Does it matter
shorebird said:
I have a friend with a typically fast (210mph) Witman Tailwind powered by an stock 160HP O-320 w/carb, Light Speed electronic ignition and Catto prop. He finds that flat out, full throttle at 5000' turning 2900 rpm he sees his timing running right at 25 degrees.
I'm sure that many of you guys are running 180+hp O-360's equipped RV's w/electronic ignition (w/digital timing readout) and fixed pitch props. Can anyone tell me what their timing is indicating for an O-360 operating under similar conditions as mentioned above. Thanks
It should be the same, regardless of airframe or a few more cubic inches / HP. My logic is this, as long as you are over 75% power you will not see any timing advance from the base line 25 degrees BTDC. Klaus does not advertise the MAP vs RPM vs Adv info but we can assume nothing will happen above 75% power.
At 2900 RPM and WOT at 5,000' is more than 75% power on any fast homebuilt plane.
I am sure like other EI, the LS Plasma has a fixed MAP x RPM curve or formula to base timing advance. Any time flying at full RPM, WOT below 8,000' or even a little higher you will be above 75%. With the RV's good ram air we can make 75% power up to 9,000' at full RPM. So the installation, i.e., the air box and exhaust does have some effect. As far as the LS Plasma, all it cares about or knows is the MAP and RPM regardless of altitude. At 5,000' and WOT you will be above 75% power unless you have a severely restricted induction.
Someone pointed me to the Electro air timing curves on their website. It may be of interest.
http://www.electroair.net/technical.html.
Playing with this Electro air data and the numbers you provided, I would guess timing adv would be about 26.25 degrees just from the RPM. You don't say what the MAP is for your friends hot rod, but it could be as high as 24" may be 23"? So the advance from MAP would only be 0-2. So the total would be 26-28 degrees. On a RV limited to 2,700 RPM the advance would be about 1 degree less, close to 25 degrees.
The Electro air looks like it peaks at 45 degrees max adv. The Plasma is about 42 degrees I recall. Using the Electro air data again, at 2,900 RPM, the map would need to be down around 15" to get max Adv. That would be somewhere at altitudes in the low teens (WOT).
The conclusion is with normal power settings, most of the timing advance happens at altitudes above 8 or 9 thousand and continues to advance into the low teens, where it maxes out. Clearly at low altitudes you can throttle back to say 20" MAP for example; with a fixed prop you RPMs will go down as well, which according to the EA graph lowers timing adv. Lets say you are at 20"/2200. The advance would be about 32-33 deg. Of course this is all guess work and does not apply to the LS Plasma, but it is good for illustration. The Electro-air and LS Plasma EI are different of course, but they should have similar timing advance curves or scheme.
