In Chapter 9, Camshafts and Valvetrain Events, I spent considerable time explaining a Chevy big-block’s needs in the way of valve event timing. In Chapter 2, Pistons, Connecting Rods and Crankshafts I also discussed the advantages of increasing the lifter bore diameter to achieve greater lifter velocity and acceleration to maximize valve lift. Now it is time to put this information to good use by applying what was learned to make better hardware choices.
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Cam and Valve Lift
Before I delve into valvetrain optimization, let’s revisit the subject of intake valve lift and define a primary target. As I have previously stated, approximately 0.100 inch of lift is needed for every 100 hp. That figure proves to be very close for a typically well-developed 10.5:1 street engine. It applies to almost any displacement whether it is a 454 right through to a 700-inch-plus build.
When you increase the compression ratio and reduce internal losses, the engine’s thermal and mechanical efficiency are improved. (See Chapter 2, Pistons, Connecting Rods and Crankshafts; Chapter 3, Lubrication Systems; and Chapter 4, Cylinder Heads.) In fact, more than 100 hp per 0.100 inch of valve lift can be achieved. If you have an engine with really good 24-degree heads, a 15.5:1 CR, skinny-piston rings, a good oil pan and evacuation pump, etc., it can mean as little as 0.090 inch of lift per 100 hp. However, within the realms of a typical street-performance engine you should still figure that your build needs something at or near that 0.100 per 100 hp number.
It is possible to lift titanium valves to as much as 0.950 while still using a 0.842-inch lifter diameter as well as a stock cam journal diameter and a 3/8-inch pushrod. As convenient as that may seem, it’s on the ragged edge. When targeting this much lift, it’s best to take a sounder engineering approach and go with larger-diameter cam journals, lifters, and pushrods.
You really cannot afford a valve-train breakage, so if you are building a budget-oriented race engine with a stock-diameter cam journal, you should limit lobe lift to 0.485, or maybe 0.510 if the cam has a duration of 280 degrees or more at 0.050 tappet lift. With, for example, Comp’s Ultra Pro Magnum rockers, a 0.485 lobe would, after lash, result in 0.860 net lift at the valve. On a really well developed 15.5:1 engine in the 550-ci range equipped with good 24-degree heads, that is enough lift for more than 900 hp.
In the past, I have made a case against using hydraulic roller cams unless you are prepared to spend what it takes to get good hydraulic rollers. Cheap ones on the market will cause a build to lose 75 to 100 hp. Hard to believe? Yes. But since my first Chevy big-block book was published, I have had at least a dozen semipro or pro engine builders contact me who have experienced this, and the power loss was usually nearer to 100 than 75 hp! If you are going to use a hydraulic roller, make sure it is fully functional. Here are the ones I can recommend: Morel, Comp, Crower, and Crane.
Lunati offers Morel hydraulic roller lifters in three performance grades: street, street/strip, and race. Prices increase accordingly from street to race. I have not tried the street performance rollers, but the other two types work very well and are quality pieces, so expect to pay accordingly.
Comp has a short-travel race- grade lifter (PN 15854-16) that also works well. They are of the 0.300 tall-link-bar type that fit either an early flat-tappet block or a block originally equipped with dogbone factory hydraulic rollers.
I did some Spintron testing back when with Crower’s top-of-the-line hydraulic roller, and it ran flawlessly to 8,000 rpm with a pretty aggressive profile cam.
Crane hydraulic lifters (PN 13532-16) are highly functional and made of 8620 steel billet. These are also of the 0.300 tall-link-bar type that fit either an early flat-tappet block or a block originally equipped with dog-bone factory hydraulic rollers.
The reasons for enlarging lifters are covered in Chapter 2, Pistons, Connecting Rods and Crankshafts; now it is time to look at what it’s worth. The most relevant example is the use of a 0.904-inch-diameter flat hydraulic lifter (stock Chrysler or American Motors) instead of a 0.842 lifter. A back-to-back test here is costly, but it can be simulated with rocker ratios. To replicate the use of a bigger-diameter flat lifter, you can select rockers with lift characteristics that simulate the same rate of valve opening. The diameters of the two lifters determine what is possible. Performing this move on a relatively low-buck build in the 575-hp range produces about a 20-hp increase.
If you are after hydraulic rollers with a larger roller, I am unaware of any hydraulic 0.904 lifters that have a larger roller than is typically used for a 0.842 lifter. Unless the roller is larger, there is little point in using a larger lifter body.
As the demand on the valvetrain increases, so does the potential gain. On an all-out race engine, there can be an 80-hp difference between stock lifter bores and cam diameter and, for example, a big Jesel lifter installation with lifters having more than a 1-inch diameter. If you are targeting high valve lift for a race engine, any serious effort in the valvetrain area should use the biggest lifters and cam core possible.
Be aware that installing a 1-inch- diameter lifter is not something that can be easily done to a stock or even many aftermarket blocks. This size lifter is usually the domain of a DRC Pro Stock block. Most blocks, such as from Dart, can normally go to a 0.937 lifter, which utilizes a 0.850-diameter roller.
