Originally Posted by Inygknok
ok, im cracking my head with the compressor map thing, and ive come to wonder, wat the hell does the Engine are flow thing exactly mean? anybody got any idea wats meant with an Engine's Air Flow (measured in lb/min - pounds per minute).
I will hit on a little of this because there are some great sites on the web which have great detail.
Think of an engine as an air pump. As the engine rotates it sucks air through the air filter, the piping, the throttle body, manifold, the head, the valves, and into the cylinders.. Everything in this list restricts that air flow. Think of it as a garden hose. If you turn the water on a small amount of water comes out. If you pinch the hose less water comes out and pressure builds up behind the pinch. Water and air both operate the same way and are governed by the laws of physics. The only difference is that air can be compressed and water canít. You canít put more water into a five gallon bucket then five gallons. But you can compress air hundreds of times, and put 100 or 200 or 500 psi in that same bucket. (Really good bucket, ok).
Back to the engine. We have a certain volume of air going into an engine. This volume is determined by the bore and stroke of the cylinders, the restrictions of all the items I mentioned above, and the speed the engine is cranking. Now if weíre really good at math, (you know that stuff you really hated in school), we can calc the volume of air each rev pulls into the cylinder.
If we know the volume then a simple little formula tells us how many lbs/per/minute of air an engine would suck in. We call this air flow. Air flow is the number of molecules of air, flowing past a certain point in a certain amount of time. Now thatís a crock pot full of molecules so we convert them to lbs.
In an engine when the fuel ignites it is converted to a gas under great pressure. After the power stroke the engine goes to the exhaust stroke and all these gasses are forced through the exhaust valves and down the tailpipe. Here we have the same problems we had at the intake. The valves are small and restrict the gas flow, The head and header has turns and bends and restrict the gas flow, the catalytic convertor (invented by Satan) restricts the gas flow, the muffler deadens sound and restricts gas flow, even those cute little things we put on the end of our tailpipe can restrict the flow of gases, even the size of the pipes restricts the gas flow.
Remember the engine is an air pump. We want it to suck as hard as it can and blow out as hard as it can. This is ninety percent of tuning an engine. We must remove all of those restrictions that we possible can. We put in big air filters, bigger pipes. Throw out the cat, install headers, free flow mufflers. Anything which will improve air flow is a good thing. Except, as the valves open and close in an engine shock waves are setup in both the intake system and in the exhaust.
In the modern engine the shock waves are used to reinforce the main wave as it enters the cylinder. Here is how it works.
As the intake valve slams closed the air that was moving into the cylinder comes to a dead stop and bounces back to the throttle body or hits the walls of the intake plenum and bounces back toward the valves. If we can time this wave to arrive at the valves the second the valves open, we can force a little more air into the cylinder. The same thing happens in the exhaust, only we want the wave to suck on the valves.
Smack the water next time your in the bathtub, you will see a wave go out to the bathtub wall and return to your hand. Same thing happens in an engine. A really smart tuner understands how to use this to get the most out of an engine.
Ok here comes the turbo part. What if we had a way to get more air into an engine. Why not just pump it in. Now instead of working with atmospheric pressure (naturally aspirated) we grab a turbo and use it to force air into the cylinders. Now how much we can force in depends on the engine itself. We must calc how much air the engine can stand at maybe 2500 rpm, at 50% rpm, and 100% rpm. We do this thru a process called air flow mapping. Itís just a graph which shows air flow through the engine vs rpm. You need to make several based on number of lbs of boost you want. In the turbo air flow output is determined by the pitch, speed and angles of the blades.
Weíre talking super computer math stuff here. Thatís why most turbo manufactures have application charts available on the web. All you need to know is engine size, and the boost you want to run. Check the Turbo City website and other turbo manufactures for application charts.
Ok fellows it has been 40 years, since I studied this stuff at UT. Please feel free to correct where necessary.