Budman997

The single most important thing affecting Tach readings is the "Air Density" at the time. Air density is controled by several factors, Altitude, Tremperature, Humidity, Barometric pressure being the foremost. What that means is comparing your readings to someone elses in another part of the country on any given day will not give you a true indication of what your engine is doing in comparison to his.
Case in point! Here in Phx. AZ. nearly all Q-500 and QM-40 Pylon racing fast times and records are set due to great air density when we hold the races each Jan. in cool dry weather. You can Tach a Nelson first thing in the morning after the evening dew has dried out and then again, same prop, later in the heat of the day and the readings will be several hundred RPM's different.
I have a Thunder Tiger 40 that runs in the Q-500 standard class where we are required to use the C-2 APC 9/6 prop. I have Tached it as high as 17,200 RPM at times here and seen times when it will only get to 16,400. I attended a race in Whittier Ca. a few yrs. back and Mike Delponte flew my ship and we set fast time and handily won all three heats and the best it would tach over there that day was 15,800.
So don't throw away your engine because you can't reach figures some guy in Timbucktoo tells you he is achieving and remember most guys using tuned pipes don't know what they are doing and running a pipe way too long for ultimate performance.