With so many of our discussions lately revolving around the valvetrain, we thought you might like to finally learn how to get the best performance from yours. High-performance valvetrains can take years to perfect, and even longer to prove reliable. And like everything else related to performance, ingenious valvetrain components don't just happen overnight. First, some engineer must put an idea down on paper, and then some machinist builds it, and then a racer runs it in his engine until it either breaks or he wins races with it. That's the simple, yet expensive way of doing things. But, since most of us don't have an engineer or machinist in our back pocket, we have to rely on what someone else has already learned to make sure our stuff doesn't break. And if we can win a few races in the process, all the better.
The canted, or "splayed valve" head concept is not new, but its application to small-block Chevys is. Notice in this side view how both valves are tilted 4 degrees in toward the center to allow clearance for a larger valve in a small cylinder bore.
IT STARTS WITH THE CAM
There is actually a lot more to camshafts then we could ever explain in these few pages, so that's why we're going to mostly concentrate on solid roller cams this time. First of all, keep in mind that the cam's lobes are designed to do only one thing: push the lifter up smoothly to open the valve, allowing the lifter to follow the lobe back down to close the valve without bouncing it off the seat. It's much more difficult than you might think. And the good-old flat-tappet cams of the past were never the most efficient ways to move a lifter, either. But, they were cheap, and that kept them popular in all circles--until now. Flat tappets are becoming increasingly harder and harder to find, and their prices keep going up. There are a lot of reasons for this--not the least of which is that flat-tappet lifters can cost you horsepower. Lifter friction, shape, and diameter can all limit the opening rate a cam designer can grind on his lobes for a flat-tappet cam. The larger the lifter diameter, the faster it can rise up the lobe, so consequently, imaginative racers have been putting bigger lifters in their engines to "cheat" the system for years. Then came roller cams. The OEMs haven't put a flat-tappet cam in their engines for 20 years, so why should you? If it were still an affordable way to do things, the cost-conscious OEMs would've stuck with flat tappets forever. So where did the big push for roller lobes come from? That's easy to answer: it was all about reducing friction. Because friction costs power, and since they're always looking for the best, most reliable, and cheapest ways to make power, the OEMs choose to reduce friction inside the engine. But notice that they've stuck with hydraulic roller cams, and it's been the aftermarket that's had to lead the way in solid-roller cam development.
This is the COMP Cams Endure-X roller lifter designed specifically for street and marine use. The groove directs pressurized oil to the lobes and roller bearings to keep it alive at low rpm. COMP has also developed an affordable tool that anyone can use to cut an oiling groove in each lifter bore to accomplish a similar effect with any lifters.
So what makes today's solid-roller cams better than yesterday's? Nolan Jamora, of Isky Cams comments, "Roller cams are really not much better today; it's the other valvetrain parts that have made the difference. The new technology in springs and roller lifters is the key. You have rollers like our Red Zone Lifters that oil right to the roller bearing to help their longevity, and also, the new springs designed for more endurance and longer life. So you no longer have the roller lifters bouncing on the cam when its been run for a long time on the freeway because the springs do a much better job or keeping lifters following the profile of the cam, which gives you better performance and longevity."
Sure, there've been new lobes designed, but the V-8 engines we use haven't changed that much, and they still respond well to some of the oldest roller grinds. That's not to say that a better cam hasn't be ground for parts like the latest sets of aluminum heads that didn't exist 30 years ago. But, there has definitely been a movement toward slightly less-aggressive solid-roller lobe designs to help them live on the street. "A true mechanical street-roller camshaft has relatively gentle ramps that allow for decent street use with reasonable valvespring pressures. A race-oriented roller cam has considerably higher acceleration rates, both positive and negative. They also have considerably higher valvespring load requirements to keep the valvetrain together. That's fine for a racer, but they won't give the sort of valvetrain life that a street-driven engine needs. The less-severe ramp designs that are used on street-roller cams also have less side loading of the tappet, which helps," Mike Golding of Erson Cams noted. Also helping to that extent have been advances in metallurgy, which has led to the development of much better valvesprings. These springs can make a roller-cammed engine live indefinitely on the street. The old problem was that at low rpms (read: street idle), there's very little oil splashing onto the cam and around the top of the engine. That lack of oil means there's little-to-no lubrication and cooling effects on the cam, lifters, and springs. And since all of these parts tie together, if one's not getting enough oil, they will all suffer. "Just make sure to realize that you need to keep the valvetrain wet. Restricting the oil supply to the top end may work in a drag motor, but street motors need oil. From the roller tappets to the rocker arms, and even the valvesprings need plenty of oil for lubrication and cooling." Golding went on to explain when we asked him what was the most important aspect of running roller cams on the street, "In the old days, the really aggressive roller lobes required stiff springs to control valve motion. These high-pressure springs created a lot of load on the lifters and cam. And like we said before, there's very little oil on those parts for lubrication and cooling. So consequently, roller lifters would often fail in street engines due lack of oil at idle. And when a roller lifter goes away, it's not a pretty sight."
