The Advantages of a 7,500 RPM Rat Motor with Hydraulic Roller Lifters
Words: Jeff Smith; Photos: Steve Brule and Jeff Smith
“Wait… what? Spin a big-block Chevy to 7,500 rpm with hydraulic roller lifters? No way. Big-blocks have big heavy valves, springs and bad rocker geometry. That kind of rpm will kill that hydraulic roller lifter.”
We’ve all heard that kind of talk. But the headline’s true. Steve Brule at Westech spun this Rat motor up several times with no signs of distress. Plus, in a similar test he pushed a small-block to 8,500 rpm with Crane hydraulic roller lifters. But as is usually the case, there’s more to the story than just bolting some parts together and spinning the engine up on the dyno. Let’s look at the basics and then we can deal with how Crane and Westech put this whole high rpm hydraulic roller big-block package together.
Let’s start by saying that rpm alone does not guarantee horsepower – but it certainly can break parts if the engine isn’t designed to handle the load. This goes beyond just building a valvetrain that can take this kind of rpm abuse. It’s essential to start at the core of the engine with a good steel crank, high quality connecting rods, and top-of-the line forged pistons. In this particular big-block Chevy, Westech built a 496ci Rat with a forged 4340 steel Scat crank, 4340 steel I-beam rods, and a set of 12.5:1 compression JE forged pistons to create a reliable rotating assembly. These parts have to be high quality because the g-forces experienced by the rotating assembly increase radically with rpm and you don’t have to be a mechanical engineer to know that big-block pistons are heavy. That’s why they call ‘em big-blocks.
On the top end, Westech’s Steve Brule added a set of AFR 325cc, rectangle port aluminum heads that feature stainless steel, 2.30- and 1.88-inch intake and exhaust valves where the intake flows 372 cfm at 0.600-inch valve lift which is more than enough to make decent power. Then it was time to select the valvetrain, which we will get to in a moment.
Assuming that our Rat is capable of sustained rpm, the next logical question is with mechanical roller lifters that can easily support this kind of rpm, why deal with the potential hassles of hydraulic lifters? The reason is that mechanical roller lifters come with their own inherent issues. Pit talk, bench racing sessions, and internet forums overflow with failed mechanical roller lifter stories. The essential problem is that with high spring load, the needle bearings eventually fail, exiting the roller lifter causing massive internal engine damage. It was these catastrophic damage issues that led to solutions such as roller lifter bushing conversions. At least when bushings wear out, they don’t spew dozens of tiny bearings through the engine.
In discussions with engine builders, racers, and several cam companies, who deal with mechanical roller issues, excessive spring pressure and mismatched valvetrain components are most often mentioned as leading causes of mechanical lifter difficulties. But another, less talked about contributor is lash. All mechanical roller cams require some amount of clearance to account for heat expansion. For street engines that spend perhaps as much as 50 percent of their life cycle at idle, this clearance contributes to impact loads that hammer those tiny roller bearings. This is no small concern. In the Bosch Automotive Handbook* under the heading of roller bearings there are lists of advantages and disadvantages to these bearings. The first disadvantage listed is “Sensitive to impact loads”. This is, in part, due to the extremely small contact points created by the tiny roller bearings. Tight lash camshafts offer improvements in this area by reducing this clearance, but impact loads related to lash remain an issue for street engines because of the increased number of impact cycles the lifters must endure.
Given this situation, let’s investigate the advantages and disadvantages to hydraulic roller lifters. The major advantage to hydraulic rollers is that the clearance between the roller and the lobe is eliminated, so impact loads are radically reduced. The first criticism of hydraulic lifters is that the hydraulic parts cannot handle the spring loads necessary to accommodate high rpm applications. That’s a two-edged sword because, as you will see, the springs used in our 496ci big-block Chevy were mild in comparison to typical mechanical roller springs and yet were still capable of accurately controlling this Rat motor all the way through 7,500 rpm.
