[DanSumo]

Hey fellow riders & wrenchers,

I’m a new rider and have successfully rebuilt the bike twice with good results thus far. Recently, I bought and installed a high compression piston (13.5:1) in my 2007 crf450r. I’m led to believe that I need a new variant of spark plug to get the most out of the HC piston. I’ve been doing some digging online to see what others have done in this situation and can’t find any leads. Does anyone else have experience in this area? I know of the diagrams that break down the variety of spark plug configurations but surly there has to be tried and true reference for such a popular modification, right? If you have or know of any recommendations please do get back to me on this! Thanks for your time.

[JimDirt]

Welcome to the site !!

You should not have to change the plug , the "recommended" plug will run fine on the higher compression piston , the ONLY reason you would want a different heat range on the plug would be if you are running at high RPMS the majority of the time , then you would run a colder heat range , but not some "special" plug that is different from what the manufacturer recommends


Your plug choices should be (I would not recommend deviating from this)
1st the Standard plug would be NGK IFR8H11 or Nippon Denso VK24PRZ11 (this should work with any gas/race gas you would use and work fine with most temperatures and pretty much any compression piston you would use *for moto , street and trail riding*)

2nd choice would be the Colder plug NGK IFR9H11 , or Nippon Denso VK27PRZ11 (for running high RPM's almost all the time , like wide open Baja type riding)

I would not use any other plug in your bike , these were specifically designed to run in these bikes in stock and modified form and what the manufacturer has proven to not damage anything

I have run 11-1 to 13.5-1 on the stock heat range plug , with no issues ever , using pump gas and also using a mixture of 1 gal. 100LL Av Gas and 1.5 gal. 93 Non Ethanol Premium

[DanSumo]

Thanks Jim!

This is very reassuring news to hear, especially your past successful experience. This seems to make a lot of sense because of how little information there was about the R&D typically required when modifying an engine away from the stock configuration. For instance with car engines it’s pretty common to have trial and error to figure out what runs best in a unique non factory tested system. I can’t thank you enough for the time you’ve spent helping me, very great-fu”e”l.

Now... I can start putting my time into figuring out this race fuel situation you speak of.