This page is a collection of my thoughts and observations about turbocharger upgrades for the 2.5L turbo Dodge Caravan & Plymouth Voyager minivans. While I wrote this page specifically with these minivans in mind, these ideas apply more-or-less across the board for all 2.2L/2.5L 8-valve turbo Dodge, Chrysler and Plymouth vehicles. If you landed here for information on another kind of car, you will need to do some extra research in order to determine if what I say applies to you. My remarks below assume you already have a good understanding of the basics of turbocharger operation; if you do not, go back and read my Turbo 101 page first. If I say something that confuses you, let me know and I'll clarify things where possible.
Now then, let's look at the stock Mitsubishi TE04H-13C turbo. Putting it mildly, this turbo is horribly undersized for serious power production on our engine. Why is it too small, exactly? There are two reasons (and one is far more important than the other): the compressor and the turbine. Any given compressor has a certain range of operational airflow, of course, and ours is designed to only flow so much air. The larger problem, though, is the turbine--it absolutely chokes our engine! Imagine running a marathon where you can inhale freely through your open mouth but you're forced to exhale through a soda straw--this is the equivalent of our engine trying to breathe through the tiny TE04H turbo!
This is a complicated question which, naturally, has a complicated answer. The reason is because there is no one turbo which is the best one for all situations. How much total power do you want your van to make? Where in the rev range do you want your power to be? Where will you drive the van, and how will you use it day-to-day (if at all)? You need to know the answers to these questions in order to form a plan of attack. Research will be in order, so be ready to do some homework!
Sooner or later you'll come to a general conclusion about the sort of turbo you'll need. Now you have to sit down and spec it out. This is nothing new; racers have been doing it for decades. In my opinion, though, there is one thing which people constantly get wrong when choosing a turbo: they go about it backwards. When designing your new turbo, move from the general to the specific: begin with the turbine, not the compressor. Why is this? It all goes back to the whole air pump thing--if the air cannot get out of the engine then you will never make very much top-end horsepower. Once you know how much power you want to make and where in the rev range it will reside, you must choose a turbine to support this airflow requirement. Now that the turbine has been specified--which also dictates the shaft speed--select a compressor which will supply the proper amount of air to match.
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.
So now let's talk 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. Quite frankly, there are only two choices for a Chrysler-style housing: .48 (stock Garrett) or .63. Unless you want to keep your van's power output very close to stock--and you wouldn't be reading this page if you did--you can safely assume you want to use a larger .63 A/R housing. But what does this number mean, exactly? Look at this 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 from the .48 to the .63 A/R, it's the area that changes; the radius is essentially identical. This is precisely why the .63 housing flows more air--the passage is larger!
Now you've decided your turbine's A/R ratio. Next, choose your exact turbine wheel. Turbine wheels are typically referred to in stages: StageI, StageII, StageIII, etc. One very important fact is that these stages are not universal! A Turbonetics StageII wheel is far different from a Garrett StageII wheel, for example. Make sure you know what you're getting when you ask for it.
What's the big deal about stages? This is how turbo manufacturers refer to the differences from one wheel to another. What changes, exactly? The shape, curvature, pitch and "overlap" of the wheel's blades, primarily. For a great example, look at the picture below. See how the stock wheel's blades "fold over" one another, preventing you from seeing through them? By contrast, check out all the open area between the blades of this aftermarket wheel.
All those open areas on the aftermarket wheel result in far less turbine backpressure, which paves the way for lots more top-end horsepower... but remember: this aggressive turbine will spin more slowly than the stock one. The slower shaft speed means the compressor spins more slowly, also. When the compressor speed slows down, your boost output falls off as well. This is why large turbines need large compressors to match!
I suggest checking with other minivan racers and seeking their advice for choosing your exact turbine wheel. Find out which stage they have, what the resulting boost threshold is, and how well it pulls on the top end. Their real-world experience can help you fine tune your exact turbine wheel selection to meet your desires.
Now that you've selected a turbine, it's time to choose a compressor to match it. You will have a variety of options. However, it won't be a case where only one exact wheel will work--instead, it's a matter of one (or two) wheels which will work best. This is where you'll need to do some math, come to grips with a few technical terms, and so on... but it still isn't outrageously difficult to do, so don't sweat it.
Speaking of technical terms, let's get right to a few of them. To understand why and how one compressor wheel flows differently than another, you need to understand the anatomy of the wheel itself. Let's take a look at the following picture:
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:
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.
Speaking of subtle differences, let's take a look at them. First, the inducers:
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? Let's take a look:
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; we'll discuss these more later on.
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.
Now that we've covered all that, it's time to mention compressor flow maps. These are simply charts which give you a feel for how one wheel's flow compares to another; in fact, they give you the entire relationship of airflow, compressor efficiency, pressure ratio and shaft speed. Don't overlook the importance of that last varible--this information will be crucial when matching up to free-flowing turbines which are spinning quite a bit slower than stock. Here's an example of a flow map:
Compressor maps always use X:Y graphs where the horizontal axis represents airflow (typically expressed in lb/min of air) and the vertical axis indicates the pressure ratio (manifold pressure divided by ambient pressure). The oval lines represent "islands" of efficiency, and the percentage figures for each one tell you the thermal efficiency of the compressor wheel at that combination of airflow and pressure. The lines which cross the islands indicate the shaft speed required to generate that amount of air flow; for example, the top line indicates a shaft speed of 126,077 RPM. These numbers are very important since a 'larger' turbine will spin slower than a smaller one.
This page isn't done yet--there's more info to come. Stay tuned!
Modified 2/19/08.