In order to appreciate what’s going on in the modern version of the hydraulic roller lifter, let’s lay down some basic parameters. The criticism of hydraulic lifters has always targeted seemingly opposite faults – they either pump up or they pump down. Neither works well but the explanations of how these problems occur will lead us directly to why current quality lifters are generationally better. Critics of hydraulic rollers point to lifter pump-down as an endemic issue that is directly related to spring pressure. The accusation is most often tied to typical production lifters that cannot handle increased spring pressure. At high rpm, spring pressure plus increased inertia loads push the small piston down into the lifter, sacrificing lift. If the lifter is lightly preloaded, the pump-down can push the lifter piston down by 0.050-inch or more. Multiply this by a rocker ratio of 1.7:1 for a big-block Chevy and this represents a loss of 0.085-inch or more. Clearly, this would lead to a dramatic drop in power. The key element causing this issue is excessive lifter piston-to-bore clearance that allows dynamic valvetrain force to push the oil out from underneath the piston.
The second hydraulic lifter issue is lifter pump up. Unlike Hans and Franz, we don’t want to “pump you up!” This really isn’t a lifter problem. For our example, let’s start with preloading the lifter with 1¼ turns. On a 7/16-inch x 20 stud, after achieving zero lash, one turn of the adjuster nut is equal to 0.050-inch of preload, making 1¼ turns equal to depressing the piston into the lifter 0.062-inch. Now at some rpm point, because of inadequate valvetrain design such as weak valve springs, spindly pushrods, heavy valves, or a combination of several variables, the spring loses control of the valve. When this happens, there is separation in the lifter-pushrod-rocker-valve assembly which introduces clearance. Now because there’s no spring load to depress the piston in the lifter – engine oil pressure pumps the piston against the retaining clip. Now that 0.062-inch of piston travel holds the valve open by over 0.100-inch (0.062 x 1.7:1 rocker ratio) and all kinds of bad things start to happen – with the loss of power and possible engine damage at the top of the Bad Juju list.
Now that we know how these bad things happen, the fix is easy. We spoke at length with Chase Knight of Crane Cams who told us that their current crop of Ultra Pro Crane hydraulic roller lifters are constructed out of high strength 8620 alloy steel, which offers an incredibly strong base from which to build a lifter body. Then, more importantly, the clearance between the piston and bore in the lifter body is measured in microns. A micron is 0.0000393-inch, which means the tolerance of the piston-to-bore is going to be very precise. By maintaining this extremely tight tolerance, lifter pump-down is minimized. Right away, this means that spring pressures approaching those of mechanical roller cams can be accommodated. Crane says that its hydraulic roller lifters can accommodate up to 300 pounds on the seat and 800 pounds at maximum valve lift.
This doesn’t mean that you need these high loads. In this particular application, despite the fact that these AFR heads use a 2.300-inch stainless steel solid stem valve – which is anything but light, this engine’s spring loads were relatively tame with 175 pounds of seat load and 530 pounds at max lift. It’s worth emphasizing that valvetrain components like springs, retainers, rocker arms, rocker studs, pushrods, and lifters all last much longer when subjected to lower spring pressures.
So now we must look at lifter pump-up issues as the next hurdle the hydraulic roller lifter must overcome. This is somewhat easier because, as we mentioned, this is more related to proper design of the remainder of the valvetrain. Knight spec’d all of the components used in the valvetrain for this test and the parts list is almost shocking in its simplicity. You may have noticed that this is not a shaft rocker system. The only thing holding those wide-body Crane rocker arms in place is a set of ARP 7/16-inch rocker studs. We also did not use a girdle to help prevent rocker deflection. A big part of the reason this wasn’t necessary is because the spring pressures are conservative. That also reinforces the use of standard Crane 3/8-inch, 0.080-inch wall thickness pushrods.
According to Knight, reducing weight is important to the valvetrain, which is why he did spec the use of a set of titanium retainers as a way to reduce the inertial forces of the valvetrain at higher engine speeds. This is because the top of the spring travels the farthest distance, so by reducing weight at the top of the spring, this reduces the total force that the spring must control. Knight says that while weight is an important factor, stiffness is also critical and that you should not sacrifice stiffness for the sake of weight reduction.
A final selection was to choose a cam lobe that would work in this engine and help support the goal of spinning this engine to 7,500 rpm reliably. Knight chose an off-the-shelf Crane hydraulic roller for the big-block. The specs are listed in the accompanying chart, but this was not a custom lobe grind but instead a cam right out of the catalog.
