Flat Tappet vs Roller - BS
I kinda get tired of reading all the negative viewpoints against the "traditional" flat tappet cam that has been, and still are, used in many engine builds. I really get insulted when the flat tappet cam choice is associated with my age and the generation that I grew up in - as if roller cams are a recent phenomenon of the millennium and later generation. Duh?
Roller cam followers (also called lifters, valve tappets, lash adjusters) have been around since the late 1800's. They were solid wheels that followed the cam lobe required regular adjustments. Early tappets had rollers to reduce wear from the rotating camshaft, but it was found that the roller pivots wore even faster and that the small radius of the rollers also tended to accelerate wear on the expensive camshaft. Tappets then developed plain flat ends, although these were slightly radiused as 'mushroom' tappets, since a perfectly flat end led to 'slamming' against a steep camshaft face and to reduce wear from the rotating camshaft, the tappets were usually circular and allowed, or even encouraged, to rotate.
In 1910, a French car builder near Le Mans, France named Amedee Bollee invented the first self-adjusting valve tappets. Bollee’s two-piece tappets consisted of an upper and lower piston held slightly apart by a small spring. A port in the side of the lifter bore allowed oil to enter the cavity between the two pistons. Development of the self-adjusting lifter continued and in the 1930s, General Motors developed its own “zero lash” tappets first used on the Cadillac V16 engine. By the 1950s, hydraulic lifters were common in most engines. But roller type lifters were still in use, they didn't fade away. Aircraft engines used them and race car engines used them, becoming popular with the Super Stock racing era.
Price - Cam & Lifters - Approximately $260.00 - No need to change the distributor gear, purchase new pushrods, or possibly install an aftermarket valley pan. New valve springs, valve retainers, and valve locks are optional on a flat tappet cam depending on cam lift - not on a roller. You don't have to use a double roller timing gear set-up, but you do on a roller cam.
With any flat tappet cam, hydraulic or solid, you will need to break your cam in upon initial start-up. Flat tappet cams are more sensitive to stiff springs than roller cams. This typically means if you have performance heads that use and take advantage of heavier spring pressures provided by dual valve springs, the inner springs MUST be removed before the cam break-in process can begin, and then they have to get re-installed afterwards, which means the whole valve train needs to be taken apart and put back together. Again, this is only on performance engines that use dual valve springs, and most performance engines WILL have dual valve springs. “The biggest mistake is not appreciating the fact that a flat-tappet does need a break-in,” warns Steve Slavnik of Lunati Cams. “Light springs used to be the standard in the industry for break-in. If you’re not an engine builder and don’t have the facilities, it’s a pain to change the springs.”
So the typical car hobbiest may look at the cost savings of running a flat tappet cam and think it's the way to go, BUT, if you can't do this operation yourself, you may have to pay your machine shop or another auto repair shop to have your inner springs removed, then break the cam in, and then remove the springs again to re-install the inner springs. This is going to cost a little additional money for this operation, which might have been offset by going with a roller cam that does not require the extra work, but many shops have an engine break-in stand
Date: 2013 Tech Tip courtesy of Crane Cams - Reasons and Causes for Camshaft Failure
Of all the damaged cams that Crane Cams has checked over the years, it says more than 99.99 percent have been manufactured correctly. Some people have the misconception that it is common for a cast iron flat tappet cam to occasionally have a soft lobe. Crane says they have yet to see a cast iron flat tappet cam that had a soft lobe.
When the cast core is made at the casting foundry, all the lobes are flame hardened. That process hardens all the lobes to a depth below the barrel of the core. That depth of hardness allows the finish cam grinder to finish grind the cam lobes with a Rockwell hardness above 50Rc. The generally accepted hardness on a finished cast cam should be between 48Rc to 58Rc.
Lobe wear, incorrect break-in lubricant. Use only the moly paste that is included with the cam from the manufacturer. For extra protection, an anti-wear additive should be added, such as Crane Super Lube.
Do not use synthetic oil during the break-in period. It is not recommended to use any type of oil restrictors to the lifter galley, or use windage trays, baffles, or plug any oil return holes in the valley. Oil has a two-fold purpose, not only to lubricate, but also to draw the heat away from whatever it comes in contact with. The cam needs oil splash from the crankcase and oil run-back from the top of the engine to help draw the heat away. Without this oil flow, all the heat generated at the cam is transferred to the lifter, which can contribute to its early demise.
