Hi there just preoreder supercharged edition a few days ago and I’m loving it!
My question is how important is MTBF, and what should the range be for different types of engines.
My most of my non turbo engines have an MTBF of 42-48k
My turbo engines have 36-43k
And my non turbo performance V8 has 30k
Are these ok? And what is the best way to increase MTBF without losing a lot of performance
Are you using kilometers or miles? If it is kilometers, than it could be improved upon. If the number is in miles, than the engines are quite good 
! Check out the engine and car sharing forums to get an idea of what the MTBFs of other engines are.
I use miles, how can i improve it without losing too much power?
If you use miles, those numbers are quite good as they are. The performance V8 could be improved. Lower RPMs help, especially in bigger engines. Using better materials is also good for the MTBF. If you would like to know all the ways, the devs and more experienced players would be very helpful.
Ok thanks
Increasing the quality of the fuel system also will help you to gain MTBF.