In ancient times (about 20 years ago), seat preparation was performed using a series of grinding stones to establish the seat angles. Generally, this involved three stones that were dressed at the specific angle required. The accepted standard valve seat for a typical performance intake and exhaust valve seat uses 30-, 45-, and 60-degree angles. The 45-degree seat matches the angle ground on the valve. The 60-degree angle is also often referred to the throat angle that directs the air (on the intake side) from the port toward the seat. Then the 30-degree top angle helps radius the flow into the chamber. Different cylinder heads can and do respond to different combinations of angles, but the classic 30-45-60 is tough to beat.
With the advent of the dedicated valve-cutting tool machines such as the Serdi, Sunnen, and others, the machinist can specify a custom-made cutter that machines all three seat angles and widths simultaneously. Not only is this faster and far more consistent through all 16 valve seats, but it also ensures far more seat concentricity, which also helps flow.
This is the stock chamber after cleaning. You can see that if there are three angles on th
Peart set up the heads in JGM's Sunnen SGM-1500 machine and efficiently cut all the intake
This photo illustrates the three angles on the intake and exhaust valve seats. A very mino
With the machining complete, all that's left to do is assemble the heads. We went with an Edelbrock valvespring to match the camshaft, since the springs were really inexpensive. There are a couple more steps that include choosing the right valveguide seals and valvesprings and ensuring that we have plenty of clearance for our new cam. The cam is a mild Edelbrock Performer-Plus with only 0.420-inch valve lift, so coil bind and retainer-to-seal clearance aren't issues. We also upgraded the intake seals to Viton-style positive seals that mount on the outside of the guide. Once the spring clearances were verified, Peart assembled the heads with a little lube on the stems, and we carried them back to Strope's shop to bolt them on the engine.
While the cam we decided to use in the 318 is very mild, it doesn't hurt to check for inst
There are several types of valve seals. We decided to use a positive-style seal (left) ins
Here is the finished head with the springs installed and ready to be painted Mopar orange
|Valve stem to guide
||Intake, 0.001 Exhaust, 0.0015
|Valve seal to retainer
|Clean and Magnaflux
|Install new guides
|Back-cut valves (8)
These are JGM prices that may not be typical of other machine shop prices, but they will give you an idea of what the operations cost.
Note that our new valve job really didn't improve the flow, which is not unusual. The change that really helped was the 30-degree back-cut to the intake valve. Note how this improved the flow at 0.200-inch valve lift the most and then fell off, actually hurting flow slightly at 0.400 inch. Since our cam's max lift is only 0.420 inch, this should be worth a little torque if nothing else.
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