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Keep the Valvetrain on Track

The key to choosing the right valvetrain components is to match the parts to the application. It sounds simple enough, yet many valvetrain component suppliers tell us there are a lot of misconceptions about what kind of parts are best for different kinds of performance applications.

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Conical valve springs are wound progressively tighter toward the top. This reduces the mass of the spring for less harmonics, and allows the use of less spring pressure for the same RPM. Dual conical springs don’t touch and run cooler (courtesy of Comp Cams).

Conical valve springs are wound progressively tighter toward the top. This reduces the mass of the spring for less harmonics, and allows the use of less spring pressure for the same RPM. Dual conical springs don’t touch and run cooler (courtesy of Comp Cams).

Valvetrain components such as lifters, pushrods, rockers and valve springs are often replaced when rebuilding stock engines. Worn rocker arms and lifters should not be reused to prevent failures down the road. If you’re installing a new flat tappet cam, new lifters are a must. Any pushrods that are not straight must also be replaced, along with high-mileage valve springs that have weakened with age.

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Those are basic guidelines for a stock engine. But if an engine is being modified to make more power or turn higher RPMs, or it is being purpose built for racing, stock parts usually won’t cut the mustard.

Rules may restrict the type of valvetrain components that are allowed in certain racing classes, but if there are no rules you can choose just about any combination of goodies that will deliver the kind of performance you’re hoping to achieve.

The key to choosing the right valvetrain components is to match the parts to the application. It sounds simple enough, yet many valvetrain component suppliers tell us there are a lot of misconceptions about what kind of parts are best for different kinds of performance applications.

The requirements for a street performance engine are very different from those for a circle track, drag, marine or diesel engine. Street engines have to be drivable, durable and affordable. Most street engines with a hydraulic flat tappet cam or hydraulic roller cam will be rev limited to 6,500 to 7,000 RPM and spend most of the time idling or cruising in the 1,500 to 3,500 RPM range. There’s no need to install a set of race only roller or shaft-mounted rockers, super stout pushrods or high-pressure valve springs because the loads just aren’t there. A set of street roller rockers, some chrome moly pushrods and a new set of stock or slightly stiffer valve springs should be all that’s needed.

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On the other hand, you don’t want to scrimp on the valvetrain components when putting together a purpose-built race engine. You want maximum valvetrain rigidity and stability for peak RPM power. That usually means installing larger diameter studs for stud-mounted extruded aluminum or steel roller rockers and a stud girdle to stiffen things up. Or, if the budget permits, possibly upgrading to a high end shaft mounted rocker system with CNC-machined aluminum or steel rockers. Steel rockers are almost indestructible and usually outlast most aluminum rockers by a significant margin.

The pushrods will also have to be replaced with larger diameter, thick-wall pushrods to add stiffness and rigidity to the valvetrain. As for valve springs, the amount of pressure required will depend on the valvetrain and how high the engine will rev. Up to 7,500 to 8,000 RPM single springs will usually work.  Beyond that you’ll need double springs, and triple springs if the engine is going to push 10,000 RPM.

Pushrod Power

Stock pushrods are often the weakest link in the valvetrain because of this component’s tendency to bend and flex. Even a mild upgrade in performance should always include a new set of chrome moly pushrods. Pushrods with thicker walls and/or a larger diameter will significantly improve valvetrain stiffness and stability.

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Performance pushrods are available in diameters ranging from 5/16˝ all the way up to ¾˝, with wall thicknesses ranging from .083˝ to .188˝ or more. Replacing a set of 5/16˝ pushrods that have .083˝ walls with a set of 7/16˝ diameter and .165˝ thick pushrods will improve stiffness 75 percent. Drop in a set of 5/8˝ diameter pushrods with .188˝ thick walls and you’ll boost stiffness over 300 percent.

There is a weight penalty with larger diameter and thicker wall pushrods, but the effect is not as great as you might think. Going from a 5/16˝ diameter pushrod to a 7/16˝ diameter thick-wall pushrod increases the weight of the pushrod from 62 grams to 145 grams. The upgrade more than doubles the weight of the pushrods. But on engines with rocker ratios of 1.4 or higher, it has minimal impact on the engine’s RPM potential.

