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13-06-2007, 10:14pm
For Those Interested I copied this from .ORG
What is a turbo?
Quite simply, a turbo is merely an exhaust-driven compressor. Imagine a small shaft about the size and length of a new pencil. Now rigidly attach a pinwheel to each end of the pencil. One pinwheel (called the turbine) is placed in the path of the exhaust gases which are exiting the engine. These gasses are 'caught' in the turbine, causing it to spin. This in turn spins the whole shaft, along with the pinwheel on the other end (called the compressor). The compressor is placed in the intake air's path; once it begins spinning, it actually compresses the air on its way into the engine.
Why is this beneficial? Well, normally aspirated engines have to work to draw in their intake air. In other words, as the intake valves open, the piston's downward movement creates a vacuum which 'sucks in' some air through the intake system. Ideally, the piston's movement would suck in 100% of the air that could fill the combustion chamber. In the real world this is not the case; the typical engine will draw in only about 80% of the total volume of the combustion chamber. There are many reasons for this--intake restrictions, valve timing, camshaft design, and much more.
Now imagine that the engine mentioned above has a turbocharger. When the turbo compresses the air it builds up pressure in the intake manifold. Now when the intake valves open, air is actually forced into the combustion chamber. (This is one reason why turbocharged engines are sometimes referred to as 'forced-induction' engines.) As you might imagine, this allows more air to fill the chamber.
Okay, so now we have more air entering the engine. To benefit from this, we need more fuel to match. On computerized vehicles such as these, various sensors will "see" this amount of boost pressure and increase the amount of fuel accordingly. Now that we also have more fuel entering the engine, more power is made. (When you get right down to it, the only way to make more power--on any engine--is to shove more of the proper air/fuel mixture into the engine.)
How do turbochargers and superchargers differ?
While they perform the same function, turbochargers and superchargers go about it in completely different ways. As has already been mentioned, a turbo is driven by the exhaust gasses which are already being expelled from the engine. So, in effect, turbos add 'free' power since their compression is created by what was already discarded.
Superchargers, however, are different: they are belt-driven. They feature a pulley whose belt is directly attached to the crankshaft, this allowing them to spin in direct proportion to the engine itself. The upside is a near absence of lag (see below); at least some boost is typically available the instant you crack the throttle. The primary drawback to a supercharger, however, is that they take power to make power. The overall result is more power than there would be without the supercharger; it's just that they aren't as efficient as a turbocharger from an energy standpoint. Other drawbacks include lower mid-range power than a turbo, lower thermal efficiency than a turbo, (sometimes) much harder to incorporate intercooling, etc.
What is turbo lag (and how do I avoid it)?
The majority of turbochargers feature a wastegate--a valve which allows some of the exhaust gas to be directed around the turbine. This allows the turbo's shaft to spin at a reduced speed, promoting increased turbo life (among other things). Think of it as a 'stand by' mode. Since the turbo isn't needed during relaxed driving anyway, this effect is harmless...
...until you suddenly want to accelerate. Let's say that you are loafing along, engine spinning 1500 rpm or so. You instantly floor the throttle. The exhaust gas flows through the turbo and cause it to spool (spin up to speed and create boost). However, at this engine speed there isn't very much exhaust gas coming out. Worse still, the turbo needs to really get spinning to create a lot of boost. (Some turbos will spin at 150,000 rpm and beyond!) So you, the driver, need to wait for engine revs to raise and create enough exhaust gas flow to spool the turbo. This wait time--the period between hitting the throttle at low engine speed and the creation of appreciable boost--is properly called boost response. Many people incorrectly call it lag, which is really something different. Lag actually refers to how long it takes to spool the turbo when you're already at a sufficient engine speed to create boost. For example, let's say your engine can make 12 psi at 4000 RPM. You're cruising along at a steady road speed, engine spinning 4000 RPM, and now you floor it. How long it takes to achieve your usual 12 psi is your turbo's lag time. Between the two, slow boost response usually causes the most complaints.
There are two aspects to consider when dealing with boost response: engine factors and driver factors. As far as engine factors go, there are many things which affect turbo lag... although most are directly related to the design of the turbo itself. Turbos can be designed to minimize lag but this usually comes at the expense of top-end flow. In other words, you can barter for instant boost response by giving up gobs of horsepower in the upper third of your RPM range. (Behold the catch-22 in designing one turbo for all uses.)
Driver factors are another matter. You basically need to understand how a turbo works and modify your driving style accordingly. To sum it up, don't get caught with your pants down! If you feel that there may soon be a sudden need for serious thrust, downshift until your engine speed is at least 3000 RPM. This way there will be noticable boost almost as soon as you hit WOT. If you are going up a hill at WOT around, say 1800 RPM and your speed is dropping, you'll need to downshift just like any other car in the same situation. Remember: turbos need exhaust gas in order to spin. Let them have some when they need it.
