Important Magneto Timing Information for L-5 Owners
Magneto timing is an important issue for anyone who owns an L-5 or other airplane with the Lycoming O-435-C or O-435-C1 engine installed (military designations O-435-1 and O-435-11) . Some of the following is a re-hash of old and hopefully well-known information among L-5 owners, with some history thrown in for flavor. If you’ve read through the old club newsletters, you already know what setting to use, but I’ll cover all bases anyway.
If you have a mechanic who insists that the timing advance should be set at 15 degrees per the military maintenance manual, be sure they read this article.
Our engines are low compression by modern standards, with a compression ratio of 6.5:1. A modern Lycoming O-235 or Continental O-520, for example, has a compression ratio of 8.5:1. It is well known that power and efficiency increase with higher compression ratios, but there are trade-offs. One of those is that tetraethyl lead (TEL) has to be added to gasoline to reduce engine knock caused by pre-ignition that occurs at higher compression ratios. With the rapid advance of engine technology and higher compression engines that became available between the two world wars, in the USA there came to exist more than a dozen different grades of aviation fuel with different levels of TEL in them. From a supply, distribution and storage standpoint this was not practical for the military. So, by 1941 three basic aviation grades were adopted by the Army Air Corps : 73, 91 and 100 octane.
73 Octane Aviation Gasoline – WWII
Glossing over the fine points, suffice it to say that 73 octane is the highest grade of gasoline that can be made without the addition of TEL. Being far cheaper to produce and safer to handle than higher octane fuels (TEL is deadly poisonous in high concentrations), 73 octane became the universal standard for army ground vehicles and low performance airplanes like the Stinson L-5. For best power and efficiency at a manageable cylinder head temperature over a wide range of altitudes and ambient temperature conditions, the magic number for engine compression ratio using 73 octane was – you guessed it, 6.5:1.
The:Lycoming O-435 was created in 1941 specifically for use on the Stinson Model 76 prototype that later became the L-5. Since this new engine was designed to run on 73 octane fuel per military requirements, the optimal spark advance for the magnetos was 15 degrees before top dead center (BTDC). This is what one finds printed on unmodified O-435-1 engine data plates and in the military maintenance manuals. Again over-simplifying a complex subject, this had a lot to do with the flash poiint and flame propagation rate of 73 octane fuel so that peak power could be achieved. This allowed easy starting and good power performance while avoiding pre-ignition (engine knock) and possible detonation inside the cylinders at higher operating temperatures.
80 Octane “Red” Aviation Gasoline
All well and good, and it would be great if we still had 73 octane fuel, but before the end of WWII, a “better” product crept into use and this was the 80 octane “red” stuff that became the de-facto standard for lower powered general aviation aircraft from the mid-1940’s through the 1970’s. The addition of lead allowing the bump in octane rating paved the way for the use of higer compression pistons and cylinders, giving a boost in performance to the new engines being designed and some of the old ones being updated. However, Lycoming was already moving beyond the two versions of the O-435 used on our L-5’s because those contracts expired at the end of the war, so there was a rapidly declining need for those engines except as replacements. Hence they didn’t get upgraded with higher compression pistons that could take better advantage of the more potent 80 octane fuel (the VO- and GO-435’s did, but that was a different application and those parts are not approved for us).
Anyway, what they could do is advance the ignition timing to 20 degrees BTDC. With a higher flashpoint and slower flame propagation properties, a larger spark advance could be used with without causing engine knock. So, on the type certificate data sheets for the O-435 that were printed in the 1950’s and 60’s, the magneto timing is listed as 20 degrees instead of 15. In fact, if operated on 80 octane at the 15 degree setting, the O-435 lost some power and ran hotter, so 20 degrees was the proper choice. During WWII, several liaison squadrons experienced burned valves and other problems when using higher octane fuel, including 80 octane “truck gas” in their L-5’s because 73 was unavailable, but they might have avoided some of those troubles by advancing the ignition timing.
