Propjockey
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K&B 12mm VS. 13mm crank (3.5 OB)
Just curious, which is the better engine and why.
Just curious, which is the better engine and why.
K&B 12mm VS. 13mm crank (3.5 OB)
- Thread starter Propjockey
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Re:K&B 12mm VS. 13mm crank (3.5 OB)
Hey PJ,
Well, here's another case where - "Bigger is better." The reason? More area to pull the air/fuel mixture into to case, up the by passes and into the cylinder.
I could be wrong, won't be the first or last time, but I think the old crank was 11mm and the new is 13mm.
J.D.
Hey PJ,
Well, here's another case where - "Bigger is better." The reason? More area to pull the air/fuel mixture into to case, up the by passes and into the cylinder.
I could be wrong, won't be the first or last time, but I think the old crank was 11mm and the new is 13mm.
J.D.
Re:K&B 12mm VS. 13mm crank (3.5 OB)
Hi Guy's,
Older crank (8906etc) is 12mm diameter with an 8.5mm gas passage. Newer 13mm (8907,8935) has a 9.5mm gas passage. As JD says bigger is always better when it comes to cranks. As an example Nova's (the power gods of 3.5cc) have 14mm cranks. BTW there is no point in putting a carby on with a total venturi area that exceeds the crank gas passage cross sectional area - just can't flow more than the crank will allow.
My $0.02
cheers
GT ;D
Hi Guy's,
Older crank (8906etc) is 12mm diameter with an 8.5mm gas passage. Newer 13mm (8907,8935) has a 9.5mm gas passage. As JD says bigger is always better when it comes to cranks. As an example Nova's (the power gods of 3.5cc) have 14mm cranks. BTW there is no point in putting a carby on with a total venturi area that exceeds the crank gas passage cross sectional area - just can't flow more than the crank will allow.
My $0.02
cheers
GT ;D
Flying Pilgrim
Active Member
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Re:K&B 12mm VS. 13mm crank (3.5 OB)
I thought the reason they increased the size was due to a problem with the bearings of the smaller cranks going out. ???Obviously, the larger area of intake passage will increase the high end rpm if carb throat is appropriate size too. Too much carb will lack bottom end acceleration. Right now with K&B it doesn't much matter what you want, you take what you can find!!
I thought the reason they increased the size was due to a problem with the bearings of the smaller cranks going out. ???Obviously, the larger area of intake passage will increase the high end rpm if carb throat is appropriate size too. Too much carb will lack bottom end acceleration. Right now with K&B it doesn't much matter what you want, you take what you can find!!
Re:K&B 12mm VS. 13mm crank (3.5 OB)
Hi guy's,
Forgot to mention the 8906 and early 8907 had the K&B black carby....later 8907 and 8935 have Gold carby which has bigger throat bore for more power. my understanding of why they went to the 13mm crank was people were asking the motor to pull higher RPM and there were problems with the 12mm cranks breaking so they made a bigger crank. As the 13mm and 12mm bearing both fit in basically the same from housing I would have guessed the 13mm crank version had less room for the balls to circulate....don't think this would necessarily be a good thing.
My $0.02
cheers
8)
ps. I haven't had any problem getting motors and parts ???and I live way out here in the land of Oz; from the states of course. ;D
Hi guy's,
Forgot to mention the 8906 and early 8907 had the K&B black carby....later 8907 and 8935 have Gold carby which has bigger throat bore for more power. my understanding of why they went to the 13mm crank was people were asking the motor to pull higher RPM and there were problems with the 12mm cranks breaking so they made a bigger crank. As the 13mm and 12mm bearing both fit in basically the same from housing I would have guessed the 13mm crank version had less room for the balls to circulate....don't think this would necessarily be a good thing.
My $0.02
cheers
8)
ps. I haven't had any problem getting motors and parts ???and I live way out here in the land of Oz; from the states of course. ;D
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