Camshafts and Valve Gear
The limitations of the Holden cylinder head have a big influence over the cam profile selection. Because the port design and the tendency for the valves to be shrouded limits the high-lift flow, it's important to get the valves opened as quickly as possible. In other words we need to get as much area under the curve as we can without making the duration ridiculously long. The downside to using such aggressive profiles is a reduction in cam lobe and lifter durability, but really if you want to get the maximum performance from the engine you don't really have much choice but to smack the valves open and put up with the shorter cam life. Like many GM engines, the Holden engine would benefit from a camshaft with bigger journals and a bigger base circle diameter - with flat tappets there are limits to the lift and valve acceleration rates that are achievable. In this respect a roller type cam would have a decided advantage, though I'm going to conveniently ignore them here as being outside the scope of these basic notes. If you have the time and money to use rollers though, it would certainly pay to discuss them with a reputable cam grinder as they will provide not only a higher peak power level but more area under the torque curve as well.
Basically we are going to have to choose beween hydraulic or solid flat-tappet cams. Common wisdom has it that any street driven vehicle should run a hydraulic cam, but there are some very good reasons to go with solids. Everyone knows solids aren't susceptible to the lifter pump-up issues that hydraulics can sometimes suffer from. But there is also a substantial performance advantage that goes with solid lifter profiles. Hydraulic lifter cam profiles need to have a short but gradual ramp just before the valve acceleration starts in earnest. The same thing happens on the closing side. The net effect is that, for a given duration at 0.050", a hydraulic cam will hold the valves off the seat for longer than a solid lifter cam will. These ramps are so gradual that no useful flow occurs, but it's enough to bleed off cylinder pressure at lower speeds. By contrast a solid lifter cam will leave the valve firmly on its seat before slapping the tappet rudely. In other words, for a given peak horsepower, a solid lifter cam will give a stronger low end and midrange than a hydraulic cam. Or looked at in yet another way, for a given low or midrange torque, a solid will give a better top end.
Hydraulic lifter cams do have a couple of advantages though; they are quiet and maintenance free, and if performance isn't the primary consideration they could be a logical choice. They also don't require adjustable rocker gear. The factory VK EFI cam is an excellent hydraulic grind for a mild engine, but of course any of the original cams will be very badly worn by now. Also keep in mind that the cams in the later engines were set up retarded a few degrees to help with emission control so if you use one you might like to install it "straight up" for stronger bottom end torque. If however performance is the priority, definitely go for the solids. They have a reputation with some for needing constant adjustment, but in reality once everything is bedded in they should rarely need attention. Millions of engines are fitted with solid lifters as standard and these often go for years between adjustments. Granted, the stresses on a high revving Holden 6 valvetrain might warrant more frequent setting, but a few times a year should be enough. Besides, adjustment is much easier and quicker to do on the 6 than say a V8, so it doesn't take long at all.
When comparing cam profiles between different manufacturers check the duration at 0.050" only, the advertised duration figures are too "rubbery" to be meaningful.
Some very very rough rules of thumb for cam selection:
Longer duration = more top end, less bottom end
More lift for a given duration = more area under the torque
curve
Closer lobe centres/more overlap = more bottom end and less on top
(but more overlap also makes for rougher light throttle running and
idling)
Wider lobe centres = better top end at the expense of low down
torque, better light throttle running and idling
Slightly retarded cam timing = more top end, less bottom
Slightly advanced = more bottom end, less on top
Did you notice the pattern there? Anything that results in the intake valve closing later will move the torque peak up the RPM scale, and vice-versa. Of the four valve events the intake closing is by far the most critical - you can shift the other three around a bit and it doesn't really make much difference to the power curve, but as soon as you shift the intake valve closing point the curve will also shift.
So what cam to use? Here is another very rough guide:
Mild daily driver, smooth enough for granny - 200 to 210deg @
0.050", 0.39" to 0.41" lift (hyd)
Slightly warmer, rough idle but still fairly civilised - 215 to
225deg @ 0.050", 0.4" to 0.44" lift
Hot barely streetable cam - 230 to 240deg @ 0.050", 0.44" to 0.48"
lift
Bigger again, maybe just barely streetable for occasional use by a
masochistic driver - 240 to 250deg @ 0.050", 0.490" to 0.5"
lift
Animal, dont even think about trying to use it on the street
-250+deg @ 0.050", 0.520" to 0.570" or more lift
The engine capacity also has some influence on cam selection; a cam that is "big" in a 202 will behave even "bigger" in a 161. Or put another way, a cam profile that produces peak torque at a certain speed in a large engine will peak at a significantly higher speed in a smaller engine. Bottom line: don't kill the bottom end torque of a small engine by using too much duration. We mentioned this briefly in the Planning section but it bears repeating: the cam profile must be selected with the available gear ratios in mind. It's very easy to overcam the engine in the pursuit of horsepower, and while the dyno numbers might look impressive it may well be impossible to keep the engine pulling hard on the track with less than six gears. Do your sums carefully - it's very likely that you'll have to use a relatively conservative cam to get the most performance from the commonly used transmissions.