Weight on the pushrod side of the rocker arms does not affect the valvetrain as much as weight does on the valve side of the rocker – especially with higher lift ratios due to the multiplier effect of the rocker arm. For every gram of weight reduction on the valve side of the rocker arm, the engine will rev 35 to 40 RPM higher with the same amount of spring pressure. Knock off 10 grams and you’ll gain 350 to 400 RPM top end.

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It doesn’t matter how the weight savings are achieved. You can use a lightweight rocker, a titanium valve spring retainer or smaller diameter steel retainer, lighter weight valves (titanium or hollow stem stainless steel), or beehive or conical springs that have a smaller diameter at the top and thus less reciprocating mass. Every gram of weight that is saved on the valve side of the rocker allows more RPMs with no increase in spring pressure.

Most Top Fuel engine builders don’t use hollow pushrods because they want a rock solid valvetrain. The pushrods have to overcome tremendous pressure in the combustion chamber to open the valves, so strength is more important than anything else. The rockers are lubricated from above rather than routing the oil up through the pushrods.

For most engines, valvetrain flex is not a good thing because it causes erratic valve operation and costs you horsepower – as much as 20 horsepower according to some dyno tests. That’s why most valvetrain component suppliers recommend using the stiffest, strongest pushrods that will fit the engine. The intake ports in the cylinder heads and the configuration of the casting will limit the maximum diameter of pushrods that can be used without interference. Using offset lifters can allow larger diameter pushrods to be used. Offset lifters may also reduce the angularity of the pushrods in some engines. The same goes for offset rockers that reposition the upper end of the pushrod left or right a bit so there is better alignment between the lifter and pushrod.

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Another way to reduce flex in the pushrods is to use an engine block with a shorter deck height. A shorter deck allows the use of shorter, stiffer (and lighter) pushrods. Tall deck blocks require longer (and heavier) pushrods, which make it more important to use extra stiff pushrods.

Top fuel requires a really stout valvetrain to handle the loads that are placed on the rockers and pushrods.

Top fuel requires a really stout valvetrain to handle the loads that are placed on the rockers and pushrods.

Weight And Spring Pressure

To optimize valvetrain dynamics, you should use no more spring pressure than is absolutely necessary to maintain valve control at the engine’s peak RPM. Doing so will reduce loading and stress on the rockers, pushrods, lifters and cam, and reduce the risk of valvetrain component failures.

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Hollow stem valves can cut valve weight up to 20 percent or more, and are not as expensive as titanium valves. Valves with smaller diameter stems can also reduce weight and reduce the amount of spring pressure required to achieve the same RPM.

Beehive springs taper slightly toward the top, which reduces the weight of the spring. The shape of the spring also allows the use of a smaller diameter and lighter valve spring retainer. Another benefit of beehive springs is less valve spring harmonics that can cause instability at higher engine speeds. Beehive springs are a good choice for street performance engines as well as many oval track engines that are limited to single valve springs. Changing from conventional, coil valve springs to beehive springs can usually allow an engine to rev 1,000 RPM higher with no other valvetrain modifications. The design of the spring, however, typically limits the maximum amount of valve lift to .650˝.

Conical springs are another alternative to consider. Unlike conventional, cylindrical valve springs, the coils on a conical spring become progressively smaller with each turn as the spring is wound from the bottom to the top. This gives the spring a tapered appearance from the side, with the top being smaller than the base. Like a beehive spring, this reduces spring mass and harmonics so the engine can rev higher without increasing spring stiffness. Single conical springs work best with hydraulic performance cams that deliver up to .675˝ of valve lift.

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Dual conical springs are also available for higher revving (up to 8,000 RPM) engines and higher lift cams (up to .800˝). Unlike conventional dual springs, dual conical springs have a slight air gap between the springs so they do not touch. This reduces friction and allows the springs to run much cooler and last longer. The manufacturer says dual conical springs exert only 150 lbs. of closed valve seat pressure with an installed height of 2 inches, but provide better control than conventional dual springs that have 300 lbs. of closed seat pressure.

A lot of racers prefer some type of dual spring setup not only for the added spring pressure that’s necessary for higher engine speeds, but also to protect the engine should a spring break. With two springs holding a valve, the second spring serves as a failsafe backup to prevent the valve from dropping down into the cylinder.

steelrocker3_tdmachineRocker Ratios

Higher rocker arm lift ratios produce more total valve lift from a given cam lobe profile. It also provides faster valve opening and closing. But the trade-off of higher lift ratios is more load on the valvetrain components, which requires higher spring pressures and stronger parts. Increasing the lift ratio from 1.5 to 2.0, for example, increases the load on the rockers, pushrods and lifters 30 percent.

