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High Performance Valvetrains: Modifications for Speed and Strength
By Larry Carley
Whether you’re building a street performance engine, a killer drag motor or a Winston Cup powerplant, the valvetrain requires special attention if you’re going to maximize reliability and performance. Most stock valvetrain components are only good up to about 5,500 rpm. Beyond that point, upgrades are necessary to handle the higher loads.
Even though the camshaft turns at only half the speed of the crankshaft (one revolution of the cam for every two revolutions of the crank), engine speed reaches a point where the springs can’t pull the valves shut quickly enough to keep the lifters on the cam. Instead of following the lobes back down to the base circle, the lifters begin to kick off their lobes. And the steeper the profile of the lobes, the worse the problem becomes as rpms go up.
When the springs can’t keep up with the cam, the lifters bang back down on the cam and 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 screws up airflow into and out of the combustion chamber and hurts high rpm performance.
If engine speed continues to increase, the point is soon reached where the springs can’t close the valves fast enough before they start to open again. The valves begin to "float" (stay open), which allows compression to blow right past the open valves. The engine begins to misfire, and if the driver doesn’t back off on the throttle he runs the risk of a valve kissing a piston goodbye.
Lighter valves reduce the risk of valve bounce and float by reducing the mass of the valvetrain. For street engines, valves with smaller stems and thinner heads can reduce unwanted weight. For serious performance engines and customers who can afford the best, titanium valves are the only way to go.
Stiffer springs are also a must for any performance engine. In fact, the springs are probably the most crucial part of the entire valvetrain in terms of rpm potential and durability.
Stiffer springs exert more force to keep the lifters in constant contact with their lobes and to overcome the increased momentum of the valves, rocker arms and pushrods at higher rpms. But, springs that are too stiff for an application can create just as many problems as ones that are too weak. A really stout 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.
The key here is to follow the recommendations of the cam manufacturer when it comes to choosing stiffer springs. Better yet, buy a cam kit that includes new lifters and springs with the cam. This way you’ll know the parts are properly matched.
For small-block street/strip flat tappet cams with up to about a .450˝ lift, you can usually get by with stock springs. Or, you can upgrade to stiffer single springs that give about 100 to 115 lbs. of seat pressure when the valves are closed, and no more than about 300 lbs. when the valves are fully open. For small-block street/strip flat tappet cams with .450˝ to about .500˝ lift, stiffer springs are a must.
For a cam with more than .500˝ lift, even stiffer “conical” springs or “double-springs” are necessary. This, in turn, usually requires additional changes including:
- Flycutting the spring seats in the heads to accept larger diameter springs, or dual or triple springs.
- Changing the spring retainers to ones that are designed for double or triple springs.
- Changing the valve seals to accommodate conical or double springs. This may require machining the guides.
- If the heads have push-in rocker studs, the studs will have to be pinned or replaced with screw-in studs.
- Replacing the stock pushrods with stronger and stiffer 4130 chrome moly pushrods (to prevent pushrod flexing and breakage).
- If the springs provide more than 350 psi of pressure when the valves are open, the stock stamped steel rockers will also have to be replaced with stronger steel or aluminum rockers.
- For highly modified/racing engines, a "stud girdle" can be installed to reinforce and steady the rocker arm studs.
A "rev kit" is another add-on that can improve reliability and rpm potential. The kit consists of extra springs that fit over the lifters to assist the valve springs and to keep the lifters in their bores should a rocker arm or pushrod break.
According to Justin Hynes of NHK Intex Corp., Schaumburg, IL, (makers of Endura-Tech springs), most NASCAR teams run dual springs with seat pressures of 190 to 200 lbs. and open pressures of 500 to 600 lbs. at .750˝ lift. Pro Stock drag racers, by comparison, typically run triple springs with seat pressures of 375 to 475 lbs. with the valves closed, and up to 1,000 lbs. open.
"Spring durability is always an issue, and it depends on the quality of the materials used to make the springs and the heat treatment they receive. We nitride all our springs to extend their endurance, but it’s a tricky process that has to be carefully controlled.
"Most NASCAR teams will only run a set of springs for one race, then replace them with a fresh set that typically costs $500 to $700. But many budget racers run the exact same kind of springs all season long with no breakage or problems at all," said Hynes.
