Selecting A Camshaft
This section is written to give you some idea of how the other modifications to your engine, and the intended use of the engine, will effect the selection of a cam profile. At Kennelly Cams we recommend you contact us to discuss your specific requirements.
Some of the main factors effecting camshaft selection are:
Engine RPM
This is arguable the most important consideration in choosing a camshaft.
What idle quality do you expect and how do you want it to sound?
What drivability do you expect at low RPM?
How important is the mid-range power, and how aggressively do you want the power to come on?
How far do you want the engine to rev out?
In brief, a shorter duration will suit lower rpm, and longer duration will suit higher.
Tighter lobe center angles (LCA) will promote strong mid-range power, but at the expense of idle quality and it will also limit how far the engine revs. Wider LCA will reduce over-lap improving idle quality and will allow the engine to rev out further, but at the expense of mid-range punch.
Tighter centers give a shorter power band with lots of mid-range punch, wider centers give a wider smoother power band.
High rpm engine suffer considerably more from friction loses. During the exhaust stroke, pumping pressure and friction can be reduced by opening the exhaust valve earlier. This means wider LCA and more duration. Opening the exhaust valve earlier will reduce the power produced by the gas pressure pushing against the piston, but this will more than be made up for with the reduction of parasitic friction and pumping loses.
Bigger cylinder capacity typically needs more duration and wider center lines to produce similar engine characteristics to a smaller engine. For example a 5 litre engine with stock internals, that you want to have a reasonable lump to the idle but run on a stock torque converter might run 218 degrees at .050" on 108 lobe center angle (LCA). A 7 litre engine with stock internals, with the same idle qualities and also running a stock torque converter, might run 230 degrees at .050" on 110 lobe center angle.
Compression Ratio
It is worth considering the compression ratio when choosing a cam. One of the first modification that a cam manufacturer might recommend is a 1 point increase in the compression ratio. This helps improve the crispness or throttle response at low rpm. That means you can get away with a bit more cam and get more mid-range power.
If you want an engine that drives nicely at low rpm and you only have 8.5 or 9:1 compression, you will need to stay mild in the cam. Increasing to 9.5 or 10:1 compression will let you run an extra 6 to 10 degrees more duration while still maintaining bottom end power.
Very low compression engines as found in older engines, like shorter duration and tighter LCA
Intake System
There are many variations of intake that will effect cam choice. Single or dual plane, carb or injected, individual runner?
Factory fuel injection on many engines can be very sensitive to cam changes. On these systems, if you really want to change cam, avoid too much overlap. Overlap will cause pulsing in the intake system that injection systems often cannot interpret.
However some other systems, usually with mass airflow sensors, can sense the change in the volume of air going into the engine at different rpm, and adjust fuel flow to suit the changed cylinder efficiency.
Single and dual plane manifolds can come in many different shapes and sizes and performance will change with each.
Probably the most important thing is that a dual plane manifold, whilst producing good bottom-end power, will not let the engine rev. There is no point running a dual plane manifold and then trying to cam it to get good high rpm power. All that would be doing is running a manifold that hurts the top end, and a cam that hurts the bottom. Net result, no strong power anywhere.
Individual runner carburetors or injection are the ultimate. This prevents intake pulses mixing between the cylinders, thus allowing very strong pressure wave supercharging of the cylinders. Because individual cylinders are not being effected by pressure waves from other cylinder, more cam duration can be used without unduly effecting the smoothness of the bottom-end power. Typically 10 degrees more.
Port Flow
The most important thing about the cylinder head flow might seem quite obvious but is often overlooked; is there enough?
If you do not have enough intake flow for the amount of horsepower you want to make you will need to leave the valve open longer to try to fill the cylinder. This extra duration will hurt bottom-end and mid-range power leaving a very compromised power curve. If at all possible fix the flow problem. As a rule of thumb, 100cfm measured at 28 in/h2o is good for about 28HP per cylinder.
The most important area to look at on exhaust flow is; how much flow does the exhaust have compared to the intake at 35% of full lift. When the exhaust valve first opens the burnt gas in the cylinder is at high pressure, and a good flowing valve will empty the cylinder quickly. So if you have good flow at this point, you can open the exhaust valve a little later yet still relieve enough gas that pumping pressure on the exhaust stroke will not cost too much power. The later exhaust opening means the exhaust gas pressure on the piston is kept high a little longer making more power.
An exhaust to intake ratio of 80% (at 35% lift) is fairly normal for a 2 valve engine, and would typically require 4-10 degrees more exhaust duration than inlet, depending on the engine size. A 4 valve engine can often achieve 110% (10% more exhaust flow than intake) or more, and can usually get away with less exhaust duration than intake.
Supercharged / Turbo Charged
Supercharged engines typically have a shorter intake, longer exhaust, and wider lobe center angle (LCA).
Supercharged engines can continue pushing charge into the cylinder longer than an NA (normally asperated) engine, and also the greater pressure differential between the manifold and the cylinder means the cylinder is filled quicker. This means they like a shorter intake duration on a later centerline, compared to thier NA counterparts.
Higher cylinder pressure after combustion means more gas to get rid of, so the exhaust valve is opened earlier than on an NA engine. This means longer exhaust duration, on an earlier centerline.
Early exhaust centerline and late intake centerline means a wider LCA. This means there is less overlap which also helps stop the pressurized intake charge being lost out the exhaust valve.
Turbo Charged engines are more complicated.
Before the turbo charger reaches an efficient speed the engine behaves much like an NA engine, albeit a low compression one. This means to get the cylinder filled efficiently and producing good amounts of exhaust gas to 'spool up' the turbo the ideal would be a relatively tight LCA. However once the turbo has 'spooled up' and is efficient the engine behaves more like a conventional supercharged engine and wants a wider LCA.
Turbo engine camshaft selection, and the overall performance of the engine, is greatly effected by the turbo selection.
It is easier to get a turbo to spool up at lower rpm by choosing a smaller exhaust turbine housing than by manipulating cams. This means the cams can then be optimised for 'on boost' performance.
Typically higher boost levels, and higher rpm, require more cam duration. The main difference between supercharged and turbo charged engines, is that turbo engines do not flow from the intake out the exhaust, at overlap, as easily as a supercharged engine, and therefore tend to open the intake valve earlier. So turbo engines tend to have a longer duration intake than a supercharged engine, but still shorter than an NA engine.
The turbo charger should be selected before the camshaft, remembering that a turbo, much like a supercharger, can restrict power if it is not big enough.