Your doctor will tell you that stress kills. The same is true for high-performance engines and it’s a safe bet that if your engine is running aggressive valvesprings and stud-mounted rockers, there’s stress and deflection going on that you may not realize. But forget nervous tension, we’ll show you an easy way to evaluate your valvetrain to see if it is experiencing undue anxiety.
This story evolved out of collaboration between Comp Cams lobe designer Billy Godbold and EFI University owner Ben Strader on a project they call Spinal Tap, an 11,000-rpm 382ci LS drag race engine. Among the things we learned from this team was a very simple technique for checking for valvetrain deflection.
|Items That Affect Stiffness Rating|
|Lifter bore clearance||Loose is bad - more deflection|
|Cam core diameter||Smaller diameter = more deflection|
|Pushrod diameter||Small diameter = more deflection|
|Pushrod wall thickness||Thicker is better - less deflection|
|Pushrod angle||Straight is best - angle loses lift|
|Rocker arm deflection||Stronger = less deflection|
|Stud deflection||Studs deflect more with big springs|
|Rocker travel across valve tip||Proper geometry = less change in ratio|
|Pushrod length||Shorter pushrods are stiffer|
When you look at a cam timing card, some simple math indicates that lobe lift multiplied by the rocker arm ratio will create a theoretical lift at the valve. Most backyard engine builders never bother to verify this number. If they do, they usually do so by using a light-duty checking spring in place of the actual spring. We’ll take that one step further to show you numbers on a big-block Chevy street engine and what happens when you put big springs on a rocker stud engine.
Taking this one more step further, many engine builders fail to consider how much deflection occurs in even a mild street engine combination using the classic stud-mounted rocker system. We will start by measuring the theoretical valve lift on a hot street big-block with an aggressive mechanical roller camshaft.
In this example, we will use a 496ci Rat engine fitted with a set of AFR oval port aluminum heads and a Comp mechanical roller cam. The cam card lists the intake valve lift at 0.734-inch using a 0.432-inch lobe lift and a 1.7:1 rocker ratio (0.432 x 1.7 = 0.734).
We installed a Comp mechanical roller lifter, a correct-length pushrod, and a Comp 1.7:1 steel roller rocker to verify the lift, using a light-duty checking spring to eliminate any variation due to deflection in the system. We mounted the dial indicator as parallel as possible to the valve to eliminate error.
Rolling the intake revealed a maximum valve lift of 0.744-inch—or 0.010-inch over what was listed on the cam card. Note that for all the static testing we installed the rockers with zero (0) lash. In the real world of a running engine, we would have included the 0.020-inch of lash. But to keep these tests simple, we did not include the lash. This additional lift could result from a slight bump in rocker ratio but likely the max lift is affected by pushrod length. We checked both a shorter and longer pushrod from our pushrod length and with a 0.150-inch longer pushrod the max lift increased by almost 0.006-inch. This is why we took the time to perform the checking spring test rather than just use the theoretical lift number.
The dual valvesprings in this engine at first were a Comp dual spring configuration that checked in with an open load at 620 pounds at a valve lift of just over 0.700-inch lift. We considered these springs too light for this cam, so we added a set of Comp 944 springs. The seat load didn’t change much but the new springs pushed the peak open load up by 120 to 740 pounds.
|Comp Mechanical Roller|
|Cam||Average Duration||Duration at 0.050||Lift||Lobe Separation Angle (LSA)|
After establishing the checking spring max lift, we cheated a little and moved our full open load test to an adjacent cylinder with the valvespring already in place, but using the same lifter, pushrod, and rocker arm. Max lift dropped from the original 0.744-inch lift to a much shorter 0.695-inch. This means we lost 0.049-inch, or just over six percent, of the potential lift due to deflection.
This brought up the question of what was bending. Since the valvetrain is a system, our lift loss was likely originating from several places as opposed to one big offender. We’ve created a short list of the potential deflection sources within the valvetrain, but we decided to concentrate on the places where we could efficiently reduce the deflection.
Our first solution took aim at the pushrods. We were using a set of performance 3/8-inch hardened, 0.080-inch-wall thickness pushrods. Several companies, including Comp and Trend Performance, offer a slightly thicker 0.135-inch-wall thickness pushrod that otherwise appear identical. We ordered a set of these same length, thicker pushrods from Trend and then evaluated the difference.
