This is one of the questions that come up a lot, the old turbo vs. supercharger debate.
Technically, anything that forces more air into the engine then it would have ingested naturally aspirated is a supercharger. Superchargers that are driven off of exhaust gas through a turbine are technically called turbine superchargers, but more commonly just called turbos.
All of the forced induction systems have the primary goal of forcing a higher mass of air into the cylinder, which when fueled correctly, generates higher cylinder pressures and more torque on the crank, which makes tires go up in smoke faster
First lets look at the turbo. Turbos are bolted to the exhaust and use exhaust heat energy to drive a turbine wheel. The shaft of that turbine is connected to a compressor on the other side, which then compresses the air and sends it back to the engine (usually through an intercooler) to produce more power. Typically turbos have enjoyed having the higher adiabatic efficiency over superchargers, especially at higher boost pressures. When you compress air it gets heated up no matter what type of supercharger, the more adiabatic efficient the supercharger is, the less it heats the air. Typically most turbos will be in the low to mid 70% efficient adiabatically. Turbos are also typically easier to design as a kit or a one off, since no belt drive has to be created.
Turbo Pros -
- Since they are using exhaust energy to drive the turbo, you can typically make more horsepower for a given injector size (I will explain this more on the belt driven supercharger)
- They make the blow off noise that a lot of people like
- Typically when sized correctly, newer turbos will provide a really nice midrange pull.
- Because of the restriction in the exhaust created by the turbine wheel, a natural EGR effect takes place which helps with gas mileage.
- Good at altitude compensation. Since they are not tied to engine RPM, they can spin a little faster to make up for a higher altitude to deliver the same pressure.
- Since the turbine is a restriction in the exhaust, it acts like a natural muffler a little bit also making things quieter.
Turbo Cons -
- The exhaust turbine blocks the free flow of exhaust gases out of the engine, typically causing the pressure in the exhaust manifold to be roughly at least 1.5-2 times the intake manifold pressure (unless you are running a really big turbine housing). So if you are running 10 psi in the intake, the exhaust manifold is at 15-20 psi. This backs heat up into the cylinder head and combustion chambers and makes it more prone to detonation.
- There is always a disconnect between gas pedal position and power. For the turbo to make power, the throttle blade has to open, intake air has to go into the cylinder, be ignited, exit out the exhaust and turn the turbine wheel. That process takes time and is the reason that turbos have a delayed response. Newer turbos like the EFR's do a really good job a reducing the mass of the turine wheel and compressor to help speed up that transition, but it is always there. Some of the OEM manufacturers are working on electric turbos which do completely elimate any lag since they can be spooled almost instantly are something to watch in the future.
- Since the turbo retains a lot of heat, things around the exhaust manifold should be protected from the heat to make sure they do not fail prematurely. Right now on the Solstice/Sky's we are having about a 10:1 failure rate of the waterpumps on the factory turbo cars vs. the stock 2.4L cars. The exhaust manifold for the turbo engine sits really close to the water pump and appears to be getting them hot and causing the seal in the waterpump to fail.
There are a couple different main types of belt driven/geared superchargers out there, positive displacement and centrifugal.
-The positive displacement superchargers are what most people are familiar with and what you see on top of top fuel dragsters. The positive displacement supercharger moves a fixed volume of air per revolution (techinically they do not, since there is some volumetric efficiency differences with the speed that you spin them, but for this conversation we will just stick with a fixed volume). Because they are a fixed volume these are the superchargers that make your engine just seem like a bigger engine when you drive it, typically also the torque curve on this type of engine looks just like the stock torque curve, just elevated. Some positive displacement superchargers have internal compression (Whipples) most do not have internal compression and just move more air than the engine can ingest and produce boost that way. This is why positive displacement superchargers typically do not do well with higher boost pressures (more than 10psi) since it causes air to leak back through the rotors, generate heat and less efficiency.
