I replaced battery terminals in my 2004 Mazda RX-8 because they were corroded and poor electrical continuity might have caused the occasional engine stall at idle. The new terminals look great, but my engine would still stall, so I investigated other possibilities listed in the workshop manual and found a likely explanation: my engine is very low on compression.
A compression test was one of the listed diagnostic items but not something I could do myself. While some auto part stores offer engine compression tester rental, they’re for piston engines. Rotary engines require a specialized testing tool. The service department of my nearest Mazda dealership doesn’t have theirs anymore so I searched for a local rotary engine specialist shop. I found Lucky 7 Racing offers rotary compression testing and took my car to their new City of Industry location.
A rotary engine tester measures pressure value for each chamber around the three-sided rotor and also engine RPM. The nature of a rotary engine means compression pressure rises with RPM. (Oversimplified explanation: centrifugal force flings apex seals outward against the housing, improving compression as it spins faster.) So the tester also normalize/compensate its readings to 250 RPM for comparison purposes. According to “Compression Inspection” section of Mazda’s workshop manual, standard compensated pressure is 830 kPa and minimum acceptable value is 680 kPa. For my engine, compensated pressure values for one rotor measured 581, 556, and 554 kPa which are well below minimum. The other rotor measured 393, 392, and 347 kPa which aren’t just below minimum, they’re less than half of standard pressure! My engine is in sorry shape.
Low compression is a common symptom of old engines as various parts wear down. For a piston engine, valves and piston rings are the usual suspects. For a rotary engine, their apex seals are usually the first things to go. Dropping below 680 kPa is cause for engine replacement under Mazda warranty but since the powertrain warranty has long since expired, this is now entirely my problem.
A new engine is not an option, Mazda production wound down years ago. A proper fix now means taking the engine apart to replace worn out bits. Lucky 7 Racing performs rotary engine rebuilds, but that is typically done for performance-focused customers who want to install more durable (and expensive) race parts to wring out maximum power. For a daily driver car like mine, its faster and easier to swap my engine core with another core that has been remanufactured at a dedicated facility. Either option will be significantly more expensive than the market value for a 2004 Mazda RX-8. In other words, it would cost more than I could possibly ever get back and makes no financial sense. Plenty of people are passionate enough about their rotary-engined cars to pour money into a hole, keeping shops like Lucky 7 busy. I will have to decide if I’m one of them. Until I make that decision, though, I will have to live with the problem.
Header image: “Rotary Man” sculpture sitting among parts and equipment at Lucky 7 Racing. His head (painted blue) is a rotor and his torso (painted white) is an eccentric shaft.
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