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Old 03-22-2012, 11:36 AM   #1
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Default Back cutting valves for improved air flow

Back cutting valves for improved air flow


This is a procedure done to help flow the air charge across the back of the valve and smooth the transition into or out of the combustion chamber.
Normally don on valves that are not fully machined or swirl polished.


Once the valve has had the face re-ground. Most common is a 44* or 45*
The valve machine is reset, and another angle is ground onto the back side of the valve towards the stem, most common is 30*



The valve on the left has the basic face ground to mate back into the valve seat.
The valve on the right has had the basic seat inked,(black) and the added angle ground. (sliver)

Most OE valves are not fully symmetrical, so the width of the valve face to seat varies around the valve, the added angle will help correct this and smooth the transition of air flow across the back of the valve.
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Old 03-22-2012, 01:04 PM   #2
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Supposedly improves intake flow almost every time, and usually not for the exhaust.
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Old 03-22-2012, 01:12 PM   #3
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Well, I have found out you gain about 3-5 cfm on both the intake and exhaust. at lifts below .400, at that point it did not seem to improve at all

This was done on a common E7TE 5.0 Ford head, same head, same cylinder, same valves
Tested on a SuperFlow 600 at 28in.
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Old 03-22-2012, 01:36 PM   #4
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Looks good. Definitely improves low lift flow. For the exhaust an additional 45deg on face really helps flow. If you are up for accuracy on your valve job you can go a bit deeper. Since I like my intake seat only .020 wide. (assuming good metals) my valve seat area gets very narrow. About .060-.100 max on the valve. Putting the actual seat as far out on the valve as possible. .010 to .020 away from the edge.

I have had cheap SS valves so bad a third angle was required for symmetry.
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Old 03-22-2012, 02:39 PM   #5
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On some vlave jobs I will lap the valves in then bring the back cut to the lap line.

.020 seat is thin on some head, and wide on others.

Since I use Serdi style cutters, the seat width is built into the blade.

But to narrow the contact patch between valve and seat, I use an interfearance angle ground on the valve.

If you push the seat too far out on the valve face, with too narrow a contact patch a couple things can happen that are not the best in the world, cupping of the valve face is one, and tuliping the valve is another.
What brings this on is improper heat transfer from the valve to the seat.

If it on a "race" head that is pulled down on a rugular basis, then no big deal, it will get caught at inspection.

But On a DD head.. just causes issues and time the car is down.
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Old 03-22-2012, 06:06 PM   #6
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all good points and all to true. Intake or exhaust widths as well. I don't build for others so not in the same class as you for sure. :-)

Seat width is always a challenge and truly can vary. Pricey but I use set materials so longevity or seat erosion is minimal. Seat location on the valve has not been a failure item for me. (hmm, Yes it can be too exposed to the chamber but way out there) Heat dissipation becomes my only real concern. Which could happen with either you described. Seat pressure is another variable of course. (I maintain a set seat psi (seat area/spring pressure) Have run .015 on 2.1 intake 200 seat pressure = no seat or valve damage after 15k miles of 4k average on hwy.

Serdi cutters I have to special order. Or as I prefer anyway, Serdi cut- then grind the finish to finalize. Lapping I have not had much luck with. It dulls (rounds) the edges of the seat. Crisp square edges seal better and last longer. imo

Would I build to that level for a customer. no way. A lot of time and care to get it exact. Probably a waste for my stuff as well.
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Old 03-22-2012, 06:09 PM   #7
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Hey bogus, just ignore me. I must be bored. Here you are putting out good information for everyone. I am just muddling your info.
Sorry
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Old 03-22-2012, 06:12 PM   #8
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We back cut ALL our valves Also. That's included in our valve jobs. Keep up the good work
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Old 03-22-2012, 07:08 PM   #9
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Justaturbo.... Its all good!

Diffrent builds need diffrent levels of detail.

A box stock rebuild on a 3.4gm head will not get the same level of detail that I have to do for say ... a set of Dart alum heads for a SBC with a .650 solid roller cam.

I do have a few Serdi blads that have been custom made too, they are kept in a seperate box, for more detailed work.


@RP.... Back cutting valves has sort of fallen to the wayside in todays machine world with the cost of stainless steel valves becoming more affordable.

Sort of like polishing the beams of stock connecting rods....

Since I have started doing machine work, I have run from diesel, tractor, stock rebuilds to some mid level performace work.
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Old 03-22-2012, 07:13 PM   #10
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Yea its something that we offer even on our 5 angle valve jobs.
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