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P-Mag timing

calpilot

Well Known Member
I have had P mags (114's) on 3 aircraft with parallel Lycoming O-360's. My current aircraft has a FI O-360 with 9:1 pistons, venturi cut intake seats, performance camshaft, counterweighted crank, and P-Mags timed to TDC. After a very informative talk with Brad at P-Mag, I retarded one mag 2.5 degrees, the other left at TDC. Running on each mag individually, I found the retarded mag lost only 1 Kn in cruise (9500', wide open, 23oo rpm, leaned to peak) but the CHT's ran 10 to 20 degrees cooler!!! I then timed both mags to 2.5 ATDC, and run about 1-2 knots les that prior, but have cooler cylinders, and lower fuel flow. Sooo, it seems that the pumped up parallel engines are near their max advance boundaries, perhaps that is why the angle valve IO-360 and the IO-390 have magneto timing at 20 degrees instead of 25!
DAR Gary
 
Thanks for the account Gary. I have been reading about this for quite some time now and at some point think I will retard my pmag a bit. My temps are fine if I watch them in climb but I would like a little more margin. I have also read somewhere that retarding doesn't lose much power up to a point. Your account seems to confirm this.
 
I have timing set at 2 degrees AFTER top dead center, jumper in. Therefore, max advance is 32 degrees vs 34, doesn't sound like much, but in cruise (9500 feet, 65 degree OAT, full throttle, 2300 RPM leaned to peak,
My #1 cylinder went from 381 to 359! I tried it three times, same result, only a loss of about 1-2 knots, lower fuel flow at peak. Sooo, I think most stock 230 - 360 engines are good at TDC, but pumped up and angle valve engines, perhaps experiment from -2 to -5 degrees after TDC. I had a long talk with Brad at P-Mag, and he has seen this a lot! I thought it was just my James cowl that caused the higher temperatures, but alas, I think the culprit is in this "pumped up" engine, when the manifold pressure is reduced due to altitude or throttle, the timing advances to the point where the fuel is attempting to "explode" rather than burn. Learn something every day!

DAR Gary
 
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Think about it. Pull the timing back, go one, maybe two knots slower, drop the CHTs. Spend some effort on drag reduction, and regain the one or two knots.

Even if the drag work takes the form of cooling mass flow reduction and CHT is pushed back up to the previous level, the engine remains significantly less stressed.
 
True! I don't think I can reduce drag much more, everything is faired, James cowl, winglet wing tips, very small gaps in the control surfaces, filled butt ribs, no antenna drag. At 9500'; standard day; wide open, 2300 rpm with my Hartzell composite "beast"; leaned to peak, I see a consistent 192 mph @ 8.2 gph. My previous RV-7 with an MT 3 blade on same conditions only produced 181 mph. (Stock cowl, everything else same in engine except for performance camshaft).
Regards,

Gary
 
Think about it. Pull the timing back, go one, maybe two knots slower, drop the CHTs. Spend some effort on drag reduction, and regain the one or two knots.

Even if the drag work takes the form of cooling mass flow reduction and CHT is pushed back up to the previous level, the engine remains significantly less stressed.
Think the same would work for a carbureted O-320?
 
Any aircraft would benefit from drag reduction and optimum engine performance. The amount of measurable difference starts with the amount of drag that can be reduced. Some aircraft, like the RV series, are fairly low drag to begin with, takes some finesse to improve on things. but take an aircraft ( certified spam can) like many single engine Cessna and piper products, and a lot can be gained by cowl redesign, gear fairings, etc. My first RV-7 at the same fuel flow as my current RV-7 was almost 10 mph true airspeed slower at altitude, improvements were mostly due to drag reduction, and efficiency due to better engine "tweaking". I helped a friend put 9:1 pistons in his carbureted O-320 RV-6, venturi cut intake valve seats, and at altitude, close the throttle just a tiny bit to cause turbulent flow in the intake manifold (better fuel atomization), and he picked up 4-5 mph true airspeed on the same fuel flow! Now, some tweaking on the cowl inlets and gear fairings, with the resultant reduction in cooling drag and parasitic drag, and I would suspect even more performance!

DAR Gary
 
Any aircraft would benefit from drag reduction and optimum engine performance. The amount of measurable difference starts with the amount of drag that can be reduced. Some aircraft, like the RV series, are fairly low drag to begin with, takes some finesse to improve on things. but take an aircraft ( certified spam can) like many single engine Cessna and piper products, and a lot can be gained by cowl redesign, gear fairings, etc. My first RV-7 at the same fuel flow as my current RV-7 was almost 10 mph true airspeed slower at altitude, improvements were mostly due to drag reduction, and efficiency due to better engine "tweaking". I helped a friend put 9:1 pistons in his carbureted O-320 RV-6, venturi cut intake valve seats, and at altitude, close the throttle just a tiny bit to cause turbulent flow in the intake manifold (better fuel atomization), and he picked up 4-5 mph true airspeed on the same fuel flow! Now, some tweaking on the cowl inlets and gear fairings, with the resultant reduction in cooling drag and parasitic drag, and I would suspect even more performance!

DAR Gary
Yeah, I get the drag issue.
I was asking primarily about the timing adjustment.

So I think I may go ahead and start with a 1° change from TDC and see if that changes anything.
 
Yes, worth a try! However, you might make sure the data is accurate, fly as is and note the density altitude, power settings, and true airspeed, also record CHT's. If you retard only 1 mag, run on the stock mag (might have to enrichen a bit) and note the above, then switch to the retarded mag, see if there is any measurable change, if things look cooler with little change in speed, you might retard the stock one to 1 or 2 degrees after TDC, then retard the other to 2-4 degrees, and try it again. Brad at P Mag talks about the "sweet spot" where satisfactory temperatures are achieved with little loss in performance. Just takes a lot of data. I'd like to know what you find!
Regards,

DAR Gary
 
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