XRAY - Model racing cars

could some one be kind enought to to help me out,i run a 19 turn stock motor,im trying to get a good setup with the gearing,im trying to get to a FDR of 6,a well known Model shop told me to go smaller on the spur and bigger on the pinion,he said i need a 78 spur and a 27 pinion,the trouble now is the motor is getting so hot that one of the brushes have heat marks on it,would it be kinder in the motor to go with a bigger spur and smaller pinion,iv looked at the setups posted but know one seems to put there spur and pinion and also dont run 19turn,
if some one has a spur and pinion combo that is down to 6 and fits also a good 19 turn stock motor recomendation,that would be great
thanks
mark

Thanks for that Hurrie. I was trying to be clever and made a mess of the question What I should have asked was...

If you are aiming for an FDR of 6.0 why does the size of the spur affect the strain on the motor (ie the motor getting hotter). The question assumes that any given spur size is matched with an appropriate pinion to give you 6.0 - I know you will not get exactly to 6.0 with every combination.
I ask because I had always thought it was the FDR that was the importnat point, not the spur/pinion combination that you use to get to that FDR.