It is fairly common to hear from people who think that simply installing a bigger camshaft, a smaller supercharger pulley, a high flow exhaust or a better intake will be “THE” answer to more performance. The often-used statement that the internal combustion engine is a simple ‘air pump’ never seems to get the point across. The engine is really a complex number of components that MUST work together to produce the power to turn the crankshaft, which turns the transmission, which turns the wheels, which move the car. It is simplistic to think that changing one component without considering the rest will result in the optimum performance increase.

A factory engine design team makes a major effort to make all things work together well, while working within cost, performance, drivability, reliability and emissions design constraints. They establish a target for performance and each component is chosen to optimize towards that point. If they target to have “X” horsepower, they can then adjust each and every component to that point. Everything from boost pressure, throttle body port size, exhaust system, valve sizes, etc. all are established to meet the target. Changing any one of the many options may indeed gain some performance, but the result is by no means optimized.

BMW Mini Cooper cylinder headGoing back to the 'air pump' analogy, let's consider the single most popular upgrade done to the Mini Cooper "S": increasing the boost pressure. There is no question that there is horsepower to be gained by simply increasing the amount of air being forced into the combustion chamber. While the factory must be conservative in their choice of components and thus will always build-in a margin for error in favor of reliability, there are real engineering limits to what we can expect from a single simple component change. Increasing boost pressure consumes horsepower and generates heat which must be dissipated by the intercooler. The capacity of the stock intercooler on the MINI is easily exceeded, negating the potential gains of increased boost pressure. The real question regarding performance enhancements, is how to know when we exceed the point of diminishing returns, where we no longer achieve optimum performance from an upgrade.

The MINI supercharger might be thought of as a multiplier. It takes horsepower to make it rotate and in return it forces more air/fuel mixture into the combustion chamber, which in turn results in more horsepower. The intent is to get more than you give.

BMW Mini Cooper S PortingBut increasing the air out of the pump is only the first step; you then have to get it into the combustion chamber, and to do so you have to pass it thought the ports of the cylinder head and then past the valves. As we increase the blower’s output, we can quickly approach or exceed the head porting design limit established by the factory. Getting back to a more optimum balance of components will have many beneficial results. If we make it easier for the supercharger to deliver the desired amount of air, it will work less and since the basics tell us the work equals horsepower, we will consume less horsepower for the better results. And if the pump is not working as hard it also can deliver a cooler air mixture, which results in a more dense mixture, which again equals more horsepower.

So what can we do to the cylinder head to regain some of the balance of components? Air flows into the valves by ways of ports. The shape and size of these ports control flow and velocity. These are the two critical components of port design. The optimum design will yield the highest flow (volume) of air while maintaining the highest velocity (speed). If you think of the port as a straw it is easy to BMW Mini Cooper cylinder head performanceunderstand that as the diameter is increased the flow will increase but with the same input pressure, the velocity will have to decrease. The result of flow and velocity might also be thought of as low RPM verses high RPM performance. You need the velocity at low engine speeds to produce the HP but if the volume is not there at the high RPM, the engine will not achieve maximum horsepower and torque. This is why we leave porting to the professional with access to the proper flow testing equipment. An amateur with a die grinder will tend to make the ports way too big, drastically dropping the intake charge velocity and ruining power (not to mention ruining the head).

When building a full race motor. it is not uncommon to give up low end performance in favor of total HP at high RPMs that can be maintained on a race track. Since most of us also want to be able to use our cars on the street or least need a wider RPM range, then we MUST balance Flow and Velocity.

This is the real ‘art’ required to get the optimum performance from a port job. As we have probably already increased the pressure from the blower (with a 15% reduced pulley) we now must modify the cylinder head to again re-establish that optimum balance of flow and velocity. The ‘art’ typically requires not just an understanding of how to do it but also often requires a number of hours of very intensive hand operation with high powered die grinders followed by hand sanding to finish the job.

Why isn’t this level of attention given at the design and manufacturing process? While the factory engineers well understand the benefits of porting cylinder heads, it is one of the engineering/cost compromises that must be made in modern production engines.

The general practice In cylinder head porting is to carefully reshape by hand both the intake and exhaust ports to enlarge them slightly while straightening out the airflow path and reducing obstructions that result in any sort of pumping loss. As you reduce the turbulence you increase air flow. This is also a balance as a perfectly smooth mirror finish port wall can result in a loss of performance if the fuel atomization is compromised. Most cylinder head tuners will leave the intake port looking rougher than the exhaust for just this reason. It is very common for a good porting facility to test both their progress and their finished product on a flow bench. This device is the best way to quantify the gains, short of putting the head on an engine and engine dyno to prove the results. A good port technician can establish a baseline on a flow bench and then be able to do many ports and heads to the exact results that can be verified on the flow test bench.

The next step in the process is to get the air past the valves. There are two levels of performance that can be considered. The first hurdle is how to get the best flow from stock valves. The more important hurdle is how to balance increased pressure from the supercharger by increasing the size of the valves. It has been proven that 50% of the gain typically found in cylinder head tuning is achieved by the simple process of doing a high performance valve job. The factory again accepts compromise for cost and manufacturability, and thus you will find single angle valve seat in a production head. These are typically a wide area 45-degree cut on the edge of the valve and corresponding seat area in the head. These are easy to do and as the valve size, etc. has been designed for x-horsepower there is no need for the factory to spend more money.

Just as we reshaped the ports of the cylinder head to improve flow, we can do the same with the seat area of the standard size valves. A ‘multi-angled valve job’, typically 3 or 4 angles will result in smoother transition for the air and greatly enhance flow, while maintaining adequate valve seating area for the necessary dissipation of combustion heat. This precision process is considered standard practice in performance tuning shops.

Multi-angled valve seats are one improvement that does not require the trial and error ‘art’ that is found in cylinder head porting. Requesting a 3-angle valve job should be considered the minimum that you would ask from your machine shop. Typical 3-angle valve jobs include an angle cut on either side of the actual sealing area of the valve seat. The high accuracy required of this technique means that the seat area is kept to a minimum (often less than half of the original seat area). The net result is that the gasses are encouraged, or funneled, through the valve-to-seat opening. The flow increase is dramatic. The intake seat width is typically about .040” while the exhaust is kept a little wider (.050”) to allow for better heat dissipation. It is critical that the corresponding cuts in the valve seats be matched and thus it is important not to switch valves between cylinders.

A 15% pulley reduction increases boost by 10%, from a max pressure of 10lbs to 15lbs. As a positive displacement pump this means you get a higher velocity of air sooner although not much increase in flow or volume. Porting and a good valve job will still leave you short of the needed improvements to reach optimum potential provided by the pulley size reduction. The next logical step is to increase the size and lift of the valves. Again, this must be done with the understanding the need to balance flow and velocity. Remember that we have said the flow is more important at high RPMs while velocity is critical at the lower RPM. Since the blower upgrade mostly produces more velocity, then in order to keep this balance the focus needs to be on flow. This is best accomplished with increased valve sizes. Another compromise that we must keep in mind is the need to keep the correct ratio between intake and exhaust flow. In the ideal cylinder head the exhaust should flow at about 70% of the intake. This balance is maintained both in port design and in valve sizes. For the MINI we have discovered that by increasing the intake valve size by 2% and the exhaust by 6% we not only can regain the ideal balance of intake verses exhaust, but the increased flow (combined with a top quality porting job and valve job) allows you to get much closer to reaching the potential benefits of the reduced the pulley size.

While all this head work provides great benefit in and of itself, a performance camshaft, with properly engineered duration, overlap and lift, will put the finishing touches on your re-engineered, high performance MINI cylinder head. With other engines, the available space above the head allows for many variations in camshaft design. Discussing these choices and the resulting performance and drivability impacts is a subject for another paper at another time. Suffice it to say that for the MINI, the physical limitations of the camshaft area on top of the head limits the performance gains and potential impact to drivability. These limitations include taller lobes interfering with the metal spark plug tubes, and the need to maintain the stock base circle diameter of the lobes because of the integral rocker-arm/hydraulic lifter design. As a result, there is very little difference in performance between the camshaft upgrades we are aware of for the MINI.

We hope this discussion has shed some light on the 'why's' of cylinder head work, and you can now decide whether to trade in your stock head for a ported cylinder head, or undertake the project yourself with the help of a local 'head guru', or leave the pursuit of serious power to more hard-core enthusiasts. Whatever your choice, you are sure to have many happy motoring miles ahead of you!