So now we ultimately come to the discussion on the idea of an electric supercharger.
First I want to note that this article doesn't apply to the few electrically driven compressors that have been specifically engineered for the purpose of providing short-term boost over a quarter mile. The Thomas Knight products are an example. Those kits are similar to a turbo or centrifugal blower driven by a high-amperage electric motor and require heavy duty wiring and entire banks of batteries to drive them. They do not hook up to the stock 12v vehicle electrical system and are not meant for street-driven vehicles as they are only capable of producing boost for very short periods and require a long time to recharge. These kits cost thousands of dollars and if you're considering purchasing one, I'm going to assume that the scope of this article is elementary in comparison to the knowledge you must already have.
So then, back to the topic of electrical superchargers. Understanding that forcing more air into the engine will make more power, it stands to reason that if we can place an electric fan in the intake path that will force air into it, we can make more power.
The problem with this idea is that most people who think of it really have no idea how much air gets pumped by the engine during its normal operation. The engine acts as a big air pump and is capable of moving huge volumes of air all on its own. With that in mind, any electric fan that you place in the path of the intake is only going to act as an obstacle to efficient airflow and not provide any level of boost whatsoever. In order to provide boost, the fan has to be able to force more air through the intake than is already being sucked and that's simply not going to happen with some off-the-shelf computer fan or bilge exhaust blower or whatever else you've found or had marketed to you.
Now that you realize how futile the idea of an electric supercharger is, consider the dangers. If you place something in the intake path between your air filter and the engine and that thing isn't made to be there, you now run the risk of having it break and send unfiltered pieces into your engine, potentially causing catastrophic failure. I would hope this is more than enough to discourage experimentation in this area.
As a general rule, if something seems too good to be true it generally is. Modifying a car to add power isn't a cheap endeavour and most of these sorts of shortcuts will only end up burning you in the end.
Interestingly, the company from whom I got that image above even has a small disclaimer on their site with a list of cars that showed no gain from their units. The list included GM's 2.4L engine in both the Grand Am and Cavalier Z24. I suspect it likely includes all cars from ripped-off customers who bothered to get the system dyno tested.
Notes:
I've had some questions posted about this that I'd like to address. First off, on the topic of whether one of these things will actually be a hindrance to the airflow, I did some calculations to determine the maximum airflow of a 2.4L engine. If you're interested in this, the thread can be found here. Please let me know if you find the link to be broken.
Secondly, someone just recently mentioned in my guestbook that these fans can be found rated to move upwards of 1000 cfm. I just wanted to note that those ratings are referring to the amount of air moved in an unrestricted environment, for which they've been designed. If you install one of those in your boat to bring outside air into the cabin, it may very well move 1000 cfm. If, however, you place it in a restricted path such as your engine intake where there simply isn't room to move that air without building up pressure, it will not build up that pressure that you want. While it may be capable of moving this much air past it, it is not capable of forcing that air to compress into the smaller space.
On Thursday, Feb. 24, 2006, a Mr. Ed. Walker left a note in the guestbook saying the following:
it is about the cfm, and not the psi. as long as your giving the vehicles motor more air than it can naturally draw in(cfm). read the definition of the supercharger in the dictionary. and our dyno results.
www.streetfreakzperformance.com
I won't address all the spelling and grammatical errors. I'll leave that up to the reader to make their own conclusions about. As for looking up the Dictionary Definition of a Supercharger... Well the reader can do it themselves and find that my reference above is quite accurate and that nothing about an electric blower fits it. An electric blower doesn't force air and doesn't compress it.
Let's look at what he's saying though. Obviously Mr. Walker didn't read the whole page but I wouldn't expect that from someone trying to bilk people out of their hard-earned money. He suggests that CFM (Cubic Feet per Minute) is the key here, rather than PSI (Pounds per Square Inch). He's saying that flow is more important than pressure.
