In our last article, we used formulae to show how cylinder head cross sectional areas, runner lengths and flow figures are used to meet horsepower requirements at given rpm set points. In this issue we now look at camshaft timing events used to compliment the flow characteristics of the cylinder head and the ability of these timing events to meet torque and horsepower targets. We will also cover the restrictions and limits faced by OEM engine designers with respect to emissions legislation and the effects these camshaft constraints have on Australia’s latest muscle machines.

Firstly, automotive manufacturers and their engineers must consider a multitude of environmental issues that the average car builder would hope never to have to deal with. These include weather variations – from the heat and humidity of the Northern Territory to the ice and snow of Tasmania. Other variables include induction noise, fuel consumption, driveline component torque compatibility and those all-important exhaust emissions.

Luckily, the car enthusiast simply wishes to extend the performance boundaries of their engine and considers the various upgrades available on a “horsepower for cash” basis – the old “bang for your buck” routine. Let us examine a Gen III (LS1) engine for example. The 5.7-litre engine, like most other late model engines (particularly the 5.4-litre Ford quad-cammer), seems to lack low-end torque. Why is this the case when most of these engines have around 10:1 static compression? Some may say that hauling over 1800 kg is a good place to start in the search for sluggishness, however, a more important issue arises on that ever-present emissions line. The camshafts in these engines have a minimal (or zero) overlap period so as to unburnt fuel from being exhausted. This then produces a delayed signal to the inlet runner by the time the intake valve opens at or near piston top dead centre (TDC). This delayed signal to the intake charge eliminates the inertial effect of that incoming charge as it races down the port at over 300 ft/sec. Camshaft overlap allows this inertia effect to work in conjunction with the negative pressure effect of the exhaust gases leaving the cylinder. Put simply, if the timing of the incoming air/fuel charge and the exiting exhaust gases is just right then for an instant the pressure in the cylinder is lower than atmospheric pressure and an even greater volume of air and fuel can be drawn into the cylinder – resulting in higher cylinder pressures and more torque and power. Factory pollution camshafts do not allow this to happen and the result is a much lower volume of fresh air/fuel mixture in the cylinder, with a resultant loss of cylinder pressure and a very noticeable lack of bottom-end torque.

Engineers try to combat this unavoidable situation by increasing the length of the intake runners to harmonise the pulses which move through the induction process and to increase the velocity of the intake charge. An example of this extended torque control is the 4.0-litre Ford 6-cylinder engine that uses a short intake runner for power production above 3500 rpm (approximately) and a long runner to boost low end torque. This arrangement is quite successful.

Back to the camshaft issue. We can now see that OEM cams shut the exhaust valve earlier and this can also build negative pumping losses within the cylinder as the piston attempts to compress the trapped exhaust gases rather than push them past the exhaust valve. Making matters even worse is the fact that these trapped exhaust gases take up valuable room for the fresh intake charge, effectively polluting the next firing stroke and reducing potential power generation.

Graph 1: Position 1 indicates the cylinder pressure as it falls and runs parallel with the primary exhaust pulse. At position 2 the cylinder pressure is actually below the inlet pressure at intake valve opening IVO. Position 2 also indicates how the exhaust scavenging wave is properly placed across the overlap period. This indicates that the tuned lengths of the exhausts are correct but not necessarily optimum. Position 3 shows the trough of the intake pulse which is not occurring at the point of maximum piston velocity. Position 4 shows the pressure buildup that should occur before the intake valve opens, this helps to keep the fresh charge in the cylinder as the compression stroke begins. Position 5 points to the exhaust velocity of .5 mach. This is the perfect figure and indicates that all is well in the exhaust system. Position 6 indicates a maximum intake port velocity of .7 mach. Position 7 shows the correct downward angle of the blowdown portion of the pressure/volume diagram.

A saving grace for the Gen III engine is its late closing intake valve which, combined with their excellent cylinder heads, is why they are capable of making power all the way to 6000 rpm in standard form. From the diagram, you will notice the 119-degree lobe separation angle. In general, this will create a flat, wide power band. In contrast, a narrow lobe separation will give strong mid range power with a shorter top end limit and a choppy idle due to increased overlap. Lobe separation is determined by adding together the actual centreline of the intake lobe and the exhaust centreline (normally at maximum lift) and dividing the sum by two. As we move the intake lobe to take advantage of earlier valve opening this, in turn, allows air movement into the cylinder earlier, thus increasing torque. Closing the intake valve later has the effect of allowing the cylinder more time to fill completely at high rpm. However, if the intake valve is closed too late it will pump some of the incoming intake charge back up the intake port (known as reversion), bleeding off cylinder pressure and reducing torque at lower rpm.

See Diagram of Gen III Performance camshaft relative timing events below.

Horsepower gains with the Gen III are significant within the same powerband using an improved cylinder head design and a performance camshaft – however, emissions are guaranteed to rise.

