Guys i need your input.. Looking at the specifications, 421B seems to be a better engine.. But i haven't heard much about 421B as compared to P5..My local track is quite technical with only 1 40 metres straight..

I run the P5 in my MBX-5 at our track with is fairly long and have no troubles keeping up with any car on the track the P5 is awesome for fuel economy i get up to 10 minutes on a tank not running it lean either. the 421 heaps masses of HP but it is very peaky (another mbx-5 at our track has one.

while i cant say dont get the 421 i can definately recommend the P5 especially with the price difference

P5 Hands down.Expecially on a technical track.I dont think there IS a better motor.
Les

If i get the P5, do i get the turbo or non turbo? Any changes needed e.g adding/removing of shims? And what plug do i use? Also how does P5 compare with OS V-Spec?

Non Turbo.Use a plug you can find local.They will help you with what works.
Les

Get the P-5 non turbo.The v-specs are nice motors ,but the P-5 is more reliable and it has way more bottom. People are also having problems with the outer bearing on the v-spec.

Is the 421b's bottom end comparable or close to the p5's?

the 421 is a top end motor where as the P5 is a broad range motor with heaps down low but still a nice amount of top end

Go with a p-5 that motor is bomb proof. non turbo is a better choice because turbo plugs are sensitive to vibration. just match the motor to a good pipe and youll be fine.