"Stories"
My good friend runs a Velocity with the angle valve Lyc and 10 to 1 compression. He also runs dual L/S Ign's. With the base timing set at 18 degrees. Other than one cracked cylinder, he has had no problems.
Now, I hear all sorts horror stories from all manner of people. Including some engine builders at Sun N Fun. The Elec ign folks all say "no problem" the engine builders are all over the map on this one. Including dire warnings of "blowing a cylinder off".
However, I am currently operating an (exhibition experimental) Extra 300L with a angle valve 10/1 compression BPE engine with no problems. I would like to install an electronic ignition to provide better starting and more efficient cruise.
The aircraft is equipped with EFIS with EGT/CHT.
My specific question is whether anyone here has used electronic ignition with advance on a 10/1 Lycoming angle valve? If so, what were your results?
Thanks!
Franklin
The KEY word in your comments is "Stories". For some reason in the aviation world "stories" become urban legend and myth. Now not saying that high compression and timing advance don't add up to more pressure or temp in the cylinders. I can say just out of hand the combo of higher compression and even timing advance means less detonation margin.
The thing about myths is there is always a thread of truth, but often the total story is "busted". Most EI including LS have very conservative advance schedules or mapping, so the pressures are lower, advance is safe, even with high compression, especially with a very conservative 18 degree base timing.
Cylinder crack? It could be just a defect in the cylinder? It could be a weak point that if at 8.5:1 CR and mags would have lasted longer or forever.
The key is so much in the hands of your friend and how he manages his engine. First does he watch his Oil and Cyl temps like a hawk. Does he run it around well above 75% power mostly. Does he ever lean above 75% power (personally I would make it 70%). Does he run it hard and hot and than chops the throttle. Does he do long idle descents. I am not alluding to shock cooling since that is somewhat of an urban legend, but normally it is bad form to do the above for many other reasons besides possibly cracking cylinders.
A cracked cylinder is a hard call. Piston, ring or valve issues (damage, melting) could be a sign of detonation. A cylinder crack? How many hours on the Jug? If its low time it could be a defect. If it was a 2nd time around rebuild it could just be metal fatigue.
Personally I built my dual LS O-360 with stock compression for reliability. You can expect an engine you milk more power out of than stock to not make TBO. A little compression bump tends to be more efficient fuel econ wise but 10.0:1 CR is getting a little high. My advice, replace the jug and go fly.
SHOULD YOU GO TO EI WITH YOUR HC ANGLE VALVE?
No, is my gut feel. EI really pays most dividend at high altitude (low power) cruise. It should like you are not just boring straight and level holes in the sky. You're right, starting should be improved with EI, but why is it hard to start? The money and expense to install EI may not be worth it? You can turn the advance off and just run it straight Zero degree for start and fixed timing after. The hotter spark does pay dividends for both starting and flying, adding a little power. If you go that way, I would just need one unit. I like the LS more for the "ultimate" ignition, especially for an all out acro plane. The first EI gives 90% of the benefit efficiency wise, the second is just the last 1% or 2%. EI is at its best advantage for lower power cruise econ (where the timing advance kicks in). For starting benefit only one EI is needed. Magnetos are not totally old fashion or with out merit. If you put EI on and "de-tune" you timing, you lower the HP gain you got from the compression. For an acro plane I'm tempted to say leave the mags and try to figure out a way to improve starting. I know SkyTec has a new top-O-line geared starter that has more crank power but uses much less electrical power. Bottom line the more power you squeeze out of an engine, logically there may be some loss in TBO. For racers and competition acro, max TBO is not an issue.
