> Is this any concern at all?
I guess it's theoretically possible. But it seems unlikely in practice.
For one thing, the only time both valves are open is near TDC, but 4 and 6
cylinder engines almost never stop at TDC since there is no compression
going on at that point.
For another, the exhaust pipe is wide open while the path back to the intake
is narrow and convoluted. Being an explosion, the flame lasts for only a
tiny instant. Very improbable that it would both feed back through the
intake that far, and last long enough to light any solid or liquid matter.
I once had a summer job driving a tractor for a corporate farm. Discovered
that leaving the throttle open and turning the key off would produce an
impressive exhaust backfire, several seconds after turning off the key. It
also blew an interesting smoke ring; so I did it fairly often. Never saw
any indication that it was getting back into the intake (which would have
blown oil all over as it had an oil bath air cleaner).
-- Randall
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