If the rules allow it, do it! By the time we finished everything that could be done while staying legal, these heads were up by 14 hp over a set of "as-new" heads.
2. A top cut of 30 degrees by somewhere between 0.010 and 0.030 inch wide is usually used for most applications, but recent studies indicate there may be a small advantage by using a 37-degree by 0.020-inch-wide top cut.
3. A bottom cut that is 60 degrees by about 1/8 to 3/16 inch wide followed by a 75-degree cut to blend into the cast surface of the port will get the job done.
If you want to get fancier here, there is still room for improvement-if the shop you are working with has the appropriate seat and guide machining equipment.
Let's move up the ladder a rung or two and talk radius approach (intake) and departure (exhaust) forms here. If the metal is available, adding a radius on the port side of the seat can pay dividends, especially on the exhaust. Although these fancier seat designs can pay off, they need a seat and guide machine that is more up-market than what many of the low-cost job shops have. That is not to say such machines are by any means rare, but you need to make sure that the shop you choose has a suitable machine. Such machines as the Sunnen and Serdi are relatively common. Until just a few years ago, the Serdi was often seen as the state-of-the-art machine and was most used by the top pro racing shops. These days, there is a new player on the block when it comes to generating high-tech seat designs-the single-point-tool CNC Newen machine. I recently used one of these machines at Advanced Induction in Concord, North Carolina, and was very impressed with its accuracy and versatility. If you can find a shop with one of these machines, be aware that they can design a seat right on the machine and cut it to within about a tenth of one thousandth afterward.
Part of the seat design involves the form on the valve. A 30-degree back cut, such as the one between the 45-degree seat and the back face of the valve, is essential to strong low-lift flow.
Take a look at the seat shown in Figure 2. This was one I used on a Dart head. Normally, these heads come with a good multiangle seat. Such seat designs are used because they combine function with cost-effective production. However, as good as the out-of-the-box seats were, there was still room for improvement. With an appropriately back cut valve, this seat was worth 2-6 percent more than a good multiangle seat in the lift range from 0.050 to 0.350 inch.
On an air-starved engine (which most low-cost engines end up being), this is rarely worth less than 5 hp and as much as 15 hp at the other end of the scale. These bigger gains usually happen on engines that are running an effective exhaust header system. If the headers are sized to produce a good scavenging wave but the exhaust port is not flowing too well or maintaining a good outward-going exhaust velocity, then the scavenging effect is reduced. By cutting a good seat with a healthy radius and keeping the throat of the exhaust port on the small side, the scavenging is markedly improved. Result-bigger power gains from an effective seat.
Cup car head porter Dave Mitchell cut the seats on a set of Ford heads to an early-style Cup car radius design and increased the flow all the way up to 0.700-inch lift, where it was a full 12 cfm up on a conventional multiangle valve job. | 
The CNC Newen machine is where the high-performance industry is going at present. Here, Phil Odom of Advanced Induction cuts a set of high-tech seats that were designed right on the screen next to the machine head. | 
Here is a Newen CNC-machine-generated seat form that proved effective on a production-style head. The top cut (green) is at 37 degrees by 0.020 inch wide. The seat (red) is 45 degrees and 0.060 inch wide. The form below this (yellow) is a radius that joins the seat at 50 degrees and ends at 80 degrees. The blue section is an 80-degree cut that blends into the port. |