Vizard's View: Avoiding Flat Tappet Cam And Lifter Failure
Even if the cam company warranties the cam and lifters, a lobe/lifter wipe out is an expensive burden for the engine builder.
By David Vizard
There is nothing worse than building a cost effective engine for a customer, shipping it, and then having it come back a short while later with the installed flat tappet cam wiped out. Even if the cam and valve train was warranted by the cam manufacture, you, as the engine builder, will bear the brunt of the rebuild cost.
In many cases, it's a little more than just replacing the offending parts as hard iron debris will have gone a round of the engine and basically set the scene for further failure down the road. My intent here is to show how to minimize the flat tappet cam failure primarily seen with domestic V8s.
Numerous reasons exist for cam/lifter failures and, not surprisingly, numerous steps to prevent such failures. Basically, the preventative measures fall into two groups: those done while the engine is being built and those done at or after customer hand-over.
Just to simplify things I am going to discuss most of what is covered in terms of hydraulic cams. However, most everything also applies to solid cams, especially those high acceleration ones used for limited lift race classes. Let's make a start at the potential source of the problem - the cam profile itself.
Selecting the Profile
Flat tappet cams are great at opening valves quickly. Contrary to popular belief they can out-accelerate a roller by a hefty margin if the cam designer so chooses it to. There are safe profiles, and there are power orientated profiles, and just about everything between. The key to knowing which profile you may be selecting is determined by its "hydraulic intensity." This term, coined by Harvey Crane, describes the rate of lifter rise imparted by the profile. This number, for a hydraulic cam, is found by subtracting the 50 thousandths duration figure from the 6 thousandths duration figure.
Most factory cams have a hydraulic intensity in the region of 70°. Originally such large, and consequently low intensity, figures were the result of mild profiles brought about, in part, by cam cores having far less surface load bearing capabilities than current cam cores. Be aware that some cheap cams are ground on cores, which cannot support high surface stresses. If you feel the need to use such cams understand that you will need to apply extra diligence to the selection of other parts in the valve train and to the care of the valve train during subsequent assembly. My advice here is buy cams from a company that uses top grade cores.
If nothing else leads to its downfall, a profile ground on a good core can have a hydraulic intensity of 50° - 55° and be a pretty safe bet in terms of reliability. There are, however, many flat tappet cam profiles to be had, which offer substantial increases in performance by pushing the hydraulic intensity boundaries. All the big cam companies such as Crane, Crower, Comp, Lunati and Isky have them and, to stay competitive, they keep pushing the envelope. Lunati's new Voodoo range is a prime example here and recent tests in a big block Chevy showed they certainly delivered as promised.
The problem of wiping out flat tappet cams could be prevented by going to a roller design. Two problems here: one is that a roller calls for more money; and two, unless the roller has a seat duration longer than about 270° to 275°, the flat tappet cam is likely to out-perform it.
In case you doubt the validity of this just check out the hydraulic intensity and lift figures of Comp Cams Xtreme Energy range of rollers and flat tappet cams. Although a crude comparison, the numbers indicate that until about the 270° mark is exceeded, a flat tappet cam is likely to give more opening area under the lift curve. If you want to impress a customer with the power you can build into a relatively low cost unit the higher intensity cams from any reputable company are the type to use. Now let's see how to make them safe.
Taper and Crown
In reality, there is no such thing as a flat tappet cam and lifter. The lifter has a crown on it, typically between .050? and .100? radius. This runs on a cam profile, which is tapered across its form. The cam profile itself does not run centered on the lifter but is offset to one side. The combination of this offset and the cams taper and the lifter crowning causes the lifter to rotate. At the end of the day it is the lifter rotation (which considerably reduces the rubbing speed) that saves the situation from a sure disaster.
To make sure that the system works as it should first measure each lobe on the cam you intend to install and check that it has at least one thousandth taper across the lobe (1.5 to 2.5 are typical). For a Chevy the largest dimension should be toward the back of the cam. On any others the largest dimension should be on the side of the profile that runs toward the outside of the lifter diameter.
