High Comp engine rebuild

[sixer3]

well, i've decided to do my rebuild on my 400ex at 12:1 compression, i've got most of everything ready for install, j.e 89mm piston, web cam, hd rockers and springs, runnin a 450r carb for now, hd head studs, cometic gaskets. head is in for boring, and race porting and stud install

a few things i was wondering and dont know a whole lot about are:

1- oversized valves (i.e. Kible White +1mm)- would i gain anything with these and what exactly are they going to do for me?
2- should i avoid problems in the furture by upgrading my crank and rod, and would the hotrods be enough strength upgrade from stock or do any of you reccomend something different?

also any other suggestions and info is greatly appreciated

[400exrider707]

If you have the dough definitely upgrade the rod.... I would go falicon personally.;)

[GPracer2500]

Big valves allow more air to get into the engine. One downside is that with some engine configurations you may loose some bottom end because the larger valve opening can't maintain the same intake/exhaust velocities at lower RPMs. That's speaking generally though--I'm not exactly sure if that is a significant issue on the 400EX head.

[sixer3]

ya, i guess that makes sense on the oversized valves sense....i may try it further down the road

te reason i was going to go with a hotrods is there is a band new hotrods crank and rod for like 190 shipped off ebay

but im not going to waste 200 if a falicon is going to be stronger 12:1 is quite a bit of pressure for a 400ex top end

[1fst400]

hot rods makes a heavy duty, and standard duty rod. I have the heavy duty in my 400, with 12:1 piston. works flawlesly.

[nakomis0]

A rod if you got the money. I been running the stocker rod for 6yrs on about 11:1 comp. Runs great but I know that things gotta break soon. Its my only worry.

O/s valves would be good, anything involving air flow.

Sounds like your gonna have a nice setup.

[sixer3]

yah, i do a lot of dune riding mainly and of course like to race up and down the hill and at the drags, i told em i wanted a mid-range port...hopefully i'll be happy with that

im also tryin out a buddies velocity(ebay) air intake kit basically an aluminum tube to replace the air box and boot, im going to see if i can gain any power from it

[wilkin250r]

Originally posted by GPracer2500
Big valves allow more air to get into the engine. One downside is that with some engine configurations you may loose some bottom end because the larger valve opening can't maintain the same intake/exhaust velocities at lower RPMs. That's speaking generally though--I'm not exactly sure if that is a significant issue on the 400EX head.

An 89mm piston will bump the displacement to 440cc. Combine that with the high compression, he'll have PLENTY of bottom end.

Me, personally, I say oversized valves are a great idea on almost any engine. For the smaller bores, it allows better flow to reach higher RPMS and attain those peak HP numbers. For the big-bores, they're flowing so much more air than the stock displacement that intake velocity is automatically increased.

[sixer3]

yah, i definately wont have a problem with bottom-end i'll be usin 20" paddles so im going to play with gearing quite a bit before i settle down, its going to be funnn!