INTRO
This post is to help answer a few questions that some of the newbie’s have about turbos. By all means this is not the be all and end all on the subject. It's pretty general but gives newbie's a good idea of where to start witha turbo!
IMPORTANT
There are two questions to ask yourself when beginning a turbo project.
ie how much power/kw do you want to produce?
This is different from superchargers because you are no longer fixed to a boost level; with the help of boost controller on a turbo you could change psi levels thus hp levels.
Second question to ask yourself is how much do you want to spend; this will also determine your horsepower level(to a degree).
Let's dispell a few turbo myths
MYTH
Another myth that should be dispelled is that “psi” is not what makes power; it is the speed and the amount of air that the turbo huffs out into the engine. “Psi” is a measure of restriction. For example motor A has a turbo that puts out 15psi and makes 400hp but motor B has the same setup as motor A but B has ported and polished upper/lower intakes and heads. Motor B’s turbo puts out the same 15psi but it makes 450hp. This is because motor B has less restriction in the “air path” and allows more air to go into the cylinders. So the key is not to make more power with more boost but to make more power with less boost and lessening the engine’s restriction to hit your horse power goal. Doing it this way will help make things last longer too.
Let’s say you where going to make your own kit and do all the fabrication yourself, if this is the case then you should make a list of all the parts needed. If you’re buying a kit well then this will give you an idea for what to look for in kit that suits your needs, because not everyone will have the same hp goal.
MAKE A LIST
When adding a turbo to your motor!
1) Make a list of all the parts you need to finish the project,
2) Make a list of thing that need to be done.
3) Make diagrams and drawings too, (like: exhaust routing, oil lines with fittings, etc, etc..)
There is no real sense in me trying to recommend parts you should get as every single engine is different - not too mention the wants and desires of those modding them :)
SELECTING YOUR TURBO
COMPRESSOR
Turbo selection is your biggest concern. You will have to choose a turbo that will enable you to produce your desired power level. There are numerous turbos to choose from that are capable of producing a range of different power outputs.
Fast Facts
A small turbo will reach boost quickly but will run out of steam at high rpms, where as a larger turbo will come on boost at higher rpm but will produce more power in the latter part of the rev range. There are also turbos in between to get what you’re looking for. I’m going to try to explain compressor maps with out too much math, just the basics. But to get the most out of the car not just highest hp but the most efficient set up possible it is necessary to do further research and to do the math.
In this pic find the displacement of your engine on the x-axis, and then draw vertical line. Every time it intersects one of those solid lines, it represents engine air flow on the y-axis at different RPMs at 10psi, obviously the dashed lines is the same but at 15psi.
<img src="
http://www.modifiedcarforums.com/ima...nfo/data4.jpg" alt="turbo compressor maps AR ratios">
So now you have a set of values of air flow requirements at different rpm levels for a fixed boost level, next you look at different compressor maps. A compressor map is like a topographical map of how efficient the turbo compressor is at different pressure levels and different air flow levels.
Example of compressor map:
<img src="
http://www.modifiedcarforums.com/ima...nfo/data5.jpg" alt="turbo compressor maps">
The
x-axis is the engine air flow requirements; the
y-axis is the pressure ratio also called boost pressure. All compressors have a definite combination of airflow (x-axis) and boost pressure (y-axis) at which they intersect at a specific efficient range. On the map 2.0=15psi and 1.68=10psi. Take 10psi for example; draw a line horizontal line (red line) from 1.68 on the y-axis all the way across the compressor map. Then draw the vertical lines from the 5 different airflow values (blue line) from the x-axis to the drawn horizontal line. Now make sure that all the intersections fall under a 70% or higher island, 70% or higher are ideal for an efficient setup it will also gain quicker boost response. And try not to get it to the left of that dashed line, your turbo wont work there. Also as efficiency drops, the heat will build up, even if the boost stays the same. In a twin turbo setup, divide the total airflow requirement in half, and then select a map that satisfies those conditions. Also when engine displacement increase, a given turbo still blows the same amount of air, but a boost gauge pressure reading will be lower.
TURBINE
Picking a turbine is a little bit trickier than choosing a compressor, choosing a turbine will help customize the turbo for your particular setup. Here are 2 pics to help you out, keep in mind that they are very general suggestions. Use these to help pick a turbo, not to justify buying the turbo.