Solid Roller Lifters
For solid roller cams, the simplest “big lifter” solution is the 0.904-inch size. Roller and flat lifters are available in 0.875-inch diameter, but you should use 0.904 because it’s no more effort to enlarge the lifter bores to this size. The variety of 0.904 lifters available is fairly extensive. Comp, Crane, and Jesel all produce 0.904-inch roller lifters. All of them use a roller larger than the 0.842’s 0.75 to 0.76 roller. These lifters are also offered with a centered or offset pushrod location so pushrods have enough clearance in the head casting when the ports have been widened at the pushrod pinch point. Before selecting a lifter, consider all of the lifter options that these companies offer by spending time with their catalogs or on their websites.
Most 0.904 lifters use a 0.850 roller for a small but useful increase in the lifter acceleration. Even with a stock-diameter cam and base circle, the 0.904 lifter’s bigger roller should allow 0.015 inch more lobe lift before the same limiting side loading is seen. This increase allows a 0.500 lobe lift to be used in place of a 0.485. At the limit, valve lift up to a net 0.915 (about 0.935 gross) could be had, and that should be enough to service about 960 hp. Even if you do not change your cam, the bigger roller provides greater lifter acceleration off the seat and delivers a performance benefit.
The next step up from a 0.904 lifter means you are moving into the high-buck arena. Crower’s Groove- Loc lifter employs a locating pin to act as the lifter guide in the lifter bore. I have used them in a small-block. They work well and are a little less expensive than the Jesel equivalent.
However, when it comes to all-out high-tech lifters, Jesel is the specialist. The company manufactures a range of big-bore lifters going from 0.904 to more than 1 inch. Its top-of-the-line keyway-guided lifter rides in a bronze bushing. The lifter itself has a DLC (diamond-like car-bon) coating and, while highly functional, the whole setup is definitely not “budget.” This is what a typical Pro Stock engine uses. However, Jesel does make what is best described as a “sportsman” lifter of a more conventional design. They are still expensive but they are of aerospace quality.
As you have seen, a cam’s physical dimensions can affect its performance and reliability. Now is a good time to quantify dimensional effects.
On a dynamically sound lobe of a given duration, the amount of lift is restricted by the cam lobe and the size of the cam bore it must pass through during assembly. Sure, you see stock-diameter cams with lobes having as much as 0.550 lobe lift, but they are pushing the limits. The way to get higher lobe lifts while retaining some semblance of reliability, in addition to big roller lifters, is to increase the cam diameter.
The stock cam journal is 49.54 mm (1.950 inches). The most common oversizes are 55 mm (2.165 inches) and 60 mm (2.362 inches). The larger sizes allow lobe lifts of as much as 0.650 inch.
From the detailed discussion in Chapter 9, Camshafts and Valvetrain Events, you remember that the cam card does not necessarily provide the definitive timing setup. This is because the cam’s optimal timing is not a function of the cam itself. Instead, it is governed by the low-lift-flow characteristics of the head and by any pressure influences, positive or negative, within the intake and exhaust system.
Many engine builders assume that the stated cam timing is indisputably accurate. In reality, it is nothing more than a good starting point based on an educated guess. Making horsepower is the number-one criteria for most engine builders and the only way to do that is to put the engine on a dyno and adjust the cam advance/retard until best results are achieved. Doing so shows just how often the best out-put does not occur within 2 or 3 degrees of the cam manufacturer’s recommended setting.
So, the forgoing means you need some sort of cam adjustment. Sure, you can use offset cam buttons. Going this route is a cheap and totally inconvenient way to get the job done, especially when it comes time to adjust on the dyno. Multi-keyways are also a cost-conscious alternative, but at the end of the day you really need some means to adjust the cam timing that is user-and dyno-friendly.
A flat-tappet cam is mildly loaded toward the back of the block against the thrust face by the taper angle (usually about 0.001 inch across the width of the lobe) on the profile. A roller cam has a profile with no taper and therefore no thrust toward the back of the block.
A thrust button of some type is needed to control end float and consequently to stop the ignition timing from jumping all over the place. This usually takes the form of a nylon or aluminum button located in the end of the cam. It is sized so that it just touches the timing cover and consequently stops end float.
Rockers and Ratios
Buying rockers seems like a straightforward job. Just look through the ratios recommended for the various cams listed, and buy the best for the job that falls into your price bracket. Although seemingly simple enough, there is more to it than meets the eye. Real-world tests show that in practice, rockers do not deliver a constant ratio.
Figure 10.25 shows how a rocker’s advertised ratio and installed “working” ratio can differ substantially. You know that lifting the intake valve as fast and as high as possible is the way to go for an under-valved big-block. The question most likely to be asked here is: Just how much difference can there be between one brand/design of a 1.7 rocker compared to another 1.7 rocker?
The answer, as shown in Figure 10.31, is way more than you ever suspected.
If you are using more cam lift with a stock style rocker be aware that it can run out of slot length. It is less than obvious when this occurs. When the engine is turned over it is hard to feel a small contact issue because the cam lobe has a lot of leverage over the rocker. Even a minor collision results in breakages so check for this very possible problem.
