Remote-Mounted Turbocharging vs. Supercharging
With fuel prices quickly heading towards $5.00 per gallon, purchasing a turbocharger and getting better fuel mileage is the obvious choice over a supercharger and a loss in fuel mileage but here are some more differences to be aware of when considering turbochargers vs. superchargers:
Turbo systems use flow and exhaust pressure instead of a belt driven pulley and are therefore more efficient at generating horsepower and torque.
Boost is easily adjustable with an electronic switch to match your performance needs. No pulleys or belt changes are required.
Turbo systems are quieter when driving around town. Turbo noise isn't heard unless the boost is on.
Turbo systems can increase gas mileage compared to stock mpg numbers because they make the engine more efficient and don't take power from the engine to make power.
Turbo systems are more reliable than belt-driven superchargers because there are fewer mechanical and moving parts.
Remote mounted turbo systems leave more room under the hood making it easier to perform normal engine maintenance.
Remote mounted turbo systems do not increase under the hood temperatures.
Remote mounted turbo systems run cooler because the exhaust coming in is cooler and the tubing coming from the turbo cools the boost charge before it gets to the intake manifold.
- Use belts driven by crank shaft to increase boost to the engine. It takes horsepower away from the engine to drive the supercharger.
- To change the amount of boost you must change the size of pulley used by the supercharger.
- Superchargers in general are noisy because the gear drive is always engaged, even at idle.
- Superchargers generally decrease gas mileage because they are using the engine's power even when the supercharger is not producing boost.
- Superchargers can throw belts causing damage to other components in your engine compartment.
- Superchargers are installed in the engine bay making it difficult to do regular engine maintenance.
Turbocharger vs. Supercharger
The turbocharger and the centrifugal supercharger use the same style of compressor to pump the air into the engine. The main difference is the way that they drive the compressor.
The supercharger uses a belt driven off of the crank shaft to turn the compressor. This has 2 main characteristics:
#1 - It takes horsepower away from the engine to drive the compressor. It will rob power any time the engine is on, just like running your AC, so fuel mileage is typically worse. At WOT when the supercharger is working hard, depending on how much boost you are running, it can take anywhere from 50 to 100+ hp on street applications. The result from this is that you are putting a substantially large demand on the engine and it's components to make additional horsepower that you aren't ever going to see at the tires.
#2 - Next is going to be the way the boost acts because the compressor rpm is tied directly to engine rpm. By design, the centrifugal compressor wheel is not a positive displacement compressor. So this means that it gets more efficient as the rpms increase. This causes the compressor to make more boost as rpm's increase. The higher you rev the engine, the more boost the compressor will make. So you may not have any boost at 2000 rpm, a couple pounds by 3000, 4 psi by 4500 and then the full 6 psi (if that is your max boost) by 6500 rpm. Then when you shift the car and the rpm drops back down to 4500, the boost also drops back down to 4 psi. This is what is called "Supercharger Shift Lag"! This is why you typically don't see a big flat hp and torque curve and the superchargers produce big peak numbers but don't perform so well in the 2-4k rpm ranges.
Because of the belt drive, any change in boost level requires a pulley/belt size change. Running higher boost many times requires that you run a 'cogged' type of belt rather than the standard ribbed belt. This increases the cost and the noise that the drive produces and is obviously something that you can't do 'on the fly'. Larger increases in boost levels also may require changing the compressor head unit which is going to be very costly. The throttle is very responsive and boost is nearly instantaneous but only to the amount that the supercharger is going to produce at that rpm. In other words, stabbing the throttle quickly produces 3 psi at 3k rpm but then you have to wait until the speed of the vehicle picks up and the engine reaches 6500 before you see the full 6 psi.
Most centrifugal superchargers are quite noisy, even at idle due to the complicated gear drive/belt drive systems so make sure that you want your vehicle to produce this kind of noise before making a purchase.
The roots style superchargers are a positive displacement so they will have full boost anywhere from about 1500 rpm to redline. Like the centrifugal they are crank shaft driven so they do suck a fair amount of power off the crank, more so than a centrifugal so you can expect less hp per pound of boost and lower fuel mileage. They are typically a less efficient method to compress air so they put a lot of heat into the air charge. By design they are hard to build a large efficient intercooler into the system so this heat dramatically affects the power that the system makes. Heat creates more volume in the air charge which basically puts less air in the engine and the air that does go in is hot - bad on both accounts if you are trying to make power. Belt driven systems are capable of different boost levels (within fairly small limits without changing compressors) but do require a belt/pulley size change to raise or lower the boost level. These systems tend to have instant throttle response and instantly go to full boost which produces big bottom end power, moderate midrange and poor high rpm performance mainly due to the heat and parasitic loss. Great for burning the tires at a stop light but not very good for a good hard pull on the freeway.
Turbochargers are very similar to the centrifugal supercharger compressor, however they are driven from spent exhaust leaving the engine. They are not 'free' horsepower but they are way more efficient and cause much less of a parasitic loss than the belt-driven alternatives. As the turbo puts more air into the engine, the engine produces more exhaust, which spins the turbo faster to put more air into the engine. This process is defined as the 'spooling up' of the turbo. You can think of the snowball effect logically in your mind as the above process continues to build more and more boost but it actually all happens in a fraction of a second.
Turbos typically do not produce boost under 2k rpm but above that they can spool up and produce boost. This allows the compressor to go from zero boost to full boost in a fraction of a second and sometimes within 100 rpm, depending on what gear you are in. Due to the efficiency of the system, they typically get a very high hp per pound of boost. We've seen as high as 50 hp per psi on some systems but 25 hp per 1 psi is more typical. The torque curve of the turbocharger is what is really impressive. They typically will have a huge broad torque curve that gives you massive power and full boost in the 3-5k rpm range, which is where you spend most of your time driving.
