Found what I needed.
 

Hi guys,I recently purchased a OS 1.40RX engine and it came with the OS header and what I was noticing is that the engines exhaust port opening is bigger than the headers inlet.My question,is there a reason for this.I am a mechanic and have hopped up alot of engines and we allways match up the headers to the exhaust ports,so I don't know if the same rules apply to these engines.One more question what would be the best pipe to use to get the most power at the highest rpms,not worrying about noise regulations.

RE: OS 1.40RX-P Header
 

They don't.

never heard of a "P" before.

but just use that header, it doesn't care....trust me, I've run 12 of them into the dirt.

-Mike

RE: OS 1.40RX-P Header
 

I think it is designated P because of the fuel pump.