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Camshaft manufacturers are still perfecting a tri...
Regardless of what type of cam goes into an engin...
Everybody loves the idle sound of a full race cam...
Camshaft & Valvetrain Technology Overview
The basic technology that’s been used to actuate the valves hasn’t changed in decades, whether we’re talking pushrod engines or overhead cam engines.
By Larry Carley
Camshaft and valvetrain technology is a topic we’ve written much about over the years. A rotating eccentric lobe on a camshaft still opens the valves. In the case of an overhead valve engine, the cam lobe pushes a follower or bucket tappet to open the valve.
With a pushrod engine, the cam lobe pushes a lifter, pushrod and rocker arm to open the valve.
In the 1980s, Renault introduced its exotic “pneumatic valve springs” as an alternative to ordinary mechanical coil wire valve springs for Formula One racing. With this setup, the valve springs are replaced with small bucket style cylinders that are pressurized internally with nitrogen gas.
The air springs can handle engine speeds up to 16,000 rpm or higher, which is beyond the capacity of traditional metal coil springs. Pneumatic springs are now the norm for Formula One racing, but it’s unlikely many of our readers will ever see this exotic technology on a drag strip, circle track or street engine any time soon.
For over a decade now, automotive engineers have also been playing around with various types of electronic valve actuation. Their goal is to replace the camshaft, lifters, pushrods and rockers with fast-acting electronically-operated solenoids.
A “camless” engine does offer many potential advantages over a mechanical valvetrain: infinitely adjustable cam timing and duration for optimum low end torque and high speed power, no springs, pushrods or rocker arms to fatigue, wear out or break, no frictional losses generated by a rotating camshaft, timing chain, belt or drive gears, or lifters or rockers, and the weight savings that would be gained by eliminating the camshaft and all of its related mechanical components. A camless engine would be a cinch to tune because all you’d have to do to change valve timing would be reprogram the ECU.
Sounds too good to be true, right? Well, in spite of all the news reports touting the coming age of camless engines, we have yet to see anything outside a test lab let alone on a race track or the street. It will likely come someday, but not as soon as the press releases would have us believe.
Valeo has been a pioneer in the development of camless engine technology, and they have signed up a number of auto makers as potential partners if and when their camless engine technology is ready for production. But it’s not there yet. The challenges have been to develop a reliable fast-acting camless valve control system that is affordable, doesn’t break or require megawatts of electrical power to operate.
Others have been developing electrohydraulic valve actuation systems, primarily for heavy-duty diesel engines. The idea here is to use fast acting hydraulics to open and close the valves. As with other camless engine technologies that are under development, electrohydraulic valve actuation is still on the test track.
This brings us back to the reality of today.
Camshaft manufacturers are still perfecting a tried and proven technology that has been around for a long time and is not going to be displaced by anything that is radically new or different for many years to come. NASCAR and many forms of circle track racing are still committed to flat tappet cams. Flat tappet cams still give many drag racers and street performance enthusiasts a lot of bang for the buck.
Solid roller cams are the hot setup for many drag racers, while hydraulic roller cams offer many advantages for street performance engines and even drag racers, too. So for now, performance engine building is still based on choosing the right camshaft and valvetrain for a given engine, cylinder head and induction system combination so it will deliver the kind of power and performance that keeps your customer’s smiling and coming back for more.
Though overhead cam engines such as Ford’s family of modular V8s have grown in popularity in recent years (especially Ford’s new 5.0L Coyote engine), good ‘ol American pushrod V8 engines are still the norm for most performance applications. Big block and small block V8s dominate drag strips, circle tracks and most forms of professional racing. Even on the street, a pushrod V8 will still have the edge unless a late model Camaro SS comes up against a Mustang GT, in which case the Mustang will likely come out ahead (Editor’s note: the author drives a Mustang)!
Street performance engines can be more of a challenge to build than a dedicated racing engine for a variety of reasons. Street engines have to operate at a variety of engine speeds and loads, from idle to cruise to full throttle bursts of speed.
Street engines also have to be drivable, which means some degree of idle quality and low end torque, and they have to be durable enough to run tens of thousands of miles without any major repairs. And if we’re talking a street engine for a late model vehicle in a state with emissions testing, the engine also has to be emissions legal and capable of passing an OBD plug-in or tailpipe emissions test.