Turbochargers typically get better fuel mileage than a stock vehicle so really are a great option with no real downsides. The 'lag' in the system also brings the power in just a little smoother than the superchargers do. This small cushion makes the turbocharger much easier on drive train components and typically keeps the tires hooked up rather than causing them to break loose. Out of all the power adders, the turbocharger produces the most power with the least abuse to the engine and drive train and can typically put more hp at the tires with all other conditions being equal.
The turbo system can also produce different levels of boost which can be switched 'on the fly' and typically are capable of turning the boost and hp way up without any major component changes or additional financial investment in the system.
So depending on what you are looking for, each system is capable of producing lots of added power to your vehicle. Understanding the differences allows you to pick the right system for your needs and for your changing needs down the road.
Engine Horsepower vs. Wheel Horsepower
The Real Story of Parasitic Supercharger Power Loss
The question is frequently asked, "How much boost is safe to run on a stock engine?" This is an important question that requires more than a simple answer of how many pounds of boost. Most people think of boost rather than of horsepower. The question really should be, "How much horsepower can I run without damaging the engine?" Because of the efficiency differences from system to system as well as many other factors involved, the boost can almost be an irrelevant part of the equation. Obviously, the more boost pressure you have to stick into the engine to receive the desired horsepower output, the harder that is on the engine itself. The optimal goal is to achieve the most horsepower output with the least amount of pressure in the cylinders.
When we talk about horsepower output, there is another large factor that needs to be considered - the difference between actual engine horsepower and wheel horsepower. What is important to the driver of the vehicle is the wheel horsepower (as measured on a chassis dyno) because that is what actually gets your car down the road. Things like drive-train loss and other 'wasted' or 'lost' horsepower are removed, leaving you with only the resultant power to the ground - which is really the only thing that counts for the cars actual performance.
On the other hand, what is important to the engine is the amount of work it has to do in order to produce the desired wheel horsepower. This actual engine horsepower can even be broken down farther. You have the flywheel or crank horsepower (as it would be measured on an engine dyno) and you have the 'real' or actual engine horsepower that the engine is required to produce (which can be calculated by the fuel consumption). This is the actual power demand, forces, and 'stress' that are placed on the internal engine components (rods, pistons, crankshaft, head gaskets, etc.) that when exceeded, cause engine failures.
So the optimal goal is to get the most wheel horsepower (actual performance) with the highest degree of efficiency so that the strain on the engine stays below the failure point. Choosing the power adder that produces the most horsepower with the least amount of parasitic loss is the way to achieve the highest wheel horsepower with the least amount of boost pressure required. This is where the efficiency of the turbocharger truly shines. With almost no parasitic loss, the turbocharger is able to let nearly 100% of the engine's power get to the wheels.
On the other hand, superchargers require that some of the engine's power be used to drive the supercharger. Depending on design and boost levels, this draw can be as much as several hundred horsepower in high boost applications. Even in a lower boost street application, a highly efficient supercharger system may still draw 50-75 horsepower. The net result is that you will have to run 2-4 more pounds of boost pressure and tax the engine with the added stress of an extra 50-75 actual horsepower, as well as supply the extra fuel to feed that 'wasted' horsepower. In recent independent third party testing of two supercharged vehicles and two turbocharged vehicles, superchargers had to run 2 to 4 more pounds of boost and still did not reach the power output of the two turbocharged vehicles.
The other factor involved is 'heat'. Heat is a major factor in power production. The laws of physics demand that producing more pressure creates more heat. The intake temperatures of the supercharged cars, with their higher boost levels, are much higher than the intake temperatures of the turbocharged cars. Horsepower is lost at a rate of approximately 1/2 HP per 1 degree F that the intake temperatures rise. The higher cylinder pressure created by the added boost pressure in the superchargers, combined with the added heat in the intake charge, actually lowers the threshold for detonation (the engine will tend to detonate at higher cylinder pressures and with higher intake temps) and detonation is the one factor that will destroy an engine faster than anything else. Consequently, the timing must be reduced to avoid the risk of detonation. Reducing the timing reduces the power as well as induces more heat into the cylinders and drives up the exhaust gas temperatures.
So what does all this mean in real world terms? Turbocharged cars produce more wheel horsepower with less stress on engine components. You can use this benefit in two ways: 1) you can run the same power as the supercharged car with less boost and much less risk of engine damage, or 2) you can run more boost and produce more wheel horsepower with the same or sometimes even less risk of engine damage. Turbochargers are by far the best choice when you are looking for the most horsepower per pound of boost.
The above graph appeared in "Battle of the Boost", Hot Rod Magazine in the August 2003 issue. Turbo systems are the clear choice if you are looking to generate usable horsepower between 2500 and 5000 rpm. "Given equivalent vehicles, the turbo would easily motor away from the centrifugal in an acceleration contest......The turbo offered massive midrange torque production, the only system to exceed 600 lb-ft. Need more convincing? At 4,000 rpm, the turbo was more than 100 lb-ft. stronger than either the Roots or centrifugal." Richard Holdener, "Battle of the Boost"
With a little searching on the web, you will be able to find many people with personal experience and knowledge about the advantages and disadvantages of turbochargers and superchargers. For example, check out this guys personal website, www.twinturbostang.com/turbos, where he addresses some of the "Myths" of turbocharging. We strongly encourage our potential customers to do as much research as possible, because we know that they will choose STS once they are fully informed.