<img src="
http://www.modifiedcarforums.com/ima...nfo/data1.jpg" alt="turbo compressor maps AR ratios">
<img src="
http://www.modifiedcarforums.com/ima...nfo/data2.jpg" alt="turbo compressor maps AR ratios">
The turbine’s job is to spin the compressor fast enough to produce the airflow requirements at the desired psi. Basically small turbines spins faster then larger ones, which will have little lag, but will have more backpressure which will restrict exhaust flow. However, a bigger turbine will make more power then smaller ones because the turbine wheel’s overall diameter and the turbine housing outlet’s inside diameter will determine the turbines ability to generate the “shaft power” needed to spin the compressor at the airflow rate required to create the boost level or power level. But the trade off is that it will have more turbo lag.
Next to look at is the A/R ratio. Which is area over radius; this basically determines where the turbo starts to spin. Take a look at pic below
<img src="
http://www.modifiedcarforums.com/ima...nfo/data3.jpg" alt="turbo compressor maps AR ratios">
The turbine looks like a snail shell, unwrap it and looks like a cone. The end of the cone is cut off leaving a hole, the cross sectional area of this hole is the A in A/R ratio. The hole size at the end determines the velocity at which the exhaust gases exit the turbine scroll and hits the turbine blades. Smaller the hole, the higher the velocity, but greater the restriction to exhaust gas flow. The R in A/R ratio is the distance from center of the cone’s cross sectional area to the center of the turbine shaft (see pic above). A smaller R means faster rotating speed of the gas to the turbine; a larger R will give the turbine shaft a greater torque to drive the compressor wheel, imagine like using a cheater bar at the end of a ratchet to loosen a nut off a bolt, longer the cheater bar the greater the torque.
Now we know what A/R means. A high A/R will be slow off the line but once it starts to accelerate it will be smooth and linear. A lower A/R will give a quicker response.
INTERCOOLER
There are 2 basic options you could go here, air-to-air, air-to-water.
With air-to-air you have tube and fin and bar and plate. Tube and fin is cheaper generally then bar and plate, but the trade off is that you have more pressure loss in a tube and fin then a bar and plate. A bar and plate also flows better.
There is also a difference between cross flow and vertical flow and size too. Generally a cross flow that stretches across your bumper will cool the air more but will lose more flow and pressure since it spends more time in the intercool. The vertical flow like the ATI Procharger style will not cool the air as much as a cross flow but it will loose less flow and less of a pressure loss, since the air spends less time in the intercooler. But when picking an intercooler either way try to get one that looses less then 2.0psi across the core.
There is also that water squirting thing, but I rather have an intercooler that never runs out.
WASTEGATE
More about wastegates here
Wastegates are used to control the turbo, without it a turbo will spin out of control and will over boost a motor. Wastegate’s have a spring-loaded valve that is operated by a diaphragm assembly which is actuated by a boost signal. The valve releases excess exhaust pressure when a specific boost level is reached, thus controlling the turbine wheel’s speed and keeping boost under control.
<img src="
http://www.modifiedcarforums.com/ima...nfo/data6.jpg" alt="turbo wastegate">
There are internal and external wastegates, there really are no big differences they both do the same job. An internal wastegate are pretty easy to use, the external ones require more thinking. External wastegates come in different sizes that should be matched to how much power you’re putting out. Too big a wastegate and it may open early making it hard to reach the desired boost levels, and it could leak more pressure then you want, so when it closes again your turbo will have too spool up again. Too small a wastegate and the engine may over boost and it will hard to control.
Wastegate size for power level
32mm up to 400hp
38mm up to 500hp
42mm up to 900hp
51mm up to 1400hp
Mounting a wastegate is very important too. Can’t stress that enough. To the get the best performance form the wastegate the exhaust gases should not have to drastically change direction when headed towards the turbine in order to flow through the wastegate; this means a split Y configuration is BEST. A wastegate mounted at a right angle to the turbine inlet pipe is the WORST. Also the wastegate dump should connect back to the turbine outlet pipe at least 18 inches away from the turbo; this will help with performance of the wastegate.
