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I didn't know if there was anything like this so I thought I'd help out. Without further introduction, here you go.
To understand why and how one compressor wheel flows differently than another, you need to understand the anatomy of the wheel itself.
Two key parts of a compressor are the inducer and the exducer. The inducer (sometimes called the minor diameter) is the part of the wheel that first takes a "bite" of ambient air. The exducer (sometimes called the major diameter) is the part of the wheel that "shoots" the air--now compressed--out of the turbo. Just remember that the inducer is where the air comes in and the exducer is where the air exits. Got it? Good.
You need to understand those two terms in order to grasp the concept of trim, a bizarre bit of tech-speak which is often thrown about. Trim is simply a term to describe the size of a specific compressor within a family of wheels. It can be expressed in abstract ways (such as when Turbonetics says they have P-trims, Q-trims, etc) or you can use the actual numeric measurement (50 trim, 57 trim, etc). Here's how you calculate the measurement:
Trim = (minor diameter / major diameter) ^2 * 100
So now we have a way to perform some math and get a number. What does it all mean? Generally speaking, the larger the trim the more flow the wheel will have. Nevertheless, one should not rely solely on a trim measurement when selecting a compressor wheel! Find out specific wheel measurements (inducer and exducer), understand how subtle differences will affect airflow and response, and then choose a wheel accordingly.
What happens when you upgrade to a larger inducer while retaining the same exducer? The most notable change is more airflow capability; since the turbo is taking a bigger "bite" of air in every revolution, it can obviously "spit out" more air as well. Gee, more airflow aounds great... so why not go to the biggest inducer you can find? Because that creates two main problems, one much more important than the other. The smaller problem--really it's just a nuisance--is the turbo will now have a little more lag during spoolup (because the bigger wheel weighs more, plus it has to do more work with each revolution, etc). While this extra lag might not be noticed on a dyno--all the bystanders will be oohing and ahhing at the huge top-end horsepower such a turbo would produce--it would make for dissatisfaction in your day-to-day drive and could even cause you to lose a drag race to a car with less peak horsepower but more area "under the curve" due to his turbo that spools sooner. The real trouble with a large inducer increase but no exducer increase, though, is it makes the turbo much more likely to surge. Surge is the situation when the compressor "spits out" more air than the engine can swallow, which causes a backup of air at the intake and it actually creates reverse-flowing pressure waves that can be very damaging to the turbo. You want to avoid surge at all costs.
Okay, so maybe we won't go hog nuts wild with the inducer. How 'bout the exducer?
When you upsize the exducer without modifying the inducer, the exact opposite effect happens: your spoolup time is reduced. Why does this happen? Remember that a compressor "spits out" the air in a radial fashion. The larger exducer gives a higher wheel edge speed for a given shaft speed, and that higher edge speed means the compressed air exits at a higher speed than before... and thus it builds boost faster. Another effect of this upgrade is an increase of the compressor's pressure ratio capability without a significant increase in its maximum flow rate.
So now let's tie it all together. If you want more power with similar response, look for an upgrade of both diameters. The larger inducer will net you more airflow and thus greater power capability, while the larger exducer keeps boost response within reason and lessens the chance of surge.
(Stop and take a deep breath--you've digested a lot of info.)
Turbines are actually a very simple science. The turbine powers the compressor because it is a physical restriction in the exhaust flow. The more it restricts (ie: the smaller the turbine) the faster it spins the shaft... but the more it chokes the engine and robs you of top-end horsepower. The less it restricts (ie: the larger the turbine) the slower it spins the shaft... but the less it chokes the engine and the more top-end horsepower you can make. That's the key to understanding a turbine.
Now that we understand this we must understand sizing. When outlining the details of your new turbo, move from the general to the specific. Thus your first decision will have to do with the A/R ratio, but what does this number mean exactly? Well look at this nifty picture
Select the point where the turbine housing begins and measure the cross-sectional area A at that one point. Now measure the distance between the center of this area and the center of the turbine wheel--that's the radius R. Do some division and you come up with a measurement. Now move to a different point in the turbine housing and do it again--the calculated ratio remains constant because the housing constantly gets smaller in diameter the closer it gets to the turbine wheel. When upgrading to a bigger A/R, it's the area that changes; the radius is essentially identical. This is precisely why a bigger A/R flows more air--the passage is larger.
06-13-15, 01:49 AM
