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10 Things You Need To Know About Rotary Tuning

Are you a piston guy or a rotor guy? If the former applies to you then you might find this article irrelevant and rather boring. However, if you identify with the latter then this might well be the most important article you’ll ever read.

Right from the outset we need to clarify that this is not a specific, DIY-type “how to tune” walk-through feature, rather an overview of general principles that can be used as a guide to the fueling and ignition requirements of a rotary engine.

The Basics

Much like its piston equivalent, a rotary engine is an internal combustion engine, utilising the same four stages; intake, compression, combustion and exhaust in each cycle.

That’s where the similarities between the two end. Rather than pistons, rods and crankshafts a rotary engine uses rotors spinning on an eccentric shaft. This effectively divides the oval-shaped rotor housing into four sections, each dedicated to one stage of the combustion cycle. Each section is sealed with an Apex seal located on each tip of the rotor.

Ask any rotary enthusiast and they will quickly cite a list of advantages this design has over the conventional, piston design. Most have proven to be true and are the reason why the rotary, despite being abandoned by car manufacturers, still has such a strong, cult following.

Rotary-engine

Compact design and fewer moving parts. There are only two types of moving parts in a rotary engine; the eccentric shaft and the rotor. So for example a triple rotor (20B) engine will only have four moving parts. In comparison, a piston engine can have over 50 moving parts. This makes a rotary engine not only more compact but also less complex with fewer parts requiring maintenance, lubrication or replacement.

Less vibration, higher RPM. As the only moving components in a rotary are rotational parts, it produces considerably less vibration than a piston engine. Doing away with camshafts, rockers, timing belts, gears and most importantly valves and valve springs means a rotary engine can also achieve much higher RPM.

Despite having plenty of street and race cred, rotary engines are often labelled as unreliable and difficult to tune. The truth is that with a quality engine management system and a good tune, a well-built rotary engine should provide you with many years of peak performance.

10 things you need to know about rotary tuning

1. A rotary has half the available processing time of a piston engine. It means you have to inject the required amount of fuel and charge the ignition coil in half the time (approx 10 milliseconds at 6000rpm) compared to an equivalent piston engine. It’s important to check that your ECU is capable of interfacing with the factory Mazda crank angle sensor, supports staged injection, allows you to tune the end of injection angle and ignition split angle as well as supporting the rotary engine cycle itself.

2. Remember that as the revs climb the demand on your fuel and ignition system increase. Make sure your fuel/ignition system is capable of providing enough spark and fuel to the engine at high rpm. Your fuel/ignition system needs to be bigger/more powerful than on an equivalent piston engine as it needs to provide the same amount of fuel/spark in half the time.

3. Running lean and engine misfire are the enemies of a rotary engine – avoid them at all costs.

4. Rotary engines use two spark plugs per rotor, a leading and a trailing spark plug. The leading plug (mounted lower in the rotor housing housing) ignites up to 95% of the air/fuel mixture providing most of the power. Trailing plug (mounted above the leading plug) fires 10-15 degrees after the leading plug and completes the combustion providing a more uniform flame front across the rotor face.

5. Ignition Split Timing is the angle the trailing plug fires after the leading plug. The lower the split timing the more chance of engine damage. Typically, setting your entire split map at 10 degrees will yield good results with the least chance of engine damage.

6. Remember that your rotary ignition map won’t look like its piston engine equivalent. Don’t chase the power by adding ignition timing as the rpm increases. The timing should only increase a few degrees across the whole rev range.

7. Diagnosing ignition problems on rotaries is difficult as a misfire is not clearly audible. A quick way of diagnosing an ignition problem is by disabling the trailing spark plug by unplugging its ignition module and putting the engine under load. If there is a problem with the leading plug, a run on a dyno will reveal an ignition misfire.

8. Most rotary engines like to idle at at Air/Fuel ratio of 13.5 – 14:1. As the engine approaches atmospheric pressure, target A/F ratio of around 11:1 is recommended. On boost, your A/F ratio should be around 10.5:1.*

9. Most rotary engines should be happy with an injection angle of 270 degrees under cruising conditions and 330 degrees at the rev limit.

10. Don’t lose power at the top end of your RPM range. Providing everything attached to the engine is sized correctly and working properly, your power curve should come up to full power and hold close to this power until the rev limit. If the power starts dropping before max RPM check your injection angle.

*Keep in mind that these are general guides only and each engine will need individual fine-tuning.