After you have finished reading what I have said about the positives of a fast off-the-seat intake rocker action, be sure to heed the cautions I cover at the end of this section on rockers.
The rocker ratio charts in Figure 10.35 help you to make a rocker decision that can easily amount to a 25-hp gain over one made without them.
To demonstrate the characteristics you can most likely expect from the rockers, tests were done with a moderately hot street hydraulic roller cam having a lobe of 230 degrees at 0.050 tappet lift. This is a typical duration I use for what I would call a strong running and totally streetable build.
One pushrod length was used for all the tests recorded on the chart. For the purposes of convenience I performed a pushrod-length check for lift characteristics on the principle rockers, which is discussed later.
Now let’s look more closely at the chart in Figure 10.35 and analyze what can be purchased most cost effectively to best suit the engine’s needs.
Stamped- and Cast-Steel Rockers
Although the stock GM rocker is rated at 1.7:1, it is far from that figure during the initial valve-opening phase. The measured initial ratio is only marginally more than 1:1.
Granted, the rocker almost reaches its advertised ratio at high lift, but you should never lose sight of the fact that a Chevy big-block’s out-put is very sensitive to intake valve acceleration. Any build employing a rocker with such a low initial ratio could be giving up as much as 20 hp. The Comp CC1211 rocker is like the stock rocker, so it’s a stamped-steel piece intended for a tight-budget performance rebuild. In that context, these and similar Crane rockers do very well. Comp’s 1.72-ratio Magnum roller-tip rocker is listed next on the chart. This entry-level ball-pivot/roller-tip rocker is intended for builds that have a maximum of 375 pounds of spring load at full lift. Excess spring forces are demanding and even destructive to the ball pivot.
The Magnum-style rocker from Comp Cams gets my “best buy” recommendation based on what it costs versus what it delivers in performance and reliability. Although advertised at 1.72, the ratio delivered is considerably more. With the short test cam, the off-the-seat ratio is so much higher that the valve, at TDC, was opened 52 percent farther than the stock factory rocker. These rockers are a great deal, especially for short-cammed, street-driven big-blocks.
Some application-specific recommendations for optimizing their use are in order. As mentioned in Chapter 9, Camshafts and Valvetrain Events, the exhaust valve is more sensitive to duration than it is to off-the-seat acceleration. This means that if both intake and exhaust are equipped with Comp’s Magnum rockers, for a dual-pattern cam the exhaust lobe of the cam needs to be no more but preferably as much as 0.020 less lobe lift than the intake along with about 4 to 8 degrees more duration. If you’re doing a low-buck build with a stock stroke, these rockers work well with Comp’s Thumper cams for engines from 427 to 480 ci. These cams are ground on a 107 LCA, which is close to perfect for most engines in the displacement range just mentioned. That’s particularly so when using production factory heads with minimal modifications.
Such a combination still gives the Comp Cam’s Thumpr rompety-romp idle while producing good output. Also, this setup works very well with an entry-level nitrous kit. For example, a 125-hp nitrous-oxide kit with this cam/rocker combination typically cranks out an extra 140 to 150 hp.
Opt for a single-pattern cam if you are looking for the best deal in idle quality and mileage as well as a strong top-end output The best plan here, by far, is to select a cam from COS-Cam specific to your application. It takes no more than a phone call and costs the same as ordering from a cam supplier. Pro engine builders have reported that the COS-Cam selection often produces up to 40 hp more than they had otherwise seen. I have said it before: Remember, the wrong cam costs just as much as the right cam.
Aluminum rockers are by far the most popular type for a performance valvetrain so it is worth looking at them in detail.
Comp Cams: Their budget die-cast High-Energy rocker is intended for use with typical street-build springs, which are no more than about 400 pounds over the nose. The low ratio, compared to the advertised ratio of Comp’s High-Energy rockers delivers what might look like a convincing argument for not using them on a build intended for maximum out-put. The reality here is that it is a near-perfect exhaust rocker for many common cam specs.
As discussed in Chapter 9, Cam-shafts and Valvetrain Events, a dual-pattern cam works better with a lower-ratio rocker in the exhaust. Usually about 0.1 is enough, but with certain relatively common cam grinds, the exhaust rocker ratio needs to be lower by at least 0.2. Here is why: For the production of a catalog dual-pattern cam, many cam grinders simply use a longer and higher lobe-lift intake profile as an exhaust lobe. This can be recognized by the fact that the lobe used for the longer exhaust also has a greater lift than the intake.
This is one of those times when using a significantly lower ratio rocker on the exhaust delivers better results. Such a rocker selection typically smooths out the idle and allows the engine to make more torque from just off idle to about 500 rpm short of peak power. At that point, higher-ratio exhaust rockers often deliver a little more top-end output, but it’s nothing compared to what they lose to the lower-ratio rocker over the lower 75 to 80 percent of the RPM range.