Wastegates can bet set at fixed boost levels with a specific spring rate inside the wastegate. So a wastegate with a 6psi spring will open its valve only after the turbo has reached 6psi. This is not that great since the turbo has reached 6psi but the upper intake may see less then 6psi since there will be pressure loss in the intercooler.
Or you could get a boost controller. This tricks the wastegate to staying shut longer to raise boost pressure. But let’s say you have 6psi spring in the wastegate, with a boost controller you can raise the boost higher then 6psi, but you will not be able to lower the boost lower then 6psi.
BLOWOFF VALVE
It always a good idea to get a blowoff valve, not just because it makes a cool pshhh sound, but because it release pressure in the intake tube. When the throttle blade slams shut, there is pressure wave created that bounces off the shut throttle blade and goes back to the turbo which can damage the blades on the compressor. I believe the best place to mount this is before the intercooler, since that is the place where most of the pressure will be (also a louder psshhh). Try to pick a blow off valve by not which one sounds the best, but which works the best, I’m sure the best sound is not worth bent compressor fins.
OIL
Oil is big a thing, turbos don’t need that much oil, so you could run 3–an line to a turbo, and you could get oil restrictors to further restrict oil. It’s a good idea to use an oil restrictor, because you don’t need or want to put a lot of oil in there, sometimes it will bypass the seals and leak into the turbine and will cause a lot of blue smoke out the tail pipe, and it may cause draining problems.
Also use ½ id hose lines for oil drain, I would personally use nothing less then 5/8 id hose for drain. If the drain is too small the oil will build up in the center cartridge and leak to the turbine and/or the compressor.
TIPS
Also if you want a really cool sound from the turbo, mount your filter right on the turbo, with no pipe at all; it makes a cool whistle/sucking noise, sounds like a plane taking off when under full boost.
Also a good tip to remember is that for the turbine to work well, you want pressure before the turbo and no pressure after the turbo.
MAF SETUP
There are 2 different ways you can set up your MAF, a suck through or blow through. This will also dictate if you’re going to have a blow off valve a bypass valve.
a. blow off valve will relive the compressed air in the atmosphere
b. bypass valve will relive the compressed air back into the compressor
in the pics
1) turbo compressor
2) air filter
3.a) blow off valve
3.b) bypass valve
4) intercooler
5) MAF
6) throttle body
<img src="
http://www.modifiedcarforums.com/ima...blow_thru.jpg" alt="blow thru turbo setup">
That is the Blow through system, because air is being blown through the MAF. As you can see air enters the air filter, gets compressed by the turbo then the air gets blown past the BOV (blow off valve), through the intercooler, through the MAF, then finally past the throttle body. When the throttle body is shut, it will cause a vacuum in the upper intake manifold. A rubber hose is connected from the upper to the back of the BOV; the BOV will sense this vacuum. Inside the BOV is a spring and valve, on one side of the valve is boost pressure, and on the other side is vacuum. Once the valve senses high pressure on one side and vacuum on the other side, the spring will compress and the valve opens releasing pressure. Once the valve is open it will release the compressed air into the atmosphere. Then when the pressure difference tends towards equilibrium the valve will shut. The BOV works the same as a bypass valve or BPV.
In this blow thru system, the BOV has to be mounted before the MAF. This is because the BOV vents to the atmosphere, if it was mounted after the MAF, the BOV would release air that has been metered, if it vents metered air, then it will throw off the fuel and will make things rich, since the air that should be there isn’t there.
<img src="
http://www.modifiedcarforums.com/ima...suck_thru.jpg" alt="suck thru turbo setup">
This is a suck through system, because air is being sucked in through the MAF by the turbo. Which one of the differences from a blow thru system.
In a suck thru system the MAF is put in front of the compressor. But the BOV still as to be mounted on the pressure side of the intake pipes, for it to do its job, which now has to be converted to a BPV with the help of that “green” pipe. The BPV can not vent into the atmosphere otherwise it will vent metered air. So it has to vent back into an intake pipe before the turbo and I believe after the MAF because you don’t want to meter air twice.
As far as which setup is better, I hope someone with more tuning insight can answer that. Because I’m sure it will come down to which is more easily tuned. But I like the blow thru system its easier to fab up and the louder “psshh” is a plus too.
phew - that is finally finished!
Rich
Basic Turbo Info (must read)
Linear Mode
