The basics of a three angle valve job:
On the inlet valve, the lap marks should be placed on the outer edge of the 45° valve face. This takes full advantage of the diameter of the valve. When the lap marks sit inside the outer edge of the valve, the overhanging portion of the valve serves as an obstruction to flow. The top cut on the seat (usually 30 degrees), establishes the outer edge of the seat and helps turn flow into the combustion chamber. A 60 degree bottom cut sets the width of the 45°seat.
The 45° exhaust seat is deliberately kept inboard of the outer edge of the valve – otherwise hot exhaust gas will eventually erode the edge of the valve.
The diagram below shows the three angles: 30°, 45° and 60°:
After the valves have been lapped, a backcut is placed on the back of the valve, close up to the inner edge of the 45° seating area. This further improves flow around the head of the valve:
Look at a stock replacement cylinder head and you’ll find they have a basic 45° angle cut on to the seat. Lap in the valves and inspect the valve face to see exactly where the seat is coming in contact with the valve face. Even on aftermarket heads that come out of the box with a three angle cut, when you lap in the valves you’ll see the angles haven’t been placed for optimum performance.
The graph below shows the results of a flow test I carried out on a brand new CNC 044 cylinder head (40mm inlet valve).
The red line shows ‘out of the box’ flow with the factory fresh 3 angle seat.
The blue line shows the improvement by recutting and repositioning the 45° cut to the outer edge of the valve. I also added a 30° backcut to the valve.
At .500″ lift there’s a 15cfm increase in flow. Depending on the engine specification, that translates into a potential power gain of at least 15bhp.
If you’re curious as to what difference backcutting the valve makes, here’s another test. This time using a band new stock 1600 twin port head (35.5mm inlet valve).
Red line: straight out of the box with no mods
Green line: 3 angle valve job
Blue line: 3 angle valve job plus 30° backcut on the valve
Overall gain in flow @ .500″ lift: 14cfm
This illustrates how even the mildest of engine builds will benefit from the addition of a decent valve job. On a mild 1776cc engine build, this modification alone could account for an increase of 10bhp or more.
Most people associate increasing the flow of a cylinder head with having to enlarge the ports. However, if the percentage flow increase found by opening up a port is less than than the percentage by which you’ve increased the port’s cross sectional area, you’ve just decreased port velocity. With a valve job alone, you increase flow with no modification to the port. Therefore any flow gains comes hand in hand with an increase in port velocity. It’s a win-win.
Although the term ‘3 angle valve job’ is the commonly used term, nowadays many machine shops use four, five or even more angles when cutting valve seats. My first piece of valve seat cutting equipment consisted of individual grinding stones – one used for each angle. Today I use preformed carbide cutters, where all the angles are cut in one operation. Cutters are available in a variety of configurations. The advantage of using a carbide cutter is absolute consistency from seat to seat, with all seats being cut to exactly the height. Many of the blades I use also feature radiused blends from the bottom cut into the port and also above the top cut, blending smoothly into the combustion chamber.