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Hello all,


I am not a mecanic and am learning about alot of things trying to keep my alfa on the road. Thus this may seen like a very stupid question but please feel free to set me straight.

I have been reading alot about alfa engines and how engines work in general as part of my education about all things automotive. I have a 79 alfetta sedan with Spica and the air pump system. Stock except for Motronic pistons. I have read about the drain on the engine that the air pump uses and the pros and cons of removing/disabling the system. I have disabled mine and while I did not notice a great power improvement it did make the engine alot quieter.

In my reading I have discovered forced injection (turbos/supercharging) and learned that basically they are air pumps that force air into the intake side of the engine and with extra fuel devivery can greatly increase power. That got me thinking as to what type of flow the stock air pump puts out and what if instead of sending that air to the exhaust you instead were able to get that air into the intake side. With that a couple of questions:

1) what type of volume or air does the air pump put out?
2) how does this compare to superchargers/turbos (I would think alot less)?
3) to get any type of benefit I would assume that the intake system would have to be presserized or any forced air could be forced out of the air box. Correct?? if so is there any type of one way air flow valve that could be installed in the intake system to accomplish this?
4) given the presumably low air flow would there be any benefit of trying this or would the draw on the engine to run the air pump outweigh or equal any power increase?
5) would this need any modifications to the spica pump?
6) any other comments or sugestions I should consider?

Thanks in advance.

Leonard

79 Alfetta Sedan
 

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I am not an expert at all, but besides all other technical obstacles, I would think the pressure would not be anywhere near the same league as the pressure you would build with a turbo compressor....but then again, i know very little about turbo's ......or air pumps for that matter
 

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I am not an expert at all, but besides all other technical obstacles, I would think the pressure would not be anywhere near the same league as the pressure you would build with a turbo compressor....but then again, i know very little about turbo's ......or air pumps for that matter
Thanks for the input - that is what I was thinking in question #3 above. Do you know how this is done in a "normal" supercharger/turbo? I would assume there is a regular freshair intake that passes some type of one way valve to have the engine run before the turbo spools up and then after that one way valve the turbo adds its forced air. With a superchager this seems more straightforward in that the supercharger is inline with the fresh air intake and thus that "obstruction" would stop the pressurized air from flowing out the air intake plumbing.

I guess the question then beomes what amount of air does the engine use at any given RPM point (a) v. how much air does the air pump put out at those same points (b). If (a) is less than (b) then in theory the engine should make more power disregarding the power loss to run the pump. Since I believe the air pump uses 3-5hp (about 2-4% of engine hp) in reality the air pump would have to put out at least 2-4% more air than normal.

Thanks again - this is more of an academic exercise for me to see if I am understanding the workings of an engine
 

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Off Hand, I Don't Think This Would Work

The amount of air ingested by an engine can be estimated by taking the displacement per revolution (1/2 the engine volume) times the engine speed. You will get some pretty high numbers. To boost the engine by 1 atmosphere (15 psi), you need to add that same amount of volume at that pressure.

An air pump adds a small volume of air to the exhaust. It is nowhere near what is required to add any significant pressure to the incoming charge.

BTW, a turbo is in series with the intake. When it is not making pressure, it is a restriction in the intake path. A supercharger is similar. There is no bypass valve to move the air past the turbo/supercharger when not in use.
 

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it might be about right for a lawnmower. but nowhere near the air needed for a car
 

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it might be about right for a lawnmower. but nowhere near the air needed for a car
and

BTW, a turbo is in series with the intake. When it is not making pressure, it is a restriction in the intake path. A supercharger is similar. There is no bypass valve to move the air past the turbo/supercharger when not in use.

Thanks for the replies. I am confused about the turbo though. I thought the turbine is on the exhaust side (fed from the exhaust flowing through it) and then that pressurized air gets sent to the intake side through hoses (sometimes going through an intercooler)

I could be wrong as I am still learning.

As to the size of the air pump - thats what I thought - way too small to even get the regular ammount of air into the engine. Just got excited when I read "air pump" when reading about turbos/superchargers and I said "I have one of those". I guess it works like a supercharger though in theory. neat!

Leonard
 

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BTW, a turbo is in series with the intake. When it is not making pressure, it is a restriction in the intake path. A supercharger is similar. There is no bypass valve to move the air past the turbo/supercharger when not in use.
The turbo is not a restriction "When it is not making pressure...".
If you did before and after the turbo pressure measurements with a sensative pressure gauge (read manometer or other low pressure gauge, not a boost prssure gauge), it would show minimal pressure differential.

Remember, an engine running at low loads (idling or cruising) doesn't actually make that much power to do the job. Therefor actual airflow into the engine is actually quite small. Small enough that the (spinning) compressor wheel isn't any where near enough of a restriction.

Bypass valves/blow off valves on turbo engines are more for exhaust emissions than just about any other reason in cars with hot wire airflow meters and help prevent damage to vane/flap AFM from reversion.

SC bypass valves are there to prevent cavitation of the SC when the throttle body is before the SC. Cavitation of a pump occurs when the pump can pump more fluid out of the outlet than can fill the inlet. In air compressor speek it's call Free Air Delivery.
Cavitation of a pump causes noise, erosion of pump parts (rotors and housing) and heat build up.
 

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and

Thanks for the replies. I am confused about the turbo though. I thought the turbine is on the exhaust side (fed from the exhaust flowing through it) and then that pressurized air gets sent to the intake side through hoses (sometimes going through an intercooler)

I could be wrong as I am still learning.

Leonard
Hi,

Asking is the best way to learn. Both the turbo and supercharger have their impellers in-line with the air flow. The main difference is in how the impeller shaft is made to spin.

In a turbo the shaft is connected to another impeller (yes, two of them in total) in the exhaust stream but these are totally separate. The exhaust only provides the energy to spin the shaft, nothing more. In the supercharger the energy comes from a belt connected to the engine itself.

That's also why the differences is size and types. The turbo can spin to amazingly high speeds in the 100,000's of rpm so it can be pretty small to move that much air. The supercharger is limited by the rpm of the engine and the size of the pulleys so it needs to be a lot bigger to move the same amount of air.

The other thing is that the supercharger is spinning whenever the engine is turning so the extra power is always available (say about 1/6th of full speed at idle if it idles at 1,000 and spins to 6,000). The turbo needs the exhaust gases to be flowing quickly so at idle it is really not doing much at all and might be at 1/100th or less of full speed. That's why you get turbo lag in some systems.

(It's also why you get some systems with both so the supercharger handles the low rpm range and then the turbo comes in to give a kick at high rpm.)

Both can use an intercooler because the act of compressing air creates heat and hot air can't hold as much fuel vapour so won't burn as well. The turbo just needs it more because it is closer to the hot exhaust and the turbo housing gets very hot.

I'm not an expert either but just some things I've picked up. Hope it helps.

Andrew
 
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