Piston compression 10:1 or 11:1

[eprovenzano]

I'm in the process of rebuilding the top end on my 99 400ex. It was still on the OEM piston and rings. The bike would smoke like a two stroke, so I knew it was time to freshen up the top end.

My question is which piston compression should I go with a 10:1 or 11:1 compression piston. I prefer to keep it close to stock, so I'm leaning towards 10:1, plus I don't want to use race fuel. Upon inspection during disassembly, the cylinder bore is fine; it just needs to be honed. There was definitely oil getting into the combustion chamber through the valves, as the carbon build up was pretty bad. So I'm going to have the valve seals and seats repaired at a local shop. I'm going to stick with the original cam.

Any other suggestions are welcome.

[togup]

With a stock motor I dont think youll notice much difference with a 10.5-1 but with honing piston and gaskets it will be inexpensive and get you back to riding pretty quick

[eprovenzano]

If I go with an 11:1 piston, can I run it on pump gas? I assume the increase in compression will give me a little more power.

[sandspanker]

If you want to run pump gas (super) go with the 10:1 piston.. will build less heat and overall will be better. i went with a 10:1 with my 416 and i run pump gas with no problems.
:macho

[honda400ex2003]

I like my 10:1 alot. i also run 93 if i can find it or 91 mostly. I would also be able to run 89 if needed for a short time to make it home or something i supppose. steve

[eprovenzano]

After further consideration Im going to go with the 10:1 piston.

[mcgrath]

I have an 11:1 comp and run 93 octane and 87 octane if need be. I usually only run the 93 but have never had any problems running 87. No knocking, no heat issues. I have put on about 1000 miles this year since rebuild with 0 problelms. I would use a 10:1 personally but used an 11:1 just because thats all that was available when I needed it. 10:1 is stock and dependability is key to me.

[tri5ron]

Originally posted by mcgrath
10:1 is stock and dependability is key to me.
inaccurate info here.
10:1 is not stock.

[mcgrath]

Yeah right on tri5ron my bad. I was actually just reading your thread about the xr400 gaskets. If the info on that post is accurate I guess maybe thats why I have never had any issues with an 11:1. I thought 10:1 was stock but apparantly its a claimed 9.1:1?

[sportracer07]

How much power if any does an 11.1 piston give you if everything else is bone stock.

[Glamis400ex]

When I had my 400ex and built it to a 416 and then a 426, I had 11:1 piston and I ran 50/50 super pump and VP c12 race gas...but I was running it in the sand dunes. The guy I sold it to only run trails and such and says it runs good on straight 91 pump.

If you do run 11:1 you WILL need HD studs, no way around it. Also, you won't reap the big benefits of a hi comp piston without a cam. They work together.

[eprovenzano]

I'm going to go with the 10:1 piston. The bike is only trail ridden, and basically gets the snot beat out of it... After talking to my son (it's his quad), he mentioned I rarely get a chance to fully open it up, at least for any extended period of time. So he too would prefer to keep the bike bullet proof as many times he could be 20+ miles away from the truck. So after further consideration we will keep it close to stock. We’ll go with a new top end, have the valves cleaned up, and do some other maintenance items.

In the end, the goal is to go for a ride, and always being able to finish the ride without any issues. These bikes are so good on the trails, and so reliable, I don’t give it a second thought when he heads out for a ride.

[rooster300ex]

I would go with the 10:1, because keepin everything stock isn't gonna give u much power gain. Them 400ex's are grunty enough for trails in stock form.