Crane Cams: Crane Energizers are die-cast rockers. They deliver an over-all ratio a little above the advertised ratio and are a cost-effective rocker to consider for a strong street build. They work well on both intake and exhaust with a single-pattern cam.
Crane’s no-frills 1.7 (PN 13774) is a good rocker for springs up to about 550 pounds over the nose and works well with a large single-pattern cam. The Crane Gold race rocker is a 1.8 and I am lumping the 1.7 and 1.8 together here as their lift characteristics make them a really well matched pair for cams with 4 to 8 degrees more exhaust duration and about 0.010 to 0.015 less lift (up to about the same lift as the intake lobe). Also, when using a short, fast-opening single-pattern flat-tappet street cam (with up to about 275 degrees of “off-the-seat” duration), rockers with these ratio characteristics deliver well when used with decent-flowing heads.
PRW: You probably have never heard of PRW. It is best known within the engine rebuild community, for which it is an active supplier. I have used a number of their “split ratio” rocker sets (1.7 and 1.8 are listed in the chart). The ratios they deliver work well with a dual-pattern cam and the price is competitive.
Harland Sharp: This company was the original manufacturer of roller rockers. I have only used a couple of sets of their rockers, but I can report that they are well made and durable. The chart shows only 1.8, but the 1.7 has similar but lower ratio characteristics.
Scorpion: This company is fast becoming the rocker company. Every year their range increases by a large margin. At the time of this writing, I have used only their very high-ratio rockers to boost the lift on short-duration street-oriented valve-trains in engines of more than 490 inches. Experience to date indicates that they work well when used with cams having a lobe lift of up to 0.380 and duration figures around 244 to 248 at 0.050. When used in conjunction with shorter profiles from Comp, Crane, or Lunati, they allow the production of a strong top end without any compromise in the low end, thus making for an excellent street driver with great track performance.
Heavy-Duty Cast Stainless Rockers
Only PRW and Comp rockers are shown on the chart because that is where my experience lies. Crower also makes a very nice-looking stain-less steel rocker in various ratios from an advertised 1.6 to 1.8. I have used this company’s small-block rockers and I can report they are high quality. As the chart shows, PRW rockers deliver power-producing potential in valve lift. Even the 1.7 ratio exceeds 1.8, and the 1.8 ratio is bordering on 1.9. If you are anticipating using a relatively short cam, which in itself limits lobe lift, these rockers are a great choice to get the lift required to make a good top end while still retaining good low-speed manners.
Finally, Comp’s Ultra Pro SS and XD SS are serious rockers that can deal with the sort of spring loads typically used in a competition engine where valve lift is in the region of 0.900 or more. When building a serious engine requiring full-lift spring loads of 800 to 1,000 pounds and retaining a stud-mounted rocker, these are rockers for serious consideration.
It is easy to understand why a faster off-the-seat ratio is good for a typical Chevy big-block’s output, but there are downsides to its use. The faster the valve is lifted from the seat, the faster it is put back down. Excess valveseat closing velocities cause power-robbing seat bounce.
For a true street profile cam, this is not likely to be an issue. If you are building a high-lift valvetrain for a high-RPM engine, you must select cam profiles that have dynamically smooth closing ramps. This is yet another reason to go the COS- Cam route.
You must also take into account the effect that faster valve opening and closing has on other aspects. One commonly overlooked “domino effect” is swapping rockers with a low initial ratio for ones with a high initial ratio.
When using a solid cam, the lash is not intended to set a clearance of a specific measurement between the rocker tip and valvestem but to set the gap between the lifter and the cam profile. The intent is to set the lash between cam and lifter to absorb clearance at the end of the tappet ramp.
For example, assume that the tap-pet ramp is 0.010 inch high and the rocker ratio is 1.5:1. The lash, excluding expansion, is 1.5 x 0.010, which equals 0.015 at the valve. If, for example, the 1.5 rockers are swapped out for 1.8 rockers, to retain the correct lash at the lifter/cam profile interface, the lash must be increased to 0.018. If this is not taken into account, the advantage of faster lift rockers can often be largely canceled.
Lash is not the only factor that comes into play. Remember that a big-block really needs intake lift; lots of it and applied as fast as mechanically possible. Let us assume that the cam timing was perfect for the rockers originally installed on the intake. In such a case, simply swapping an initially lower-ratio rocker for higher off-the-seat-ratio rocker almost certainly reduces output. Why? The faster initial opening makes the engine think that the cam has been advanced because the intake part of the overlap triangle has increased in area.
However, the cylinder filling process does not know how high the valve is lifted at any particular point; it only recognizes a change in airflow. This means that the cam needs to be retarded to restore the same exhaust to the intake overlap triangle balance that existed prior to the rocker change. During valvetrain tests, this situation always comes about and must be addressed.
Here’s an example: The test engine was a 730-hp street unit. The cam was a solid street roller. The valvetrain setup (with a 0.100-long valve and a 0.05 lash cap) was optimized in terms of lash and cam advance. A set of advertised 1.8:1 intake rockers was used up to this point (not shown on the chart because of trouble with this brand). These rockers had an initial off-the-seat ratio of 1.55 and ended up at well over 1.8:1.