What's an intercooler and how does it help?
To answer that question, a discussion of thermodynamics is involved. Turbos, as has been mentioned, compress an engine's intake air. By laws of physics, compressing air also heats it. For an engine, heating the intake air is a bad thing. For one, it raises the combustion chamber temperature and thus increases the chance of detonation (uncontrolled combustion which damages your engine). Another bad thing is that air expands as it is heated. So in other words, it will lose some of the compression effect and the turbo must work harder to maintain the desired level of compression.
Thus enters the intercooler into the equation. An intercooler is a heat exchanger--sort of like a small radiator except that it cools the charge (your intake air) rather than the engine coolant. Now that the charge is being cooled, two benefits appear: combustion temperatures decrease (along with the detonation), and the charge becomes denser which allows even more air to be packed into the combustion chamber. Exactly how much heat is removed varies greatly; some factors include the type of intercooler used, its efficiency, and its mounting location. From what I've seen, getting your intake charge temperature within 20 degrees of ambient is excellent; consider this a practical limit for a street-driven car (meaning you might get closer but not without spending tons of money).
There are two types of intercoolers: air-to-air and air-to-water. Air-to-air means that as the charge passes through the intercooler, the intercooler itself is cooled by air flowing through its fins. Picture your car's radiator but substitute the intake air where the coolant goes and you'll have a rough idea of how it works. In an air-to-water intercooler, the intercooler is cooled by a liquid rather than air; this liquid has its own radiator placed where it can receive airflow, hoses connect this radiator to the intercooler itself, and the liquid must be circulated throughout the entire system.
Each type of intercooler has its strength and weakness. Air-to-air units tend to require longer ducting to route the air from the turbo through the intercooler then back to the engine; this extra tubing might increase lag slightly on some engines and may also present interesting packaging challenges. Air-to-water units, however, can have significantly shorter intake plumbing; the intercooler can be placed in hot underhood areas where no airflow is present since the liquid coolant circulates to its radiator. This allows for simpler installation but at an expense of reduced cooling efficiency. Note that both kinds cool better when air is flowing through the intercooler (air-to-air) or the radiator (air-to-water); both kinds can benefit from the installation of a fan for low-speed operation.
Which type is better? Depends on your goal. From where I sit it seems that air-to-water intercoolers are used either for convenience--to eliminate the possible ducting nightmare of the intake--or for drag-only vehicles where a "one shot" setup uses ice to actually drop charge air temps below ambient... for a very short while. I think it is telling that a number of street cars which featured air-to-water intercoolers from the factory--such as the GMC Syclone and Typhoon--are almost always converted to air-to-air units when upping performance is the goal. Check out an issue of Turbo magazine; you'll see these cars with huge air-to-air units mounted below the front bumber (or else behind the grill and in front of the radiator). There's a message here somewhere....
For very detailed technical information on intercooling, I recommend you visit this Buick-oriented web page. There are math formulas and lots of technical explanations which will really open your eyes to what makes for a good intercooler.
Can I mount more than one intercooler?
Sure you can; your limits will be defined by the room you have to work with and your budget. If you try this, should you mount your intercoolers parallel or in series? The correct answer is simple: in parallel. ALWAYS. Mounting intercoolers in series doubles your pressure drop, which is very bad, while mounting in parallel cuts your pressure drop in half while also allowing for more thorough cooling. Twin intercooling will cause great results; a racer's rule of thumb states you can never have too much intercooler. Here is a web page showing how one FWD Mopar fan set up parallel intercoolers in his Daytona Turbo Z.
Can I make my air-to-air intercooler more effective?
Certainly! What can be done? For starters, maximize airflow through the intercooler. This means remove anything between the incoming air and the intercooler's fins--the A/C condenser, funky ducting, or anything else that actually impedes airflow. If your intercooler isn't directly in the path of air, relocate it so that it is. If you are unable to move it around, create some sort of shroud/airdam to redirect air through the intercooler (tin or plastic should be great for this).
Another idea for you creative types is to make a mister. Get a windshield fluid reservoir, mount it where it will stay cool, and fill it with water. Now run the output tube to the intercooler. Mount a few spray nozzles aimed at the front of the intercooler's core, then join them to the output line with tees and such. Rig up this reservoir pump to a switch or button inside the car so that you can momentarily enable it when desired. The water evaporation will help draw even more heat off the intercooler, further lowering the temperature of the intake air that flows through it. You can get really fancy here; I had a friend that rigged the on/off switch to the throttle body so that the mister would activate at WOT. You can decide how to do it, but this is a neat little trick for just a few bucks.