Twenty-five or so years ago, there was a trend towards cam profiles with more aggressive lift rates, higher lifts and shorter durations. The idea was that with these profiles high outputs could be maintained without narrowing the powerband as dramatically as the traditional cams did. The cams were marketed using terms such as "high-intensity" and "high-energy", and the inference was that the shorter duration could be compensated for with faster lift rates. Or, in other words it didn't really matter whether the area under the curve came from lift or duration, the engine would make power proportional to that area. Unfortunately it isn't that simple, but there are some that persist with this line of thinking even today.
To put it simply; to make peak torque at a particular rpm level requires a certain amount of duration, and no amount of additional area under the curve will drastically change this. Certainly, faster valve acceleration rates will help fill the torque curve either side of the peak, but they won't help much at lower rpms and may even hurt low-end torque. Anyway, what I'm getting at is that lift and duration are definitely not interchangeable, and if it takes say 305deg of duration to make the engine peak at a particular speed, well, that's what it takes.
When you're choosing how much lift to use it basically comes down to a trade-off between performance on one side and durability and expense on the other. For a regularly street driven car I'd probably limit lift to about 0.5" in the interest of long component life. For an unlimited competition engine the output will keep increasing with valve lifts right up to 0.650" or more, but you'll need some pretty high quality parts to run these lifts for any length of time. Roller cams in particular (because of the heavy lifters and high acceleration rates) require springs with very high rates and these need to be very high quality. And if the spring rates are astronomical so are the prices... Valves, seats and guides also take a pounding with the high lifts and spring rates. If that's what it takes to be competitive then there isn't much you can do but run the big lifts, but be prepared to pay for it..
When you get the cam, make sure you get all the related components from the same manufacturer so that you can be sure the springs, retainers and lifters etc. are compatible. You may need to make the spring seats bigger/deeper for big springs and high lifts. If you aren't sure how much material is in the spring seat area do a practice run on an old head first or cut an old head up to see how much meat is in there. Be aware that the 12 port heads ran a shorter installed height than the 9 port heads, and that often means that 0.1" offset retainers are necessary even with some street cams. For more extreme cam profiles it may pay to use longer valves (eg. V6 Buick valves) in order to make room for longer springs.
Drive the cam with an alloy or steel gear, but for chrissakes don't use those damn straight cut gears - their only virtue is their cheapness and the constant whining is irritating. The claims that the helical gears sap power are drivel. The only time I would consider straight cut gears is on a very high rpm engine with extreme valve spring pressures. These engines tend to have a fair bit of torsional vibration of the camshaft and with helical gears they rattle against the thrust faces and scatter the spark timing a little. In extreme cases they can break the retainer plate. For anything not so radical (and for any streetable engine) I'd stick with helicals though.
Don't try to be clever with cam timing; at least to start with set it exactly where the cam grinder intended it to be run; you can experiment with minor variations later. If you need to reposition it more than a few degrees for best performance you've got the wrong cam... Carry out all the usual precautions ie. check for coil bind, retainer to guide clearance, valve to piston clearance, rocker slot, etc etc. As well as all these checks, you'll also have to check for valve to deck clearance because of the chamber overhang. The press in rocker studs are good enough for a mild engine, but use screw in studs for setups with higher lifts and heavier springs.
The factory rocker setups are fine with any reasonably streetable cam, though you'll need some form of adjustment for solids. Roller rockers are nice to have with bigger cams, say over 0.45" lift, and the shaft mounted jobs eliminate stuffing about with guideplates. If you run ball-mounted rockers (or stud mounted rollers) on a head that originally had aluminium rocker bridges you will need guideplates and hardened pushrods. Standard rockers are 1.5 ratio, you may find rollers in other ratios or adapt rockers from other engines (eg Cleveland). Be aware that higher ratios increase the loading on the entire valvetrain so while they definitely improve performance there is a tradeoff in durability. If you decide to try the cheap Chinese rollers carry a couple of spares in your toolbox, and pray that when they fail you catch them before they dump a million needle rollers throughout your engine.
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| Adjustable shaft-mount rockers like these Street Terra units allow some flexibility when setting up the rocker geometry |
There is a difference in the length of pushrods used with ball mounted rockers versus the later non-adjustable rockers, so be careful when mixing components from different engines. You should also take the time to get the rocker geometry right; with so many things like deck height, cylinder head dimensions, cam profile etc. being altered it's quite likely that a non-standard pushrod length will be required. At the mid-lift point, an imaginary line drawn from the valve tip contact point through the centre of the rocker pivot should be perpendicular to the valve stem. Shaft mounted rockers have an advantage here as the pivot post can be machined or shimmed to correct minor discrepancies.
Holden lifters should be used, even though there may be others (eg. Chev) that will fit. The reason for this is that the other lifters may not have the relieved section in the right position on the body, and could possibly cause a major loss of oil pressure.
Follow the manufacturers cam break in procedure closely and make sure you use an oil with sufficient zinc additive to protect highly loaded flat tappets, eg. Rimula Super or Valvoline Racing. Most modern petrol engine oils aren't suitable, and using them will lead to cam and lifter failure.
Keep in mind that many of these cams will provide insufficient vacuum for brake boosters and so on - you may need to provide a separate vacuum source such as an electric pump or an alternator from a Japanese diesel with a built-in vacuum pump.
One more rule-of-thumb: if you can't choose between two different cams always go for the milder one. An overcammed street motor will be slower than it should be, and more painful to live with.