just got my 040 piston for my 400ex.didnt ask when i ordered it,but its 11:1 comp.if i run 94 octane gas will this be ok? or do i need straight racing gas or a mix of 94&race gas? also will be using a xr400 cam.rev box&160main and40 pilot jets.i talked with golden west cycle and they thought this combo would work well.i would like some other input before i put this back together and find out i`m screwed>

 

It should run on super pump gas, but if it pings hard when floored dont run it.. a slight ping when lugging is ok but nothing more. some use the 250 cdi box for more rev's but with your pistin i wouldt run it that fast.. give it a shot
Joe

 

you should be fine with 93 octane gas. It will run better with a 50/50 split. You don't need cam2 straight. What are you using for a pipe?

 

using supertrapp ids with 7 discs,stock head pipe .i ported the head myself with a dremel tool.i took the piston and jug to the local machine shop, there going to mill the piston down a little to get the comp around 10.5:1.also ordering a 16tooth front sproket to get a little more from the top end.

 

I am wondering how milling your piston would increase your compression ratio? Or maybe I interpreted you wrong and you are decreasing your compression. As far as increasing your compression with a different piston, I think it would be better to mill the head. Please explain this to me, because I have no experience changing compressions with different pistons.

 

i`m having the piston milled down a little to decrease the comp level. it should work out around 10.25 or 10.5:1 .i thought 11:1 was a bit to much. the 11:1 piston is domed shaped with 4 vavle releifs. the stock piston and 10:1 piston are flat across and have 4 vavle reliefs