The stiffer pushrods improved the valve lift by 0.004-inch, with maximum lift improving from 0.695- to 0.699-inch. This change could not have been easier since all we had to do was swap the pushrods. Trend also offers a dual tapered pushrodwith a 7/16-inch body that tapers to the smaller ends. It appeared these could clear the guideplates if we decided to run the engine, but a quick test revealed only a 0.001-inch improvement. Since these fully tapered pushrods are more expensive, we decided to stay with the 0.135-inch-wall 3/8-inch Trend versions.
The next place to logically look would be the rocker studs. Even though we were using high-quality ARP steel, centerless ground studs, with 740 pounds of force working against a 1.7:1 rocker ratio, this represents a force of well over 1,250 pounds trying to bend that stud.
We had a big-block stud girdle from Comp Cams that had worked on an earlier big-block project, but it didn’t fit our AFR heads. So we put a call into Air Flow Research and they supplied a stud girdle kit as well as some new AFR valve covers. With the stiffer Trend pushrods working in conjunction with the AFR stud girdle, our test revealed valve lift nudged from 0.699-inch to 0.700-inch, which was only 0.001-inch. This was disappointing since an earlier test had produced a greater improvement. The combination of the stiffer pushrods and stud girdle improved the lift a total of 0.006-inch. This was certainly stiffer but not what we had anticipated.
|Valve Lift Deflection Test|
|Test||Checking Spring Lift||Loaded||Lift Loss||Improvement (inches)|
|* Baseline with 0.080-inch pushrods|
|** Added Trend 0.135-inch-wall thickness pushrods|
|*** Added AFR stud girdle|
|**** Replaced stud girdle system with Crower shaft rocker system and Trend pushrods|
The next step was to evaluate a shaft rocker system. There are several reputable companies selling shaft systems, but we decided to contact Crower Cams to test one of their stainless steel shaft rocker systems. A shaft system is an especially good idea for big-block Chevys with its compound angle valve position.
The shaft system requires a different pushrod length as well as adjustments to the height of the rocker stands. Because this system needed different length pushrods to maximize the shaft system, we ordered a new set of 0.135-inch-wall pushrods from Trend. Once the pushrods and the shaft system was installed, we again ran through our lift check procedure. The first thing we noticed was that our maximum valve lift with a checking spring changed from 0.744 with the stud-mounted rockers to a significantly taller 0.766-inch—or 0.022-inch more maximum lift.
We did a little math and determined our rocker ratio was not 1.70:1 as stamped on the rockers but calc’d out to 1.77:1. I spoke with a Crower engineer, Shane Pulido, who told me that Crower makes the ratio 1.72:1 but in some cases it can be more. They do this to compensate for angular changes in the pushrod coming off the lifter that reduces the lobe lift slightly.
Nevertheless, this isn’t a bad thing as we increased lift and improved valvetrain efficiency (which is the whole point of this exercise). But this also means we can’t in fairness directly compare the results with the stud-mounted system since the peak lifts were different. As you might imagine, the Crower shaft system did reduce deflection. Checking maximum lift with the rocker shaft working against the 740 pounds of valvespring, we saw 0.731-inch of maximum lift, meaning we still managed to improve the maximum valve lift with a loss to deflection of 0.035-inch. The best version of the rocker stud system lost 0.044-inch, which means the shaft system is clearly stiffer and more stable.
Another interesting result of this testing produced another small, but significant, improvement. While measuring the deflection with the rocker stud system, we also noted the travel of the rocker arm across the top of the valve stem of 0.110-inch. Generic specs for this travel call for an idealize rocker travel distance of around 0.080-inch with a narrower pattern preferred. We probably could have reduced that 0.110-inch number close to perhaps 0.090-inch with careful pushrod length changes, but time constraints got in the way.
When we measured the Crower shaft system, even with 0.030-inch more valve lift under load, the pattern across the valve tip narrowed to 0.060-inch. This is nearly half the distance travelled by the rocker stud system. In the photo of the pattern you can see that it is skewed slightly toward the exhaust side of the head instead of in the middle.
|Test||Peak HP||Peak HP Gain*||Avg. HP||Avg. HP Gain*|
|*All gains are compared against Test 1|
This wear pattern across the valve tip is called scrub—which is another word for friction. Keep in mind that even with a roller tip on the rocker arm, once loaded, it does not roll across the valve tip but rather scrubs across the tip. Decreasing the distance the rocker travels across the valve tip’s face reduces friction and heat—all of which are enemies of power.
With stud-mounted rockers, changing pushrod length directly affects the rocker arm contact point on the valve stem. But with a shaft system, the contact point is determined by the position of the shaft mount. Pulido suggested elongating the holes in the intake rocker shaft because we needed to move the shaft mount closer to the intake side (inboard). This would place the rocker arm contact point closer to the valve stem centerline and minimize the thrust angle on the valve stem, which will also reduce valveguide wear.
