Pushrod engines have been dubbed as antiquated technology by some, but for many engine builders these engines are far from suffering the fate of the horse and buggy. In fact, with both GM and Chrysler keeping the pushrod engine alive in their current product lineups with the Gen IV, Hemi and others, pushrods and lifters are still very important words in the engine builder’s vocabulary. Overhead cam (OHC) engines may be the current darlings of the OEMs but overhead valve (OHV) engines are what keep the lights on in the rebuilding industry.
For many engine builders the pushrod engine is the bread and butter of their business. The technology has been around a long time and while it has evolved, the basic design has stayed the same. But is this technology going to continue much further on into the future? Most of the pushrod and lifter suppliers we interviewed agreed that pushrod engines are going to be around in one form or another for quite some time.
“People keep predicting the demise of the pushrod engine but we see it hanging around for quite a while,” says Bill Skok of Elgin Industries. “We keep saying we’re going to be the last ones making buggy whips! We still see more life in it, and especially with Chrysler coming out with the Hemi.
Sure, there are more sophisticated engines now, but a lot of rebuilders won’t be able to rebuild the newer OHC engines but a pushrod engine they can. It’s still more economical to build pushrod engines.”
In many cases, OHV engines, where the camshaft is below the piston and pushrods actuate lifters or “tappets” above the cylinder head to operate the valves, are less expensive to manufacture than an OHC engine. They also offer some benefits to automotive engineers that the OHC engine doesn’t.
Pushrod engines are more compact, thereby allowing OEMs to use more sleek hood designs and lower aerodynamic drag. Pushrod engines also develop peak horsepower at lower rpms and produce better low-end torque than most OHC engines.
There are downsides, however, including the fact that the mass of the valve train creates inertia in pushrod engines causing them to be susceptible to valve train separation. But the pushrod engine has been able to overcome many of these obstacles thanks to improved pushrod and lifter designs.
Pushrods are considered by many to be the weakest link in the valve train. They transfer and redirect the upward motion of the lifters, which move in one direction, to the rocker arms, which move in another direction. Pushrods are also prone to “deflect” as engine rpm increases and can result in valve train separation. While mild steel stock pushrods are generally sufficient for stock replacement applications, performance applications require significantly stronger pushrods to withstand heavier valve spring loads and aggressive cam profiles.
“Everything is a bit different now,” says Elgin’s Skok. “We have made some pushrod design changes over the years and we do quite a few swedged rods where we take a larger diameter tube and swedge down to a smaller end, giving it more column strength. We also use different heat treatments, putting different skin heat treatment on the rods. We use different materials from regular steel balls to tool-steel balls for better wear characteristics. So there are many things happening as these engines evolve and the pushrods have sort of evolved with it. They really are a link in the valve train. Many people refer to it as the ‘weakest link’ because if anything breaks they’d prefer it to be the pushrod.”
According to Engine Pro’s Ron McKey, pushrods have to be stiff, strong and the same size as the other pushrods. “The trend is to go with stiffer pushrods to compensate for higher valve spring pressures that are coming along with the bigger camshafts,” says McKey. “You’re better off having a stiff pushrod that doesn’t deflect than a light one that does. The stiffness of the pushrod is a very critical issue. Weight is still a factor but you have a lot more problems if the pushrod is flopping around than if it weighs a little more. When the pushrod bends it deflects and then you get separation in the valve train (valve float).”
Experts say that while valve train weight is important, it’s not equally important: the pushrod side is not as critical as the valve side. Looking at the rocker arm as a fulcrum, the retainer, keeper and valve combination, because it is controlled by the spring, is much more critical than anything on the other side of the rocker arm.
Another critical factor with pushrods is determining the correct length. This has to be one of the most common questions asked of suppliers. Because tolerances have also decreased significantly in late model pushrod engines leaving less margin for error with regards to pushrod length, say experts. If you have changed anything in the engine you will want to check that your pushrod length is correct for your combination.
“Engine tolerances are about 70 percent tighter than what they were even 10 years ago,” says Elgin’s Skok. “The rebuilder really needs to look at the stacking of tolerances when he’s rebuilding an engine. We offer special length pushrods for these engines to help the rebuilder take up the tolerance. We’ve worked very closely with quite a few rebuilders in this area, so we know tighter tolerances are a big issue with all late model pushrod engines.”
However you set up your process it must have consistent results for the correct pushrod lengths. For a lot of shops that want to reclaim pushrods, this creates a problem because it’s a link to the valve train. According to our experts, you have to set up your process so you can order a pushrod in one of the custom lengths. And then you also have to realize that when you’re machining and maybe taking off a little less or a little more there has to be a process to compensate for the irregularities.
