Pmag a or b curve

miyu1975 · #1 08-26-2012, 06:22 AM

Help me understand the timing advance in the pmag. I have dual pmags on my IO 360 a1a. I am still in phase one with about 16 hrs now. I have been running the pmags with the jumper in or A curve, which is recommended by the book to start with. I am considering going to the b curve, but I don't understand the differences or advantage and the manual doesn't really discuss it.

Kyle Boatright · #2 08-26-2012, 06:36 AM

The simple answer is that the B curve increases the upper limit on spark advance at power settings where cylinder pressures are <relatively> low, like high rpm, low MP operation.

The B curve should give better power for the same fuel burn under those circumstances. The downside is that by extracting more energy from the fuel, more heat is generated, which translates into higher CHT's. So, if you would be comfortable with your cruise CHT's increasing a few degrees, the B curve is worth trying.

RV10inOz · #3 08-26-2012, 05:05 PM

Kyle, have you any data to support that?

I would like to see some dyno data showing the HP achieved at various levels of ignition advance given a fixed RPM and say 24" MP, or whatever is available.

Quote:

The downside is that by extracting more energy from the fuel, more heat is generated, which translates into higher CHT's.

I do not believe this is the case. The reason the CHT is up is due to the Peak Pressure being higher AND closer to TDC. Theta PP advances with the advanced spark timing.

The juggling act is between the torque produced (area under the curve) after TDC less the amount used (area under the curve) before TDC.

There is an optimum, and I wonder just how well some of the Pmag's achieve this.

Thanks in advance (pardon the pun) if you are able to get your hands on it.

DB

N941WR · #4 08-26-2012, 06:19 PM

The A curve starts the timing out at around 25* BTC, same as a standard mag and allows it to move out to 34* BTC under reduced power.

The B curve takes those numbers and shifts them 5 degrees.

You can connect a PC or our EICommander to them and program your own B Curve.

The B Curve should provide you more power and better fuel economy but you should see a rise in CHT?s along with a corresponding drop in EGT?s. I only recommend you try the B curve, if you have a full EMS so you can watch your temps on all cylinders.

RV10inOz · #5 08-27-2012, 05:01 AM

I understand the CHT and EGT shift quite well, but when the spark timing is changing matters with respect to power made Vs lost..

Has anyone done some Dyno tests to show the low power differences for the PMAG advance, and the torque produced?

I am interested to see the results.

Omega232Devils · #6 08-27-2012, 05:55 AM

When I switched to the B curve, my EGT & CHT both dropped! This is opposite what is put out in the 'gouge' about the two curves. For me, switching to the B curve I was able to get all four cylinders comfortably LOP! Running on the left side, I was able to get the lower temps, lower fuel flow and only minimal power loss. YMMV

Dan

Dean Pichon · #7 08-27-2012, 07:32 AM

I am running dual P-mags in my IO-360 AIA. I have only run them without the jumper (B-curve). The CHTs have not gone up relative to the Lasar electronic ignition previously installed. I have not tried running with the A-curve and am reluctant to try it since it seems to work well now... I would actually like to see higher CHTs as I cannot get them above 320F. I believe this is a common issue with -4's

Dean Pichon
Bolton, MA