Im thinking to put a 415 bore kit in my 400ex my mechanic say this kit will give me A good
boost in overall power, and provides approximately a 20% increase over stock horsepower. Cylinder
boring required (no re-sleeving).
He says all I need is a 415 bore kit $235 and an high rev CDI $117
is this al a really need or does it need more

2001 400ex
fmf megamax 2
nerfbars
twistthrottle
k&n air filter
renthal handlebars

 

Simply boring to .080 over will not increase your HP 20% over stock.

Raising the compression, changing the cam, and increasing the bore size should make a significant difference, then changing the carb would be the next mod.

I'd seriously evaluate your need for the high rev CDI. I don't "need" one, and I run a ported 416 with 10.8:1 compression, an FCR 39mm carb, and an XR400 cam.

That also sounds like about double what a 87mm piston should cost (unless you are going over 11:1, then piston prices double). The cost on the piston & rings should be approx $100 - $125, gasket kit should be about $25, boring should be around $30.

I spent about $100 on my machine work from a local race (car) engine builder, which included boring, head porting, chamber polishing, header pipe "porting", head dis/re assembly and a valve job.

 

Gabe,

I've been thinking about the same mods to my engine that you listed. I have a few questions if you wouldn't mind answering them. What brand piston would you suggest? Where can I purchase the piston? Did you modifications give a huge gain over stock? Any other information you could give would be greatly appreciated. Thanks in advance for your time and help.

 

I HAVE A 426CC KIT BY PRO FAB, VERY HAPPY...IF YOU GET A BIG CAM, GET THE HARD FACED ROCKERS, OR YOU WILL ONLY GET ABOUT A YEARS RIDING OFF OF IT....I GOT THE 12:1 WIESCO PISTON, FULL PORT...AND THE CDI, I DID "NEED" IT..I TRIED TO RUN IT STOCK, IT KEPT REVING OUT QUICKLY....DOES THIS HELP?

 

If you're not hitting the rev limiter with a modified 400EX, then something may be wrong with your setup. Even a good pipe will make the bike bang the limiter pretty quickly.

A good quality piston kit (JE, Ross, Arias) (including rings, pin, circlips and headgasket) will set you back at least $200. A Wiseco piston will be less than that - the material is cheaper and they're designed to be replaced more often, thus the lower cost. The compression ratio does not determine the cost of a piston. A 10:1 piston and a 13:1 piston cost the same if they're built by the same manufacturer.

A 415cc kit (10.5:1 to 11:1) with pipe and different cdi will be about a 20% increase over stock hp on the 400EX. The reason this small displacement increase makes a pretty big hp increase is because of the design of the 400ex head. The stock head flows more cc's through it than the 400cc engine demands. Increasing the cc's to 415 gets the thing better matched to the stock head, thus allowing the whole package to work better together.

This is such a good setup that some lighter riders are faster on a 415cc kitted bike than a 440cc kitted bike (bore only, not bore and stroke, which IS faster).