Even though the valvetain plays a supporting role in the overall performance potential of an engine it can still be a make or frequently break proposition for engine builders. In a street performance application in particular your customers are usually quite proud of their vehicles and spend a lot of money on appearance. But they also want the engine to sound and perform as good as it looks.
Most stock valvetrain components can only function normally up to about 5,500 rpm and usually cannot accept more than .500? lift. Past that point, upgrades are necessary to handle the higher loads. Rockers, lifters and pushrods as well as valve springs are upgraded to lighter, more durable materials to squeeze out as much power as possible from an engine package.
Valvetrain designs may vary slightly in layout from engine to engine but the task is the same: the main objective is to control valve timing so the pistons can pull enough air and fuel into the cylinders to produce maximum torque and horsepower. The amount of variation involved in this most basic function is almost infinite.
The camshaft is the heart of the valvetrain. Precisely timed in relation to the crankshaft and pistons, the cam spins around and around at half the speed of the crankshaft, but high engine speeds can sometimes be too much for the stock valve springs to control. When this happens the valves can’t shut quickly enough to keep the lifters on the cam. Instead of following the cam lobe profile, the lifters begin to jump off the lobes. And the steeper the cam profile, the worse the problem becomes as engine speeds climb higher.
Another thing that happens when the valve springs can’t keep up with the cam, the lifters land harder on the way down and begin to bounce slightly, causing the valves to also bounce as they seat. In addition to increasing wear and the likelihood of fatigue failure, valve bounce also disturbs airflow into and out of the combustion chamber and decreases high rpm performance.
If engine speed continues to increase, the valve springs soon won’t be able to close the valves fast enough before they start to open again. This leads to a situation where the valves “float,” which allows combustion pressure to blow past the open valves. The engine begins to misfire, but the situation can become worse. If the engine continues to accelerate, the valves will likely hit the tops of the pistons and destroy the valvetrain and possibly more.
One way to guard against floating the valves is to reduce the mass of the valvetrain. For street engines, lighter valves with smaller stems and thinner heads can reduce unwanted weight. For your customers who want the best, and money is no object, titanium valves can be a good way to lighten things up, but the cost is not always justified in a street application.
While a lumpy idle may sound really cool it’s not always the easiest to handle on the street. All-out race engines are designed to operate most effectively in specific rpm ranges; usually the powerband is only separated by a few hundred to a thousand rpm. However, in a street performance engine your customers will complain if the engine doesn’t pull from down low while racing streetlight to streetlight as a well as being able to handle some high rpm conditions. If your customer is running the vehicle in track events as well as driving on the street, then you will be challenged even more. First and foremost, it depends on what the customer wants.
In order to understand all the variables that affect the valvetrain, you must first know what you are trying to achieve. You need to know as much as possible about the purpose of the engine you’re building. Whether it’s just being driven to church on Sunday or to the track, you need to have a clear picture of what your customer expects from the performance build to ensure the proper selection of valvetrain components among the other items in the build.
For performance customers who just want to add an aggressive cam, it can be a difficult task explaining to them everthing else that goes along with it. If you go with camshaft durations at .050? at 224° and above, experts say you need to replace springs and go with anti-pump up lifters and so forth.
One valvetrain modification that can make sense for street performance customers is converting from flat tappet cams to hydraulic roller cams. The extra cost can be justified when you create more horsepower and torque as well as increasing the longevity of the engine. Flat tappet cams are susceptible to wear especially during break-in, and with reduced levels of zinc and phosphorous additives in today’s oils, many engine builders and performance enthusiasts are switching to roller cams and lifters.
Engines that use roller cams have an extremely broad powerband compared to the relatively narrow powerband of the flat tappet lifters, which means you make more horsepower and torque over a wider rpm range due to increased area “under the curve.” This means the rollers on the lifters allow the cam lobe to open and close the valves quicker, increasing the flow and efficiency of the engine. By using the correct components that match the cam, a hydraulic roller cam can idle smoothly while making great low-end horsepower and pull strong all the way through the rev range.
Some of the other advantages to a roller lifter cam is that it reduces friction, which generates less heat and wear. A roller cam can also handle a much steeper ramp on the cam profile than a flat tappet cam. This allows the valves to open more quickly and reach maximum lift sooner.
Of course, this isn’t meant to imply that roller cams are perfect for every application. There are many flat-tappet camshaft and lifter combinations that offer some of the best performance value for the money.
If you are still messing around with stud rockers, girdles and polylocks you may want to rethink using these antiquated parts that don’t really work anymore in a modern performance engine. A good way to add value to your next performance engine build is to switch to shaft rockers. These components are not as expensive as some may think and offer big benefits for your performance customer. Here are some of the benefits of shaft rockers according to one manufacturer:
More Power – Reduced valvetrain friction equals more power, regardless of valve lift or rocker ratio. Shaft rockers are more stable at high rpm ensuring accurate valve timing events. Shaft rockers are mounted to the head using a steel stand that positions the roller directly over the valve tip while the stud rocker is aligned by the stud location and a .080?-wall tubular pushrod and guide plate which one would you rather have in your engine? In many cases the valve, stud and pushrod guide are not actually in line, as found with most small block Chevy intake rockers. This misalignment was generally compensated for in the stamped pivot ball rockers, but became a serious liability to the stud mounted roller rocker as it had no way to correct the misalignment with their single plane trunions.