There are a couple of things to watch when installing valve springs. One is height. This assures the springs have the required pressure to keep the valves shut. Height is checked by measuring the distance between the spring seat in the head and the retainer on the valve stem.
Most performance valve springs are closely matched, but if adjustments are needed it can be done by shimming the valves to equalize pressures. Shims are made of hardened steel, come in various thicknesses and are usually serrated on one side to prevent rotation (the serrated side faces the head). The thicker the shim, the more it increases spring pressure. Don’t overshim, though, because doing so may lead to coil bind with a high lift cam or rocker arms.
Always check the spacing between the spring coils when the valves are fully open. High lift cams and/or rocker arms push the valves further open, so it’s important to make sure there’s still some room between the coils at maximum lift. This can be done by inserting a feeler gauge between the coils. A minimum clearance of .060˝ is usually recommended, though some racers run tighter clearances.
If you’re using double-springs that generate more than 350 lbs. of force when the valves are open, the stock stamped steel rocker arms should be replaced with stronger aluminum rockers.
Two alternatives here for upgrading valvetrain performance are aftermarket hardened steel retainers or lightweight titanium retainers (aluminum retainers are generally considered too soft). For the street, steel retainers with stock 7° keepers work fine. For racing or high rpm roller cams, titanium retainers with 7° or 10° keepers are best. Some keepers have an extra step inside that reinforces the bottom of the retainer and reduces the risk of the valve pulling through at high rpm.
When the valve locks are installed around the valve stem, their edges must not touch each other. They should clamp against the valve stem to hold it securely.
Keep in mind that the design of the retainer affects the installed valve height and spring tension. Be sure to check the clearance between the retainer and the top of the valve guide/seal at maximum lift. A minimum clearance of .060˝ is usually recommended. If there’s less, the valve guides will have to be machined down to provide the necessary clearance.
The stock lift ratio for a small-block Chevy V8 rocker arm is 1.5:1. For a big block Chevy V8, it’s 1.7:1. Bolt-on aftermarket high-lift rocker arms with higher ratios are a simple way to increase the net lift of an existing cam without the expense and hassle of changing the cam. Installing a set of high-lift (1.6:1 to 1.9:1) rockers on a small-block Chevy will add more horsepower with virtually no loss in low speed torque, idle quality or vacuum. But high-lift rockers alone won’t provide as much of a gain in performance as a hotter cam/rocker combination.
The trend today is to use high-lift rockers to achieve greater lifts rather than relying so much on the cam lobe. Dan Jesel of Jesel Valvetrain Innovation, Lakewood, NJ, says high-lift rocker arms allow the valves to open and shut more quickly. This increases effective duration as well as lift for better performance without increasing lifter travel.
"Ideally you want a square lobe on a camshaft, one that opens the valves quickly, holds them open at maximum lift, then closes the valves quickly. Increasing the lift ratio of the rocker arms has that effect on valve breathing and is a great way to add performance.
"If you bolt on a set of 1.95 rocker arms with a stock Corvette cam that has a .295˝ lobe lift, you increase the valve lift to .575˝, which is a nice improvement for a street engine,” said Jesel.
For street motors, rocker arms with roller tips are a good upgrade over stock stamped steel rockers because they reduce wear on the valve stems and guides. For performance motors, they’re a must. Aluminum rockers are a must for serious performance, but they’ve also proven to be durable enough for street motors, says Jesel.
Most aftermarket aluminum rocker arms feature needle bearing fulcrums and roller tips to reduce friction. The less expensive ones are usually die-cast while the strongest ones are extruded or forged from 7075-T6 aluminum billet bar stock. Some rockers are also shot peened to increase durability even more.
Installing a set of rocker arms is a simple bolt-on job, but there are several areas that need to be checked when valve lift is increased. More lift means less valve-to-piston clearance, less spring retainer-to-valve guide/seal clearance, more valve spring compression (and possible coil spring bind), possible interference between the pushrods and cylinder head, and between the rocker arms, cylinder head and valve covers.
Measure for coil bind and retainer-to-valve guide clearance at maximum lift. As a rule, the valvetrain should have clearance to travel an additional .060˝ beyond maximum valve lift.
