You guys are giving bad advice. I fly a Glastar with an O-320 E2A in
it. I have flown dozens of hours at density altitudes above 15000 ft.
And that is with the plane loaded to GW. With the plane loaded lightly
I have had it above 21000 density altitude (which was right at 18000 ft
MSL) using autogas, just to see how high I could go. I have flown
across the Rockies half a dozen times at VFR altitudes of 13500, 14500
and 15500, depending on which way I'm going. Those flights were at
density altitudes typically 2000 to 3000 higher than the MSL altitudes.
The Slick mags never misfired even at 21000 ft, but the plane was at
it's service ceiling of 100 fpm there and I'm sure any misfiring would
have resulted in a descent. I enjoy high altitude XC flights using O2
and a pulse oximeter, the air is smooth, cool and the plane runs great.
Don't blame your engine for limits imposed by your airframes.
Justaguy