Before we get started on the turbo, it would be helpful for you to have a basic understandings of a couple things. First of all, turbochargers make torque, not horsepower. Horsepower is a function of how much torque the engine has at a given RPM (ie. It is speed related). In order to increase HP without increasing torque, you will need to increase the RPM. Most of the wear, tear, and abuse in a engine is going to come from increasing the RPM because of a simple law of physics:  Force increases with the square of the speed increase. In simpler terms, as you double the speed of an object, it's force increases fourfold. These are the forces that tend to tear an engine apart not add power so be aware of that when you design and build your engine. They are also the same forces that require you to spend the big bucks on the expensive high RPM parts.  

A much safer and cheaper way to make the car go faster is to increase it's power output while staying in the same RPM range. This can only be done by increasing torque. With a properly sized turbo you could double the torque of the motor at a given RPM while only increasing the peak force on the engine 20% or so. Yes it sounds far fetched but here's how it works:

Keep in mind that the pressure in your combustion chamber is a combination of the how much pressure your piston created when it compressed the fuel mix and the pressure from the burning mix. This fuel mix will burn in your combustion chamber at a certain speed depending on mixture, pressure, and other factors, but for simplicity sake just be aware that it does not burn instantly. In a combustion engine the peak pressure is reached near the top of the stroke when only a small portion of the fuel mix has burned. After that point the piston is accelerating downward and the cylinder pressure drops off rapidly while the fuel is still burning.

In a turbo engine under boost, you may have twice as much fuel mix in the combustion chamber, but since it does not all burn at the same time this additional pressure does not add much to the total cylinder pressure that would have existed in a normally aspirated engine. Now as the piston is accelerating downward there is more burning fuel in the combustion chamber and this burning fuel mix pushes harder than a normally aspirated motor. This is where the real power increase in a turbo takes place.  At about 90 degree crank angle the turbo engine's fuel mix is pushing on the piston 3 or 4 times harder than a normally aspirated engine would push. The pushing pressure is still less than the peak pressure which occurred near the start of combustion so it does not create the "overload" to the engine that most people would expect.

So you see that while the peak pressure in the cylinder has not doubled, the average pressure pushing on the piston over the entire stroke has doubled. This higher average pressure translates into more torque at the rear tires at the same RPM.

In the above explanation I have attempted to summarize a fairly complex topic. For a more in depth explanation I invite you to look in the "Maximum Boost" book below which has a fairly good explanation of the entire process.

If you've read the previous sections, the above may also help you further understand why the racers use low compression motors with turbos and why turbo cams have different valve timing. In short, you want as much fuel/air mix as possible in the combustion chamber and you want it to push on the piston harder and longer.

As an added bonus, a properly designed turbo car will be more drive-able at low speeds than an equally fast non-turbo car with the same size engine. Remember turbos like relatively docile cams. The low overlap turbo cam will provide better low speed driving and more low speed torque than a high overlap "race" cam.  When you hit the gas and the turbo kicks in though . . . watch out.

The point where the turbo comes in depends on a lot of things, cam, compression ratio, design of intake and exhaust manifolds, and the turbo itself. Different snail shell housings change the "Turbine Map" of when and how it pushes air. The lower the AR number(.42, .48, .6, .8 etc.), the less CFM it takes to get it going. The disadvantage of too small of an AR number is that you will exceed the limits of what it can push at higher engine RPMS. You want to match the output of the turbo to that of the cam profile of your engine. A good example is say you have an engine that you don't want to go above 5500 RPM. Well, for a VW 2276 with an Engle Turbo grind of their 120 cam, using the Chrysler T-3 off of a 2.2 Liter Daytona, the engine will be into full boost by about 2400 RPM (12 PSI) and by the time you get to a little above 5000 RPM, the engine starts to quit pulling so hard, kind of like someone put a nail in the tach. What we found out to be was the exhaust was starting to back up and couldn't get out fast enough, to produce more on the intake. This is where the "Pumping" losses take over. To correct this, say I now want my engine to go to over 6000 RPM (providing that the rest of the engine stays together), all I need to do is change the exhaust snail shell from an AR of .48 to .6. What I just did was slide the turbine map upward. Now what happens is I don't make full boost until about 3300 RPM but hang on all the way through 6000 RPM!