With all the parts in place, the first test we ran on the dyno was with the lifter preload set at a ¼-turn. This produced an impressive run that made 744 horsepower at 6,500 rpm and we spun the engine all the way up to 7,500. Notice in the graph that the horsepower curve dips slightly from 6,500 to 7,200 rpm. This could indicate that the valvetrain was actually suffering from the onset of valve float or perhaps either lifter pump-up or pump-down. Knight suggested that we change the preload on the hydraulic lifters from ¼-turn (roughly 0.012-inch) to one (1) full turn. As you can see from the graph, this is represented in the HP2 curve which makes as much as 17 more horsepower at 7,100 rpm and the curve presents a clean, gentle curve all the way to 7,500 rpm.
The power numbers themselves are really not what we were looking for in this test. What was far more important was the shape of the curve as presented on the graph. The nice, gentle shape of the power curves reveals that the valvetrain was not in distress throughout this entire run. This also shows that the lifters were doing their job. What we did not do was run a set of mechanical roller lifters in this engine. Had we done this, it’s entirely possible there would be a slight increase in power.
When Brule increased the preload on the lifters from ¼ to one full turn, this pushed each hydraulic roller piston deeper into the lifter body. This is important because this reduces the height of the column of oil inside the lifter body. It is inevitable that a tall column of oil will contain a small percentage of air trapped in the oil. Under high rpm and load, the air compresses and allows the piston in the lifter to collapse slightly, which reduces valve lift. By setting the preload to one full turn, this reduces the height of the column of oil roughly in half (about 0.050 to 0.060-inch).
By reducing the height of the column of oil with more preload, we also logically reduce the amount of air in that column and therefore reduce the amount of piston deflection. This means the system retains the valve lift. In the graph, you can see that this simple change in preload was worth a solid 17 hp at 6,900 rpm and then the horsepower numbers came back closer together near the top of the rpm band. This indicates that the valvetrain was in some sort of distress around 6,900 rpm which caused the load to increase at that rpm. Likely had we changed to a set of mechanical lifters, the power might have increased, but it would have been likely in the single digits.
Another obvious advantage to employing hydraulic roller lifters is the fact that checking lash will become a thing of the past. For street engines and bracket racers, this reduces engine down time and makes at least the automotive side of your life a bit less complicated. We’re already seeing a movement of some bracket racers to convert to hydraulic rollers and companies like Shafiroff Race Engines offering race engines with hydraulic roller lifters. Think about it…
*Reference: Bosch Automotive Handbook, 5thEdition, published in 2000, page 289 under the heading “Rolling Bearings”
|Crane Camshaft||Adv. Dur.||Dur. 0.050||Valve Lift||LSA|
Valve Spring Load Chart
The following generic chart is a starting point for selecting valve springs for serious street engines. These are not one-size-fits-all numbers but rather recommendations that you can use as guidelines when selecting a valve spring. This chart also deals with traditional valve springs.
|Small-Block Street Engines||Seat Load (lbs)||Open Load (lbs)|
|Big-Block Street Engines||Seat Load (lbs)||Open Load (lbs)|
|Crane hyd. roller retro-fit camshaft||139851||Summit Racing||$490.00|
|Crane hydraulic roller retro-fit lifters||13532-16||Summit Racing||N.A.|
|Crane dual 1.550-inch valve springs||99896-16||Summit Racing||256.00|
|Crane titanium 7-degree retainers||99678-16||Summit Racing||376.00|
|Crane 7-degee locks for 11/32 valve||99097-1||Summit Racing||32.40|
|Crane Gold Race 1.7:1 wide rockers||13763TR-16||Summit Racing||456.00|
|Crane pushrods||13642-16||Summit Racing||179.20|
|Crane Pro-Series steel billet timing set||13984-1||Summit Racing||168.00|
|AFR 325cc aluminum heads||3250-1||AFR||2,089.00|
|Fel-Pro head gaskets||1144||Summit Racing||63.97|
|Fel-Pro intake gaskets||1275||Summit Racing||16.97|
|Edelbrock Super Victor2 intake||2896||Summit Racing||530.97|
|Holley 1050 Dominator HP carburetor||0-80903HB||Summit Racing||1,246.95|
|Lucas Semi-Syn. Assembly Lube||10153-1||Summit Racing||5.97|
|Lucas Synthetic Racing Oil, 0w-30||10605-1||Summit Racing||13.97 (ea.)|
Airflow Research (AFR)
Holley Performance Products
Lucas Oil Products
Westech Performance Group