Crower cut one of its HIPPO (Hi Pressure Pin Oiler) lifters for us to show the channel (arrow) that directs a constant stream of pressurized oil right to the lifters roller bearings.
HIGH-TECH LIFTERS AND SPRINGS
One new approach to solving the exploding solid-roller lifter dilemma is from Schubeck Racing. To solve the problem, Schubeck, has developed a new roller lifter that does away with the roller axle and needle bearings completely. Roller-X Lifters are unique because their roller wheels float on oil in their bearing "cage" as they push against the cam. It's the same technology that's kept main and rod bearings from rubbing cranks and burning into oblivion since the internal combustion engine was practically invented. Sometimes, something old can be something new again.
To further combat roller lifter problems on the street today, newer springs have been designed to run with much lower pressures, yet can still control the valves. That leads to less loading on the lifters and translates to longer life at idle. Also, most major cam companies have further addressed the low-speed oiling problems by adding a pressurized oiling groove in their roller lifters that directs oil to the cam and roller lifter bearings. COMP Cams has even developed a tool that allows anyone to cut a tiny oil pressure groove in the lifter bores of their block to accomplish the same effect. Either way gets more oil to the cam and lifters so they'll live.
These Jesel dog-bone roller lifters feature an aluminum body with a coated steel sleeve for lightweight. The steel dog bone gets bolted to the block to guide and retain them. These lifters are not only the high end of rollers, but are also about the lightest you can get.
VALVE LASH CONCERNS
Back in the heyday of musclecar performance, if you wanted the most powerful engine on the dealer's lot you had to get it with a solid-tappet cam. That meant valve lash adjustments would be in order, which, at the time, no one seemed to mind. Today, no one wants to adjust their valves anymore, and we can't blame them, either. It's a hot, messy, tiresome, back-breaking process. Typically, valves will go out of adjustment for two reasons: either something's wearing in the valvetrain and is about to break, which creates more lash, or thermal expansion increases the lash when the engine's hot, which causes someone to think their valves are out of adjustment. This is normally the case. The truth is, if you've properly locked down the rocker arm's adjusting nut, they shouldn't move--ever. And as long as you're checking your valve lash with the engine at a consistent temperature every time, they should all be within spec. Remember, aluminum expands greater than steel. And with a lot of today's engines running aluminum heads, they're going to expand enough to add lash at the valves. It's ideal to set your lash cold, subtracting .002- to .004-inch for expansion every time you do it. Besides, lash is a great tuning tool, and you may find some power by moving lash around, so don't be discouraged to try a hot-lash adjustment, just remember that it won't read the same the following morning.
If you're getting serious about cams, then it's also time to get serious about installing them correctly. This Powerhouse degree wheel kit contains all you'll need to degree any cam.
VALVE MOTION DEFINED
All cam lobes incorporate something that's called "clearance ramps," aka "lash ramps." The clearance ramp is an area following the base circle on the lobe that allows the lifter to begin its rise smoothly, without being tossed right off the lobe like Evil Knievel jumping over some old school busses. On solid lifter cams--both roller and flat--the clearance ramp also keeps the lifter from literally smashing into the cam as it takes up the lash and begins to lift.The process goes like this: Picture the lifter on the base circle of the cam just before starting to rise up the lobe and open the valve. The lifter must first overcome tremendous initial valvespring pressures, the weight of the valve, and the mass and friction of the rocker arm, which have all been sitting completely still until this instant. Rocker-arm ratio also plays a big part here, and the higher the ratio, the harder it is to get things moving. The clearance ramp is designed to allow a smooth transition from the base circle to the lobe, and without it, solid-tappet cams would die a quick death. Interestingly, on hydraulic-tappet cams, the clearance ramp is considerably less, due to the fact the there's no lash to take up. That also means that hydraulic lobes will actually move the valve quicker off the seat, but they must slow the lift curve down much sooner to prevent valve float (see: "Running Solid Tappets on Hydraulic Lobes"). The exhaust lobe takes the worst beating on its clearance ramp since cylinder pressure is extremely high when the exhaust valve starts to move, and there's no way to bleed pressure off except to open the valve which is why most engines will bend exhaust pushrods first.All of this leads to the best way to build power in any engine--by using the lightest, stiffest valvetrain parts you can afford and then topping them off with the correct spring.