Another area not previously touched upon is area under the curve, or lobe area of the camshaft. With the rapid progression of modern technology, camshaft manufacturers have developed methods of accelerating the valve train faster (and safer) and holding the valve open longer for the same advertised duration. This design effectively makes the camshaft bigger than a cam of the same advertised duration that has more gradual opening and closing ramps. Put simply, this means that the faster-opening camshaft will hold the valve off its seat for exactly the same number of degrees of rotation, however, it will hold the valve open further off the seat earlier in its opening phase and later on its closing phase. So, although the more aggressive cam has kept the valve open for the same duration as the less aggressive cam, its average valve lift is greater and therefore more air has been allowed to flow past the valve. Therefore it is the duration above .050-inch lift that becomes important when comparing these two camshafts. Most cam manufacturers will be able to supply duration figures at 0.200-inch lift and this will clearly illustrate the differences.

Next, let’s look at a performance only head and camshaft package in the context of our previous discussions, namely: cylinder demand flow points; the importance of overlap in the induction/scavenge cycle and the effects of exhaust tuning.

When we begin to fill the cylinder and the piston moves down the bore, it accelerates to a point and then begins to slow down as it approaches bottom dead centre (BDC). At the point of maximum piston speed the cylinder head is exposed to maximum flow demand. If we can match the air flow through the cylinder head with the maximum flow demand of the cylinder it is possible to achieve 100% volumetric efficiency – ie: the cylinder is completely filled. If, after this point, we can ram extra air and fuel into the cylinder it is possible to achieve greater than 100% volumetric efficiency. How is this done?

Graph 2: This is Dynomation’s example of a low speed untuned engine tested at 3000rpm. The scavenging wave arrives at point 1, which is before the overlap period of the camshaft, robbing potential filling of the cylinder. Point 7 is the compression wave which always follows the scavenging wave. The compression wave causes reversion of exhaust gas into the cylinder at point 3, polluting the incoming charge. Point 6 shows the intake ramming wave which has arrived too early creating a reversion at point 5. Points 2 and 4 show the typical slow port velocities that occur at lower rpm. Graphs and descriptions from Dynomation’s Wave-action Simulator software manual.

When an engine moves air and fuel into the cylinder and exhaust gas out of the cylinder it creates pressure waves. These pressure waves comprise high pressure points and low pressure points (suction) as they pass down the intake tract and out the exhaust pipe. By using these pressure surges and troughs to our advantage it is possible to scavenge the gas from a cylinder or fill an empty cylinder more efficiently. We have all seen V8 Supercars belch flames from the exhaust pipe as they slow for corners and this happens when the engine’s pressure wave tuning is out of phase. In the tuned power band the exhaust rushes from the cylinder head to the exhaust opening at which time a low pressure reversion wave returns back up the pipe to the cylinder head (at the speed of sound). By tuning the length and diameter of the entire exhaust system this low pressure (suction) wave will arrive at the cylinder during the overlap period, creating an induced signal that effectively pulls more fresh air and fuel into the cylinder at a higher velocity than would normally occur. As this low pressure pulse works in conjunction with the point of maximum piston velocity it creates an inertia effect which continues to ram air/fuel into the cylinder even though the piston has started to slow down. This situation creates significant power gains when the various events are in phase, however, when this occurs out of phase the low pressure event from the exhaust system is actually strong enough to pull the incoming air/fuel mixture through the cylinder and into the hot exhaust system where it burns. This is why V8 Supercars throw flames at lower rpm.

These same pressure waves also exist in the intake runners and can be used in a similar way to pressurise the cylinder and achieve greater than 100% volumetric efficiency. If a high pressure pulse in the intake runner can be timed to arrive at the valve during overlap then it too will effectively ram a greater amount of air/fuel into the cylinder than would otherwise be possible in a naturally aspirated engine. If this high pressure pulse arrives just as the low pressure pulse from the exhaust begins to scavenge the cylinder then the ultimate cylinder filling environment has just been created. Timing is everything when we are chasing the ultimate amount of horsepower per cubic inch. This illustrates how naturally aspirated, carefully tuned engines can produce horsepower and torque figures approaching those generated by low boost supercharged engines. 3 hp per cubic inch is not unachievable in high end naturally aspirated race engines.

Engine analysis software such as Dynomotion can be very helpful when it comes to phasing these intake and exhaust pulses to maximise horsepower and torque over the widest possible rev range. Obviously, accurate measurements must be made of all the engine’s critical dimensions and combined with accurate camshaft timing events. Such software allows an engine to be “built” on computer to achieve a given output before a spanner is even picked up. It does however, take years of experience to translate the software data into real world power but a grasp of these tuning effects can make a good engine exceptional.

Crow Cams LS1 Camshaft Profiles

Standard LS1 Camshaft

300kW LS1 Camshaft

Screen Shots from Dynomation’s New Windows Software

The graphs on the previous pages were reprinted by permission: The new release of Dynomation’s Windows version is currently available from ProRacing Sim. LLC, 3400 Democrat Road, Suite 207, Memphis, TN 38118, 901-259-2355, www.ProRacingSim.com. The new version of Dynomation has been in development for almost two years and several million dollars have been spent in speeding up the programme and making test results much more accessible to the user, not to mention the addition of many new features.

Re-printed from Volume 12 Number 4 of Perth Street Car Magazine with permission.