Which .40 size engine?
If the plane stays below the 3.5lbs I would put on a OS 32sx. Yes they cost more than some other 32s. But it is money well spend.
I have my OS32sx on my 4lb menace. and it pulls it out of a hover w/ ease.
I have had mine on a MS Menace, a SPAD, Tower Hobbies extra special, trainer, 2 other scratch build fun flys. And the only time the motor dead sticks is when I fun out of fuel.
It never loads up in the midrange. I haven't touch my low in since I bought it 3 years ago. when i bought it after 12oz of fuels ran thru it I leaned out the low end 1/4. Since then I haven't touch it.
I have crashed this motor hard. And it still comes back for more punishment.
throttle responce is instantly w/ a 10-3, 10-4, 10-5, or 11-3 APC props. throttle is very linear. Motor never hestistates or bogs down in the midrange. Will idle for minutes then go to full throttle with no hestistaton.
Sometimes motors cost more because they are worth more.
I started off flying w/ motors that were cheaper, and they gave me a lot of head ache and pain. I have decided I am not going to spend my time working on motors, trying to get them set.
When I buy a new motor I judge it against my os32sx. And so far only 1 meets the quality, power, throttle responce, linear transition. And that is my YS 63 4 stroke.