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.

 

Back in the days before hardened valves/seats (for unleaded fuel) we used to do a 5 angle valve job. 30* & 45* on the valve, and 29.5*, 44.5* and 60* on the seat.
This allowed for a very narrow seating surface and improved air flow into and out of the CC.
Having been out of the business for +/-25yrs, I don't know if that would be practical these days though?

 

it hasnt changed much, alot of valves are being produced back cut, 30* with a 45*(44* at times) on the valve

now a 5 angle Vjob is oll on the seat, 15* to 30* to 45* (seat)to 60* and then a 80*

this is given that the bowl is big enough and the cutter is small enough to get in there and do the work

seat contact is where it can get tricky, to large and the valve cools to fast and the deposits build up, too narrow and the valve does not cool enough and the bernt valves happen

 

Valve cooling was less of an issue when using leaded fuels and vehicles had carburetors, unless running it extremely lean to begin with of course.
Now, the loss of that form of lubrication and electronically controlled fuel injection (much more efficient) has different issues that need to be considered.

 

the lead additive was also used to absorb some of the impact of the valve to seat. the Old heads, before harden seats, used to pound out the exhaust seat alot, ford 360/390and the early SBC.

I do not see many pounded seats nowadays.