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High lift ratios may also result in valve spring coil bind or contact between the valve spring retainer and guide, so make sure there is adequate clearance if you are increasing the lift ratio with higher lift rockers (a minimum of .060˝ is usually recommended).

In recent years, there has actually been a trend back to somewhat lower lift ratios than those used in years past. Lift ratios up to 2.25 and higher are available from many rocker suppliers. Replacing a set of stock SB Chevy 1.5 rockers with 1.6 rockers is usually good for 10 to 15 horsepower. But as the rocker ratio goes up, you need stiffer springs and pushrods to maintain valvetrain control.

An alternative approach that has been gaining ground in recent years is to use less rocker ratio and a camshaft with larger lobes. The size of the cam bores in the block limit how much lift can be delivered by the cam lobes, even with a reduced base circle, so the solution has been to enlarge the cam bores and use a cam with oversized journals and larger lobes. A camshaft with 50 mm journals can achieve about .440˝ of maximum lift at the lobe while a cam with 60 mm journals can deliver .590˝ of lift.  Some Top Fuel Hemi engines are now using cams up to 82 mm in diameter, and even larger cams may be on the way.

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If the cam lobes do more of the heavy lifting, the rockers do less and require less spring pressure. The result is improved valvetrain stability and power. Aftermarket blocks are available with larger cam bores so you can install a camshaft with bigger lobes. Some of these big lobe cams are available with split “clamshell” bearings that allow the use of smaller journals with a big lobe cam to reduce friction and heat.

length_graphic_manton_pushrodsValvetrain Geometry

Another important aspect of performance engine building is valvetrain geometry. By this, we mean the relationship of the rocker arms to the pushrods and valve stems. The rockers are the fulcrum points that translate the upward motion of the lifters and pushrods into a downward motion to push the valves open.  It’s a simple task but one that requires the correct geometry to minimize valvetrain friction and side loading against the valve stems and guides.

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Valvetrain geometry is one of the last things you establish when putting an engine together.  You don’t really know how the geometry will line up until the heads are mounted on the block and the rockers are in place. At that point, you need to use an adjustable pushrod to figure out the correct pushrod length for the head/block/rocker/valve combination you are using.

Ideally, you want the point of contact between the roller on the tip of the rocker to be more or less centered on the valve stem as the valve opens and closes. If it is off-center too far, it will increase friction and side loading on the valve stem.

The end of the rocker arm follows an arc that depends on the length of the arm from the pivot point to the tip of the rocker. The contact point at the tip of the rocker should start its travel slightly offset to the intake port side of the valve stem when the valve is closed. The tip then slides towards the center of the valve stem when the valve is 1/3 to 1/2 open.

Here’s where things get tricky. The contact point on a shoe style rocker (no roller on the end) or a stamped steel stock rocker will continue to slide across the top of the valve stem toward the outside as the valve reaches the full open position, then slide all the way back as the valve closes. With a roller tip rocker, the geometry is a little different. The contact point of the roller will start out offset slightly to the intake side of the stem when the valve is closed, move toward the outside side of the stem as the valve reaches 1/3 to 1/2 open, then roll back towards the intake side again when the valve is fully open. With either type of rocker, you want to align the rocker so the contact point remains as close to the center to the valve stem as possible as the valve opens and closes.

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Longer rocker arms with longer pivot lengths and/or higher lift ratios follow a broader arc that reduces scrubbing across the top of the valve. The difference between a 1.5 ratio rocker and a 1.6 ratio rocker is about 9 percent less sweep across the tip of the valve. So that’s 9 percent less friction and 9 percent less side thrust on the valve stem.

Rocker arm manufacturers can move the pivot point and change the pivot length of a set of rockers to optimize the geometry, even with the same lift ratio. Offsetting the rocker pivot points allows better valvetrain geometry and high lift ratios. This can be done with offset stud mount rockers, pedestal mount rockers or shaft mount rockers.  Changing the installed height of shaft-mounted rockers can also change valvetrain geometry, so height adjustments should be made before measuring the pushrod length.