Increased Reliability – Because the longer pivot length rocker does not side-load the valve as much, shaft rockers are easier on valve guides, valve seats and valve tips fewer comebacks for the engine builder. Shaft rockers simply rotate to take up valve lash stud rockers slide up and down the stud taking up the valve lash before they open the valve. Example: If your valves are lashed at .030?, your stud rocker is sliding up and down the stud every time the valve opens. Just imagine this slide-hammer effect at 7,000 rpm! And when valve float occurs, the entire weight of the rocker is loaded onto the valve tip until the system resets.
User Friendly – Lashing the valves with stud rockers and the attending stud girdle is a pain in the neck. First, you loosen the stud girdle, lash the valves, and then re-tighten the stud girdle. If the studs are the least bit out of alignment, tightening the stud girdle changes the valve lash. Shaft rockers hold lash much longer and are easier to lash properly. On some vehicles like 5.0 Mustang racecars, the valve covers are very difficult to remove between rounds, so having a vehicle that can go the entire race weekend without adjusting the valves is a big deal. Most racers have found they can remove shaft rockers from their stand to change a valve spring and return them without adjusting the lash again.
Additional Benefits – Shaft rockers also offer other benefits. They are more adaptable to constantly changing cylinder heads, they enable pushrod offsets to clear wider ports, and have assorted pivot lengths to accommodate a wide range of ratios, 1.5 through 2.25!
So what’s this upgrade to shaft rockers really going to cost an engine builder? If you price out a set of aluminum roller rockers, quality studs and guideplates, and a stud girdle, the cost is very similar to the value-priced shaft rocker system.
Many of the newer performance engines have roller lifters rather than the conventional flat tappet or solid mechanical lifters. With rollers you can get by with a more aggressive cam profile because of the roller but you have to watch your valve lift, experts say.
The critical thing to remember is that anything over .500? valve lift may have a greater potential for the valve spring to bind. Generally, you need about .060? clearance on the springs.
When installing a cam with more than .500? lift, it is absolutely essential that clearance between the valve spring retainer and guide be checked. Do not attempt to operate an engine with less than .150? retainer-to-guide clearance. If you are using valve seals, check the clearance from the top of the seal rather than the top of the guide.
When using a flat tappet camshaft and high pressure valve springs with more than 130 lbs. of seat load or 330 lbs. of nose load, at least one manufacturer requires a 30 minute break-in period using only the outer springs. Install the inner spring only after the break-in period. Only after the break-in period should the inner spring be installed. Following this procedure will greatly reduce any chance of camshaft of lifter failure.
Proper pushrod length should be determined when converting an engine originally equipped with hydraulic flat tappets to an engine using longer-than-stock retrofit hydraulic roller tappets. In this case you must also use shorter than originally equipped pushrods.
It is possible to install a high performance hydraulic (non-roller) camshaft or a mechanical flat tappet camshaft in a block originally equipped with a hydraulic roller camshaft. Matching lifters, pushrods, timing chain and, in some cases, rocker arms must be used to accommodate this conversion.
Here’s how to determine correct pushrod length: starting on the base circle of the cam adjust your “Adjustable Checking Pushrod” out, including your lash settings at the valve or preload on the lifter (generally .030? – .060?). Make sure the rocker tip never rides off the tip of the valve, favoring the center of the valve through its travel. For longevity on your guides and seals you are looking for the shortest sweep across the valve tip. For increased performance, you want the pushrod side of the rocker tilted back with the roller tip of the rocker off center towards the intake side of the engine. Also you will want at least .040? clearance between the edge of the retainer and the under side of the rocker.
Stiffer springs are a must for most performance engines. Experts say that springs are just about the most crucial part of the valvetrain in order to gain the most rpm potential and durability.
Stiffer springs apply more force to keep the lifters in steady contact with their lobes and to overcome the increased momentum of the valves, rocker arms and pushrods at higher rpms. Springs that are too stiff for an application can create just as many problems as ones that are too weak. A really stiff set of springs will increase cam lobe and lifter wear. They may also be too much for the stock rocker arms, pushrods, spring retainers and keepers, and exert more force than the stock parts were designed to handle.
When upgrading cams it is important to go by the recommendations of the cam manufacturer when choosing stiffer springs. Another way to ensure you get the right components is to buy a cam kit that includes new lifters and springs with the cam.
For a complete listing of valvetrain suppliers, click on the High Performance Buyers Guide and search valvetrain.