Will I have any problems with the stock dpr8z Honda plug as far as piston to plug clearance? and do I need to run a colder plug?
I have white brothers cam,porting,426 bore,dmc pipe,fcr carb and am fixing to crank it but I want to use the right plug.

 

i personally do not think you will have a problem with that plug. i am currently running the ngk dpr9z although i do not know if it made any difference or not. i just wanted to try it. i raced a friend of mine on the exact same bike and we were close up until the end of the race where i had more top end. my whole bike is stock except i am running that plug. my friends bike is totally stock also. the only thing different is that he had his spark arrestor off and i did not. i don't know why but i did have a top end advantage over him. that spark arrestor could of made that difference.

 

What fuel are you running? I was using a DPR9EA in my 250x on 110 octane. It is a big bore at 10.5:1. You must stay D (thread size). If you had an easy way to "bump" the engine over to check to see if the protruding tip (the P in the plug #) contacts the piston, which I doubt it will. I recommend staying resistor (the R) type with the 400 ignition. Heat range, good question. I usually say 1 point colder with every 1.0:1 increase in compression. However, I warm it up one point for every 5 octane increases. So, factory 9:1 compression to 13 raise plug number from 8 to 12 (They only go to 10, I know this is part of why you should raise compression). Since that motor will be burning at least 105 octane verses stock 89, lower it 3 points. So the 9 is right. The E is the ground electrode style. I am going to change to a V tonight. This is actually a reverse of the splitfire V. The A is the center electrode style. I personally like the G plug because the CANNOT be fouled. Had to try it, overly rich RM125 at 24:1 mix oil designed for 50:1, didn't foul. My switch though is going to go with an X. Just as important as plug is the gap. DON'T just pull it out of the box. Gap it at honda specs for gas. If this is going on alcohol, forget the information above and use the D9VX plug gapped at 18-20.
Hope this helps
Richard
P.S. You will LOVE that FCR (39mm?). If it has a flat spot at any rpm you can easily adjust it out. The huge accerator pump gives awesome response.

 

the stock plug will work just fine and the spitfire plug makes zero diff on the dyno .......