Another stumbling block that’s often encountered when building a street performance engine is cost. Many customers have a very limited budget and can’t afford the best camshaft and valvetrain setup that money can buy. They might want a big hydraulic roller cam with high lift lightweight steel shaft rockers and titanium valves, but all they can afford is a budget flat tappet cam, lifters and valve spring set. Consequently, the camshaft and valvetrain components you choose to use in a customer’s engine may be influenced as much by their cost as their performance potential.
Too Much Cam
The one piece of advice virtually every camshaft manufacturer we contacted for this article offered was this: Don’t overcam the engine. Everybody loves the idle sound of a full race cam with lots of valve overlap and duration. It sounds really bad (in a good way) but it can run really bad, too (in the worst way), if the cam is mismatched for the application.
“Bigger is not always better,” said one of cam suppliers we interviewed. Their advice? Be realistic with your performance expectations and understand that cam selection/design is always a matter of compromising or “give and take.” Before choosing a particular cam, contact several camshaft manufacturers and talk with their specialists about which particular cam they would recommend for the engine you are building.
A lot of variables have to be considered when choosing a cam, and many manufacturers have highly detailed application forms to account for everything from engine displacement, carburetion and compression to type of transmission, gear ratios, differential ratio, tire size and vehicle weight.
All of these things have to be taken into account so the engine will develop peak power in an rpm range that provides the best all-round performance and drivability. A cam that makes peak power from 4,500 to 7,500 rpm would not be the best choice for a typical street application where you want peak power in the 1,500 to 4,500 rpm range.
Most cam suppliers have a broad selection of cams (both flat tappet and roller) that are designed for specific kinds of applications. They’ve done their homework so don’t try to second guess their expertise unless you are working on the cutting edge of developing the latest and greatest racing engine technology.
Many cam suppliers do offer custom cam grinding to whatever specifications you want, but in most cases there is probably an off-the-shelf cam profile that will do exactly what you want it to do without the cost and risk of a custom grind.
Besides looking at the lift and duration numbers when evaluating cam specifications, you also need to consider the lobe separation angle. Less lobe separation makes for a narrower power band and moves the peak torque to a lower rpm. It also reduces intake vacuum while increasing effective compression (thus increasing fuel octane requirements to prevent detonation).
Hydraulic or Solid Lifters?
For street performance and even some types of racing, hydraulic lifters are a usually preferred because they eliminate the need for periodic valve lash adjustments. However, the rev limit for a typical set of stock hydraulic lifters is usually around 6,200 to 6,500 rpm. If you want to rev the engine higher than this, you either need solid lifters or modified performance lifters that can safely handle higher rpms without pumping up or collapsing.
Roller or Flat Tappet Cam?
Roller cams have a couple of advantages over traditional flat tappet camshafts: they reduce friction, and they can be ground with more aggressive cam lobe profiles to make more power. You can also swap roller cams without having to replace the lifters. A roller cam’s main disadvantage compared to flat tappet cams is its higher cost.
A roller cam is no more expensive to manufacture than a flat tappet cam (unless it is being CNC machined out of billet steel), but the roller lifters it works with are more complex and costly to make.
The use of roller cams is also against the rules in certain forms of racing. You can’t run a roller cam in many dirt track and circle track classes, stock eliminator drag racing (older cars), some types of marine racing and truck pulling, or vintage road racing. Consequently, you may have no other choice than a flat tappet cam.
Regardless of what type of cam goes into an engine, you want that camshaft to last. Most cam failures are due to lubrication, installation or break-in issues rather than manufacturing defects. One cam manufacturer said that of all the damaged cams they have inspected over the years, more than 99.99 percent were found to have been manufactured correctly to specifications with no defects.
Soft cam lobes that are not properly heat treated to achieve the correct hardness of 48 to 58 Rc can and do happen occasionally, and a soft lobe can allow rapid wear and premature cam failure. But such failures are the exception provided you are buying your cams from a reputable supplier who has a good handle on quality control.
Roller cams do not require an initial break-in period, but flat tappet cams certainly do. Use the extreme pressure moly paste lubricant that is included with the cam from the manufacturer to lubricate the cam lobes and the bottoms of the lifters. Roller cams only require engine oil to be applied to the lifters and cam. Also, apply the moly paste to the distributor gears on the cam and distributor for all camshafts. The oil should also contain adequate levels of ZDDP anti-wear additive to provide ongoing protection.