Skok says pushrod length is critical for both small shops and larger production engine remanufacturers and consistency is the key. “Even if you need to have a custom rod made …you dial in your 3.4Ls, for example, and you find you need minus .030″ or maybe plus .060″ or plus .043″, and you figure that’s the way you’re going to set your process up as a production engine rebuilder. You need a pushrod that will hold the same tight tolerances as the OEMs. Aftermarket guys need the same thing, so you need to set up your processes the same way. And it’s very difficult to hold those tolerances, but it’s going to be very critical in going forward for the industry when shops start doing more and more of these late model pushrod engines.”
Pushrod suppliers offer a wide range of custom length pushrods that are often carried in stock. “We have two different diameter rods in our ‘shelf’ pushrods: 5/16″ and 3/8″,” says Engine Pro’s McKey. “And then we keep a pretty big range of lengths in stock. From 6″-11″, we make a pushrod every .050″. So that’s about a 5″ range of pushrods measured in .050″. They are all made of 4130 alloy steel with .083″ walls and carbon nitrided to 60-62 HRc, so they’re a hard pushrod, too.”
Clevite Engine Parts’ Gary Wertzbar says your selection process should include paying attention to what you modify and by how much. If the camshaft is ground down too far, it may require a different length push-rod than stock.
“We offer custom length pushrods because when engine builders get into the performance work such as modifying the head in order to get the lift they want, you have to grind the base circle of the cam down,” says Wertzbar. “You need to maintain a geometry so that the nose of the cam is not higher than the cam bearing. To do this you would need to take the base circle of the cam down but if you take it down too much you have to use longer pushrods to compensate for that variation. And we offer a .060″ longer pushrod than stock. Here again you need to know what the geometry of your cam is from a stock cam and then you will know how much longer you need the pushrods. For the most part, until you get into the higher performance applications, you can use a fairly stock pushrod.”
Manufacturers repeat their warning that the margin of error is not what it used to be, and the shops reclaiming rods are going to have a difficult time down the road. The mileage on cores is higher so there’s more of a wear issue, which takes up more of your tolerances too. Your rocker arms, pushrods, lifters and mating components all need to be replaced in many cases.
“Lifters are always a huge topic of conversation in the aftermarket because of supply and price,” says Enginetech’s Hunter Betts. “It’s been a real issue. There’s only one engine being manufactured at the OE level anymore that uses the flat tappet, and that’s the Jeep 4.0L. Everything else uses hydraulic rollers, and since about the 1990s everything switched to roller lifters at the OE level.”
As the OEs have moved away from the flat tappets to the roller lifters, the price of the component has jumped significantly too. One of the things that have been difficult for the aftermarket says Betts is that many rebuilders are reusing the roller cams and lifters now when they rebuild an engine. This is because the newer pushrod engines have steel roller cams that don’t wear, and they’ve got hydraulic roller lifters that don’t go bad at the same rate as the flat tappet components. However, Betts and others caution that it is risky to rebuild an engine with 100,000-plus miles and reuse the cam and lifters when you’re going sell it back to the customer as a like-new engine.
Whether it is mechanical, hydraulic, flat tappet or roller style, the lifter is one of the most important components in the engine. The lifter determines how the camshaft will react to the rest of the engine combination. Depending on the type of application, valve train separation may become a serious issue. Experts say increased spring pressure can be a solution, but that depends on whether the lifter can handle the valve train load or not.
While flat tappet solid and hydraulic lifters were very common in the ’60s and ’70s muscle car era, and still used in some economy pushrod engines, the biggest trend today is a roller lifter. There are two main advantages to a roller lifter. One is that it offers reduced friction, which is always a good thing. The thinking here is that a wheel rolling on a cam lobe generates much less friction, heat and wear than a flat faced lifter rubbing metal-on-metal against a cam lobe.
The other advantage to a roller lifter is that from a performance standpoint, it can handle much steeper ramps on the cam lobe than a flat faced lifter, which allows the valves to open more quickly and reach maximum lift sooner.
“Gradually more and more people are going to roller lifter cams,” says Howards Cams’ John Steely. “The solid lifters are still very popular among racers and even street performance enthusiasts, surprisingly, but we do sell a lot of roller stuff as well.”
Clevite’s Wertzbar agrees that solid lifters are still a popular choice but also sees the roller lifters becoming more popular among their performance customers. “The demand for the roller lifter still isn’t great,” says Steely. “But we are seeing increases in demand for them. For the most part, the Saturday night racers are still running mechanical or hydraulic flat tappets. For the weekend racer on a budget, going with roller lifters is a lot more expensive, but the benefit to that is you can go with a higher lift camshaft because there is less stress on the component.
