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I thought this was an informative article about turbos.

More Turbo Tech
Turbos and Backpressure, A Turbos Main Weakness

By Mike Kojima​

Another interesting affect of a turbo is that although the turbine recovers waste exhaust gas energy from the expansion of the hot exhaust gas, the kinetic energy of the flowing exhaust gas and the acoustic energy of the exhaust gas, the working turbine also causes an increase in exhaust gas backpressure. This increase in backpressure can hinder the engines volumetric efficiency. A typical streetable turbo system has more exhaust backpressure than boost pressure and the power gains from such systems are due to the increase in the density of the intake charge, not due to increases in volumetric efficiency (volumetric efficiency is the amount of intake charge inhaled during the intake stroke vs. the actual displacement of the cylinder expressed as a percent, the larger the percent the better. This was discussed in detail in a prior installment of this series). This is because the smaller turbine housings and turbine wheels used to assure a quick spool-up time are also by nature more restrictive to exhaust flow.

We will explain the mechanics of this in more detail a little later. Racing turbos, the latest generation of medium sized turbos and turbochargers for engines where throttle response is not too much of an issue (fixed industrial engines like generators, long haul trucks and aircraft) have free-flowing turbines with less exhaust pressure than intake pressure and have improved volumetric efficiency. This condition is called crossover and is what every turbo system designer strives for. In crossover, VE percentages as high as 110% are not unheard of. Unfortunately, some of the design features, such as a large exhaust housing A/R and large, high trim turbines that can create a free flowing turbo, can also contribute to turbo lag, something that is not desirable in a street driven car that needs a wide dynamic power band.


Excessive backpressure is hard to manage in a boosted 4-stroke engine. Excess backpressure causes what is known as reversion. Reversion is when hot exhaust gas gets pumped backwards into the engine during the overlap period, something that can happen if the turbo has excessive backpressure. Reversion can cause the engines internals to get excessively hot as cross flow of the cool intake charge during overlap. This is one of the ways an engine cools itself internally. Hot internal parts can trigger uncontrolled combustion and engine destroying detonation. Because of this, it is sometimes not a good deal to crank the boost up on an engine that has a small, high back pressure, quick responding turbo. This is the kind of turbo that usually comes on a factory turbo equipped car. This is a good reason not to go crazy with a boost controller on a factory turbo'd car. A little more boost, perhaps 4-5 more psi might be tolerated well but trying for 20 psi could be flirting with disaster.

On small turbo'd cars with a lot of backpressure, camshaft overlap should be kept to a minimum. This means that the stock cam usually will work best. To deal with the problems associated with backpressure and reversion, the engines tuning must also be compromised with richer mixtures and more retarded timing than what would normally be optimal for the best power. Even on full race turbocharged cars with low back pressure turbos, camshaft overlap should be several degrees less with more lobe separation angle than on an equivalent naturally aspirated motor, unless physical measurements indicate that the engine is in crossover in the engines operational range.


Because of the backpressure and VE issues, the correct turbo size to the application is very important when designing a turbo system. A small, quick spooling turbo can be restrictive causing a great deal of backpressure and reduce VE at higher rpm. This means that it should be limited to lower boost levels. A big, free flowing turbo can be laggy and unresponsive making it unpleasant for street driving but will produce awesome power at higher rpm. To combat high backpressure and possible reversion, the compromised tuning needed to prevent destruction with an over-boosted small turbo will also reduce power. In our opinion if a small turbocharger is running a backpressure to boost ratio of more than 1.8:1, a supercharger has a good chance of outshining it due to compromised tuning, reduced VE and other reversion related issues. Fortunately for us, it is easy to design a reasonably unlaggy, powerful turbo system with a ratio of less than this.

Next month we will get out of the world of the theoretical and get practical. I'll help teach you on how to size compressors to your engine.

Until next month,

Happy Motoring!

Mike
 
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