Correct break-in procedure. After the correct break-in lubricant is applied to the cam and lifters, fill the crankcase with fresh non-synthetic oil. Prime the oil system with a priming tool and an electric drill so that all oil passages and the oil filter are full of oil. Preset the ignition timing and prime the fuel system. Fill the cooling system. Start the engine. The engine should start quickly and run between 1,500 and 3,000 rpm.
If the engine will not start, don’t continue to crank for long periods, as that is very detrimental to the life of the cam. Check for the cause and correct. The engine should quickly start and be run between 1,500 to 3,000 rpm. Vary the rpm up and down in this rpm range during the first 15 to 20 minutes, (do not run the engine at a steady rpm). During this break-in time, verify that the pushrods are rotating, as this will show that the lifters are also rotating. Increased spring pressure and/or increased valve lift through the use of higher ratio rocker arms can hinder lifter rotation during cam break-in. If the lifters don’t rotate, the cam lobe and lifter will fail. Sometimes you may need to help spin the pushrod to start the rotation process during this break-in procedure.
Spring pressure. 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. . Mechanical and hydraulic flat tappet cams and their lifters gain performance and durability with lower spring pressures. They also favor lighter valvetrain components, stiffer pushrods to minimize deflection, and stable rocker assemblies that promote smooth operation.
The use of stiffer, lighter valvetrain components cannot be overstated. Valvetrain designers pay close attention to the moment of inertia required to activate the rocker arm and the component stiffness necessary to transfer cam motion accurately. The top half of the spring travels a much greater distance than the bottom half and that’s where control problems and spring surge initiate. A lighter retainer minimizes the force the spring has to control allowing it to function more effectively. Mismatching of parts by an individual or an engine builder can set the cam & lifters up for failure.
Spring coil bind: It is recommended that the spring you are using be capable of traveling at least .060″ more than the valve lift of the cam from its assembled height.
Retainer to seal/ valve guide boss interference. You need at least .060″ clearance between the bottom of the retainer and the seal or the top of the valve guide when the valve is at full lift.
Valve to piston interference: Minimum recommended clearance: .080″ intake and .100″ exhaust.
Rocker arm slot-to-stud interference: The slot in the bottom of the rocker arm must be able to travel at least .060″ more than the full lift of the valve.
Oil Viscosity. Another critical consideration that few consider is oil viscosity. Heavier oils tend to hinder hydraulic lifter function. The oil must be able to hold pressure within the tight internal lifter clearances and navigate the passage in the refill valve effectively to maintain control of the internal piston. If the oil viscosity is too heavy, the bleed down rate of the lifter may be too slow and the lifters may retain too much oil - they can pump up at higher RPM's and over extend the valves. You can end up with valve float, misfiring, loss of power, or even engine damage if the piston strikes the valve. Each manufacturer uses different tolerances between the lifter body and plunger, so it is best to consult the manufacturer for oil viscosity recommendations. They may state: "Do not use oil heavier than 10W30." Do not fill or pump up hydraulic lifters before assembly. They may hold the valves open causing difficult starting and/or engine damage.
Distributor gear wear. The main cause for distributor gear wear is the use of high volume or high-pressure oil pumps. We don’t recommend the use of these types of oil pumps. If you do run these types of oil pumps, you can expect short life of the cam and distributor gears, especially for low speed running, in street type applications. If you must run these types of oil pumps, you can increase the life of the gears by adding more oil flow over the gear area to help cool off the point of contact. Distributors that have end play adjustment (up and down movement of distributor shaft and gear) should maintain a maximum of .010″ end play to help prevent premature wear.
Camshaft end play. Pontiac engines use the thrust plate and cam gear to control the forward and backward movement of the cam. The cam is held forward against the thrust plate if you are using the OEM set-up.
Broken cam. A broken cam is usually caused by the cam being hit by a connecting rod, or other rotating parts of the engine coming loose and hitting the cam.
