Question on longer vs shorter intake valve back cuts

[BradH]

I've got a spare set of valves where the intakes were already given 30* back cuts for the original 45* seats before I got them. When the time comes to narrow the seat widths on the valves by finishing the back cuts up to the 45* seats, it's going to result in a significantly longer back cut width than I see typically (and much longer that what the valves in the heads have now).

No big deal?

Not ideal?

It depends?


Just wondering if I should key an eye open for some uncut "out of the box" intake valves, instead, if I find myself needing to use those spare valves. Thanks - Brad

[PRH]

I’m going with “D”....... all of the above.

[BradH]

You're killin' me Dwayne...

[Walter R. Malik]

You could always change the angle of that back-cut; may even work better.

OR, add a second back-cut to narrow that back-cut.

[gruntguru]

Keep applying different angles until you have . . . . a radius.

[mag2555]

There's 2 things that take place when you back cut a valve to a different angle them it was made.
1) you get a change to improve low lift flow by forming a Venturi by means of the angles use on the seat ,and those used on the valve.
The angles used and the width of these angles to max out flow needs to be done on a flow bench, but in general valve back cuts of 35 to 26 degrees work best .

2) the side effect of using a back cut and in peticular one that narrows the seat of the valve is that you can end up sinking the valve by the same amount that you narrow the seat by.
This can be tolerated some on the Exh side with some heads, but not on the Intake side in terms of loosing air flow.

[BradH]

You could always change the angle of that back-cut; may even work better.

OR, add a second back-cut to narrow that back-cut.

I'd asked on here sometime ago about multiple back-cuts for 45* seats and didn't get positive feedback on the idea. I believe they've been used successfully on steeper seats, though.

[Abbottracingheads]

Don't over think it. Leave the backcut and seat widths the same width. The sharp edge of the backcut will help shear fuel for more power.