First of all, do you know about turbos?
Turbocharger Basics
Compressing Air - Wastegates - Performance/Efficiency - Raising boost levels - Hybrids and other upgrades
What is a turbo?
A turbocharger is a device that compresses the engine's intake air. The energy to compress the air comes from a turbine in the exhaust flow which via a shaft drives the compressor wheel (impeller) in the intake. Because the energy that drives the turbine would otherwise have been spat out of your exhaust as heat, a turbo is an efficient means of producing extra power. Superchargers also compress intake air, but they are driven directly by the engine (via a belt for example) which means they sap some of the engine's power. Below is the ST205 schematic, to illustrate the principles (the wastegate and VSV are explained a little further down).
Compressing air
Compressing air makes it hotter, and less dense. Now as the whole point of compressing the air is to get more of it in the cylinders, this is bad. For this reason almost all turbo cars cool the air down after the turbo by means of an intercooler. Interestingly a common misconception is that the turbo heats up the air because it is physically connected to the hot exhaust. In fact the vast majority of the heat comes from compression.
It's often said that the boost pressure alone determines the power increase. Not true. It is the AMOUNT of air that you can get in the cylinders that determines the power the engine will produce. All the energy to drive the car is locked up in the fuel. A certain amount of fuel needs a certain amount of air to burn - if you can fit more air in, you can burn more fuel.
The pressure of the air is linked to it's temperature, amount and volume:
pressure x volume = amount x temperature (well with a constant stuck in there as well but you get the idea). Since the volume of your engine isn't going to be changing, it's easy to see that the amount of air you can fit in is proportional to the pressure (bigger pressure = more air) and inversely proportional to the temperature (lower temperature = more air).
But enough equations, all you need to know is that a high pressure AND a low temperature are ideal for getting a large amount of air into the engine.
Wastegates
If the turbo was as simple as I have described above, it would just compress the air more and more until something went pop! In fact it is kept to a predetermined boost level by the wastegate. This is a port in the exhaust stream just before the turbo which can be opened to let gases bypass the exhaust turbine and therefore keep the amount of boost produced to a set level. On the CT series turbos found on our cars this wastegate is "Internal" i.e. it is built into the turbine housing (bigger turbos need external w/g, see below). Attached to the intake side is an actuator, which at a preset boost level pushes a rod which opens the wastegate in the exhaust side. There is a slight complication to this simple explanation with the CT series turbos (and others as well), the VSV. This is basically a switch which allows the ECU to have some minor control over boost. See here for an explanation.
Without the wastegate working properly and allowing enough gas to bypass the exhaust turbine, a phenomenon called "overboosting" occurs where the boost level can run dangerously high. Sometimes fitting a very large bore high flow exhaust can cause this with the stock wastegate.
Jargon: Lag, Ceramic turbos
All turbos have something called "lag", which everyone has no doubt heard of. This occurs because once you open the throttle it takes a while for the exhaust gas stream to get sufficiently big to spool up the turbine. The bigger the turbo, the greater the lag.
On some models Toyota has attempted to address the problem of lag by making the exhaust turbine of the turbo out of lightweight ceramics rather than steel (simple! light wheels turn easier). This is what is meant by "ceramic turbo". One downside ot this is that the ceramic parts have been known to shatter at high speeds (i.e. high boost pressures, for example over 20 psi starts to get a bit hairy!).
Jargon: Twin-Entry
With 4 cylinders, the exhaust gases don't form a continuous stream so much as a set of pulses as each cylinder fires. This pulse stream comes down the 4 exhaust runners and hits the turbo. Toyota decided this wasn't optimum conditions for it's engine so to smooth out the stream and get better low-end performance and torque, they designed the exhaust manifold to pair up the runners of 1,4 and 2,3 cylinders before they got to the turbo. So there are 2 entry points to the turbo, hence the name.
Turbo performance and efficiency
You hear many numbers thrown around describing turbos, but perhaps the most misleading are the hp numbers. "This is a 500hp turbo" doesn't really mean anything. You can guess from it that it's a pretty big beast, but there's no way you can guarantee 500hp from it. You will also hear turbos described by the size of impeller wheels, exhaust turbine wheels, A/R ratios etc.
A hefty flow of (relatively) cold, dense air is the aim with a turbo. Generally the bigger the turbo the easier this is. This is because the larger turbo is a more efficient compressor of air at the sort of values we are looking at (say approx 7 - 30 psi, which is approx 0.5 to 2 bar). The shaft of a turbo spins very fast (typically > 100,000 rpm at stock max boost on a CT20b), and the bigger turbo's shaft can spin slower to flow the same amount of air.
It's a widely believed myth about turbo sizing that (say) 15 psi on one turbo is 'equal' to 15 psi on another. This isn't true. A large turbo will be able to flow a lot more air than a smaller one at any given pressure. And it's amount of air that counts.
Most manufacturers are cagey about releasing flow figures for their turbos. Toyota is no exception regarding the CT series. This makes it difficult to compare different manufacturers models. It is useful if the manufacturer provides a compressor map (see here for example), this is the best way to describe a turbo and see it's characteristics.
Jargon: Compressor size
When hybrids are talked about, numbers like "57 trim" are often used. The 57 bit is the visible size in mm of the impeller (called "compressor inducer") that you see when you look down the intake. The actual wheel extends beyond this into the housing (this size called "compressor exducer"). For comparison, the 'trim' of a standard ST205 turbo is 47mm, whereas the trim of a good all-round hybrid without too much lag might be 50 - 54 mm.
