Tach time shift after changing cruise RPM

[Raymo]

Mostly posting this for others if they decide, like I did, to increase my cruise RPM from 2350 to 2450 after flying a while. I did this while flying cross country and discovered later that the tach time on the display lost several hours (e.g. went from 150 back to 130).

The biggest hassle was that I didn't realize when it happened so I don't know exactly how many hours were subtracted for maintenance purposed (oil changes/samples).

 

[mmarien]

I was thinking that didn't make sense. But it does. Tach time is an estimate of engine revolutions. It's just displayed as time. Your times don't add up though. 150 hrs x 60 min x 2350 RPM = 21,150,000 revolutions / 2450 RPM / 60 = 143.9 hrs.

Export your settings and search your *.DFG file for tach_time. Mine is 120278375 revolutions at 2400 RPM which calculates to 835.3 hours which is about right.

I'm not sure how Dynon calculates it but if it's just time, then adjust the tach_time in your *.DFG file to reflect your new RPM.

tach_time / 2350 x 2450 = new tach time. The cross country you did wouldn't make a lot of difference. It's just an estimate and assumes that you cruise at 2350 (or 2450). I cruise at a lot of different RPM's so my tach time is at best a guess.

While we are on the subject. I don't use tach time for anything that I can think of. I record air time. What is tach time used for?

 

[Raymo]

The numbers given above were just examples. I don't know where the tach was when I changed the cruise RPM and I flew 20+ hours before realizing the change happened. I suppose I could estimate, though, which is better than nothing (thanks for the tip).

Tach time is used for most engine maintenance items (e.g. oil changes, accessory OH, etc.), which is why I don't often look at it either and don't have it on my 20% screen, which is why I did not notice the change.