Hourang- You can in fact supply an engine with more air, increasing power, without compressing it. Consider this, a normally aspirated gasoline engine operates with a vacuum of some value measured in the intake. This is a result of the demand for flow that exceeds the actual flow. A supercharger
or turbo can increase flow from the value of manifold vacuum up to the point of 0 lbs. of boost. Anything under 0 lbs. has not yet been compressed, yet the additional flow increases performance.

 

Also, Roots blowers do not compress anything. They only transfer air from one side to the other. No internal compression whatsoever. Boost from this type of blower is created in the intake as a result of the supply of flow, from the blower, being greater than the demand for flow by the engine.

 

hrm i guess im not understanding it correctly then.

 

you are right, the roots blower does not compress the air, BUT the air is compressed in the intake. i dont think its possible to fit more air into a cyclinder without compressing it, if someone can prove me wrong id like to hear it. one other note, a twin screw blow DOES compress air in the blower itself.

 

interesting find here:
Interestingly enough, even when not making boost, the spinning rotors improve the volumetric efficiency of the engine to the point where you can maintain high cruising speeds at lesser throttle openings, and in normal driving around town, you will notice that the vehicle is much livelier even when not making boost. This phenomenon can improve gas mileage under certain circumstances, although typically on an overall basis fuel economy will decrease about 3%. This isn't much of a factor. If your car was getting 20 mpg before the blower, that means you will be getting 19.4 mpg after the blower installation but with a 40 to 50% increase in horsepower.

 

OK. You say that a Roots blower does not compress the air? I've had several of them. If you inspect the side of the blower where the air is pulled in, and check out the side the air is pushed out, there is a large size difference. The outlet side is much smaller then the inlet side. While it's true that the SC is simply transfering air, it is having to push air through the outlet side of the blower. This compresses the air, since the outlet side is smaller. As said, it air is compressed even further in the intake, and while it's being pushed through the intercooler. My T-Bird SuperCoupe had a complex series of pipes to flow the air through the IC. It is even possible to port the outlet side of a blower to increase potential power output. A Roots blower (Eaton, etc) creates more heat while pushing air then a centrifugal SC. Some people don't even use an intercooler with that type of blower, even though it would benefit themwith more power output. As far as power loss while not in boost? Very, very small. I also understand about the gas mileage. My 99 Lightning got better mileage while steady cruising at 70 then did my friends 99 non SC 5.4 F150.

 

steve, the difference in the output and inlet sizes will not compressor air, it will only slow it down or speed it up. the roots blower does not compress the air, the air from the roots blower is compressed in the intake after the blower. a centrifical or twin screw s/c produce less heat because they are compressing the air in the supercharger which reduces heat.

 

If air is being slowed down, it is being pushed under pressure. This is the exact definition of being compressed. Like I said, I've had a half a dozen engines with Eaton blowers. Ever work on one? Ever touch the top of one after it's been running? They get hot. Even if they are not in contact with the hot engine. The act of compressing air makes them hot. On many applications with a Roots blower, the blower is not on top of the intake. As I've tried to explain, there are pipes between the blower and the intake (on some applications). The air actually has to travel through about 6 feet of pipe and the intercooler, before it even goes into the intake. So how in the world can you say it isn't being compressed until it reaches the intake? Sorry, but you can't convince this user of superchargers.

 

The parts quote for my cams (I need 4) $3200. KB kit $3600. SC makes more power/$ than anything but Nitrous.

 

I don't think your statement is accurate. Its simple physics, Boyle's law I think. Compress a gas and it gets hotter. Why? Any gas contains energy proportional to the movement and interaction of the gas molecules. Put more gas into a smaller space you get more heat.

Don't make me write a long dissertation on Adiabatic efficiency. The centrifugals are far less efficient than roots, twin-screws, etc.

No, I'm not a rocket scientist but I did stay at a Holiday Inn Express.

 

 

my bad i worded that wrong, with the air being compressed in the supercharger is produces less heat than the roots type.

 

A ceiling fan moves and subsequently heats air. Does that make it a compressor? I say no. The issue is the defination of a compressor. Here is the best one I can offer. A compressor is a mechanical device that captures gas of a given pressure, compresses the gas, and releases the gas at a higher relative pressure. Adiabatic efficiency is the measure of the heat differential created by doing such work. Centrifugals are the most efficient of all the superchargers, the Roots are the least. The screw types fall somewhere in between. An increase in boost does not necesarrily mean an increase in performance. An increase in density does. 10 lbs. of hot air from a Roots may offer no more power increase than 3 or 4 lbs. of much cooler air from a centrifugal. I apologise to the original postee for the hijacking of this thread and the debate that has ensued. Perhaps we could move on....

 

slowing down air doesnt compress it, im sorry in order to slow the air down you have to take it from a small tube to a bigger tube, that in no way compresses air, there is more space for the air to travel in so it slows down. same is true for speeding it up by making the tube smaller, this only speeds the air molecules up becuase they bounce back and forth between the tube walls faster, the air molecules still take up the same amount of space. compression is when you take something and make it more dense by making the space it takes up reduced. a centrifugal does this by taking the incoming air, and with the rotor fins it compresses the air to the outside of the rotor, much like in one of those amusement rides that spin you around and you get compressed against the wall, the faster it spins, the more your compressed against the wall, ie more boost/pressure.

sorry you misunderstood what i said, i never said it didnt get hot, i said that all superchargers compress air in some way, which produces heat, but laws of thermodynamics kick in favor of supercharger designs with an internal compression ratio (centrifugal and twin screw) because they do less work on the incoming air charge.

those tubes are not to compress the air, they are for cooling(it may compress the air based on the tube design though). it is mostly compressed in the intake, well some compression is done in the head intake ports too. also, the roots superchargers can carry some of the compressed air in the intake back into the supercharger because it gets trapped by the rotating lobes that are exposed to the hotter air in the intake manifold. Now, the tubes you are talking about cool the incoming air(from hot supercharger) and reduce what i just mentioned because the air the lobes trap are cooler in those 6ft of tubing than the intake manifold. also why you see intercoolers in that tubing.

im not trying to pursuade anyone into believing anything, i try my hardest not to post invalid information and i am just proving it. if anything i say is wrong i appologise and am glad to be proven wrong as it helps me learn. believe what you want i guess.

 

Right back at ya
A roots blower makes boost right off idle, you could even set one up to boost AT idle. That means NO WAITING for boost. The centrifugal doesnt do a thing until its reved up a little. Look at a dyno plot between the 2. I would rather have a broad torque and power curve than a peaky one. A centrifugal makes great PEAK numbers, but who is driving their car or truck above 3-4K all the time? Peak means nothing. Power under the curve means everything. Especially in a truck.

Efficient to me means the power is always there. But then mabye the centrifugal is more efficient, after all, its only working half the time.

Better go tell those Top Fuel drivers they are losing power in their cars turning the blower. Notice they dont have centrifugal setups either.

Fuel mileage is dependant on the driver. There are truck drivers out there that rework their big diesels to make sometimes double the HP and torque as stock, yet get the same or better mileage as stock. Ever wonder why?