yes the revo is nitro m8 its only the larger scales that are petrol i/e any thing bigger than a 1/8th will be petrol like the fg marder hpi baja the mcd and hormans are all petrol
the ek4 is just a converted eb4 buggy with a few more shocks added and a big engine wich is a pig to tune and ur lucky if you get ten mins run time because they still use the fuel tank designed for the 21 engined buggy
if you go 4 a 28 with more torque you could also go up in size on the clutch bell wich would also give better top end speed with out loosing any bottom end speed because you have more torque