I seem to remember seeing where Mickey "One-Lap" had gotten right at or a little over 500cc's with a 350X engine. However he has earned the "One-Lap" reputation....

My $.02

Matrix

 

Although I have a T.C. Racing powered 400ex now,
I raced with a 350x powered machine for 3 years
both locally and at nationals.TC bored it to 368, the
legal limit at the time for the 4 strokes. I've had two of these quads, and as for as comparing the 350 to the 400. A well built 350x motor (not
mickey one lap) but a TC motor is very reliable and will blow the plastic off a stock 400ex.
Compared to my TC 400ex , a comparably built 350
is stronger in the bottom end but the 400 shines
in the upper revs. Depending on riding style , one could be very competitive on either bike.The
4 stroke A class was won by quad powered by a TC
350x bored to the new class limit of 416cc. There
were several 400exs with the same displacment and
even some YZ400 motors running in the class. That
was at the first GNC national in Georgia this past
weekend. SO the 350x deserves a little respect. One
of my 350s was featured in the March 97' issue of
Dirtwheels , it was the two page spread with all
the chrome. Its also on atv connection under tech
tips in trx conversions a few pics.
Well my fingers are worn out
later Pat