This is a great comparison showing how two cams with the same lift and duration can perform totally different, thanks to the area under the curve. Cams with more aggressive ramp profiles feature more area here and will open the valve quicker and close it later. Solid cams can have more area under the curve than hydraulics and roller cams can have even more area still due to its lifter design.
Ironically, the weight you reduce in some valvetrain parts can be detrimental to durability, specifically in the area of pushrods, where the common trend has been to run as light a pushrod as possible in hopes of accelerating the lifter more easily. With today's ever-increasing spring technology it's been determined that this is now a bad thing. As we mentioned before, the pushrod has to overcome tremendous pressures, and it's been proven that using a thick, stiff pushrod is a better way to make power. Besides, the Spintron has shown that the only weight reduction that's super critical to making power is on the valve side. Titanium components are the way to go here, and today you can get every single part on the valve's side of the rocker arm made from Ti. All of them will make more power for you, but they're expensive. Professional racers will use complete Ti valvetrains because they can afford it and, ironically, the parts may actually last longer due to the reduction in overall component weight compared to steel. Anywhere you can reduce weight at the valve will generally lengthen the interval between component replacements, especially where valvesprings are concerned.
The valves are a restriction in any engine. Larger-diameter valves flow better, but the bore diameter limits how big the valves can be. According to Reher-Morrison, the intake valve should be about 52 percent of bore diameter and the exhaust valve should be 78-84 percent of the intake valve diameter in performance engines.
OVERLAP IS WHERE IT'S AT
Valve overlap is hardly ever mentioned, but is just as critical to making power as lift and duration are. Overlap is simply the amount of degrees that both the intake and exhaust valves are open at the same time. All engines and cams have some overlap and certain engines will respond differently to more or less of it. An engine with an efficient intake tract will not need as much overlap to work well, while a restricted intake tract is stingy with its air, and an engine equipped that way would suffer from a cam with too much overlap. When you boil the valvetrain events down to their most basic levels, it's the closing of the intake valve that has a huge effect on power. If you close it too late you'll lose power because as the piston moves up the bore it'll force the incoming fresh intake charge right back out due to an effect known as reversion. If you close it too soon you'll choke off the engine from all that fresh air that is rushing into it. In a perfect engine we would close the intake valve precisely at the same time the air/fuel column stops moving in. Determining that point is difficult and the entire intake tract-from the air cleaner to the cylinder head and even the exhaust system-all have an effect on when the air stops moving in. Engine vacuum is also directly related to overlap, and our street engines need vacuum. Typically, with street and mild performance cams, the lobe separation will directly affect overlap. A wider separation figure (i.e. 114-116) means less overlap, while a narrower separation provides just the opposite. But giant race cams can have a wide lobe separation angle like 114 and still have a lot of overlap because the intake valve begins to open very early--sometimes way before the exhaust valve has closed on the exhaust stroke. That's what makes a hard-core race motor sound the way it does. You're hearing the intake and exhaust tracts working together to give it that raspy, wicked tone, especially at idle. See our story on cam lobe separations in this issue ("Separation of Power," pg. 56) to find out a bit more on how overlap can affect power.
Cam companies are working hard to produce more and more streetable solid-roller cam combinations because the tappet manufacturers are slowly reducing production of flat tappets to the point of possible extinction in another decade. By then, it'll be almost impossible to find a flat-tappet cam. In fact, the only real reason flat-tappet cams have lasted as long as they have is because of cost. They're much cheaper to run than any roller cam. But, just as the costs of our cars has increased by a factor of at least 10 in the last 30 years, so will the cost of building engines. Luckily, the aftermarket knows the value of affordable parts and is constantly looking at new sources for roller cam blanks and roller tappets that will be cheaper and more reliable on the street.
Most racing valve jobs consist of multiple angles. The 30-degree cut on the intake valve smoothes the transition on the seat, while the 45-degree cut creates the seal. A sharp corner on the intake margin helps reduce intake flow reversion, while a smooth radius on the exhaust margin helps flow around the valve on its way out of the cylinder. | | |