|Winding up the Wick
Written by Big Affari
(Note; this article does not relate to race cars, drag cars or extreme street cars, this is for the daily driver and the enthusiast looking for efficient, reliable power)
If you own a near standard turbo car the temptation is there. Buying a simple boost controller and turning up the boost pressure. Many tell us that the only way to gain horsepower in a turbo car is to add more and more boost. But what are the consequences, is it really that easy and how long before it all goes BANG!!!!
In the very early days of turbochargers some cars were not even fitted with a wastegate, they relied on low rpm limits or pop off valves to keep boost pressure under control. For a number of reasons this is not very smart. Even when the now famous WRX hit the market many were quick to disconnect the wastegate actuator line and let the boost literally spiral out of control. Today in the modern world we have learnt a lot about the turbocharger and exactly how they work. A term not often mentioned by the average turbo car owner is compressor efficiency. Standard turbos on your average hot four or rotor are designed to be both driveable and fuel efficient as possible, as well as maintaining reliability. Although manufacturers set boost levels on the conservative side, adding more than 5psi will often have the turbo operating outside its efficiency range.
By increasing boost we are increasing the pressure of the quantity of air being supplied to the engine. Gas Theory tells us that the more we pressurise air the greater the temperature will rise. Not only that but the increased shaft speed required to pressurise the air will also increase both mechanical wear on the turbocharger but also increases the volume of air being pressurised by the turbocharger. This means the cars intercooler is now required to cool air that is now considerably hotter than the air would be at a standard boost level. Now while the average intercooler can handle an increase in temperature, too much heat will mean the intercooler will become heat saturated, meaning it will be gaining temperature faster than it is able to dissipate it. This means that for the engine to prevent detonating it will have to retard the timing, this can dramatically effect power output and along with the higher temperatures adds to exhaust temperatures and increases the thermal load on the engine.
This is not to say that you cannot increase boost on any engine. What it does mean is operating a given turbo in its optimum efficiency range before the law of diminishing returns kicks in. It is actually possible to be gaining lower HP figures with increased boost on exactly the same engine and auxiliaries. This is because of the reduced oxygen because of the greater heat, the retarded ignition timing and the increasing back pressure in the exhaust system. Not to mention the possibility of destroying the engine.
So how does this relate to you? Next time your car is on the dyno try slowly adding more boost into the engine. With a standard turbo you will notice that the higher the boost pressure you go to the increase in horsepower will be reduced for every psi you add to the point that you are adding boost with next to no gain. This not only helps you determine what is the optimum boost pressure for your turbo and engine package but also the rough horsepower figure where you will have to consider a turbo upgrade. Whether this comes in the form of a high flowed standard turbo, turbo upgrade or even a move to a very large turbo and external wastegate setup will depend on the power gains you require. This process is all about finding a turbocharger that operates in the meat of its efficiency range when providing the power you require.
Remember that like any engine it is only as good as its weakest component, so ensuring that everything is up to scratch is just as important. But it does make you think about whether that 30psi you are pumping through your standard CA18DET is really making you much more power and just how long the whole thing is going to last before it goes BANG!!!!
If the interest is there, next time I will discuss upgrading turbos, A/R ratios and engines with the ability to run such large boost safely.