what's the best 420cc big valve combo???

125ccCrazy

125ccCrazy

Well-Known Member
#1
#1
what is the biggest/best combo of intake and exhaust valves that will work in a 420 head without any welding??? I see Parsons have 40mm int and 33mm exhaust head ready to bolt on as does NR I believe but 7mm seems like a huge difference in valve sizes....I would think 40/35 would be a better combo if they would fit or maybe 38/35...... stock is 35/32
 
125ccCrazy

125ccCrazy

Well-Known Member
#3
#3
The Restore Kid said:
33 is pretty big.. :shrug: It's easier to push air under pressure, than it is draw under vacuum..
Click to expand...
thats true but 7mm difference seems a bit extreme..you have to be able to get all the air and spent gases out as efficently as pulling the air and fuel in.... after some measuring and thinking about it I'm a considering 38/34 set up if the exhaust seat can be opened up enough to accept the 34...this would be a more even swap and not go to extremes..
 
roccosbike

roccosbike

#4
#4
The Restore Kid said:
33 is pretty big.. :shrug: It's easier to push air under pressure, than it is draw under vacuum..
Click to expand...
I have to agree with this unless your building a blower motor.You want to gulp air in,it has no problem getting out being pushed.JMO...They have new sbc heads with 2.125" intake and they still use the 1.600" exhaust valve.
 
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PacMule

PacMule

Member
#7
#7
If your looking for power 40mm is what you want,and that is still not enough. Why? You have to understand its 420cc and the head is limited,the next is a multi valve head. Look at some current motorcycle heads,you will understand. Its about percentage of valve area to bore size,that why multi vaves are the answer.
 
125ccCrazy

125ccCrazy

Well-Known Member
#8
#8
looking at a 40/35 set up and just need to know if this is do able without welding or a lot of other special work to fit the seats in this head.... This engine has the basic billet upgrades, nothing crazy, 360 Isky cam, pop up piston, 5/16 push rods, Champion rockers.....after the head this engine will be done...it will be all I want and plan to do..
 
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