For the next test, the intake rockers were swapped out for Crane’s Gold 1.8:1 race rockers. They had an off-the-seat ratio of 1.785:1. Both sets of intake rockers lifted the valves to within 0.005 inch of the same peak lift after establishing the best pushrod length in each case. The Crane rocker lifted the intake 0.020 inches more at TDC than the baseline rocker. Optimal intake lash with the baseline rocker was 0.017 when the engine was hot.
The first move was to simply replace the baseline rocker with the faster-off-the-seat Crane rockers. After very carefully lashing the valves to 0.017 the engine was rerun. The result of the rocker change while all else remained the same was a torque reduction of 8 ft-lbs and a reduction of 8 hp at peak. If this had been a typical back-to-back test, the logic to support a fast-off-the-seat rocker would have been voted a failure.
The next move was to reset the lash to 0.019 to accommodate the higher initial ratio. The following dyno pulls showed a slight reduction in output up to about 300 rpm before peak torque, and from there on, an increase in output. Peak torque increased by about 6 ft-lbs over base-line and peak power rose by 3 to 4 hp.
At this point, to compensate for the faster opening the cam advance needed to be addressed. From pre-vious experience, a cam retard of about 11⁄2 degrees (this is the justification for always testing with a belt drive) would get the job done. After this change, the dyno showed peak power had climbed to 748 hp while peak torque increased by 9 ft-lbs.
This whole excise demonstrates the value of rapid intake opening and the need to time a cam to suit it. It also demonstrates why you should not automatically accept the advance recommended by the cam company.
Pushrod length selection is frequently detailed no further than establishing a length that gives an even “sweep” pattern across the end of the valve (Figure 10.39). That may be good for reliable trouble-free operation of the valvetrain, but it falls short of what could be done for maximum output. Depending on the rockers, the optimal sweep length is not necessarily the one for best lift.
With some rockers, pushrod length affects TDC and/or full lift by an amount that can show as a 10-hp difference on the dyno. Others are not so fussy. Comp’s Magnum roller-tipped rocker is fussy about pushrod length for maximum output.
Although the TDC lift point may only change by a couple of thousandths, full lift can change by as much as 0.025 and that can be a 10-hp change in output for a 600 to 625 hp engine. There is no point in giving away 10 hp, but there is a problem here. This rocker often delivers a greater lift as the pushrod gets longer, but the roller tip moves off the sweet spot in the center of the valve. When this happens (the Magnum is not the only rocker with this characteristic) a compromise is called for. Because this type of rocker is intended for use with a relatively soft spring, you should set the pushrod length in favor of valve opening rather than a centrally located sweep patch.
Because of their length, a Chevy big-block’s pushrods are run closer to their limit than might be expected even though they are 3/8-inch diameter. Although stock pushrods, if they are the right length, are all right for stock and near-stock spring loads and RPM, this situation does not hold true when valve lift, spring loads, and RPM increase.
If you use top-of-the-line 3/8- inch, 0.080-wall pushrods, such as those from Comp’s High-Tech or Crane’s Pro series, you can assume that they will open the valves, even with spring forces of 900 pounds over the nose. However, at these loads and at the RPM likely to be called for, the dynamics fall far short of optimal. For most engine builders, a Spintron is not available. So, with this being the case, knowing how to build a valvetrain with suitably stable dynamics at the desired RPM is a must.
Most cam companies have three tiers of pushrods, which could be described as stock replacement (first), sportsman (second), and pro or custom (third). If you use a second-tier pushrod, a 3/8-inch by 0.080-wall pushrod is good to open loads of about 550 pounds. I have used a 3/8-inch top-tier pushrod to about 700 pounds over the nose with a well-sorted valvetrain in terms of spring, valve weight, and cam profile. However, Billy Godbold, Comp’s valvetrain dynamics wiz, put the “safe” number for a 3/8-inch 0.080-wall pushrod at a more conservative 650 pounds over the nose.
If required spring loads are more than 650 pounds, consider upgrades on even the best 3/8-inch by 0.080-wall pushrod. The first to consider is a 3/8-inch pushrod such as Comp’s High-Tech 3/8-inch by 0.135-wall versions. They are almost 40 percent stiffer but also have about 35 percent more mass. Although this extra mass looks like a damming consequence of increased stiffness, it actually proves to be not nearly as bad as it seems because the valvespring sees anything on the pushrod side through the leverage it has due to its ratio.
With a 1.8 rocker, the pushrod’s “effective” mass seems to be about 35 percent of what it really is. In rough terms this means an increase of 30 grams at the pushrod or lifter looks like the equivalent of 10 grams at the valve. In round terms, you gain almost 40 percent in pushrod stiffness but only pay a penalty in effective weight/mass of about 3 to 5 percent if the overall valvetrain is considered. Utilizing the thicker 0.135-wall pushrod’s greater stiffness raises the bar to about 750 pounds of usable over-the-nose spring force.
