Even with a restrictive two-barrel...
Even with a restrictive two-barrel carburetor, modern Late Model Stock engine packages are delivering 8,000-plus rpms in some situations. Keeping the valvetrain under control at those rpms requires real attention to detail.
In circle track racing the Late Model Stock classes have a now-classic set of rules that mostly hasn’t changed in more than a dozen years. The heads must come from the manufacturer, typically the Chevrolet Bow Tie 23-degree and Ford N351, and must be iron with no port work besides a three-angle valve job. Valves must be steel and the stock size. The cam has to be a flat tappet solid, the carburetor is a restrictive two-barrel (usually either 350 or 500 cfm), and the intake is an aluminum dual-plane stock-style intake that legally, you aren’t allowed to do anything to beyond dust it off and bolt it up.
But the Late Model Stock class is also normally the headliner at asphalt racetracks all across the country. And as such, it draws the most dedicated race teams that put the most effort into winning. So even though the rules package is pretty limiting, competitive engine builders have been finding ways to continually push the engine package to ever higher limits. And the one area with some of the most innovations is the valvetrain. As the camshaft gets more radical, more and more steps have to be taken to help keep the valves under control as well as making sure everything lives.
With the idea of helping racers—no matter the class—gain some insight that may help them improve their own engine program, we spent some time at race engine specialists KT Engine Development to get the latest advancements in Late Model Stock valvetrain technology. Not every idea will work in every situation, but we’re betting there are at least one or two things here that apply to any class you race.
Conventional Late Model Stock...
Conventional Late Model Stock rules don’t specifically limit compression. Instead, they require flat-top pistons and have a minimum volume for the combustion chamber. Compression equals power, so to maximize it, make sure the valve pockets in the piston tops are no larger than necessary and that the combustion chambers are all right at the minimum size (normally 62 cc’s). If you get much variation after cc’ing the combustion chambers, you can sometimes deck the head so that the smallest chamber is slightly undersized to bring the larger chambers closer to the minimum and then dish the valves on a lathe to open the smallest chamber back up a bit.
Valves have to be steel with...
Valves have to be steel with a standard 0.34375-inch diameter stem. The maximum valve size is normally Chevy standard 2.020 inches for the intake and 1.625 for the exhausts. One trick you can do, however, is used undercut valves. It’s hard to see here, but the darker portion of these Ferrea valves is actually 0.3125-inch. This is the portion of the valve stem that stays below the valve guide. The smaller diameter not only reduces the valve’s weight but also creates room in the lower portion of the port to allow for more air and fuel to enter the combustion chamber.
On the left is a standard...
On the left is a standard valvespring. On the right is a valvespring that has gone through a surface treatment process to polish it. The spring isn’t coated, but the polishing process smoothes the surface to reduce or eliminate any imperfections that can create an originating point for a crack to develop and cause a spring failure. Also notice how the edges of the spring on the right at the very top where it contacts the spring retainer have been chamfered to eliminate any sharp edges.
Here’s one concession to economics....
Here’s one concession to economics. Sometimes you may be allowed to use titanium retainers, but these are steel. The titanium units are lighter but they are quite a bit more costly and wear out more quickly. Some new coating processes help increase the lifespan of titanium retainers, but that only adds to the cost. Well made steel retainers, however, don’t add significantly to the overall valvetrain weight. No matter which you use, do make sure that the retainer fits the spring tightly and that there are no sharp edges on the spring where it contacts the retainer, which can lead to a failure.
Notice how the installed valves...
Notice how the installed valves aren’t sunk into the chambers, but instead the seats are cut so that they sit right on top. This helps get the air and fuel moving as soon as the cam lifts the valve off the seat and improves overall flow into the chambers.
The installed springs are...
The installed springs are set up to have a very light opening pressure (approximately 160 pounds) but then ramp up quickly to 400 pounds of pressure when fully opened. The 7⁄16 rocker studs are the largest that will fit to help reduce rocker arm deflection.
Just about everything in the...
Just about everything in the valvetrain comes right back to allowing the camshaft to be as aggressive as possible. With flat tappet cams this can be difficult because the extra sliding friction between the cam lobe and the lifter tappet makes extra aggressive designs hard on parts. One way to help this is a treatment called “nitriding” which makes the cam both harder and more slippery. Engine builder Ken Troutman uses a custom grind developed with the help of Comp Cams and not only did he not want to share the cam specs—which we can understand—he didn’t even want pictures taken of the thing. So instead, this is an off-the-shelf nitrided cam from Comp. Eagle eyed readers may also notice it’s for a big block, but you get the idea.
As the lift rates on the racing...
As the lift rates on the racing cams get extreme, the flat tappet lifters want to start digging into the face of the lobe instead of sliding up and over. As a result, lifter technology has made some pretty significant advancements. The idea is to make the lifter as hard and slick as possible so that it will slide with no deflection to limit the chances of grinding against the lobe. This lifter looks pretty standard at first glance, but it’s constructed from tool steel and given a DLC (diamond-like carbon) coating to make it extremely tough.
A less expensive option are...
A less expensive option are these Ferrea lifters Troutman also sometimes uses. Instead of the DLC coating these lifters have an anti-friction coating that helps the face of the tappet slide up and over the cam lobe. You can also see the pressurized oiling hole in the face of the lobe that injects a steady stream of oil between the tappet face and the cam lobe for extra lubrication.
Here’s a comparison of pushrods....
Here’s a comparison of pushrods. The pushrod on the left is the standard 3⁄8-inch-diameter steel unit. It works well, but the larger 5⁄16-inch pushrod helps limit deflection and improves valve control. A smaller pushrod may be lighter, but it flexes. The result is that the valve opens later and the lifter can lift off of the cam lobe—neither of which is good. If you experiment with tapered wall pushrods, you can sometimes get even bigger units to work.
The total valve lift on this...
The total valve lift on this Ford Late Model stock that Troutman is building is 0.550 inches on the intakes and 0.575 on the exhausts, which is necessary to help the unported heads flow more air. One way to get such extreme valve motion without stressing the flat tappet cam too much is to use high-ratio rocker arms. Here, Troutman mixes 1.65:1 ratio intake rockers with 1.60:1 exhausts to get the valve motion he wants.
When working with large valve...
When working with large valve movements, it becomes critical that the rocker arm’s roller tip is centered over the valve stem. If the movement isn’t centered over the valve it can press the valve stem into the wall of the valveguide which reduces the efficiency of the valvetrain and can lead to a broken valve. If you’re mixing and matching rocker arms with different ratios, be prepared to use two different pushrod lengths in your race engine.
A rocker stud girdle can significantly...
A rocker stud girdle can significantly improve valve control. Without it, the stability of the rocker depends on the stiffness of the 7⁄16-inch rocker stud. But the girdle ties all the studs together, and since all the rockers aren’t activated at the same time it means the strength of multiple studs come into play every time a rocker arm is rolled forward to open its corresponding valve. CT