This article was first published in 2003.
When you're piecing together an exhaust system that flows well the principles are pretty easy - big pipe diameter, mandrel bends, the least number of mufflers/resonators/cats and, of course, opt for mufflers of the straight-through variety. But what if you want an exhaust that flows heaps and remains fairly quiet - legal at least? Hmmm, add heaps of resys and mufflers 'till it shuts up you say. You could, but it's better to pick your muffler(s) carefully. Follow the lead of new car manufacturers - wherever there's a space set aside for a muffler, make sure you use a muffler large enough to gobble up the available space. There's no point having a small resonator-come-muffler in there - bigger, heavier mufflers are generally the quietest. This theory can also be seen in the brand name Japanese aftermarket 'road systems' (not the canon-style race systems), which are widely recognised for their hushed notes.
Speaking of irritating noises, when you're adding a large diameter PVC cold air feed to an airbox it's possible to come across intake resonances at certain loads and rpms - sort of a howl from the airbox/cold air feed assembly. If changing the length of the feed pipe and altering the entrance to the airbox doesn't improve things, you might want to get rid of your PVC pipe for a length of spiral wound flexible pipe. While the flexi pipe doesn't flow as well as the smoothly contoured PVC pipe, its convoluted inner surface knocks out any harmonics on-route to the airbox. This kills the resonance and - assuming your flex pipe is an 'overkill' diameter - the trade-off in flow won't have any noticeable affect.
Measuring exhaust backpressure can be a real eye-opener and it's essential for quantifying the gains of exhaust mods. The words "too hard" may come to mind at the thought of this, but - depending how you approach it - it's not hard at all. One of the best places to measure overall exhaust restriction is at the oxygen sensor fitting at the beginning of the system. Depending on the thread of the fitting, a 3/8-inch brass male barbed fitting may screw snugly into the fitting and all you need to do is connect a positive pressure gauge and go full throttle testing. If your sensor fitting won't accept a 3/8-inch brass fitting the next easiest thing to do is find a bolt that will screw into the sensor fitting, drill a hole through the centre and weld a short length of metal tube to the end. Not as pretty, but functional.
While 'screwing up the boost' was something once seen as dangerous as jetty jumping at low tide, perceptions have probably swung a little too far in the opposite direction these days. Instead of simply blowing harder into the mouth of the engine, step back and think about improving the overall efficiency of the turbocharger system - the whole caboodle. Rather than jumping at the wastegate, we always first turn our attention to letting the engine breath freely - quite different to forcing it to breath harder - intake air temps and the engine management. After tending to these areas you might not feel the need to up the boost and that's good news for turbo life, the intercooler and engine.
It's a basic process that many car owners seem to not understand - how to wash your car. The most important thing to remember is not to touch the paint with a sponge until the car has been high pressure sprayed. This gets rid of all the dust and grime that would otherwise be swirled into the paint upon sponging.
Speed sensitive power steering systems are, of course, configured so that anyone can get in and drive without fuss. No matter how physically weak the driver is, they should be able to take control of the vehicle. This is all well and good, but - often - the outcome is steering that's too light for true car enthusiasts; we like to have a bit more weighting. Those vehicles on which a solenoid is pulsed to vary power steering fluid flow, it may be possible to add a resistor to the wire feeding the solenoid. This reduces the effective amount of solenoid duty, thereby altering the fluid flow throughout the system; the upshot is a firmer steering weight. Note, however, a high power resistor - or potentiometer - is necessary along with quite major heat sinking.
For more on this topic, check "Modifying Speed-Sensitive Power Steering"
As the cars we modify become more and more complex, the need for in-depth servicing manuals grows. Not only are manuals useful for working how things are pulled apart (and put back together!), they give the appropriate tightening torques of all fasteners, descriptions of how systems work, how sensors should respond during bench tests and - perhaps most importantly - comprehensive wiring diagrams. These diagrams are essential if you want to hook up, say, an air-fuel ratio meter that reads off the factory Lambda sensor or a high load input from the airflow meter to trigger an intercooler water spray. In Australia, Gregorys publishes a range of service and repair manuals for specific cars - at an average price of around $40 - and, in most instances, you can purchase the complete set of workshop manuals direct from the service division of dealerships. This usually cost several hundred dollars, but if you're serious about hands-on it's well worth it.
Note that you can often find wiring diagrams on the web - be careful that these are from the same market as your vehicle. The diagrams for, say, a UK spec Impreza Turbo might be different to those of an Australian WRX.
And since you'll likely be spending a lot of time working through your car's wiring it's wise to invest in a good multimeter. Even the most basic multimeters display volts, resistance and amps, but - to be widely useful - you need one that also measures duty cycle, pulse width and - using a plug-in probe - temperature. These features will enable you to measure injector 'on' time, wastegate solenoid duty and intake air temps to name just a few.
If you're fairly new to the electronics scene, a multimeter of the auto-ranging type is generally easier to comprehend. It's also a good idea to have a good look through the instructions before buying a particular unit - the instruction booklet supplied with some units are pretty hopeless unless you're already a guru.
If you're filling your tank of your modified streeter in a city or metropolitan area its advisable to stop by a servo with the highest available fuel octane. Why? Well - if nothing else - it's good insurance to help protect against detonation. You won't necessarily make any more power - not unless an active knock sensor was previously retarding ignition timing - but it's a great way of optimising the longevity of an engine. You might, however, find you can make more power by adjusting timing and/or boost to take advantage of the extra RON rating. Oh, and it's a good idea to buy petrol from a recently established outlet that has a large turn-over - generally, their tanks are in better condition and the fuel is fresher.
You often hear people talking about how quickly their electronic boost control system lets boost pressure rise; this is one of the perceived benefits over a simple pneumatic control system. There's one sure-fire way to find out how quickly your boost can generate boost irrespective of the boost control system - run it without the wastegate actuator connected! Simply pull the hose of the actuator, plug the hose end and take to the road - or dyno - for some testing. Select, say, second or third gear and floor the loud pedal at the lowest revs the engine will happily accept. With the services of an assistant, record the boost pressures at 500 rpm intervals.
Note. Needless to say, running your engine with its wastegate hose disconnected is an easy way to destroy your engine and/or turbo. You must - absolutely must - keep an eagle-eye on boost pressure as revs climb and be prepared to snap the throttle shut as soon as boost reaches your predetermined ceiling. Tremendous care is necessary.
The boost pressure curve that you can now plot using your newly acquired data is a great reference. It can show you how much - if any - quicker your turbo can spool up compared to your existing control system.