Angle-milling your cylinder...
Angle-milling your cylinder heads can have a multitude of benefits-namely, it produces power by increasing compression while at the same time improving valve angle. If you look closely, you can see that this cutter is only removing material from the right side of the head (exhaust side).
Angle-milling cylinder heads is one of the best ways to take existing equipment and make it work better. Almost all racers understand the concept-angle-milling is essentially decking a head on an angle so that more material is removed from the exhaust-port side than the intake side-but fewer racers understand the many ways it can be beneficial. "If it is done right, everything's a plus," explains Jonathan Leagon of Leagon's Racing Heads. "You stand the valves up, you improve the intake-port angle, you improve compression, right on down the line."
Of course, a good head specialist is going to make it sound easy. Essentially, it is. The toughest part is realizing that a good angle-milling job isn't just cutting one angle. Once you cut the angle on the deck of the head, you have changed the geometry of the intake face, the exhaust face, the valves, the bolt and cooling holes, and just about everything else. Once you understand how all these other factors are affected, you're on your way to more power without sacrificing durability.
Since everything is based on the angle at which you deck the head, that's the first to be cut. To start, you need to know how much you are cutting. You can usually get by with a couple of degrees without having to do major work to the bolt holes or moving the dowel pins and still get a good increase in power. Extreme angle changes require more work, including moving the dowel pins, recutting the bolt holes, and fiddling with the water passages.
Luckily, there's a simple formula to determine how much material must be removed to get the desired change in valve angle you want. The only information you need is the width of the head (the widest point that still makes contact with the block) and the desired change in the angle of the valve. The formula used is a simplified version of one requiring sine and cosine calculation, but it is just as accurate and tells us what we need to know.
This illustration shows how...
This illustration shows how angle-milling helps improve valve angle. This is the general layout of a Chevrolet 23-degree head, but the idea works the same with all pushrod engine cylinder heads. In the drawing on the left, the dotted line across the deck of the head represents what's going to be cut. On the right, you see how the angle of both the valve and intake port in relation to the block change for the better after the cut is made.
W = Width of Head
A = Angle Change
C = Cut Depth
C = 0.0175 x W x A
So, if we had a Chevrolet 23-degree head with a maximum width of 7.625 inches and we desired to make a 1-degree change in valve angle (from 23 to 22), the formula would be:
C = 0.0175 x 7.625 x 1
C = 0.133
This value is the cut amount in thousandths of an inch. We should remove 0.133 inch of material from the exhaust side of the head, angled down to nothing at the intake side of the deck, to stand the valve up 1 degree.
Conversely, you can also use this formula if you know how much you want to cut and wish to know what the valve angle will be. Using the same variables, the formula is:
A = C / (0.0175 x W)
A = 0.133 / (0.0175 x 7.625)
A = 0.997 (or approximately 1 degree)
It's important to note that this change is always a reduction in valve degrees. Also, this is the same angle of change at the head's intake face. To make the head fit properly with a standard intake, you must return the intake face back to its original angle. Be aware that when you have done this, the intake will sit lower than it did before. If you are allowed by the rules, it's a good idea to go in and port-match the transition between the manifold and intake ports in the heads.
The more aggressive the cut,...
The more aggressive the cut, the more the holes on the outside of the deck of the head move toward the combustion chambers. Do not cut so much that they intrude into where the gasket seals the combustion chambers.
This illustration uses the...
This illustration uses the same dimensions we used in our formulas. As you can see, the cutting plane removes nothing on the edge of the head where the intake face meets the deck. That is the zero point.
As you deck the head, the...
As you deck the head, the intake face angle increases (from perpendicular to the deck). This, too, must be angle-cut back to its original angle in order to correctly seal the intake manifold.