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."
A "go/no go" gauge will show if your valve guides are worn or loose.
* Check concentricity and perform a pressure test. The valve seat must be concentric-as round as possible-for the valve to seat properly and obtain the necessary seal in the combustion chamber. A concentricity gauge is a must. Once the angles are cut in the head, use the gauge to determine where you need to grind the seats to true them up. Go beyond what the guy down at the local garage or machine shop might be doing with heads off street car engines.
Once any concentricity issues are addressed, check to see that the valve is seating at the desired point. Lap the seat with a fine compound to see that the valve is seating right on its outside edge. Vacuum testing the head to make sure the valves are seating properly is critical. If you have a pressure leak, then you've got a problem-recheck the concentricity, the angles, etc.
* Preventive maintenance is crucial. Valve adjustments and spring checks should be part of your valve maintenance routine. If the valves are too tight, the engine won't run efficiently. A loose valve will pound and actually start to bounce, thus destroying the seat.
"Naturally, once the valve job is done and the engine has run, it's always critical to routinely each week adjust the valve lash and stay on top of that," says Poe. "Oftentimes that will tell you if you have something going wrong. If the valve adjustment starts closing up, then you feel like the valve is either sinking in the seat or a valve stem could be stretching or you've got a problem somewhere. It's critical to stay on top of the valve adjustment every week."
Lapping the valve with a fine compound will tell you if the valve is seating properly and
Keep fresh valvesprings in the engine. Gauge the springs when they're new so you'll have a baseline figure to compare to when you check pressure after each run. "That way you can tell if one starts to fall off," Dorton says. "Usually, they won't all go at one time. One will go, then another one. It's critical to check every one because, generally, they won't all fall flat at once, just one here and there."
A valve job should be done in the range of 1,000 laps for short-track racers, although some engine builders may go as high as 2,000 laps. The key is to not take anything for granted when doing a rebuild. Even if the heads run like you think they should, or the engine runs like you think it should, don't take the valve job for granted. And, preferably, make sure the same person is doing the work each time. If you're taking the heads to an outside shop, make sure they're checking the concentricity and truing the seats as needed. Remember, a valve job for a racing engine should go above and beyond a valve job for a street car engine.
Valve maintenance is crucial to peak engine performance.
* Stick to the basics. Avoid the temptation to get radical when machining heads. A 30-degree top angle, 45-degree seat angle and a 60-degree bottom cut are standard. "A lot of guys are playing with steeper seat angles, which tend to work good for a little while, but they tend to be detrimental to the seat," says Poe. "It hammers the seat out pretty badly when you start increasing the seat angle just to gain a little additional flow."
"You can experiment with different valve configurations, but in general we find an exhaust valve with a radius on the chamber side and a flat intake works the best," Dorton says. "Check with the valve manufacturer on recommended minimum margins or valve thickness. It is not recommended to thin valves to even valve heights in the cylinder head. The seats in the head should be machined to the same height plus or minus 0.001-inch."