What is a turbo?
Quite simply, a turbo is merely an exhaust-driven compressor. Imagine a small shaft about the size and length of a new pencil. Now rigidly attach a pinwheel to each end of the pencil. One pinwheel (called the turbine) is placed in the path of the exhaust gases which are exiting the engine. These gasses are 'caught' in the turbine, causing it to spin. This in turn spins the whole shaft, along with the pinwheel on the other end (called the compressor). The compressor is placed in the intake air's path; once it begins spinning, it actually compresses the air on its way into the engine.
Why is this beneficial? Well, normally aspirated engines have to work to draw in their intake air. In other words, as the intake valves open, the piston's downward movement creates a vacuum which 'sucks in' some air through the intake system. Ideally, the piston's movement would suck in 100% of the air that could fill the combustion chamber. In the real world this is not the case; the typical engine will draw in only about 80% of the total volume of the combustion chamber. There are many reasons for this--intake restrictions, valve timing, camshaft design, and much more.
Now imagine that the engine mentioned above has a turbocharger. When the turbo compresses the air it builds up pressure in the intake manifold. Now when the intake valves open, air is actually forced into the combustion chamber. (This is one reason why turbocharged engines are sometimes referred to as 'forced-induction' engines.) As you might imagine, this allows more air to fill the chamber.
Okay, so now we have more air entering the engine. To benefit from this, we need more fuel to match. On computerized vehicles such as these, various sensors will "see" this amount of boost pressure and increase the amount of fuel accordingly. Now that we also have more fuel entering the engine, more power is made. (When you get right down to it, the only way to make more power--on any engine--is to shove more of the proper air/fuel mixture into the engine.)
How do turbochargers and superchargers differ?
While they perform the same function, turbochargers and superchargers go about it in completely different ways. As has already been mentioned, a turbo is driven by the exhaust gasses which are already being expelled from the engine. So, in effect, turbos add 'free' power since their compression is created by what was already discarded.
Superchargers, however, are different: they are belt-driven. They feature a pulley whose belt is directly attached to the crankshaft, this allowing them to spin in direct proportion to the engine itself. The upside is a near absence of lag (see below); at least some boost is typically available the instant you crack the throttle. The primary drawback to a supercharger, however, is that they take power to make power. The overall result is more power than there would be without the supercharger; it's just that they aren't as efficient as a turbocharger from an energy standpoint. Other drawbacks include lower mid-range power than a turbo, lower thermal efficiency than a turbo, (sometimes) much harder to incorporate intercooling, etc.
What is turbo lag (and how do I avoid it)?
The majority of turbochargers feature a wastegate--a valve which allows some of the exhaust gas to be directed around the turbine. This allows the turbo's shaft to spin at a reduced speed, promoting increased turbo life (among other things). Think of it as a 'stand by' mode. Since the turbo isn't needed during relaxed driving anyway, this effect is harmless...
...until you suddenly want to accelerate. Let's say that you are loafing along, engine spinning 1500 rpm or so. You instantly floor the throttle. The exhaust gas flows through the turbo and cause it to spool (spin up to speed and create boost). However, at this engine speed there isn't very much exhaust gas coming out. Worse still, the turbo needs to really get spinning to create a lot of boost. (Some turbos will spin at 150,000 rpm and beyond!) So you, the driver, need to wait for engine revs to raise and create enough exhaust gas flow to spool the turbo. This wait time--the period between hitting the throttle at low engine speed and the creation of appreciable boost--is properly called boost response. Many people incorrectly call it lag, which is really something different. Lag actually refers to how long it takes to spool the turbo when you're already at a sufficient engine speed to create boost. For example, let's say your engine can make 12 psi at 4000 RPM. You're cruising along at a steady road speed, engine spinning 4000 RPM, and now you floor it. How long it takes to achieve your usual 12 psi is your turbo's lag time. Between the two, slow boost response usually causes the most complaints.
There are two aspects to consider when dealing with boost response: engine factors and driver factors. As far as engine factors go, there are many things which affect turbo lag... although most are directly related to the design of the turbo itself. Turbos can be designed to minimize lag but this usually comes at the expense of top-end flow. In other words, you can barter for instant boost response by giving up gobs of horsepower in the upper third of your RPM range. (Behold the catch-22 in designing one turbo for all uses.)
Driver factors are another matter. You basically need to understand how a turbo works and modify your driving style accordingly. To sum it up, don't get caught with your pants down! If you feel that there may soon be a sudden need for serious thrust, downshift until your engine speed is at least 3000 RPM. This way there will be noticable boost almost as soon as you hit WOT. If you are going up a hill at WOT around, say 1800 RPM and your speed is dropping, you'll need to downshift just like any other car in the same situation. Remember: turbos need exhaust gas in order to spin. Let them have some when they need it.