Flat tappet solid lifters have improved say experts, but how much? Howards Cams is one of a few suppliers that offer solid lifters with an electrical discharge machining (EDM) hole in the face for added lubrication. “The Direct Loop lifter with the EDM hole works best in two extremes: for circle track racers who use very fast or high ramp rates on the cam lobe, which requires fairly healthy spring loadings to handle the fast ramp rates and high rpm. Some additional oiling is needed because of the heavier load ratings,” says Steely. “And it helps in the extreme opposite end for the street guy with the solid lifter who doesn’t turn any rpm. There’s not enough rpm to oil the lifters and cam because the camshaft is oiled by splash. If you’re only revving at 1,500-2,000 rpm there is a good chance it’s not getting enough oil in a healthy solid lifter camshaft.”
Hydraulic lifters generally don’t work very well in performance applications, according to some experts, because you’re limited by the valving in the lifter. A hydraulic lifter will only handle a certain amount of spring pressure or loading. If you’re not limited to a hydraulic lifter, as some racing classes require, then it’s definitely an advantage to use a solid tappet or roller.
In the stock replacement environment there are other issues going on according to Enginetech’s Betts. “Right now our customers need to build a less expensive engine. They are looking to save money on engine components and lifters are a big part of that. Lifters and the matching cam have become a big percentage of the cost of an engine. A flat tappet lifter might cost $1.50 and a roller lifter will cost $6, and when you’re talking about 16 per engine, it adds up quickly. And there’s a much bigger difference with cams (roller vs. flat tappet).”
There’s no doubt that the cost to build a roller lifter is greater than the flat tappet lifter, and since the flat tappet is essentially obsolete in the production OE environment, manufacturers are not eager to tool up to make them.
“Lifters were sort of a commodity in the past,” says Betts. “It’s the tightest tolerance component in the engine. The tolerances inside a hydraulic lifter are about .0002″ of an inch (that’s two ten-thousandths) clearance, which is extremely small, and they have to be perfect or the lifter will fail. A flat tappet hydraulic lifter has to be forged, heat-treated, and machined at a maximum tolerance of .0002″ yet it sells for less than a soda? Come on!” Betts exclaims.
According to Betts and others, the only quality made lifters are manufactured in the U.S. right now. Foreign made lifters have had quality control problems to this point, which is why some of the major lifter suppliers will only sell the U.S. lifters. It’s not worth the risk for both supplier and engine builder to save maybe 5 or 10 cents on a part that has a higher risk of failure. U.S. made lifters have a very low failure rate of maybe 4 out of 1 million, says one expert.
Ten years down the road and whatever the OEs may have come up with by then, the pushrod engine is still going to be in abundance, experts believe.
“I would agree that there is always going to be a market for pushrod engines because there are so many in service now and the amount of racing classes that mandate a pushrod engine should keep demand for pushrods and lifters fairly high,” says Clevite’s Wertzbar.
From an engine builder’s perspective, you want to know what’s coming down the road so you can plan accordingly. Your bread and butter is what’s here and now mixed with earlier engine technology but then you need to be prepared for what’s coming next. But the pushrod engine is not going to disappear.
|Common Misconception of Valve Float
Source: Crane Cams
“Valve float” is a common term for a situation best described as “valve train separation.” This occurs due to inertia load imparted into the valve train by the action of the cam lobe against the follower. Flex in the valve train (the majority of which is located in the pushrod) is the prime contributor to valve train separation. The initial loads imparted into the pushrod cause it to bend (somewhat like a pole vaulter’s pole) and then return to a straight configuration. This unloads a sharp energy pulse to the rocker arm, which transfers it into the valve/valve spring assembly. This often results in “valve lofting,” which causes the valve to operate in a different path than that described by the lobe profile. At the same time, the lifter without any load against it can also be launched off the opening ramp of the lobe, and then, as load is re-established, either strike the nose of the lobe and eventually damage it; land on the closing ramp; or land on the base circle with significant and often damaging impact. If “lofting” can be controlled (by design or good fortune and the lifter lands gently on the closing ramp), it adds to area under the curve and more power. If it is uncontrolled (which happens the vast majority of the time), it can be
Many people tend to think that the “weight” of the rocker arm is the cause of valve float. If the rocker is rigid and properly designed, it should contribute very little to valve float. Weight in this case is not the prime issue, but rather the “moment of inertia” of the rocker design. “Moment of inertia” is the affect of where the mass of the rocker arm is located relative to its center of rotation. One rocker can be much heavier than another and still have a smaller moment of inertia because of where its mass is located; so weighing rockers to determine their affect of valve float is really not effective at all. (FYI: “mass” is a measure of a body’s inertia; while “weight” is the affect of gravity on “mass.” “Moment of inertia” is unaffected by weight, but is affected by where “mass” is located relative to the center of rotation).