The next upgrade in pushrod stiffness is to increase outside diameter by utilizing a parallel (nontapered) 7/16-inch by 0.125-wall pushrod. If stud-mounted rockers are still on the build list, appropriate guide plates are needed. Comp has a 7/16-inch guide plate for just such situations. The pushrods work with shaft rockers, but if they are used, a better choice is a 7/16-inch double-taper pushrod. It is lighter and virtually as stiff but has a higher lateral excitation frequency so, typically, it is dynamically superior. With 7/16-inch pushrods, you can go to about 900 pounds of over-the-nose spring loads.
There are occasions that call for something special; for that, you need to contact a custom pushrod company. Trend Pushrods is the industry standard in one-piece pushrods. Owner Bob Fox pioneered much of the technology that you see in modern pushrods. He designed the Spintron and his spin fixture bears his name. It has become an industry standard for testing valvetrains. If you don’t see what you want in your cam company’s catalog check out Trend’s offerings.
Another company well worth dealing with is Manton Pushrods. Go to their website or call for assistance in selecting the correct pushrods.
Stud Girdles or Shaft Rockers
Stud girdles tie in all of the studs and simulate (in terms of stiffness) the addition of a shaft to the rocker system. As such, they do a better job than indicated by the results on a Spintron. I have both spun and dyno tested the dynamic effect of a stud girdle on an otherwise well-sorted Chevy big-block valvetrain that is utilizing springs of about 800 pounds over the nose. Everything appeared better, but let me make it clear, it was only marginally so.
Trying to discern 3 to 5 hp in some 900 is difficult with all but the very best dyno. However, if asked whether or not a stud girdle should be used my answer is, budget permitting, “every time.” The reason is not so much the added stiffness but the ability to adjust the valve lash more accurately and consistently. Without a stud girdle the valve lash is retained by virtue of the locking action of the rocker’s polyloc setscrew. This Allen-socket setscrew tightens down on the end of the rocker stud and in so doing alters the lash you have just set.
By using a stud girdle the rocker adjustment can be locked in place by tightening the stud girdle. Unlike the polyloc locking setscrew, this does not alter the lash setting. Also it is more reliable than a polyloc setscrew because more often than you may suppose, setscrews loosen off. Being able to set the valve lash accurately and consistently is worth measurable power and that is the principal reason to use one.
At the end of the day the best way to open and close the valves is by means of a purpose-designed shaft-rocker system. If you budgeted for an all-out stud-rocker system, you are only a small financial step away from a “sportsman” shaft setup from Jesel or T&D (both make exemplary products). The way to start your shaft system purchase is to look through either (or both) company’s catalog to get an idea of what you feel your engine build may need. From there, call the company and go through the details with one of their experts.
Let’s sum up the dimensional decisions needed for rocker arm selection. First, ratios. What exactly is best for your application? Keep in mind that a flat-faced follower can theoretically impart infinite acceleration to the valvetrain, and only mechanical material limitations prevent this. However, diameter limits the velocity, and because of the small lifter diameter, a flat-faced follower brings about a valve-lift deficiency for an undervalved air-hungry engine. So, to get the required valve lift, a higher-ratio rocker is going to work to your advantage. With a roller lifter, acceleration is limited by the side thrust (pressure angle) imparted to the follower.
Remember, valve acceleration is a prime requirement to maximize output for a given cam duration. Thus, you can compensate for the rollers’ slower initial acceleration with a higher-ratio rocker. Whether you use flat-tappet or roller rocker arms, you need to opt for the highest ratio while still retaining a viable working geometry. Without making the rocker physically bigger, as could be the case for a shaft-rocker setup, the limiting ratio for an effective stud-mounted rocker is seemingly a little more than 1.9:1. Such a ratio allows you to generate valve lifts of about 0.950 inch. So for almost any build with stud-mounted rockers and targeted valve lifts up to about 0.800, valve lift should be at least 1.8 on the intake and 1.7 on the exhaust.
Second, layout. There are occasions when the layout of the cylinder head’s intake ports dictates the moving of the pushrod hole horizontally away from the port. This happens when a rocker with a centered push-rod pickup point interferes at the head casting’s pushrod hole. Some-times the option of cutting away the offending material from the head casting is not the answer because doing so would break in to the intake port. When this is the case, use a rocker with an offset pushrod pickup point. Although ordering offset rockers is easy if you are buying a shaft setup, you should know that not all manufacturers of stud-mounted rockers offer them.
Currently, only Harland Sharp and Scorpion offer offsets in the big-ratio rockers that you are most likely to need. When making the move to offset rockers, do not use any more offset than necessary. Stud-mounted rockers do not like having an offset of more than about 0.150. If the offset is greater, the uneven loading can shorten their life, especially if very strong springs are used. Most often, offset rockers are needed for intake ports that have been widened at the top, and the pushrods are touching. In that case, you either weld the heads and recut the intake port or get offset rockers. Usually the second option is the easiest to implement.
Without doubt, the valvespring is dynamically the most important component in the valvetrain. The top coil of the spring moves the same amount as the valve; lower coils move less. In practice, the mass equivalent of the spring in terms of the mass moved is about 1/3 of its actual weight. With a Chevy big-block, you need to use high valve lift and high acceleration while con-tending with a heavy valvetrain. This is a serious challenge to your efforts toward extracting power.