Another possible cause for cam and/or lifter failure are changes in engine dimensions. If you mill your heads or deck the block .010" or more (and sometimes you may not know how much has been milled by a previous owner if the engine has been rebuilt), you may want to check engine components just to make sure there are no manifold-to-port/bolt alignment problems, piston/valve clearance issues, or changes in valve train geometry. Add to this thinner head gaskets that some of us use, and the prementioned considerations can be compounded. If the head is brought closer to the cam by either milling it, decking the block, or using thinner head gaskets, you may need to shorten up your pushrod length. Having a pushrod too long can possibly cause coil binding, hold a valve open, or force the pushrod deeper into a hydraulic lifter causing damage to the lifter or putting excessive pressure on the lifter face-to-cam contact which you do not want on cam break-in. Verify pushrod length with a pushrod checking tool to compensate for any changes in engine dimensions other than stock and get your rocker arm geometry back in line.
Price: Cam & Lifter - Approximately $800.00 or more with all needed parts.
Although the roller tappets are intended to reduce friction, the rollers themselves place higher loads on the lobes since the actual contact patch between the two is smaller than that of a flat tappet setup. This load is then intensified by the fast opening rates, higher valve spring pressures, and extra tappet weight. They can also experience valve float in higher rpm applications such as 6,000 rpms or more.
Hyd. Roller lifters used in a Pontiac engine may require an aftermarket valley pan.
Roller tappets are usually taller than their flat tappet counterparts, and this extra height is accommodated by the pushrods, which need to be made shorter to compensate. So you will need to purchase pushrods.
Installing a roller cam/lifters in a non-roller designed factory engine requires that each pair of lifters be linked with some form of tie-bar so they do not turn sideways on the cam lobe.
The higher loads associated with roller cams also mean that higher-grade timing sets must be employed with these systems. Usually, this means a double-roller-type chain and sprockets.
Racing roller valvetrains typically go through springs and retainers faster than comparable flat tappet setups. Despite this, many racers consider the increased cost of roller valvetrains a small price to pay for the increased performance they provide.
A common issue with mechanical roller lifters is the impact loading on the tiny roller bearings during each valve cycle. One or more of the small rollers takes a pounding every cycle and over time they take quite a beating. This is caused by valve lash clearance that slams the rollers during each cycle. The problem plagues some race engines with excessive spring pressure too, but race engines have far fewer cycles than a hot street engine. Mechanical roller lifters on the street often suffer greater distress than race engines because racing lifters are more frequently examined and replaced if necessary.
The harder steel material of the cam itself means the distributor gear must also be made from a material compatible with the steel of the camshaft. Bronze is often the material of choice. A bronze distributor gear, which is very soft and will bear the brunt of the wear between the cam and the distributor gears are relatively cheap and easy to replace, so this compromise has been accepted for some time.
Roller cams with high lift and spring pressure are generally not recommended for street use, because of a lack of oil splash onto the cam at low speed running to help cool the cam and lubricate the lifters. This high spring pressure causes the heat created at the cam to be transferred to the roller wheel, resulting in its early failure.
Any springs that may be used must be assembled to the manufacturer’s recommended height. Never install springs without verifying the correct assembled height and pressures.
Photo 1 shows the relationship of the lifter base-to-cam. Notice how it is not flat and the cam has a built in taper so as to spin the lifter.
Photo 2 shows a couple of cam shapes. The 2 on the left are flat tappet with one having a straight ramp for faster rate of lift while the other has a more rounded ramp. The faster ramp can sometimes be the cause of valve bounce if you don't have a stiff enough spring rate to keep the lifter on the face of the cam or hold the valves closed when they slam shut. Note the shape of the roller lobe. Practically straight up, longer period at maximum lift, then straight down - Bam! The straight up ramp puts extra side loading on the thrust side of the lifter bores and can bust them if you go with a super high lift needing heavy spring pressures. This is the purpose of the Megabrace that can be installed in the Pontiac lifter galley.
Photo 3 clearly shows the side thrust difference between a flat tappet cam lifter and roller cam lifter.
Photo 4 is the Pontiac roller lifter set. Long, big, & heavy which means more weight pounding up and down and you better get matching valve springs, valve retainers, & valve locks. If the tie-bar ever let's go, you could take out an engine as the roller lifters go sideways in their bores. Not an issue ever with a flat tappet cam.