Machining heads to obtain proper valve seat angles is vital to an engine rebuild.
There exists no middle ground whenever performing a valve job on a racing engine. It must be done right or performance will suffer. Period.
Rebuild an engine, neglect to do the valve job just right, and watch horsepower fall. Eliminate the obvious-a valve adjustment, timing, jetting, and so forth-and chances are the solution to your horsepower problems will be found when you go back to the valve job you failed to do correctly.
At its most basic, a valve job is meant to improve the seal between the valve and the valve seat on the head, ensuring sufficient pressure in the combustion chamber whenever the spark plug fires. So proper valve angles and concentric valve seats are critical to performance.
"Every little thing counts," says Jeff Dorton of Automotive Specialists. "You can't take any quick steps around a good valve job. There's more power around the bottom of the valve seats to the top of the valve seats than just about anywhere else in the engine. It's that critical. If you don't have the valve seat right, the angles are off, or it's not concentric, the valve is not going to seat and it's just not going to make power. Basically, it's not going to be efficient, and you've really got to have that part of it really efficient to make the most power."
Stock Car Racing recently spoke with engine builders Ben Barnes, Darrell Poe, and Jeff Dorton to get their advice on the elements of good valve work.
Jeff Dorton machines valve seats at Automotive Specialists.
* Pay proper attention to valve guides before starting a valve job. "The first thing you do when you start to do a valve job is look at the valve guide wear," says Barnes, of Barnes and Reece Racing Engines. "You have to have the proper amount of clearance before you can even do a valve job. Once the valve guide has been serviced and everything is good there, you have to look at the concentricity of the valve seat and make sure that it's as close as possible with the machinery we use."
Watch for signs that indicate the valve guides may need attention before beginning the valve job.
"There are no shortcuts to achieve necessary results on a valve job for racing engines," says Dorton. "Straight valve guides with proper clearance are a must to start with. The big thing is not running guides that are worn out, bell-mouthed, tapered or just loose in clearance. If you have a Serdi machine, but your guide is not right, you're not going to cut a round seat. You've got to have a good valve guide to cut a round seat. That's the biggest thing."
Once heads are machined, a vacuum test tells you how effective the work will be.
* Be careful with your machine work. When machining heads, take special care not to lower the seat too far and create a lip or change the way the entrance is to the bowl. Lowering the seats can hurt flow, Barnes says. "If you keep grinding the seat lower and lower, it starts to shroud the valve. Putting a bigger valve in a shrouded chamber may shroud the valve even more and lead to loss of power."
"You can do a lot of harm by sinking the seat," says Dorton. "That's a big mistake-cutting them too deep and going and going. Quality equipment for machining seats and grinding valves is necessary. I am sure there are a number of opinions, but we have found the Serdi seat machine and valve grinder work best for us. We are able to efficiently maintain concentricity and surface finish with this equipment.
"Surface finish and concentricity must be held to close tolerances for heat dissipation of the valve and seat. The valve seat angle, as well as the angle or angles below and above the seat, play a critical part in airflow and the performance of the engine. Normally, the valve seat on both intake and exhaust valves is on the outer diameter of the valves, 0.060 and 0.080 wide. A back-cut angle, usually 10-15 degrees less than the seat angle, up to the id of the actual valve seat most always helps airflow and performance."