Whatever valvesprings are chosen, they need to have the highest possible natural resonant frequency. A spring’s natural resonant frequency is largely governed by its mass and stiffness. The primary purpose of a valvespring is to control the rest of the valvetrain. However, unless the spring has zero mass some of the force it delivers is used to control its own mass.
It may not be the right choice to select a spring that has the seat preload and over-the-nose force that the cam manufacturer calls for. For example, let’s assume a spring of 180 pounds on the seat and 450 over the nose is called for. Typically, a run-of-the-mill spring and its associated retainer weigh about 175 grams. Let us also assume that such a spring runs the engine to 7,200 rpm before valve crash.
All this being the case, a beehive spring/retainer combo of 125 grams controls the valvetrain up to 7,700 rpm or more. However, the spring/retainer combo only has 150 pounds on the seat and 400 over the nose. So does a beehive produce better power output? Because of the reduced friction within the valvetrain and the better control exercised over valve motion, the answer to that, in 95 percent of cases, is yes. As for increased RPM potential, a beehive setup returns bigger dividends for a lot less outlay than a titanium valve does when it replaces a heavy steel valve.
Comp, Crane, Manley, and Crower have a very comprehensive range of springs. PAC springs is on the cutting edge of spring design and also has a wide selection. In addition to the dimensions, poundage, and rate for their high-quality springs they also list the natural resonant frequency.
The higher the natural resonant frequency of the spring, the less likely the spring goes into surge. Chevy big-blocks have a heavy valvetrain, and surge is a major issue to guard against. Surge occurs when the cam dynamics generate vibrations at a multiple of the springs’ frequency. This “excites” the spring so that a wave motion is superimposed on the spring coils’ regular motion from opening and closing. This surge causes the spring to lose control, not just of itself, but also over the rest the valve-train. It can happen at RPM well short of terminal valvetrain crash. On the dyno, such an event is usually seen as a dip in the torque/power curve.
To control resonance, you use double- or even triple-coil springs where each has a different natural resonant frequency. This, plus the interference between springs, or a flat dampener between each coil, dampens out most of the vibrations that ultimately lead to surge. Spring performance, then, is very much tied into its mass and its delivered force. The better the material used, the smaller the spring can be. This, in turn, means a spring has less mass. It all works in your favor.
If you bought a set of assembled heads from a reputable company, such as AFR, Dart, Edelbrock, TFS, Brodix, BMP, or RHS, you probably have a pretty decent spring. If you are buying your springs to go with a specific cam, it pays to research which spring gets the job done with the minimum of spring force possible. This is especially true when using a hydraulic roller cam because the least amount of spring force results in the least lifter collapse.
In addition, a flat-tappet cam does not fare well with springs of more than about 350 pounds over the nose unless you use some pretty expensive cam facings and lifters. However, with an appropriate spring selection a flat-tappet valvetrain with high-ratio rockers can produce some very satisfying results. But keep in mind that this valvetrain can load up the cam/lifter interface more and can lead to an earlier failure if you use excessive spring loads. Any time a flat-tappet cam of any type or a hydraulic roller is involved, your search for as close to an optimal spring as possible needs to start with a serious look at beehive springs.
Any mass that moves with the valve results in the greatest inertial impact to the system. Titanium retainers reduce valvetrain mass and thus reduce the spring forces needed to run the target RPM, but they are expensive. If a good spring selection is made, it could possibly take a smaller and consequently lighter retainer. A beehive spring excels here. The small top coils only require a small retainer; a steel retainer for a beehive is usually substantially lighter than a titanium retainer for a regular valvespring.
The most critical and influential mass in the valvetrain is the intake valve, and at 165 grams, it is significantly heavier than the exhaust valve at 130 grams. Remember that the intake needs to be opened as fast and as high as possible. But this is far from the case for the exhaust. Not only is the exhaust valve lighter, it only needs to lift about 90 to 95 percent compared to the intake. In addition, it usually has about 4 to 8 degrees of extra duration in which to open and close, plus it does not need to be accelerated as fast. Indeed, it is possible to open the exhaust valve too fast and actually lose power.
From the forgoing, you can see that this is why most aftermarket heads utilize an 11/32-inch stem instead of the stock 3/8-inch stem. Doing so saves about 20 grams. But slimming the valvestems follows a very rapidly decaying law of diminishing returns. Going to a 5/16 stem can result in a valve that, during the opening phase, waves around like grass in a wind. On the Spintron, I have seen a 5/16-inch-diameter valve-tip stem move back and forth out of its line of travel by about 1/16 inch.
On the other hand, a hollow-stem 11/32-inch valve only moved about a quarter of that amount. So, to lighten a valve by any significant amount, it is best to stick with an 11/32-inch OD and opt for a hollow stem. This is cheaper than a titanium valve and more reliable for the street. If the bud-get allows, I use Ferrea hollow-stem valves as they save enough weight to allow a valvetrain to rev to 7,300 rpm that would otherwise only run to 7,000 rpm. If titanium valves were installed, that same valvetrain would run to about 7,500 rpm.
