday, August 31, 2019
When you think of a pistonless petrol car, the FD RX-7 probably comes to mind. This poster child of the 90s made its claim to fame with the twin-turbo 13B-REW, an engine that was easy to modify and often featured in popular media.
What if I told you the 13B-REW's naturally aspirated successor was just as special?
Enter the Renesis
The Mazda 13B-MSP Renesis is one of the most unique combustion engines ever to hit a showroom floor. Despite its lack of forced induction, the 6-port variant produced 232 hp, trailing the outgoing USDM RX-7 by a mere 23 hp. Not only does this make it one of the highest specific outputs of any naturally aspirated engine, it managed to greatly improve efficiency and reduce tailpipe emissions. But wait, there's more: The Renesis screams to 9000 RPM, a significant increase over the outgoing model's 8000. A high-revving engine that pulls hard to redline is truly a thing of beauty.
So, what's wrong with pistons anyway?
Felix Wankel created the first rotary engine in 1954. The engine was designed to be compact and very smooth with a high power-to-weight ratio, all while using fewer parts than a piston engine. One of the inherent quirks of a piston engine is its reciprocating mass. Reciprocating mass presents several engineering challenges:
- Valves, pistons, and connecting rods undergo huge acceleration forces when they change direction quickly. This limits how high the engine can safely rev before floating a valve or slingshotting a piston through the hood.
- Reciprocating mass exerts secondary forces on the crankshaft which could result in wasted energy and offensive vibration. To compensate, piston engines are oriented where pairs of pistons move in opposite directions. A balancing shaft may also be added for additional smoothness.
- Crankshaft rotation must be synchronized with valve movement by means of a timing belt or chain. Without precise timing, the engine will run very poorly, if at all.
Rotary engines do not have these limitations. There is no oscillation in a rotary engine, which allows it to rev to the stratosphere - the redline is usually decided by either heat or the transmission (more on heat in a bit). This also means the engine is inherently balanced.
Rotary engines have ports instead of valves, which are specially shaped holes in the rotor housing. These holes allow air to enter or exit the combustion chamber depending on the position of the rotor. Since there are no valves, there's no need for a timing belt.
If you've been a car enthusiast for any length of time you've by no doubt heard that rotaries are unreliable. "Boost in, apex seals out". "New motor last week, just needs a new motor". Some of this reputation is deserved. Much of it is not.
Early Renesis engines had several design flaws that were corrected on later models:
The ECU was not injecting enough oil, causing accelerated wear. This could be fixed with a simple ECU flash.
Under-engineered ignition coils
After release, it was discovered that the ignition coils only lasted about 30,000 miles, a part that was not listed on the regular maintenance schedule. Many engines were taken to dealers, misdiagnosed, and replaced under warranty. Unfortunately, these perfectly good engines were replaced with poor quality remanufactured engines, which would often fail simply because they were bad engines. Many engine failures caused by the ignition system can be avoided entirely by replacing the ignition coils on regular intervals.
If these engines overheat they can fail in many ways, none of them cheap. Unfortunately the temperature gauge won't move off center until the engine has reached dangerous temperatures. Fortunately, keeping up on coolant changes goes a long way.
Owners who are used to piston engines may check their oil every couple thousand miles. A Renesis burns about a quart of oil every 1000 miles by design. This oil is injected into the combustion chamber to lubricate the rotor, housing, and apex seals. Oddly enough, most manufacturers consider a quart per 1000 miles to be within acceptable spec, even in piston engines, so this oil consumption isn't particularly unusual.
Carbon buildup occurs when the engine isn't exercised enough. The solution is simple: a redline a day keeps the carbon away!
When you fire up a Renesis you've made a commitment... at least until the engine warms up a bit. These engines are prone to flooding due to low compression, a failing ignition system, or an immediate shut down on cold start.
Is it easy to work on? You bet it is!
Owning an RX-8 requires a bit of extra care, but if the owner is diligent with maintenance and cognizant of potential problems, he or she will likely enjoy a trouble-free experience. Fortunately Mazda had our backs; most maintenance on this engine is fairly straightforward.
The Renesis is mounted longitudinally in the car, which gives you a lot of room to work. You're unlikely to find yourself jacking up the car to get to that pesky bolt, then setting the car back down to reach that other pesky bolt.
I don't own an RX-8, but I looked up a few common jobs to see how much work they involved. I was impressed at how accessible most parts are.
There are no timing covers blocking the water pump. Simply remove the battery tray, water pump pulley, a few bolts and nuts on the pump itself and off you go.
Remove the engine cover, airbox, a few vacuum lines, and the tube connecting the airbox to the throttle body. The ignition coils are right under the throttle body... much nicer than my 1.8 Miata, which hides the coil pack behind the engine!
There is a TSB for bad starters on 2004-2007 models. The starter sits right next to the transmission underneath the car.
The RX-8 community is tight-knit and eager to help aspiring home mechanics service their own machines. All the problems with this engine are known and well documented, and fellow 8 enthusiasts are often willing to lend a helping hand.
Mazda engineered the RX-8 to be a pleasure to drive, from its balanced handling to its free revving engine. The Renesis is uncharted territory - every rotary before it has been thirstier and harder on the environment. Though they were abandoned by every other manufacturer in favor of the traditional piston engine, Mazda was forced to innovate due to increasingly stringent emissions regulations. The RX-8 is high on my list of cars I want to own someday, and I have this quirky little engine to thank for that.