Turbochargers! - Part 5
[ January 8th, 2009 ] By: Mark Ozimek Posted in » Technical Articles
Welcome back to the turbocharger series. Today’s lesson should be short and sweet; It is going to address some misconceptions I hear about size. And no, bigger isn’t better. I know I have covered turbo sizing to an extent in the second part, but I feel that there are some things that I have seen recently that I must comment on.
The first of those is about something called “Trim”. I have seen this used time and time again as a size descriptor for a certain turbo. It is not! A single turbo has two trims, one for the compressor wheel and one for the turbine wheel. Usually it is used in reference to the compressor.
So what is trim? Well, if you have read part two, you may recall that it is a ratio between the inducer diameter and the exducer diameter, Inducer²/Exducer² to be specific. Now, take note of that. It is just a ratio, nothing more. Yes, it does change the flow characteristics of the turbo a bit, but it has no bearing on the overall flow capabilities of the turbo, nor it’s size.
With that off of my chest, there is one other insane issue that I see crop up from time to time. Compressor wheel upgrades. It is often viewed as a cost effective upgarde to rebuild an engine’s stock turbo with a larger compressor wheel, without changing the turbine side at all. In some cases, this is actually a good idea.
However, in the vast majority, the stock turbo has a smallish turbine side to produce boost lower in the RPM band. When an even larger compressor is hooked up to that turbine, some not so good things can happen. First and foremost is compressor surge. The turbine has the potential to spin the compressor too fast and generate more boost than the compressor is capable of handling at that airflow, which leads to surge, which is extremely bad for the turbine.
The second is something that I mentioned in part four: Exhaust backpressure. A smaller turbine will result in higher backpressure, reducing the overall efficiency of the engine, and increasing exhaust gas temperatures. In other words, the engine’s power is reduced somewhat because it has more trouble flowing air, and is more susceptible to damage due to the higher temperatures.
Moral of the story: Before upgrading a turbo, make sure that both sides complement each other well. Flow capabilities should be similar on both sides.
February 16th, 2009 at 8:50 pm
A turbocharger is an air pump that is driven by exhaust gas. It pressurizes the intake to allow more air and fuel to go into the cylinder. More air and fuel means more horsepower.
The two sides of the turbo are the turbine side, which is spun by exhaust gases as they leave the engine, and the compressor side, which is driven by the turbine side and which compresses the intake air.
The size of the turbine and compressor wheels, and the size and shape of their housings, affects the efficient range of the turbo.
The turbine size has an impact on how much power the engine can produce. A large turbine will pose little resistance to outgoing exhaust gas, so the engine will be able to make more horsepower, but a large turbine will spin up to speed (”spool up”) much more slowly. A small turbine will spool up quickly, but will choke down the exhaust and limit horsepower.
The compressor side has less of an effect on spool-up, but generally a larger compressor will produce cooler compressed air, and will be able to generate more turbo boost, but will take a little longer to spin up to speed.
The moral of the story is there is no free lunch. A larger, more efficient turbo can produce more boost, but will spin up more slowly and will not work as well at low rpms. A smaller turbo will have fast spool-up but weak high-rpm horsepower.
What does this mean? Factory turbos are usually sized for around-town driving, not top-speed autobahn use, so they’re smaller turbochargers with low lag and modest maximum boost levels. The 95-99 turbo cars have very small turbochargers, but also exhibit almost no “turbo lag.” The 90-94 cars had a bigger turbo, with a bit more lag, but still very fast spool-up.
At the other end of the spectrum are monster race turbos. These turbos will require very high volumes of exhaust gas just to get them spinning. This means no usable boost until 5000 rpms or more. This kind of turbo can produce enormous boost levels and huge horsepower numbers, but the car will be practically undrivable on the street, with weak low-rpm power, abrupt on-boost transition, and huge top-end power. (Have you ever played Gran Turismo and had the Nissan Skyline with all the mods? It’s like driving that…)
March 5th, 2009 at 6:42 am
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