Pushrod length determines where the rocker starts its journey along its arc. A shorter pushrod means the arc starts higher up relative to the tip of the valve stem, while a longer pushrod means the arc starts further down. The goal is to choose a pushrod length that positions the rocker so the longest part of the rocker arc is centered over the valve stem when the valve is 1/3 to 1/2 open.

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There is no such thing as a “standard” pushrod length for a custom engine build because pushrod length depends on block deck height, head deck height, valve stem length, installed valve stem height, cam base circle diameter, lifter size and cup location, and installed rocker height and location.  Once all of these other variables have been established, you can figure out the length of pushrods that are needed to fit the engine.

If you are rebuilding a stock engine with stock valvetrain components, off-the-shelf stock length pushrods work fine. However, if you are installing an aftermarket performance cam with a smaller base circle (higher lift), different lifters, higher lift rockers, longer or shorter valves, and/or an aftermarket cylinder head or engine block with a taller deck and/or higher cam location in the block, the stock length pushrods won’t work. You’ll need custom length pushrods.

Some pushrod suppliers offer custom length pushrods in .025 or .050˝ increments from 6 to 13˝ in length. Others make “exact fit” custom pushrods to fit your specifications, and can usually ship them in 24 to 48 hours.

Figuring out the length of pushrods to use requires an adjustable pushrod. You should also install a temporary set of light pressure valve springs on one cylinder so you can turn the engine over easily (no need to install the rest of the valves or springs at this point). Install the adjustable pushrod and rotate the engine through one valve opening and closing cycle. Note where the tip of the rocker is when the valve starts to open, is halfway open, and is fully open. Then, adjust the length of the pushrod so the tip of the rocker is centered over the valve when the valve is 1/3 to 1/2 open.

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With the proper length established, remove and measure the length of the pushrod. Generally speaking, you measure the overall length of the pushrod minus the radius for the adjuster cup (if using cup style pushrods), or minus .100˝from each end to compensate for the radius of the oil hole on ball style pushrods. Some measuring techniques require the use of a “gauge ball” of a calibrated size in the pushrod cup. Check with your pushrod manufacturer to make sure you are using the measuring procedure required, because some suppliers measure things differently.

As for the type of pushrod to use, many racing applications use a cup-style pushrod, which allows the use of rocker arms with ball-style adjusters (leaves more room for higher lift ratios). One-piece, CNC-machined pushrods are the most durable, but are also the most expensive. Quality made rockers with welded or press fit end tips are also suitable for performance applications. Some include bronze inserts for added lubricity and durability. Also, make sure the shape, diameter and surface finish of the pushrods are compatible with the rockers.

We’ll wrap up this piece on valvetrains with a few words about valve lash and lifters. With solid lifter cams, a certain amount of valve lash is necessary to compensate for thermal expansion in the cylinder heads, especially iron block engines with aluminum heads. Reducing valve lash effectively increases valve lift and duration, but also increases the risk of burning an exhaust valve or losing compression if there is insufficient lash and the valves can’t fully seat. Solid lifter cams also require constant fiddling and fine-tuning of the valve lash adjustments.

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Hydraulic lifters, by comparison, can be set to zero lash and eliminate the need for periodic adjustments. They run quieter and reduce the risk of not having sufficient lash that would prevent the valves from fully seating – provided the cup inside the lifter has the proper amount of travel and the initial adjustment is done correctly. Because of these advantages, hydraulic lifters are a good choice for street performance and even some forms of racing. But depending on the type of hydraulic lifters used, the engine may be limited to 6,500 to 7,000 RPM for a street application, or maybe as much as 8,000 to 8,500 RPM for a race engine.

Thanks very much to the manufacturers and suppliers who offered technical advice for this article: Comp Cams (www.compcams.com); Elgin Industries (www.elginind.com); Howards Cams (www.howardscams.com); Jesel (www.jesel.com); Hylift-Johnson Lifters (www.toplineauto.com); Manton Pushrods (www.mantonpushrods.com); Scorpion Racing Products (www.scorpionracingproducts.com); T&D Machine (www.tdmach.com); and Trend Performance (www.trendperform.com). 

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For even more information and contacts, visit our interactive Engine Builders Buyers Guides at www.enginebuildermag.com.

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