important question ratio / n. cell Brushless x all combo
 

I would like to reach in both cases a speed of 45 MPH


is better to use 4 cells with long-ratio?

or

is better to use 6 cells with short-ratio?

Option B, assuming you have a 6s capable esc.

Option C would be to use 5s lipo- give the esc a little bit of 'electrical headroom', so to speak.

What is the vehicle/ motor & motor kv?

vehicle E-MAXX
motor T8 tekin 2250kv (2340kv real)
esc RX8 tekin

Why option B?
you please explain?

Sure.

With a higher voltage, you can/have to gear down, meaning you reduce the physical load on the motor & electronics & increase the torque (at the wheels) at the same time. This is better for the truck as you are making more power (watts) from higher voltage, rather than higher current (amps). Example:

1000watts / 22.2v (6s) = 45amps
1000watts / 14.8v (4s) = 67amps

It is better for the esc and batts to make your power from higher voltage and therefore lower current draw, rather than lower voltage & higher current draw, as more current = higher temps & shorter runtimes (for a given mah capacity).

In your case, a 3s and 2s lipo of the same mah capacity and C rating (only different voltages) would be just about optimum- you would need to gear down alot with 6s lipo & a ~2300kv motor, meaning higher motor temps from being under geared.

I would suggest the Hyperion G3 packs, they are a good price and very powerful, though ultimately it depends on your budget- I would read around and see which lipos are getting good reviews.

Thats not to say that 4s lipo wouldnt work, far from it, but it depends on your ultimate goals really; 45mph is fairly fast for a ~10lb MT, so a higher voltage would yield better results & runtime.

ok thank's

4s and 5s systems are SUPER stable on Monsters and Rx8's, you have 2x as many burnt ESC on a 6s system.. Now this data is from my experiences as well as all the posts I read so it's not scientific, but you can get a good idea on who's burning from all the posts around. If you are racing with a 2200kv range motor I would stick to 4s as a religion. If you are playing around and want to hit some speed 5s would be fine even a touch of 6s for fun. PS in the erevo the most run time of ANY setup is had w/ the 9000mah 2s lipos in series (4s combined) not sure how it fits out for the emaxx. NO 5s or 6s config w/o hacking up the truck can match this run time. Nice 5k lipo's to be a little more nimble.

My 2c