What's an intercooler and how does it help?
To answer that question, a discussion of thermodynamics is involved. Turbos, as has been mentioned, compress an engine's intake air. By laws of physics, compressing air also heats it. For an engine, heating the intake air is a bad thing. For one, it raises the combustion chamber temperature and thus increases the chance of detonation (uncontrolled combustion which damages your engine). Another bad thing is that air expands as it is heated. So in other words, it will lose some of the compression effect and the turbo must work harder to maintain the desired level of compression.
Thus enters the intercooler into the equation. An intercooler is a heat exchanger--sort of like a small radiator except that it cools the charge (your intake air) rather than the engine coolant. Now that the charge is being cooled, two benefits appear: combustion temperatures decrease (along with the detonation), and the charge becomes denser which allows even more air to be packed into the combustion chamber. Exactly how much heat is removed varies greatly; some factors include the type of intercooler used, its efficiency, and its mounting location. From what I've seen, getting your intake charge temperature within 20 degrees of ambient is excellent; consider this a practical limit for a street-driven car (meaning you might get closer but not without spending tons of money).
There are two types of intercoolers: air-to-air and air-to-water. Air-to-air means that as the charge passes through the intercooler, the intercooler itself is cooled by air flowing through its fins. Picture your car's radiator but substitute the intake air where the coolant goes and you'll have a rough idea of how it works. In an air-to-water intercooler, the intercooler is cooled by a liquid rather than air; this liquid has its own radiator placed where it can receive airflow, hoses connect this radiator to the intercooler itself, and the liquid must be circulated throughout the entire system.
Each type of intercooler has its strength and weakness. Air-to-air units tend to require longer ducting to route the air from the turbo through the intercooler then back to the engine; this extra tubing might increase lag slightly on some engines and may also present interesting packaging challenges. Air-to-water units, however, can have significantly shorter intake plumbing; the intercooler can be placed in hot underhood areas where no airflow is present since the liquid coolant circulates to its radiator. This allows for simpler installation but at an expense of reduced cooling efficiency. Note that both kinds cool better when air is flowing through the intercooler (air-to-air) or the radiator (air-to-water); both kinds can benefit from the installation of a fan for low-speed operation.
Which type is better? Depends on your goal. From where I sit it seems that air-to-water intercoolers are used either for convenience--to eliminate the possible ducting nightmare of the intake--or for drag-only vehicles where a "one shot" setup uses ice to actually drop charge air temps below ambient... for a very short while. I think it is telling that a number of street cars which featured air-to-water intercoolers from the factory--such as the GMC Syclone and Typhoon--are almost always converted to air-to-air units when upping performance is the goal. Check out an issue of Turbo magazine; you'll see these cars with huge air-to-air units mounted below the front bumber (or else behind the grill and in front of the radiator). There's a message here somewhere....
For very detailed technical information on intercooling, I recommend you visit this Buick-oriented web page. There are math formulas and lots of technical explanations which will really open your eyes to what makes for a good intercooler.
Can I mount more than one intercooler?
Sure you can; your limits will be defined by the room you have to work with and your budget. If you try this, should you mount your intercoolers parallel or in series? The correct answer is simple: in parallel. ALWAYS. Mounting intercoolers in series doubles your pressure drop, which is very bad, while mounting in parallel cuts your pressure drop in half while also allowing for more thorough cooling. Twin intercooling will cause great results; a racer's rule of thumb states you can never have too much intercooler. Here is a web page showing how one FWD Mopar fan set up parallel intercoolers in his Daytona Turbo Z.
Can I make my air-to-air intercooler more effective?
Certainly! What can be done? For starters, maximize airflow through the intercooler. This means remove anything between the incoming air and the intercooler's fins--the A/C condenser, funky ducting, or anything else that actually impedes airflow. If your intercooler isn't directly in the path of air, relocate it so that it is. If you are unable to move it around, create some sort of shroud/airdam to redirect air through the intercooler (tin or plastic should be great for this).
Another idea for you creative types is to make a mister. Get a windshield fluid reservoir, mount it where it will stay cool, and fill it with water. Now run the output tube to the intercooler. Mount a few spray nozzles aimed at the front of the intercooler's core, then join them to the output line with tees and such. Rig up this reservoir pump to a switch or button inside the car so that you can momentarily enable it when desired. The water evaporation will help draw even more heat off the intercooler, further lowering the temperature of the intake air that flows through it. You can get really fancy here; I had a friend that rigged the on/off switch to the throttle body so that the mister would activate at WOT. You can decide how to do it, but this is a neat little trick for just a few bucks.