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Discussion Starter · #1 ·
Looking for experienced "B" motor builders. Got a question for you. I'm building a frank B motor, B20b block, B16 head. I'll be cutting the seats 3 to 5 angles. Have already ground the 45s on the valves. Sources tell me to back cut the intakes(30deg.), but no one has been able to say if it will help or hurt on the exhaust? What about another angle after the 45 on the on either intake or exhaust? Anybody got any flow bench time on these motors? I've blended the bowls, smoothed out any casting seams, gasket matched them, and sharpened the split on the intake. I'm now in the process of narrowing the guides and smoothing out that area. I'll finish that with some Splash Zone epoxy. The larger bore will require some de-shrouding also. Any body out there know what angles these heads like? Thanks, Smitty
 

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Discussion Starter · #3 ·
That's a bit of an over-generalization. In some motors it helps flow, others it has no effect, and some it even hurts flow.Another line of thinking is if it helps flow on the intake to cut another angle approaching the seat, on the exhaust, the flow is in the opposite direction, so cutting a second angle between the margin and the seat (combustion chamber side) should help flow also. Ultimately the only way to really know is to test the different combinations on a flow bench. If your interested in this stuff you would enjoy reading David Vizard's book "How to Port and Flow Test Cylinder Heads. Thanks for the input.
 

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I took a class on high performance engine building back in 2003. We back cut valves on a set of SB chevy, SB ford, northstar, and a D series honda head. Back cutting the exhaust on every one of those heads showed little to no gain in CFM on the flow bench
 
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