Also check clearances between the rocker arm and its stud. Stock rockers on Chevy small-block V8s can handle up to about .470˝ of lift. More lift requires switching to "long slot" rockers or needle bearing rockers.
Maintaining correct rocker arm geometry is important, too. The tip of the rocker should rest on the valve stem, slightly off-center (toward the intake side). As the rocker pushes the valve down, it should align with the middle of the stem at 50 percent lift. If it’s not on dead center, it will push the stem sideways every time the valve opens and closes. This will accelerate guide wear and possibly lead to valve failure.
To correct a rocker arm geometry problem, you’ll have to use either shorter or longer length pushrods. Some inexpensive aftermarket cams that are made by regrinding used or stock cams require longer pushrods because the diameter of the base circle is smaller than the stock cam.
There are two things to keep in mind here: pushrod length and strength. When engine modifications to the valvetrain, cylinder heads or block are made that affect the installed height of the valves, the location of the rocker arms or valvetrain geometry, it changes the length of the pushrods, too.
Determining the "right" pushrod length may require using adjustable pushrods. Once the proper clearances and geometry have been established, you can measure the length of the pushrods, order the right ones and install them in the engine.
Pushrods must also be strong enough to overcome valve spring pressure without flexing. Stiffer springs, therefore, require stiffer pushrods. Most performance pushrods are 4130 chrome moly steel. The diameter and wall thickness of the pushrod should be chosen to match the springs.
For street engines with single springs, 5/16˝ diameter moly pushrods with a wall thickness of .080˝ should be adequate. For dual springs and/or heavier valvetrains, 3/8˝ diameter moly pushrods would be a better choice. For a serious high rpm drag motor, 7/16˝ to 9/16˝ diameter moly pushrods with up to .120˝ wall thickness are recommended.
Pushrods and rocker arms also tend to wobble around quite a bit at high rpm, a condition that is usually made worse by stiffer springs and/or a high lift cam or rockers. Installing "guideplates" that fit under the rocker arm studs can help keep the pushrods properly aligned at high rpm. Just make sure the pushrods are hardened so they won’t wear excessively when they rub against the guide plates.
Always check pushrods (even new ones) for straightness before installing them. If you see any wobble when the pushrod is rolled on a flat surface, it is no good and needs to be replaced.
Performance engine builders should always remember to use new pushrods whenever they are installing new lifters and/or rocker arms.
If you’re installing a new cam, replace the lifters too. Never use worn lifters on a new cam unless you want to wipe out the lobes in short order. Use plenty of assembly lube on the cam lobes to protect them when the engine is first started, and run the engine at 2,000 rpm or more for 20 minutes after the initial start up to break-in the new components. Don’t let the engine idle during this critical break-in period.
Jim Hill of Crane Cams, Daytona Beach, FL, says cam selection is what determines an engine’s power curve and personality. A typical street engine might never see more than 6,200 rpm while a racing engine will usually turn up to 8,000 to 9,000 rpm or higher. So you need to pick a cam and lifter combination that matches the application.
For street engines, hydraulic lifters are usually better than solid or mechanical lifters for several reasons. Hydraulic lifters automatically maintain zero valve lash so they don’t have to be readjusted periodically. They’re also quieter. But for all-out performance, many racers prefer solid or mechanical flat tappet lifters or roller lifters because they can handle higher rpms without pumping up.
If a customer wants hydraulic lifters, one option is to install those that have a higher leak down rate than the stock hydraulic lifters. A higher leak down rate lifter can improve idle vacuum, idle quality and low-end torque somewhat by reducing effective valve lift. They can also reduce pump-up at high rpm to improve performance. But high leak down hydraulic lifters can be noisy.
For serious racing, a roller cam is the only way to go. Roller lifters, which come in both mechanical and hydraulic varieties, have several important advantages compared to flat tappet lifters. Rollers reduce friction and lobe wear, which improve durability and save horsepower.
Roller lifters can also handle steeper cam lobes which means they can open and close the valves more quickly to improve breathing for the same amount of lift and duration as a flat tappet cam. But roller lifters also require fairly stout springs (450 lbs. or more of pressure when open), and are expensive compared to flat tappet cams.