Another no-no is to reuse old lifters on a new flat tappet cam. You can reuse old roller lifters provided they are in good condition on a new cam, but never old flat tappet lifters on a new cam.
Most cam manufacturers do not recommend using synthetic oil during the break-in period. They also do not recommend using any type of oil restrictors to the lifter galley, or installing windage trays, baffles or plugging oil return holes in the valley. Oil flow is needed not only for lubrication but also for cooling and drawing heat away from valvetrain components.
Prior to starting the engine, the lubrication system needs to be primed so that the oil pump, filter and all of the oil passages are full of oil. Once this has been done, the engine can be fired up and run from 1,500 and 3,000 rpm, varying engine speed up and down within this range for 15 to 20 minutes. Do NOT let the engine idle or run at a steady rpm during this critical break-in period.
Make sure the lifters and pushrods are rotating as lack of rotation can lead to cam failure. Substituting lower ratio rocker arms and/or lighter valve springs for the cam break-in process can also reduce the risk of premature cam failure. The lighter valve springs can then be replaced with stiffer ones after the break-in process has been successfully completed.
Normal recommended spring seat pressure for most mild street-type flat tappet cams is between 85 to 105 lbs. More radical street and race applications may use valve spring seat pressure between 105 to 130 lbs. For street hydraulic roller cams, seat pressure should range from 105 to 140 lbs. Spring seat pressure for mechanical street roller cams should not exceed 150 lbs. Race roller cams with high lift and extreme spring pressures are NOT recommended for street use. Why? Because the cam may not receive enough splash lubrication at idle and low speed to lubricate and cool the cam lobes and lifters.
One “fix” that’s become popular in recent years to deal with the issue of lobe wear when running higher than normal valve spring pressure is to install lifters with a small oil dribble hole in the bottom. On some the hole is centered in the bottom of the lifter and on others it is slightly off-center. The idea is that oil inside the lifter will dribble out of the hole and help keep the lobe coated with oil.
It’s certainly better than no supplemental lubrication for the cam lobes. If you are installing these type of lifters in an engine, you should disassemble, inspect and clean the inside of the lifters before they go in the engine. The reason for doing so is because the Electrical Discharge Machining (EDM) process that is used to burn the small hole through the bottom of the lifter often leaves residue inside the lifter. Another alternative is to use lifters that have several evenly spaced shallow flats or grooves machined into the side of the lifter to route oil down to the cam lobe.
Be sure to check installed spring heights, spring retainer to valve guide clearances and spring coil binds before you turn the engine over. There should be at least .060˝ of clearance. Also check valve to piston clearances. Minimum recommended clearance are .080˝ for intake valves and .100˝ for exhaust valves. Overlooked mechanical interference problems can bend and break valvetrain components.
Another check would be rocker arm slot-to-stud interference. As you increase valve lift, the rocker arm swings farther on its axis. Therefore the slot in the bottom of the rocker arm may run out of travel, and the end of the slot will contact the stud and stop the movement of the rocker arm. The slot in the rocker arm must be able to travel at least .060˝ more than the full lift of the valve. Some engine families, like small block Chevrolet, have stamped steel rocker arms available in long and extra long slot versions for this purpose.
Camshaft end play is another dimension that needs to be measured. Some engines have a thrust plate to control the forward and backward movement of the cam. The recommended end play on these types of engines is between .003˝ to .008˝. Many factors may cause this end play to be changed. When installing a new cam, timing gears, or thrust plates, be sure to verify end play after the cam bolts are torqued to factory specs. If the end play is excessive, it will cause the cam to move back in the block, causing the side of the lobe to contact an adjacent lifter.
If a camshaft breaks, the cause may have been a connecting rod hitting the cam (insufficient clearance). When this happens, the cam will usually break in more than two-pieces. Sometimes a cam may break in two pieces after a short time of use because of a crack or fracture in the cam due to rough handling during shipping, or some time before installation. I
f a cam becomes cracked or fractured due to rough handling, it will generally not be straight which is why it’s always a good idea to check cam straightness before it goes in the block. Also, check for binding when the cam is installed in the block. It should turn freely with no resistance.