Jargon: A/R
A/R is a number often used to describe turbos. It is basically the area (A) of the cross section of the intake pipe divided by the radius (R) of the internal bit of the impeller wheel. A 'big' A/R turbo (say > 1.0) wil flow LOADS of air, at the expense of lots of lag.
Turn it up!
Turning up the boost is the quick cheap and easy way to get more power. This is the fact giving rise to the envy of all your mates with a normally aspirated engine struggling to find 5 hp with filter/exhaust mods!!
Thing is, the stock turbo is designed to work well and efficiently around it's max boost area, i.e. 8 psi for the CT26 on the ST165,ST185 or 14 psi for the CT20b on the ST205. When you get far above the stock levels (say 50-60% on top), the turbo becomes less efficient at doing it's job. Basically you can get free power up to a point, but after that for every psi you increase the pressure the increased temperature of the air is negating any gains you are making from the pressure increase (remember that equation earlier?).
Say you have wound up your ST185 to 15 psi where stock was 7 psi. Now at 3000 rpm the turbo is happy flowing air at 15 psi. However you complain that at higher rpms the boost falls off. Why is this? Consider the situation at high engine speeds, say 6000 rpm. Now since the engine volume hasn't changed, but the crank only takes half as much time to complete one revolution, then it follows that the engine needs twice as much air as at 3000 rpm, i.e. you're asking your poor little stock CT26 to flow twice as much air as it was doing at 3000 rpm. It can't flow enough to maintain 15 psi so the boost drops off.
Hybridising and upgrades
If you decide you need more power, then you can only whack up the boost so far before it starts to become so inefficient that although you get a few more psi, the air is so hot that it cancels out! Now you need a bigger turbo. There are two paths to go down - hybridising or entirely different style of turbo.
Hybridising
A hybrid is basically an upgrade to your current CT series turbo. Specifications vary, but generally what is done is to bore out the intake side and fit a bigger impeller. Depending on spec, a different exhaust turbine may also be fitted, or the existing one clipped (see below). In addition it is likely that they will also fit a stronger shaft, and fit better oil seals ("staggered gap" for example) and better bearings ("360 degree thrust bearings" are often mentioned).
Increasing the impeller wheel size basically makes your turbo a more efficient compressor. This means that it can compress and flow more air cooler, hence providing a greater potential for power. Say the average CT26 on a 185 is capable of 14 psi, it is not unusual to find a hybrid that will be happy at 25 psi.
However this comes at a price. Since the wheel is bigger and heavier, there is more lag as it needs a greater flow of air to spool up. In addition once the flow going in gets sufficiently large, it can't all get past the exhaust turbine blades on it's way out and the exhaust side becomes a bottleneck for the whole system. When they fit larger compressor wheel sizes (say a 57mm impeller), often the exhaust wheel is 'clipped' i.e. material is removed from the edges of the blades to allow some exhaust gas to pass and remove the restriction, allowing free boosting. The downside to this is that clipping creates even MORE lag, since at low rpms some gas can still get past the clipped wheel.
So as you can see getting a hybrid isn't just a matter of picking the biggest one you can get. It's important to decide what your power goals are, and what your driving style is before you take the plunge.
Different turbo
Once you have decided that you need BIG power, and your CT26 or CT20b just ain't up to it then you need to get a different design of turbo. This will necessitate a new manifold and DP, as well as possibly custom oil/water lines etc. Some manufacturers do complete kits. Many larger aftermarket turbos also have external wastegates, separate from the turbine housing itself. These are the same in principle as the internal ones on the CT turbos, but allow a much greater control of boost when you start getting into high flow situations. With a full 3" or 4" exhaust it is unlikely that a small internal wastegate could control the boost level well enough to stop some overboosting.
Larger turbos can be had quite cheaply, for example a typical spec Garrett T3 or T4 (depending on your horsepower goals) might not cost much more than a hybrid. The sky really is the limit here, but remember the bigger the turbo, the more the potential for power but the greater the lag.
One way that manufacturers have devised to avoid this lag is to have the central housing (Central Housing Rotating Assembly or CHRA) running on ball bearing races instead of the traditional centre section ("dual cartridge ball bearing" or just "BB" as they are referred to). Examples of this include the Garrett GT series (which are also rebadged by HKS), Apexi's rebadged IHI units or the Innovative Turbo units from. By using the freer ball bearing shafts spoolup time can be reduced. Again, this is offset by cost considerations - a typical spec Garrett GT30 (good for 500 hp plus on a correctly tuned motor) is up to £1000, then add in the cost of manifolds etc etc...
If you REALLY want hp though, the big aftermarket turbo is the way to go. Don't forget however that once you have a big turbo, the bottleneck in your system may well be elsewhere. For example, you rob a super high-flow turbo of some of it's effectiveness by still having the stock cams on there, as they cannot let enough air through to let the turbo do itself justice. Some longer duration cams are required to sort that out.
Rest assured that this process of matching turbo to cams, exhaust, in fact all major components has been thought through pretty thoroughly by Toyota. The stock system is the way it is because it is a perfect balance at the boost pressures required between backpressure on the exhaust side, manifold (boost) pressure etc etc and changing this takes you outside the scope of what Toyota originally intended. If you want consistent high power you need to think about the whole engine, not just the turbo. But that's another story, and will be told another time
Discussion area for Tercel, Paseo, Starlet, and Sera
. but see the other thing is am i going to have to move around some things in my engine compartment to make room for it? I know a bit about turbos but this was a big help thanks
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