Pay attention to what kind of motor you want. From the example above you can build a good play motor with good bottom end and loads of fun at the top, or, by changing the cam, and other engine components to handle extreme RPMs, you can build an 8000 RPM engine at the cost of loosing bottom end. So again, are you a drag racer, or all around play car?

Remember, the turbo charger was typically an afterthought to the auto manufacturers so it will be stuck wherever they could find a little bit of space. Before you bust your knuckles taking it off, pull off the air intake and put your fingers on the shaft. It should rotate smoothly all the way around without the blades rubbing on the housing. Now move the turbo shaft side to side. A little tolerance is built into the oil bearings so it should have slightly noticeable play. For sleeve type bearings, look for less than .022" side play which is a noticeable wiggle, and less than .008" end play which is not very noticeable. (A match cover is about .015" thick to give you an idea) If there is too much play, the turbo is either shot or needs a rebuild. Also check for signs of the blades rubbing on the housings, chips and rough edges on the blades and check the turbo outlets for signs of oil leaking past the seals.

Disconnect the wastegate rod and check the operation of the wastegate valve. Look for signs of major cracks on the wastegate port and check for a good sealing valve. Small cracks are to be expected on the wastegate port but if they are opening then scrap the unit or buy it for parts. 

Pull off the oil lines and look for heavy deposits of charred oil in the ports. Heavy deposits may indicate that the turbo has had a rough life. Watercooled turbos will probably be in better condition.

The turbo will usually be tucked away in a tight space and the exhaust nuts may be rusted tight. Now would be a good time to spray the nuts with penetrating oil and look for other parts in the yard. When you remove the turbo, keep the oil lines on because they will help keep the dirt out. Whether you use them is up to you, however keep this in mind: That small line on the top of the turbo is the life blood of the turbo. You do not want a cheap or worn line to fail.

Ford Laser, Capri

pre 91 Mazda 323, 626 or MX6, RX7

87-88 Thunderbird*

Subaru**

84-86 Mercury Lynx

84-87 Isuzu Impulse

85-90 Isuzu I-Mark

85-90 Chevy Spectrum 1.5L (RHB521)

One distributor has told me that the bearings are too small on these and that Garrett Turbos are easier to get parts for.

Good for 73-208 HP

Turbine Wheels: VJ20 - 1.8L Mazda GTX

12-R - 1.6L Mazda

Subaru: 15-R or 20-R

RHB models have been replaced by RHF

*Late 80s T-Bird has a higher flow compressor wheel. Could Boost 2.3L to 13-15psi to redline

**Early WRX models had slightly larger RHB52

Diagrams and Crude Maps

88-90 Pontiac Sunbird GT 2.0L

1990+ Ford Fiesta 1.6L Engine 8.3:1 CR

Pontiac J-2000 

Chevrolet?

Some 88-89 Volvo 740,760 others used TD05

Volvo 940 2.3L used TD04H-13C-6

Volvo 850 T5 used TD04HL-15G-7

Smaller and faster spooling than T03. Exhaust Flange interchangeable with T03

Uses small shaft and small bearings.

1990 and later Saab 9000T

89-90 Pontiac Grand Prix 3.1L V6

85'-87' computer controlled single port wastegate actuator.

used on Non-intercooled cars 7.5-10PSI boost stock

Watercooled

(The compressor AR numbers may be reversed)

1989 switched to bigger exhaust outlet and better wastegate actuator

Exhaust housing marked with M4 (some 1987-1988s) is less prone to cracking than M3

Watercooled

Turbo II

(50 Trim??)

Stock engines rated at 225hp

(rare)

Watercooled

probably best for a 3 Liter

Further Reading:

Links: