Monday, December 3, 2012

When Going Faster becomes a Passion: Competition Upgrades for Your MINI (Part II)

Competition Upgrades for Your MINI

High Performance Cylinder Heads

  The cylinder head is the portion of the engine that channels the fuel-air mixture into the cylinders and the exhaust gases out of the cylinders, and controls the flow of fuel and exhaust with the valves and valve springs that are installed in the cylinder head. The cylinder head also includes the upper portion of the cylinders where the fuel/air mixture is compressed and the explosion takes place that pushes the cylinder downward.

  The design of the cylinder head, and the quality of its manufacturing determine how easily air can enter the engine and how efficiently exhaust gases can be removed from the engine. The design also determines the pattern of the compression and explosion in the cylinders.

  For the owners who want maximum performance from their MINIs while still using them primarily as daily drivers, additional increases in horsepower and torque can be achieved through substituting a high-performance cylinder head.

  In high-performance cylinder heads, the passageways through which the air and gases flow are engineered so there is a minimum of obstruction. To further improve flow, the manufacturing process is as precise as possible, and then is finished off by hand-polishing, so that the surfaces of the air and exhaust passages are very smooth to reduce turbulence in the flow. Finally, the shape of the upper portion of the cylinders is altered to increase the explosive power of the fuel-air mixture. One product that has proven its benefits is the Stage 1 Performance Cylinder Head, available through some aftermarket MINI suppliers. Because of the complexity of its design and intricacies of its manufacturing, this cylinder head isn’t cheap, but it has been shown to add 20 to 30 horsepower to the engine’s performance.

  The Stage 1 head sells for about $1900, and there is a $1000 core charge, which is refunded if the original head is returned to the supplier within 30 days.

  Installation also requires a head gasket kit and new headbolts, which will add about $200 to the costs. Professional installation is required so the total cost will be around $2500.

Exhaust Header

  Breathing is still the means to the biggest improvements in horsepower and torque. Air has to get into the engine, and the exhaust has to get out. Because of environmental protection requirements in many states, we've left until last one of the critical links in that path: the exhaust header.

  The exhaust header is that set of curly pipes that connects the exhaust side of the cylinder head to the exhaust pipes and muffler. Four pipes coming off the exhaust side of the head, one for each cylinder, channel the exhaust gases from the cylinders into common pipes that flow into a single pipe and into the exhaust system. On the MINI, the engineers have attached the catalytic converter directly to the bottom of the exhaust header. The cat then attaches to the “cat-back” portion of the system. (In many other cars, the cat may be further back in the system). The design of the exhaust header can make a big difference in how smoothly exhaust gases flow out of the cylinders. Any constraints on this flow and the pistons have to do more work as they push upward in the exhaust cycle, which means a drag on horsepower and torque.

  As with many other parts of the engine, manufacturing costs and engineering constraints have prevented MINI engineers from designing the high-quality system that would fully optimize exhaust flow. Fortunately, aftermarket manufacturers have jumped into that gap, making a stainless steel exhaust header that includes a catalytic converter which is a bolt-in replacement for the factory part.

  Tests on one of the better versions of this performance component show that replacing the exhaust header can produce a significant increase in torque, especially in the mid-RPM range. In practical terms, that can mean better acceleration out of corners and higher speeds on entrance into the straights, which means lower lap times. Because the catalytic converter is an integral part of the emissions control system on modern cars, in most states it is illegal to remove that portion of the system or replace it with a non-factory substitute. As a result, it isn't legal to replace the exhaust header on MINIs that are going to be used on the street.

  However, if you’re building a car that is designed primarily for competitive events and isn’t used on the street, putting on a high-flow exhaust header is a sensible and relatively inexpensive performance enhancer. If you only use the car occasionally for competition and don’t mind doing a little wrench work before the big weekend, the bolt-in design of this system means that it can easily be swapped in for competition and then removed for street use. Since many of the MINI aftermarket headers include a catalyst, you don’t even have to fret about increased emissions. The aftermarket header won’t produce any greater emissions out the tailpipe than the stock header. With a catalytic converter attached, a high-performance exhaust header is available for about $750, and you can install it in your garage with standard tools. A new gasket, costing less than $10, will be needed when you make the exchange.

Next Installment: Competition Upgrades for Your MINI (Part III) - Snappier Response

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