For a top-of-the-line street build and a good race build where the budget is a factor, the best way to go is to use a titanium intake and a steel exhaust. The fact that a steel exhaust is used has no negative impact on the power potential of the valvetrain as a whole. However, longevity is more reliable.
Cam Master Program
Although I was a research engineer for General Electric in the 1960s, I started studying the factors affecting optimum cam/valve events. Initial progress was slow because determining valve events was fraught with difficulties, including the obvious need for a lot of computational capability. By 1980, I had my own dyno, and cam tests became almost a daily routine. Then certain key aspects began to fall into place. Also, cam guru Harvey Crane was a friend, but more important, he was a mentor.
In about 1985, Harvey began to take a serious interest in my work. As the founder of Crane cams, he was obsessed with the achievement of smooth mechanical dynamics. He worked with noted mathematician Dr. Gary Mathews of the University of Florida in Gainesville who pioneered work in this field. The work these two men did set industry standards. Harvey had a 60-plus-year career in the cam business and he trained countless people in the art of cam dynamics, including me. But knowing how to open and close valves rapidly and smoothly is only half the equation here if the goal is optimal output. This brings us to a very important aspect of cam design.
All of the mechanical dynamic skills in the world amount to nothing if the valves are opened and closed at the wrong moments.
Thanks to Harvey, we have a significant number of highly competent engineers who can, with math and computer programs, develop profiles that are dynamically extremely good. But what about the valve events, or as I call it “the gas dynamics?” I remember when the subject got around to valve events Harvey would say, “Use the WEW method.” And what does WEW stand for? Answer: Whatever works!
So we had plenty of cam-profile engineers, but apparently none who could compute the power-critical moment at which the valves should open and close. It was my hope to fix this so I made it my mission to become a gas dynamics expert.
In 1985 Harvey funded me for a very comprehensive and expansive cam test program at my shop in Riverside, California, to (we hoped) fill in the holes then existing in the fledgling Cam Master program. Tens of thousands of combinations (literally) were tested in many engines over a period of 12 years.
After another couple of years, a prototype Cam Master program was produced. For the next 5 to 6 years, Denny Wycoff, a retired drag race champ and engine builder, field-tested Cam Master by servicing racers through his Engine Machine and Supply. The results were nothing short of outstanding. On every single back-to-back test, the Cam Master cam won. It didn’t matter whether it was Pro Stock or Super Stock; the valve events developed by Cam Master reigned supreme.
Try this little experiment if you think that the cam company you deal with must have the expertise to sell you an optimal cam: Call up the company’s cam desk and get a cam spec for your engine. Then a week later, have a friend call for a cam spec for your engine. After another week, have another friend call. You will almost certainly get three distinctly different cam specs. Can they all be right? I think not. It doesn’t matter how many times you or your friends call, Cam Master returns the same answers every time for the same engine spec, regardless of who answers the phone.
So why am I emphasizing cam selection so much? Well, I see my job as one of bringing the latest functional performance technology to my readers. As is so often the case with cams, readers are fed rehashed outdated twentieth-century technology that informs just sufficiently to ensure failure. Cam event timing is something I lecture on at universities. It is tech you could not get from top Pro Stock engine builders, let alone a camshaft company. So why go to the method I am about to detail here? Why not do it the well-received high-tech way? The simple answer is: The method of cam selection described in my previous Chevy big-block book is high-tech compared to other methods out there. However, the cam selection process in that book is limited to just one manufacturer’s profiles. The method I am about to describe uses the cam library of all reputable cam grinders. This delivers the best cam that current technology allows, it is made from at least ten times the number of grinds, and it keeps the cost down.
The COS-Cam Idea
I wanted to make Cam Master power available to the general public, so the COS-Cam idea was born. The COS-Cam (Computer Optimized Spec Cam) works for finding a cam of optimal spec for a particular application. In contrast, if you call your favorite cam company for an optimal cam, you get whatever profile in their library looks best. But what if a competing cam company has, for your particular application, a better set of profiles, do you think that cam tech guy you called will tell you to go down the road and get your cam from a competitor? No, that just doesn’t happen. Cam Master computes the cam required and COS-Cam uses those results to search a library of cams and cam profiles from all the reputable cam grinders and selects the best cam/profiles.
Because it takes someone with capable engineering skills, only a limited number of COS-Cam–licensed companies offer it. In the United States, about a half dozen top-performance companies have COS-Cam sales capabilities. As I write this book the only U.S. license so far has gone to Pro Stock racer/engine builder Terry Walters at TWPE.
If you have concerns as to the cost of a COS-Cam spec’d cam, put them aside. Unless you have an all-out race situation, the cost is equal to or even less than you would pay for a cam from a cam manufacturer.
Also, here is something that COS-Cam does that no other cam supplier does: give you a guarantee that your COS-Cam will outperform whatever you currently have or your money back.
Written by David Vizard and Posted with Permission of CarTechBooks