[NoDOHC]

I have found that it is more about your manifold vacuum than anything (Idle AFR).

It has been a little while since I did this, so I have tried to indicate the accuracy of the numbers listed.

Ignition Timing (BTDC) Manifold Pressure at Idle (kPA relative) Idle AFR
20 -75 to -80 12.2-12.7 *These are very close
15 -70 to -75 12.5-13.0 *These are also very close
10 -65 to -70 12.8-13.2 *More of an estimate on the AFR, vacuum is right
5 -60 to -65 13.0-13.5 *Pretty accurate (ran for a while at this)
0 -55 to -60 13.4-13.8 *Ran for a few minutes only
-5 -50 to -55 13.7-14.4 *Ran for weeks
-10 -45 to -50 14.4-15.0 *Was running at 14.8, closed loop until my fuel system gave up the ghost from the E85.

This was all with a 15 degree split.

edit: I have found that rotaries like a richer mix than piston engines at high vacuums (low absolute manifold pressures). I don't know why, but my guess is that the compression ratio is low and that the large surface area of the rotor face tends to draw the fuel out of solution.

The rotary actually likes a very similar mix to a piston engine at WOT, which largely discounts the surface area reasoning.

I found that cruising at 65 mph (-65 to -60 kPA) Manifold pressure, 2700 rpm) I needed to run absolutely no leaner than 14.8:1 in order to keep from misfiring. However at higher speeds (I won't state them here) when the engine is running at significantly higher load (say -35 to -30 kPA) I could run 18:1 all day without a misfire.

This actually resulted in my peak fuel economy being well in excess of the speed limit anywhere except for maybe Montana.

The engine speed had nothing to do with it, I could run at 18:1 at -20 kPA at 1,000 RPM without a misfire, but try to run at 18:1 at -50 kPA, forget it.

I am sure that this is not new news to a lot of you, I am just trying to make the parallel.

Piston engines do this too, but will run without a misfire at 14.8 up to about -70 kPA (although most of them have significant higher compression ratios than my rotary).

I am really looking forward to the high compression rotors, maybe I can run leaner at cruise... (wouldn't 40mpg be awesome?)

To ReTed's point, I have 460 cc/min injectors in mine, so I am likely getting better atomization at idle.

Actually, the longer durations may lead to better atomization and metering precision too.

Less vacuum gives longer time... I think you are on to a good theory, ReTed!