flubyux2

head work and port work and runner alterations change the car's breathing ability and change the CFM rating or how many lb/min the engine can breath... thats how they calcualte Pumping efficiency. if an engine was Perfect, it would breath 3.0 liters of air for every 2 crank revolutions... but due to inherent design flaws, it will only flow LESS than 100% efficiency. Forced induction fixes this problem.

besides, all the design errors,flaws, and limitations can only make the flow rate of the compressor WORSE... the compressor flow rates are measured at the discharge outlet w/ no restrictions. so thats what the compressor wheel COULD flow in a perfect world... if your engine was perfect... like the 7M or a 2JZ w/ a 1JZ head.

Trip

This is something EVERYONE needs to know before upping there boost:



So? How Hot is the Air Coming out of the Compressor?

Well, I'm glad you asked. The equation used to calculate the discharge temperature is:
Tout = Tin + Tin x [-1+(Pout/Pin)0.263]
efficiency

Example: the inlet temperature is 70 deg F, the suction pressure is -0.5 psig (a slight vacuum), the discharge pressure is 19 psig, and the efficiency is 72%. What is the discharge temperature?

Tin= 70 deg F + 460 = 530 deg R
Pin= -0.5 psig + 14.7 = 14.2 psia
Pout= 19 psig + 14.7 = 33.7 psia
Pout/Pin = 33.7/14.2 = 2.373 (this is the compression ratio)

Tout = 530 + 530 x (-1+2.3730.263 ) = 717.8 deg R - 460 = 257.8 deg F
0.72

So the theoretical outlet temperature is 257.8 deg F. I sure would like to have an intercooler to cool that hot air down before it goes into my engine!

Compressors do not always operate at the same discharge pressure. The discharge pressure that the compressor produces depends on the volumetric flow into it (not the pounds of air, but the CFM of air), and the rpm that it is turning. The performance of a compressor can be shown on a graph by a series of curves. Below is a compressor map from the Turbonetics catalog attached, it is the file called H-3.JPG. [The graph is included here, and is available for download via the hotlinks provided....Ed.]



This is for their Cheetah turbo; take a look at it. The bottom of the graph shows the lbs/min of air that the compressor is moving, corrected to a standard temperature and pressure. The standard industry practice is to put this part of the graph in actual volumetric flow (such as ACFM) since the compression is constant for a given volumetric flow and compressor speed, NOT for a given mass flow. Unfortunately they didn't do their curves that way, and to use the Turbonetics curves we have to figure out the pounds of air moving and correct it from the actual inlet temperature and pressure to their standard temperature and pressure.

The left side of the graph shows the outlet pressure to inlet pressure ratio.

There are two different sets of curves in the graph; efficiency curves and rpm curves. The area where there are lines drawn is the operating envelope. It is best to operate the compressor within its envelope. It will still run if you go to the right of the envelope, just not well. To the left of the envelope, where it is marked "surge limit", the flow through the compressor is unstable and will go up and down and backwards unpredictably. This is surging. Do not pick a turbo that will operate in this area! It can be very damaging.

The Turbonetics catalog says to pick a turbo that is close to the peak turbo efficiency at the engine's torque peak while still maintaining at least 60% efficiency at the maximum rpm of the engine.

Here's how to read the graph.
Figure out the pounds of air that you are moving through the engine. In our '87 example, we were passing 41.3 lbs/min of air, at inlet conditions of -0.5 psig and 70 deg F. Now correct that flow to the standard temperature and pressure.

Corrected flow = actual flow x (Tin/545)0.5
(Pin/13.949)

Note that I am using 13.949 because we are measuring everything in psia instead of in inches of mercury, which Turbonetics assumes.

13.949 psia = 28.4 inches mercury absolute.
29.92 inches mercury is atmospheric pressure at sea level, so 29.92 - 28.4 = 1.52 inches mercury vacuum.
That is their standard suction pressure.

Their standard temperature is 545 deg R, or 545 - 460 = 85 deg F.

So we are correcting the flow from 70 deg F and -0.5 psig to 85 deg F and -0.75 psig (or 13.949 psia, or 0.75 psi vacuum, or 1.5 inches mercury vacuum, or however you want to look at it.)
Again, temperature and pressure have to be absolute.

Tin = 70 + 460 = 530 deg R
Pin = -0.5 + 14.7 = 14.2 psia

Corrected flow = 41.3 x (530/545)0.5 = 40.0 lb/min
(14.2/13.949)

So we mark that point on the bottom of the graph, and draw a straight line upward from that point.

An alternate and better way of getting airflow at less than full throttle is the use of a scan tool. The scan tool (such as TurboLink(tm)) reads the mass air sensor output. TurboLink(tm) gives this in grams per second. To convert that to pounds per minute just multiply by 0.1323. For example, if TurboLink(tm) says 18 gm/sec @ 45 mph, 18 x 0.1323 = 2.4 lb/min of air.
Correct that to standard conditions and plot that on the compressor map. Unfortunately the MAS will only read to 255 gm/sec. If you are moving more air than that, the MAS won't show it. That is why you need to go through the above calculation for full throttle air flow.

The next step is to figure out the compression ratio, using absolute pressures. Using our example, we had 17 psi boost in the intake manifold. Let's suppose the pressure drop from the turbo outlet to the manifold is 3 psi; so the actual compressor outlet pressure is 3+17=20 psig. The air pressure is 0 psig, but since the turbo is sucking air to itself the pressure at the inlet is lower than that.

Let's say it is -0.5 psig at the inlet. Then the compression ratio, Pout/Pin is :

Pout/Pin = (20 + 14.7) = 2.44
(-0.5 + 14.7)

So then we find about where 2.44 is on the left side of the graph and draw a line horizontally from that point. Where the two lines meet is where the turbo will operate.
Look at the efficiency curves, which look like circles. Our point is just a little inside the 72% curve, so when we are running at 5000 rpm and 17 psi boost with 70 deg air outside and 130 deg air in the manifold then the compressor efficiency is a fraction over 72%.

The other curves are rpm curves. Our point is above the 105,500 rpm curve, so the turbo has to spin about 108,000 rpm to get the pressure up to 20 psig from -0.5 psig. The Turbine has to provide enough power to spin it that fast.

Change any of these numbers, and the point at which the compressor runs at changes. More engine rpm means more air flow, so the operating point moves to the right. Colder intake temperatures means more pounds of air which moves our point to the right. Raising the boost probably means more air into the cylinders, but also the compression ratio goes up so our point definitely moves up and should move right. And so on.

722ish

i figured out the CFM for a wrx..@ 7200 rpm, fuel shutoff..

so


cfm = (7200 x 122) / (1728 x 2)

again, 7200 rpm, and 122 cid.... this works out to

256 or so?

exact calc... 254.16 rept...

sorry, close guess...


how possibly does my car only flow 255 cfm? i would think it would be significantly more than that?@:o