Next check the lifters. It's unlikely you'll find one wrong, because quality control on these items is very high. But you can make a quick check by just putting the face of two lifters together and holding the pair up to the light. This will quickly establish that the crown exists.
The type of lifter you choose can also be instrumental in extending the life of the valve train. More expensive hard face lifters as supplied by most cam companies are well worth it, especially if you're building a big block Chevy, which is more prone to lobe and lifter failure. And be aware that many cam companies offer a cam hardening service, which is also well worthwhile.
Added Lifter Lubing
Part of the problem of inadequate lifter/cam interface lubrication can be offset by simply using more oil at the offending sight. With a solid lifter this can be done by having a lifter with a small hole in its face, thus connecting the well of the lifter to the face.
For any V8 flat tappet cam the best way to provide additional oiling is to groove the lifter bores. This involves cutting a groove in the lifter bore that, at the upper end, connects to the longitudinal oil passage. The lower end terminates at the bottom of the lifter bore and directs a small stream of oil directly onto the approaching side of the cam. Comp Cams has a tool that does this job and at about 15 seconds a bore, it is quick and easy to use. Most importantly though, it is effective, especially on big block Chevys.
After checking cam profile taper and lifter crowning it's time to get down to assembly. Before actually installing the cam check that every lifter rotates freely in its respective bore. Without freedom of rotation the lifters will be in for a short life. Once lifter rotation is established it's time to lube up the cam profiles and lifter faces with a break-in lube. Usually there is a packet of break-in lube included with the cam but for what it's worth the difference between a good break-in lube and a top-of-the-line break-in lube is quite substantial.
About 15 years ago, I did a test using a Pinto engine as a guinea pig. What was done here was to install a cam and followers and instantly fire up the engine and turn it to 6,000 rpm with no break-in whatsoever. This pattern of events was used to test various break-in lubes.
Moly grease-based break-in lubes proved to be at least twice as effective as even the best unaided oils. At the time (remember this was 15 years ago) the clear winner for the best break-in lube was from Crane. This was several orders of magnitude better than the results achieved with moly grease.
A point to note here is that break-in lubes are just that. Some of these break-in lubes can, if used for too long a period, cause the lifter face to develop a crazed pattern.
Back in the May 2003 issue of Engine Builder I did exposé on the significant advantages of beehive valve springs. What was said then applies even more so now.
But let's look at valve springs at a broader context. The bottom-line is the lesser the spring forces used the more likely the cam and a lifter surfaces are likely to survive. For just a run-of-the-mill build almost any spring that is, at most, a little stronger than stock will do. However this doesn't mean that you can totally ignore the golden valve train rule. Namely, the best spring is one that delivers the desired seat and nose force but with the least mass possible.
For anything short of an F1-style air-spring the current king, by a big margin, is the beehive spring. The reason it works so well is that for a given delivered force it has so much less mass than a conventional spring. When used with hydraulic cams they are worth horsepower. With a flat tappet hydraulic cam, you can figure somewhere in the region of four to six horsepower more from a typical 350. Where they really shine is on rollers, and, although that's not really our subject here, you just might want to know that we've seen as much as 20 hp increase by using beehive springs in such applications.
If you are building a solid lifter race engine to a lift rule, then by its very nature, such a valve train needs a spring that can deal with the significantly higher accelerations involved. Such cams inevitably have a limited life and, unless the appropriate steps are taken, the life may be less than the break-in time. This means the selection and checking of cams and lifters is very important, as is the break-in lube involved. Equally important is the spring chosen for the job. This can make or break the installation with many of these cams.
The break-in is also critical. If a dual spring of any significantly high poundage is used, the break-in should be done with a spring that is significantly softer (i.e., say, just the outer of the pair) than what is finally used. Also the use of a lower ratio rocker, such as Comp's 1.4 ratio items, can help procure an effective break-in.
Ex-aerospace engineer David Vizard is one of the world's most widely published automotive writers. He is also Director of Applied Performance Sciences at UNC Charlotte, holds numerous patents and is a winning engine/car builder.