Pam,
Your kind and knowledgable answers beg for this request:

On the lower to mid end of the money spectrum, what performace mods do you suggest, in the proper order, for getting more out of the 400ex? Specifically talking about motor, gearing, etc. Looking for power and speed. Don't want to destroy the wallet or the Honda reliability.

Thanks in advance!

 

Thanks for the kind words, Conway!

There's good news and bad news when it comes to basic mods on the 400EX. The good news is Honda designed the engine VERY well, and the head actually flows more air than the engine pushes.

Now the (sorta) bad news. There's not many basic bolt-ons which can give you considerable power gains.

For starters, I'd probably say a K&N Filter, open up the airbox with a few holes, and go with either the Thumper Racing muffler insert or an aftermarket exhaust. This will give you about a 15% HP increase overall.

Another option (works best for tight trails and technical riding), leave everything else stock and increase displacement to 415cc (bores into the stock liner), or 440cc (requires resleeving). Either one of these kits is extremely reliable (using an Arias or JE piston), and will give you more bang for your buck.

The 415cc kit will probably cost you around $250 and will give you about a 20% increase in HP, more torque than rev.

The 440cc kit will set you back about $400 (including re-sleeving your cylinder) and will give you about a 30% gain.

Any of these mods are very reliable.

Hope this helps you out!

Pam

 

Pam,
Thanks for the info. I always enjoy reading your responses. Do either of these bore kits require valve or cam upgrades? Thanks!

Cory

 

Actually, that's the coolest thing about the 400EX, the head flows REALLY well. In fact, better than it needs to for the stock displacement! So you can go with the big bore, and leave the head stock without any problems. In fact, I recommend leaving the head stock if you're doing mostly trail riding.