Camshaft selection                                                                                    

The most difficult choice facing any RH7 owner who wants more performance from his engine, is whether or not to contemplate a change of camshaft. There is an awful lot of confusion about modified camshafts, what they do, what makes a cam 'hot', which cam is suitable for which application, whether a given cam will make an engine intractable etc. What I am going to try to do is debunk some of the jargon, explain how a camshaft works, what it does and how a modified camshaft contributes towards the performance of an engine. I will not go into detail about timing of cams, or comment on cam phasing, or cams that have different durations between inlet and exhaust.

In is imperative when selecting a cam to first understand its nature. A camshaft in a four stroke engine controls the opening and allows the closing of the valves in the cylinder head which in turn allow the fuel/air mixture into the engine prior to combustion, and allow the spent gases out of the engine following combustion. During combustion both valves must be closed to allow the ignited gases to press down on the piston, producing horsepower. Sounds simple doesn't it?

In an ideal world, the inlet valve would open when the piston is at Top Dead Centre, following the exhaust stroke, would remain open until the piston reached Bottom Dead Centre after the induction stroke, and then close for the compression stroke. The exhaust valve would then open at Bottom Dead Centre following the power stroke and remain open until Top Dead Centre of the exhaust stroke, at which time the whole cycle would start again.

The period for which the inlet valve is open is measured in degrees of crank revolution and is known as inlet duration. Again in an ideal world this would always be 180 degrees. Similarly the period for which the exhaust valve is open is measured identically and is know as exhaust duration (collectively both are simply 'duration'). Again in an ideal world free from the laws of physics, the duration would be 180 degrees.

In practice, because the fuel/air mixture inducted into the engine and the exhaust gases expelled have mass and therefore inertia, and the valves and valve train have mass, things do not happen instantaneously. If the inlet valve were to close at BDC of the inlet stroke the cylinder would be in a position of partial vacuum. In fact keeping the inlet valve open PAST BDC allows more mixture into the cylinder, even though the piston is at this time rising to compress the mixture. If the exhaust valve were to close at TDC on the exhaust stroke, spent gases would still remain in the cylinder, so the exhaust valve is left open PAST TDC even though the piston is now falling to induct new mixture.

The faster the engine runs, the more it is sensitive to the inertia of the incoming and outgoing gases. The duration of the camshaft in a normal production engine is usually around 255-265 degrees of crankshaft rotation, that is 80 degrees more than the 180 degrees of our ideal-world engine. If we ignore the effects of lift, it follows that if an engine needs to rev higher, the valves need to be open for longer in order to sustain power output.

Cam lift also has an effect on engine power, but less on engine characteristics. It follows that the higher the valve is lifted off its seat the more flow is allowed into/out of the cylinder, up to a point. The major limiting factors for valve lift are mechanical, in order to lift higher for any given duration the valve has to be lifted faster. The weight and inertia of the valve train places severe mechanical limitations on the speed at which this valve acceleration can happen without knocking nine bells out of the cam, the followers and all. If you accelerate the valve too quickly, it will lose contact with the cam follower and lobe, and then crash back down onto it and bounce when the spring exerts its authority. This will very quickly knacker the cam, follower and associated gubbins, it may also cause valve/piston contact, coil binding and general unholy nastiness.

Generally speaking 'high-lift' cams give an increase in torque across the rev range rather than moving the power band up or down. They also give good emissions (which high overlap cams do not). In the early days valve lift on pinto cams generally tended to be less (although still more than standard) with duration being longer, more recently, the tendency has been to give more lift and less duration so that emissions are more acceptable. Careful design of the lifting ramp of the cam and improvements in metallurgy have allowed these higher lift cams to operate without breaking followers or producing excessive wear.

You can see from the previous paragraphs that there is a time during the change from exhaust stroke to induction stroke when both the inlet valve and exhaust valve are open together, this period is known as 'overlap' and is measured in degrees of crankshaft rotation. Having both valves open at the same time would seem on the face of it to be a recipe for disaster. However the dynamics of the exhaust gases flowing out of the exhaust valve and their considerable momentum coupled with the momentum of the charge in the inlet port actually help to drag in mixture via the inlet valve. The incoming mixture also helps to push out the exhaust gases, this process is known as scavenging. Generally speaking, the longer this period of overlap, the less tractable the cam is at lower RPM and the hotter the cam is at the top end of the RPM range.

Cams with lots of overlap generally give good engine power at higher RPM, but give that irksome low RPM intractability known as 'off-cam' where the engine growls, spits and jumps, together with a distinct 'coming-on-cam' (very messy) at a particular RPM. This coming on cam feeling results from the harmonics of the exhaust flow reaching a critical point where the exhaust gases stop trying to exit via the inlet port (reversion), and do their proper job of exiting via the exhaust, and promoting scavenging. At low RPM when the engine is 'off-cam', the exhaust gases cause pulses in the inlet tract which lead to a phenomenon called 'stand-off' where inlet mixture is bounced out of the back of the carburettors and hangs in a mist around the inlet trumpets /filters. This contributes to the 'off-cam' feeling as the mixture then fluctuates between too rich and correct and is mixed with spent exhaust gases.

