n the previous page we looked into the basics of the cam. Now we take it a step
further and we're taking a look at making a good cam selection and points to watch out for.
A good cam selection depends on a lot, and I really mean a lot of things!
Let's have a closer look at what is involved in choosing a camshaft
The 6 cyl head is a mix between a bathtub and hemispherical design
Notice the damage around the exhaust valve and the squish area!
What kind of power do you want?
Well this is the main question! If you want lots of torque a cam with long duration, big overlaps and a
small separation angle is defenitely not what you want. You need a cam with shorter duration, moderate overlap and
a wide separation angle. In this case higher valve lift is more of importance than longer duration.
Now selecting a cam with Power AND torque will be difficult. It simply is a choice of one or the other.
However when selecting for power (long duration, high lift, normal to big overlap) you can do some concessions
to peak power and leave some torque in the lower rpm regions. This can be done by selecting a cam with
a larger separation angle. This will remove some top end power but you will regain lower rpm torque.
Another possibility is to advance/retard the cam. Advancing the cam will open the inlet and exhaust valve earlier
and also closing them earlier. It will give some extra top end power at the cost of flexibility. Retarding the cam
will cost some high end power but you will get extra torque at lower rpm.
Remember that you can't just increase valve lift without putting more stress into the valve-train. And also
coil binding is a point to watch with high valve lifts!
Type of car
Is your car a heavy car or a light one. A heavy car with a peaky engine (Long duration cam, big overlap,etc)
can be a pain in city driving because to pull all that mass it needs to have some low down torque.
When tuning a bigger capacity engine things would be less dramatic because the longer stroke and
engine capacity give you some extra torque the standard 3.5-litre lacks.
With a light car like a Westfield you can more easily use a hot cam. The engine will be driveable
with this bantam weight car.
A heavy car (like the SD1, Range Rover, etc) and a hot cam don't make a very good wedding. But there is a solution
to the problem and that is gearing...
The standard SD1 ratios are quite tall! When in production the 3.5 litre versions had one of the tallest
ratios in the world! Now with a hot cam and that tall gearing there can be a problem. When cruising on
the motorway at 70 mph and 2300 rpm the engine won't be in its favourite rev. range. When overtaking
you really have to change down to fourth or even third to get in the optimum rpm range and have good acceleration.
So if you want more powerfull accelleration and an engine that operates more often in the optimum range you
have to change the gear ratio. The hotter the cam, the lower ratio you need.
Instead of changing the gear ratio you can also select other tyres, however when tuning most people
want bigger and wider tyres with larger tyre diameters and that really doesn't help in matching the engine rev
range with the gear ratios. Low profile tyres however can help a lot here! You can experiment with their effect
at the driveline calculator page.
Fortunately there is a reasonable range of differential gear ratios available for the SD1. On normal production cars
these differential ratios were:
|3500 VdP EFI||2.85:1|
As already said a big engine inherently will give more low rpm power than a small one. So the bigger
the engine the hotter the cam you can choose without getting a very temperamental engine.
Type of use
When you drive in normal traffic a specific torquey cam or a very hot one are probably too much.
If you drive a LR or RR (and use it where it's built for!) or do a lot of caravaning, towing than a cam
giving more torque could be helpfull.
On the race track a hot cam is less of a problem because you can keep the engine "on tune"
An important thing is to select a cam which fits with your inlet setup.
If you have a restrictive inlet system, original SU system or the federal injection system, with
standard heads than these are seriously restricting flow and possible power gains. A hot cam will give
some improvement but not as much as you expected and it will be a waste of the expensive cam.
Things are already a lot better with Vitesse heads and EFI system or Holley/Edelbrock carbs.
With a hot cam a stage 1 or stage 2 head is certainly recommended. If you still want a hot cam with a
standard inlet system than you should look for a cam with higher lift (more torque) and a bit longer duration.
Be aware that when using a bigger capacity engine or running high rpm's, the valves also have to flow more air.
You can give them more lift or use bigger valves. Big capacity engines with restrictive heads, like the Rover
often have quite large gas speeds in the valve area. especially when not fully opened. This can really have a big
effect on the flow. With the valves opened 100% the flow might be good on a flow bench but at partly open the
gas speeds can be so high that flow is severely restricted. As a rule of thumb the valve will be big enough
when the gas speed is kept below 75 m/s at the fully opened position
With an open "big bore" system back pressure is less. If you want to maintain good low torque
properties you can use a cam which opens the exhaust later and closes it earlier. Because of
the good flowing exhaust, power loss will be little but you will gain in low rpm torque and
because off less valve overlap will get a less cammy engine. If you want no power loss whatever
then you can still use the hot cam you already wanted. An open exhaust system just gives you more
room to play with.
If you are stuck with a fairly restrictive exhaust system than choose a cam with higher lift
and a longer exhaust duration to offset for the high resistance. The cam should open the exhaust
valve quite early to allow the high pressure of the gases to quickly blow down in the exhaust.
Yes, this will most of the time give quite a cammy engine, but
then, nothing comes for free.
If you have a low compression engine and a hot cam, I have some bad news for you. This will simply not work out.
The reason is that with a long duration (hot) cam the inlet valve will close quite late. Now if you have a look
at the page about increasing capacity (More cu.in. 2) The calculation program there
will show you that when you close the inlet valve later you will also decrease the effective compression ratio.
A late closing inlet valve AND a low compression ratio will result in a very low effective CR. This will give
a very inefficient, temperamental engine. As a rule, with longer duration cams you also need more compression to make
it work at its optimum. This is an important point often overlooked.
Now rod length will not have a big effect on selecting a cam, but to be complete Ijust want to have it mentioned.
With a very long rod the piston will stay relatively longer at the top of the stroke. You don't believe it?, You'll
find a diagram at the right. So this will increase the time the inlet and
A very flexible valve drive with not so stiff pushrods won't be able to cope with a hot cam profile.
Wear will be very high or might even result in expensive damage. With a pushrod design you simply can't
make an engine that will rev as easily as a comparable OHC design without doing some stiffening up work on the
lifters, rods, rocker arms, etc. To compensate for it's flexibility a pushrod design will often need 1 or 2 degrees
more duration than a comparable OHC design to give the same power output.
So and now how to choose a cam from all this info?
Ehhhm, well as you see it can be made all quite complicated. And as always we try to make
complicated things easy here. So here we have a page which will allow you to enter your engine
and car data and our calculator will give you a possible cam. This will give you more insight in what is involved
in selecting a cam. As far as I can tell it is the only online calculator on the internet which can do this.
Thanks to: Adriaan Briene