More indepth explaination:
Dave
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Let's differentiate between the classic backfire, where the "fire" gets into the INTAKE manifold. If the mixture there is within combustible limits (it often isn't), the fuel/air vapor will ignite "vigorously," often seen as a gout of flame out the carburetor intake and a big bang. Since the intake manifolds are not designed to take this overpressure, the backfire may well be harmful. In the big radials this is ALWAYS due to a "too lean" mixture (not enough fuel, or too much air).
The "afterfire" occurs in the exhaust manifold, and is always the product of unburned fuel lighting off. If there's a lot of raw fuel in the exhaust manifold, a truly spectacular burst of orange fire (20 feet long, or more) can come out the exhaust stacks until the fuel is all burned. This is generally harmless, unless there is a continuing supply of raw fuel. Eventually, it may light something else off, such as the oil on a very oily engine (common on the old radials). On the other hand, if the fuel in the exhaust is well vaporized, it can also ignite "vigorously,"
causing a huge bang, often a series of them during the start. These may very well cause cracked exhaust stacks, which are deadly on a turbo'd engine, and not good anywhere. I saw this most recently on our old C-46 where a trainee got a series of about six bangs. Sure enough, the left outboard exhaust "trumpet" was found cracked almost all the way around.
Most of the above discussion is applicable to the starting. Once the engine is running, things tend to settle down and remain stable. The modern "flat" engines have fewer problems with all this, although all the above ARE possible. A stack fire on a Bonanza trying to get a start will sure singe the paint on the fuselage and cowling above, AND it may well be the ignition point for that puddle of fuel on the ground from the awful "flooded start" so many people use!
Now, at any cruise power, inflight, if you kill the combustion in a cylinder by turning both plugs off, the unburned fuel will just get blown on through, and ejected into the exhaust stack, where there is "fire" (from other cylinders) to light it off more or less harmlessly. If you turn both mags off, ALL the fuel blows on through, and there's no "fire" to burn it. It's probably a highly combustible mix, so when you turn the mags back on, and combustion begins again, the first shot of hot exhaust will probably light off everything still in the exhaust manifold, and you may get a momentary "overpressure." I don't know how much, and I'm now speculating, but I don't want to do that to my engine. I have done it, and it's a VERY ABRUPT resumption of power, at the very least. The "bang" (if
any) may be masked by the rest of the engine noise.
Note that IF you ALREADY have a dead mag, then doing a mag check will be the same as if you turned 'em both off. If you know what the engine monitor shows with a normal engine, you won't ever do that, because your EGTs will all be MUCH higher than normal if one mag fails. That's the time to ease well back on the power before you do the check.
If only one cylinder goes dead, the engine will seem like it's jumping off its mount, but that should be harmless, if corrected quickly.
Best...
John Deakin