- The centrifigal supercharger is more like a belt driven turbo. It has a centrifgal compressor that spins and is powered off of the engine directly instead of the exhaust gas. Because of the similarity to turbos, these superchargers will have internal compression and are more efficient at higher pressures than their positive displacement brothers. They typically also have a higher adiabatic efficiency than the positive displacement superchargers, typically on par with turbos. Since they do not move a fixed volume of air per revolution though, when they are spinning slowly they do not produce much boost and typically produce peak boost pressures at the engine redline. There are also several different types of centrifigal superchargers. The unit that we are using in our kit is made by Rotrex and is a small compact unit since it is capable of spinning upwards of 125,000RPMS, which helps make this supercharger more efficient in the midrange also. There are also the older styler centrifigals like Vortech, Paxton, Procharer, etc that will typically max out at 35-50,000 RPM's and make most of their power only in the upper RPM range.
Supercharger Pros -
- Instant power, no waiting for spool, no transieant response time, just immediate power. Works great with traction control since the ECM can cut throttle and add it back in and there is no delay with any of that happening.
- No restriction in the exhaust, this allows heat to get out of the cylinder head and not build up in the cylinder head and combustion chamber. Typically you can run much more static timing with a supercharger than a turbo at the same pressures.
- No connection to the engine oil or coolant system. Since the supercharger has its own oil system, there is no extra heat put into the engine oil or coolant.
- Exhaust modifications done when naturally aspirated can typically be used with the supercharger kits.
Supercharger cons -
- No altitude compensation. As you go higher up in altitude, the supercharger will produce less boost since the air becomes thinner.
- Since the supercharger is belt driven, power has to be made by the engine to power the supercharger, this means that for the same size injector a supercharger can not make as much power at the wheels since some of that power is needed for the supercharger.
As to which is best for the Slingshot -
No choice is perfect for everyone, it is kinda like blondes, brunettes or redheads, everyone has a preference though
The turbos are good for those that want to sit in 5th gear, floor it and have some boost pressure to pull them at 2000RPM. Once turbos are spun up, they can be very fun and fast on response, but still a little disconnected. The centrifigal supercharger we are using really wakes up around 3500 RPM and pulls like a train all the way to redline. It does make power below that RPM also, but the turbo will beat it below that RPM, to get the most out the centrifigal you need to downshift and run the car hard. The centrigal kit works better for the customer that has no problem shifting at redline to make peak power, drives the car hard, the instant power and response makes it a lot of fun on a track or up in the mountains coming out of turns.
If you have a chance drive them both, you will gravitate towards one or the other. If you can not drive them both, just honestly pay attention to how you drive the Slingshot and what RPM's you are at, that will point you in a direction of which to choose.
As to why we think we have not had the issues that some of the turbo engines have had -
There have been a lot of blown engines out there that we know about, and talking with a lot of those customers there have been a lot of reasons why they have blown up. Some of the early turbo kits made by some manufacturers had several design issues, which as far as we have seen have been corrected now, but some customers were beta testers unknowingly for some companies. Some customers have done stupid things installing their kits also and some engine failures really made no sense and we honestly have no idea why they failed with the information given to us. There have definitely been issues though with boost spikes and creep that have caused issues also though.
With that said, I think that there are a couple reasons why our kits have been so reliable. The first kit we installed last year ran a full year on a piggyback controller, stock exhaust manifold, stock catalytic coverter and stock exhaust at 12psi of boost without blowing an engine, anyone want to try that with a turbo? Since the supercharger is tied to the engine RPM, there is no way to cause a boost spike or creep (unless you are running the stock catalytic converter and it becomes clogs/fails). There is also no way for customers to simply change a spring on their own and overboost their setup beyond what it was tuned for and blow an engine. Unless they want to make a custom pulley to spin the supercharger faster, the boost pressure is fixed by us. Like I mentioned earlier,we keep the cylinder head, combustion chamber and pistons cooler since we allow the exhaust to exit freely from the engine. Also, since the centrifigal only makes a couple PSI around 2-2500 RPM, we do not create high cylinder pressures in those RPM's. In the lower RPM's things are happening relatively slow and if there is a little too much timing or boost you can get into trouble really quick.
So, since we allow the engine to breathe out the exhaust easier, keep a fixed peak boost pressure and are a little softer on the lower RPM range, I think that is why the supercharger kits have been happier on the stock engines without any internal upgrades. Typically we are running around 280hp at the wheels on our setups (figure 330hp at the crank) with a 1320 header and any open 2.5" exhaust which makes for a fun drive for most people
Anyway, I know that is a lot of information, hope it helps, let me know if there is anything else I can answer for you.
Dave
