Don Terrill’s Speed-Talk

Don in your buiding engines manual you have a reverse flow valve. Is the 45 degree cut on the face better than a radius? When you say a small backcut after the 45, what is that? .020? And what degree? I need Tulip valves for a crossflow head, like 24 degree.

Should the margin be wider on the intake than on the exhaust for the Z Flow?If I need a 42mm "intake" what degree backangle and radius gives the best flow for a hemi DOHC crossflow head?

For the intake valve, a face cut angle is the way to go, raduis for the exhaust. To reduce reverse, yes keep the backcut .020-.030. I'll try to get Ron Melville to answer the Z-Flow question - he coined the phrase.
https://www.speed-talk.com/shows/008_ron_melville.html

Thanks for the reply! On the intake a 45 degree face angle. On the exhaust a radius. OK

How wide a margin on intake and exhaust? What degree cuts on both valves and what degree backcuts on each for best flow. Thanks. I am going to have some made and would like to order them with your recomendation. Thank You Again!

When trying to limit reverse flow the dimensions are very touchy. You're going to have to cut up some junk valves and test - there's no one size fits all. Make the effort, it'll be worth it. Thanks, Don

with an all-out camshaft ,

Low-Lift flow increases "act" like you are increasing
the OVERLAP period and DURATION

so what happens at very low RPM , HP/Torque gets hurt
by increses in low-lift flow with all out cams

most times its BELOW the RPM range you are interested in
so it has no negative effect down low

Low-Lift flow usually picks up TopEnd ..not the very bottom end

Low-Lift Flow = similiar effect as INCREASING duration/overlap

the trouble is a valve is not a One-Way device
usually making low-lift flow increases also result in
increasing valve's ability for reversed flow


another example is the NHRA SuperStock Chrysler 318-340-360
engine .... before NHRA let any valve shape be used ,
the stock Chrysler intake valve was a Tulip-design,
it hurt low to mid-lift flow numbers , but high lift were OK

after NHRA allowed any valve shape , we went to a nail-head
design with backcut angles, low to mid-lift numbers
increased with high lift numbers the same as before with Tulips

on the dyno, the Tulips made torque sooner , came on the cam sooner,

with the nail-head valves, torque began later, but wound up as much
peak torque and with a little more Peak HP , but higher by 200 RPM

Actual DragStrip runs were faster with nail-head after rear gear ratio was increased
to compliment .....than was with Tulip valve head
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Sort of what Bill Jenkins was trying to describe in his book
and also what Phillip H . Smith's
"Scientific Design of Intake and Exhaust Systems" =>where when the
intake flow was too good for low RPM airflow demands,
you lost too much ram effect at end of stroke.
You got it back as you reved the engine higher if cam timing was OK

maxracesoftware wrote:Low-Lift Flow = similiar effect as INCREASING duration/overlap

This reminds me of a test I was working on to get the engine to "think" the camshaft was a lot more aggressive by making valve/seat shapes (step) that would kill low lift flow. I know it's been tried before without success.