First of all, I stress again that the CFM rating of those electric blowers is completely irrelevant in this application and does not reflect how much air the fan will push into the engine. That number reflects the amount of air the fan can move through it in an unrestricted environment. An intake tube with an engine powerfully sucking air through it is hardly an unrestricted environment.
An industrial exhaust fan, 4 feet in diameter and turning at 100 rpm, may be capable of moving hundreds of thousands of CFM of air but if you were to stick it to your car and create a funnel leading into your intake, it wouldn't do you the least bit of good. The same goes for these little blower fans people are hawking as superchargers. As mentioned above, just because it is capable of moving 1000 (or more) CFM of air through it in an unrestricted environment does not mean that it is going to force that much air into the engine. With air already in front of it, the fan blades will just churn through the existing air causing a restriction in flow while the engine continues to suck just as much as it was before.
In a boost application, PSI is important. CFM is also important, but only in how it is applied in conjunction with the pressure. The CFM of the whole system, engine included, is key but adding an electric blower to the system does NOT increase that CFM by any significant amount no matter how high the CFM rating of that blower is.
People will tell you CFM is important, like Mr. Walker did, and here I will provide an example of how that is true with respect to the whole system.
If you take a stock engine and add forced induction via a roots style supercharger, you will increase your manifold PSI and the engine's CFM of air consumption. By forcing more air in, you obviously move more air through. Note here that electric blowers do not force more air in because they do not increase the manifold PSI. They are not a form of forced induction. Now that you've got your boost (PSI increase), let's say you go and replace your entire exhaust system with something much better flowing. The result here is that the air can pass more freely through the engine so the CFM can increase. This may bring with it a corresponding DECREASE in manifold pressure (PSI) while providing more power. Your boost gauge will read a lower amount but your engine will make more power. This is because the supercharger is still forcing in the same amount of air but the engine is consuming it faster and making more power. If you had used a turbo, the turbo would simply supply more air to keep the PSI at the level you'd set. To get your PSI back up with the supercharger, you'd need a smaller pulley.
Now an unscrupulous salesman might spin these numbers to note that with the higher CFM you're making more power even though you've got less manifold pressure (PSI). With that in mind, clearly having an electric blower with a huge CFM rating will help you.
Let's look at that scenario.
If you take that same engine stock, there will be no pressure (PSI) in the manifold. Air is simply flowing through as fast as the engine can suck it in and blow it out. With a good intake, this is fairly efficient. Now let's add our high flowing exhaust system. We still don't have any manifold pressure, but our CFM has increased. We're making more power because the engine is more efficiently consuming the air it's sucking through. Now you add an electric supercharger. They tell you that it's got some monster CFM rating but curiously, the CFM hasn't changed or, perhaps, has dropped a little. This is because the air can no longer smoothly flow through that section of your intake. They want you to believe that the fan is helping the air along, but the truth is that if the intake was free flowing beforehand then all you've done is put in a restriction. The fan can't compress the air in front of it so it can't move the CFM that it could with no restriction. In turn, the fan itself provides restriction to the overall system, bringing down the engine's CFM of air consumption.
For a little bit of amusement for all my fans here, this is taken directly from Mr. Walker's website.
The E-Turbo from Street Freakz Performance is designed for just that. At full throttle, your performance turns to 800 cubic feet per minute, equivalent of 185 miles per hour of wind being shoved down your throttle body. Or get a twin setup, which will be 370 miles per hour!
Anyone dumb enough to think that this thing is going to provide hurricane-force winds to their engine probably deserves to be suckered out of their money. That fact aside though, the speed that air flows through a fan is entirely dependant on the rpm of the fan and the amount of restriction to airflow that exists. If you put 20 fans in a row, the air flowing through them does not accelerate faster and faster through each fan. The fans later in the line will simply have less load on them as they won't have to work to move the air. It's already flowing as fast as they'd be trying to move it. Of course, if you put a powerful vacuum at the end of this chain, all those fans would just be providing more and more restriction to the flow.