PMAG / Carb Operation Advice?

I recently installed a single PMAG, replacing a slick impulse mag; leaving one Impulse and one PMAG, on an O-320. Installation was easy, pulling new wires through the firewall fittings was the toughest part, also had to switch my RPM sensor from my right magneto to the left magneto, requiring me to pull the left magneto, which was an adventure...

Initial results seem to be a bit smoother operation and definitely higher CHTs and lower EGTs. Previously I worked very hard to get my CHTs to mid 300's while the EGTS were running in the mid 1400's. First flight with PMAG, highest CHT was back around 400 with EGT's in the 1350's at 65% power. I re-timed the PMAG to 2 degrees ATDC which dropped highest CHT to 380's at 65% power and 390's - 400 at 75% power. I was leed to believe the slick mag would be rendered useless with the added PMAG but at cruise It still runs better on both mags vs. the PMAG alone. RPM drops at runup are similar between both mags. I grounded the PMAG to an engine case accessory bolt (vac pump cover I believe).

At 7500' I was able to get smooth operation running one cylinder 50 degrees LOP, which dropped that cylinder temp (#4, hottest) in line with the others. When it comes to (trying to) run LOP on a carb, is it OK to run one cylinder LOP while others might be at peak still, or even 100 ROP? Will it get smoother by adding a second PMAG (installed breaker and pulled wire for second PMAG at this time)

Problem with a carb is it really only gets best cooling and fuel distribution with the throttle at WOT, or slightly backed off and in my plane I need to be 10,000'+ to run WOT. Usually from 6500 to 8500 I'm still backed off the wall a good inch or so.

Anyway, just looking for some advice for setting up and operating PMAG(s) with a carb.

Carb Heat

Hi Derek,

I use carb heat to run LOP with my Pmags and carb. It seems to raise the temp of the intake mixture for better vaporization and you can "steer" the mixture to the cylinders by altering the incoming airflow. hard to explain, but it works pretty well.

Send me a PM if you want to jump on the phone to discuss some evening.

Dbro172 said:

also had to switch my RPM sensor from my right magneto to the left magneto, requiring me to pull the left magneto, which was an adventure...

You can run your tach off the PMag as well.

Initial results seem to be a bit smoother operation and definitely higher CHTs and lower EGTs. Previously I worked very hard to get my CHTs to mid 300's while the EGTS were running in the mid 1400's. First flight with PMAG, highest CHT was back around 400 with EGT's in the 1350's at 65% power.

I re-timed the PMAG to 2 degrees ATDC which dropped highest CHT to 380's at 65% power and 390's - 400 at 75% power. I was leed to believe the slick mag would be rendered useless with the added PMAG but at cruise It still runs better on both mags vs. the PMAG alone.

Did you install the jumper between the serial TX/RX connectors? If not, install it and reset timing to standard at TDC.

Do you have gear fairings installed? If not, it would explain the higher CHTs. If you do, you should look at improving the baffle seals anywhere possible.

At 7500' I was able to get smooth operation running one cylinder 50 degrees LOP, which dropped that cylinder temp (#4, hottest) in line with the others. When it comes to (trying to) run LOP on a carb, is it OK to run one cylinder LOP while others might be at peak still, or even 100 ROP? Will it get smoother by adding a second PMAG (installed breaker and pulled wire for second PMAG at this time)

As long as you are making less than 65-70 percent power (>8000' density alt or pull power back), you can't hurt the engine.

Problem with a carb is it really only gets best cooling and fuel distribution with the throttle at WOT, or slightly backed off and in my plane I need to be 10,000'+ to run WOT. Usually from 6500 to 8500 I'm still backed off the wall a good inch or so.

Slightly closing the throttle helps with atomization, as does adding a little carb heat.

Anyway, just looking for some advice for setting up and operating PMAG(s) with a carb.

Click to expand...

Derek,

See responses above.

Put the jumper in at a minimum.

You could also connect EICAD (free download on EMagAir's website) to the Pmag and set the max advance limit to something lower than the default and or lower the advance shift value.

Or purchase an EI Commander from Bill R. and set the advance variables that way.

Dbro172 said:

Initial results seem to be a bit smoother operation and definitely higher CHTs and lower EGTs. Previously I worked very hard to get my CHTs to mid 300's while the EGTS were running in the mid 1400's. First flight with PMAG, highest CHT was back around 400 with EGT's in the 1350's at 65% power.

Click to expand...

The effect of advanced timing. Carefully read Nigel Speedy's recent ignition timing flight test article in Kitplanes.

When it comes to (trying to) run LOP on a carb, is it OK to run one cylinder LOP while others might be at peak still, or even 100 ROP?

Click to expand...

Peak EGT operation at low cruise power is a standard manufacturer's instruction. If power (not to be confused with RPM) is low enough, nothing done with the mixture knob will hurt a cylinder.

That said, 50 LOP and 75 ROP (for example) means a significant difference in power production between those two cylinders, which may be a perceived smoothness issue. I'd talk to Pete, and do what you can to even it out.

Thanks for the replies. Chatted with Bill R and now have some ideas to tweak the programming a bit, as others have done/suggested.