In talking with engine builder and seven-time Engine Masters champion Jon Kaase, he said that idealizing the position of that pattern does help, but he also said he’s never seen a power difference even when improving some really awful valvetrain wear patterns on the valve tip. But the position is still important to reduce valveguide wear.
It’s also worth noting that any improvement in the lift curve means you must then re-verify the piston-to-valve clearance. In our engine, the clearances decreased slightly. We found only 0.060-inch piston-to-valve (P-V) clearance on the intake side and a slightly wider 0.090-inch on the exhaust. Reher-Morrison Racing Engines (RMRE) offer a P-V clearance spec that calls for adding 0.010-inch to the existing piston-to-head clearance, which in our case is 0.050-inch, so our intake clearance is right on the edge. This can be adjusted by moving the camshaft centerline.
We decided that this also demanded some actual dyno testing, so we bolted our 496 test engine on the dyno at Westech. As mentioned before, the engine is configured with a pair AFR 300cc oval ports with 2.30/1.88-inch valve sizes. The Ohio Crankshaft stroker crank and rod package pushes a set of JE pistons that help make 10.2:1 compression. On top is an Edelbrock Victor Jr. 454-O Dominator intake with a Holley 1,050-cfm carburetor. Our test procedure was to stabilize the water and oil temperatures before every run and then to make three consecutive pulls and average those for each test. This eventually demanded loading our big-block over 30 runs to get 12 clean runs to average into four separate results.
We started Test 1 with the baseline 0.080-inch wall thickness pushrods and then Test 2 moved to the stronger 0.135-inch wall thickness Trend pushrods. Test 3 then increased valvetrain stiffness by adding the AFR stud girdle. Test 4 exchanged that entire stud-mounted system for the Crower shaft system.
|Comp mech. roller cam||11-851-9||Summit Racing|
|Comp mechanical roller lifters||96819-16||Summit Racing|
|Comp 1.7:1 Ultra Pro steel rocker arms||1620-16||Summit Racing|
|Comp 3/8" 0.080-wall, 8.600" pushrods||7906-8||Summit Racing|
|Comp 3/8", 0.080-wall, 9.500" pushrods||7757-1||Summit Racing|
|Comp dual valvespring, solid roller||944-16||Summit Racing|
|Comp tool steel retainer, solid roller||1731-16||Summit Racing|
|Comp 10 degree locks, 11/32"||611-16||Summit Racing|
|Comp BBC stud girdle||4021||Summit Racing|
|Comp checking springs||4758-2||Summit Racing|
|Trend Perf. 0.135-inch, 8.300" pushrods||TR83001353||Trend Performance|
|Trend Perf. 0.135-inch, 9.300" pushrods||TR9301353||Trend Performance|
|Trend Perf., 8.625" pushrods||TR86251353||Trend Performance|
|Trend Perf. 0.135-inch, 9.450 pushrods||TR94501353||Trend Performance|
|AFR stud girdle, V2||6202||Air Flow Research|
|AFR valve covers, black||6723||Air Flow Research|
|Crower shaft rocker system, 1.70:1||74707F||Crower|
|LSM lash tool||1T-100||Summit Racing|
We’ve outlined the dyno results in the following table and produced predictable results. As you should expect, there were no gains at the lower engine speeds but from roughly peak torque upward, we did see minor power improvements. A combination of the stiffer pushrods and AFR’s stud girdle was worth 3 hp and converting to the Crower shaft system generated a peak gain of 8.4 hp and 5.7 lb-ft of torque over the baseline. At 6,000 rpm, the shaft system was worth a solid 10 horsepower. We then averaged the power numbers from 4,500 to 6,700, which are lower but indicate measurable improvements in system stiffness.
We’ve revealed the improvements in static valve lift, but there’s also the drastic improvement in dynamic stability. Generally, most shaft systems increase the length between the shaft and valve, which lengthens the radius of the arc that the rocker scribes as it moves through the lift curve. By increasing this arc, it also reduces the travel across the tip of the valve, which reduces friction and improves stability. This becomes more and more vital as valve lift numbers continue to increase.
If you’re like us, as soon as you’re done reading this story, our guess is we would find you in the garage testing valvespring deflection with a dial indicator. If nothing else, you at least know how your valvetrain compares. The numbers just might get your attention!
|We dialed this graph in on the power curve above 6,000 rpm to highlight that there were some significant gains with all the changes. The best way to look at this is that the shaft system was worth 8 hp over the baseline stud-mounted rocker system at 6,500 rpm.|