100LL Aviation Gasoline – Our Only Choice Today
Now let’s fast-forward to the introduction of 100LL and the complete phase out of the old reliable 80 “red”. With 100LL the increased flash point and other changes in combustion properties allows a spark advance of 25 degrees BTDC and that’s what you’ll see on the type certificate data sheets published since the introduction of 100LL. Incidentally, it has more lead in it than 80 octane leaded fuel did, so it has greater anti-knock properties. It’s called 100 “low lead” because it has less TEL in it than the old 100 octane leaded fuel did.
The Optimal Timing Setting
There is a drawback with the 25 degree BTDC timing, however. With that much spark advance our engines have a tendancy to experience “kickback” when starting Most pilots have experienced this phenomenon in the L-5 and other aircraft, where the propeller is actually propelled backward by the misfire, Unfortunately, such an event can eat our expensive Eclipse E-80 starters for lunch, so most owners compromise and use a spark advance of 23 degrees or less to mitigate this hazard.
Another trick that helps is to crank the engine 2-3 blades with the mags off, then hit the switch as you continue cranking the starter. The momentum helps belay kickback. I’ve found that using only one magneto until the engine fires on multiple cylinders is a good way to go too. But we’ve drifted off subject there. Getting back on topic again, the more conservative owners still use a 20 degree spark advance, but the down side of that is the engine may not achieve full takeoff power at that setting when using the now unavoidable 100LL. Sometimes our O-435’s cannot even achieve the minimum 1950 rpm required for a static runup at that setting. A number of owners have experienced that problem over the years, but in come cases there was probably more at play than just magneto timing.
Engine Break-In
With all that behind us, we now come to the question of engine break-in that has been the subject of discussion kicked off in the “engine power” thread of the L-5 forum. Sam Taber, owner of Tab-Air whom I consider to be our resident guru on all things mechanical and the biggest friend any L-5 owner could wish for, always uses 20-22 degrees of spark advance. He just reminded me that when he was chasing down high cylinder temp problems many blue moons ago that the 15 degree setting actually caused higher CHT readings. His main problem turned out to be a carburetor jet issue though, and you can read about his views and solution to that problem in some of the old L-5 newsletters, particularly his “Carburetor Woes” article in Issue #3, Winter 2002.
For many years now, best practices for cylinder break-in has included running the engine at a steady 70-75% power through the early part of the break-in period. However, the “rough” new rings run hotter due to greater friction and if one isn’t careful the lubricating oil can start cooking off, leaving a hard carbon coating on the cylinder walls termed “glazing”. If that happens, no further break-in can occur and the cylinders being broken-in have to be removed and re-honed and the rings replaced. In an extreme case of a cylinder getting too hot during break-in, “galling” can occur. That is where the oil film completely breaks down and metal-to-metal contact occurs. When that happens, pistons and cylinders get turned into door stops, desktop ashtrays and groundschool training curiosities. We obviously want to avoid that, so keeping the CHT below 400 degrees centigrade is loudly advised.
I’m going to add that it might be advisable to avoid advancing the timing beyond 22-23 degrees, not only because it would help avoid kickback and damage to the starter, but it could reduce the possibility of engine knock and an attendant spike in cylinder temps during the critical phase of getting the rings to “seat” properly. I flew for an operator who had been in business for more than 40 years and he never allowed overhauled cylinders to be run-in at the full 25 degrees advance, especially on engines that didn’t have CHT gauges installed. To my knowledge we never had a glazing problem while I was working there even with inexperienced pilots doing the break-in flights. Take that with a grain of salt and feel free to dispute the theory behind it, and certainly discuss it with your mechanic or IA, but in my book using less than 25 degrees of advance (but no lower than 20) is a safe bet even if it does nothing more than save your $1800+ starter. If you look around the ‘net, you’ll see I’m not a lone voice in the matter, although most discussions are about higher compression engines.
Throttle Setting with the MA4-SPA Carburetor
One last thing. With the MS4-SPA carburetor, avoid climbing at less than full throttle. If you pull the throttle back to “baby” the engine, what happens is that the fuel economizer circuit kicks in, making the engine run leaner, and therefore hotter. So, either leave the throttle at full power and lower the nose to increase airflow, or level off at cruise power for awhile to keep the CHT in check. In hot climates like the southwest USA, step-climbing is a way of life to keep the CHT under control.
©James H. Gray / SOPA, January 2021