Generally speaking, when the engine is off-cam, light throttle openings minimise stand-off; because the butterflies of the carburettors are nearly closed, they minimise the exhaust pulses affect, also smaller throttle openings produce less cylinder filling and therefore less exhaust gases to give those troublesome pulses. Long exhaust primaries can also help minimise standoff by keeping the exhaust inertia from the previous exhaust stroke, and by preventing interference from other cylinders. Mapped ignition and/ or programmed injection can considerably improve the engines tractability at low RPM, as they minimise stand-off, and injection does not rely on induction gas speed to drag fuel in as it is directly injected.

Obviously camshaft choice is not the only thing which determines engine output but is the thing which most significantly changes the nature and characteristics of the engines power delivery. A race cam with STD carb, head and manifolds will give appalling driveability, and the top end power expected will evaporate due to restrictions in induction and exhaust. It is important to have an idea of how you want your engine to behave, whether you can live with the engine dropping 'off-cam' at the wrong moment before choosing a camshaft. Note also that as the power band moves up the rev range, it also narrows considerably and also pees away your precious fuel. In a car the weight of an RH7, the loss of tractability experienced with bigger cams is less of a problem, but a 'hot-cam' can make driving in traffic extremely wearing.

You can't have it both ways, if you want big power at the top end it will always be at the expense of bottom end power and tractability. The answer in the long term is of course variable valve timing which is currently seen in Hondas redoubtable VTEC screamer and Rovers VVC 'K' series engine. These give short duration low overlap timings at low RPM and increase these as RPM increases. They are the mythical holy-grail of automotive technology, the idea of variable valve timing was first put forward by Lanchester in 1903, but he did not have the available technology to make it happen.

The camshaft fitted to the 1300 (yes there is a 1300 pinto!) and the 1600 pintos is very conservative having only 256 degrees duration and 34 degrees overlap. If you have a 1600 and want a small power gain cheaply, fit the 2000 cam which has another 10 or so degrees of duration and overlap, it also has marginally higher lift.

I have set out below a general guide to cam selection which includes expected power bands, recommended carburation and other mods, notional expected power outputs and engine characteristics which I hope will help with cam selection. These are approximations only. Also alongside are examples of each type of cam from the two main suppliers Kent and Piper.

It is worth noting that the durations quoted by cam manufacturers are generally at a checking height of 10 thou or so, in real terms these durations require 4 degrees or so adding to them. Note that the first three cams are the standard offerings from Ford.

Camshaft Selection Chart

Duration/                          Typical use                 Characteristics                                                                      Power?

256/24                              Std P100 cam            AAARGH - power peaks at 4500
                                                                           Useful only as a weapon

256/34                              Std 1300/1600 cam   Std. gutless wonder for 1300/1600
                                                                           don't use in 1800/2000 

268/42                              Std 2000 cam             Slight improvement for 1300/1600 only
                                                                            for 1800/2000 std. gutless wonder

270/60                              Fast Road                    No loss of tractability, no'off cam' ,mainly extra lift
Kent: FR31,FR21,FR2                                          power band 1500-6000 std carb/DGAS/DGAV/40s
Piper: BP255K,HR255K                                      Std head/stage 1 head                                                              120+

280/66                               Fast Road                   Slight loss of tractability slight 'off-cam'
Kent: FR32,FR22                                                 power band 2000-6500 std carb/DGAS/DGAV40s                 135+
Piper: HR270/2K,HR270                                     Modified head/4 branch

290/86                                Fast Road                  Loss of tractability 'off-cam' below 2200ish
Kent: FR33,FR1,RL2                                           power band 2200-7000 40s/45s                                               150+
Piper: BP285K,HR285K                                     Modified head/4 into 1

300/90                                Road Rally                 Poor tractability, heavy 'off-cam' below 2750
Kent: RL30,RL31,RL21                                       power band 2750-7500 some stand-off, 45s/48s                       160+
Piper:BP300K,HR300/2K                                    Big valve head, long 4-1

310/105                              Rally                          Bad tractability, heavy 'off-cam' below 3000
Kent: RL32,RL1,RL22                                         power band 3000-7750 lots of stand-off, 45s/48s                      170+
Piper:HR300K,BP320K                                      Big valve head, long 4-1

320+/112                           Race                          BAD tractability,heavy 'off-cam' below 3500
Kent: RC31,RC21,RC1,WR40,GP1                    power band 3500-8000+ mega standoff,45s/48s/50s                 190+
Piper: HR320K,HR320/2K,HR330K                  Ultra big valve head, long 4-1

To calculate duration and overlap from a set of cam timing figures note that they are generally given with the inlet timing first, followed by the exhaust timing, for example:-

Inlet                Exhaust

36/76               76/36

In this example the inlet valve opens 36 degrees BTDC, and closes 76 degrees ABDC; to calculate duration, add the two timing figures together then add 180:-

E.G. 36 + 76 = 112 + 180 = 292 degrees duration

To calculate overlap, add the first inlet figure to the last exhaust figure:-

E.G. 36 + 36 = 72 degrees overlap

For Pinto production cams valve lift is around .390 inches , fast road cams are generally around .400-.460, Rally/Race cams are .450-.510, with the more radical cams, clearances between valves and pistons do need checking. Generally the cam manufacturer can advise when this is necessary. Do not confuse cam lift with valve lift, valve lift is higher as there is a ratio imparted by the followers.


Dave Andrews