Prodigy Performance 3.6 Turbo DIY Install

[KaiserBill]

One last attempt to explain from a different direction why CFM capability of the supercharger/turbo is completely irrelevant to my analysis of RIPP's claim of 40% gains at 1800 rpm. Please actually read through this and put some effort into understanding it before responding.


CFM = Cubic Feet Per Minute

That's a rate of flow of a volume of air per unit time. A volumetric flow rate. That's it. Volume/time. Pressure, mass of air, or number of molecules is completely independent of flow rate.

What is the flow rate of the Pentastar engine at 1800 rpm? For simplicity, we will ignore extra volume in the ceiling of the combustion chamber, valve overlap, etc. Let's just go by the displacement.

3.6 L total displacement, but each cylinder requires 2 rotations of the engine to intake and exhaust 1 cylinder full of air.

So (3.6 L) * (1800 rpm) / 2 = 3240 L/minute

Again... this is just volume of air. Pressure, temperature, etc., are all unspecified. It does not specify the AMOUNT of air flowing through the engine. Just the VOLUME. The AMOUNT of air depends on the volume, pressure and temperature.

So what's the volumetric flow rate of the engine at 1800 rpm with 0.3 psi boost? At 0.6 psi boost? At 37 psi boost? The answer is always 3240 L/minute. The difference is the pressure and temperature of the air, which results in a different AMOUNT of air, but the same VOLUME.

Even if there's some inefficiency with cylinder filling (intake valve isn't open long enough to equalize pressure between cylinder and manifold, for example), the end result will still be the same VOLUME of air, but at a slightly lower pressure than manifold pressure (the ECU has lookup tables of volumetric efficiency multipliers to deal with this).

What if you have a supercharger capable of flowing up to 98,000 L/min at 58 psi, and it's producing 23 psi at 1800 rpm? What's the volumetric flow rate through the engine now? Still 3240 L/minute! The supercharger does not change the volume of the cylinders. And it does not change the engine speed. Therefore it cannot possibly change the volumetric flow rate of air through the engine. It only compresses air, changing its pressure and temperature.

Therefore, two different forced induction systems producing the same boost pressures and intake air temperatures on the same engine at the same engine speed cannot possibly be operating at different volumetric flow rates (CFM, or whatever units you care to use), no matter what their peak boost or peak CFM ratings are.

Whatever, you want to believe is fine with me. However, your first formula is wrong-- your volume at 1800rpm is always 3.6L at 5400 rpm the engine still is 3.6L at 150,000rpm still 3.6L! Now CFM is also directly related to the more important number--- lbs/Ft3 measuring air-density. That formula looks like this take CFM x .076Lbs/ft3 and poof that is the magic number. I tend to use CFM because it, well, works just as long as you remember to change it over to Lbs/ft3 and then remember that is per minute. Thusly, even a compressor running at .6 PSI operating at X Lbs/ft3 will create less power than a similar unit doing .6PSI with X+2 Lbs/ft3.

Hence my Vortech examples-- at any given Manifold pressure the V-2E has the potential to produce more horsepower than its similar brother the V-2SCI unit. That little 100CFM difference if it is constant across the entire units performance band produces an extra 7.6Lbs/Ft3 of air mass per minute!!! which if you say rough equates to about 30-50hp at any rpm range for any given boost. If that is a constant 100CFM increase. Now it might not be, but it could be. Which would make your analysis totally meaningless if you didn't take that extra air-mass into account for each given boost level.

So here is the general rule of thumb!!!! And it isn't supper accurate, but if you are making 125 HP at flywheel/ by 10.5 = 12.5hp per Lb/min. So if you are at 1800 rpm and you are pulling in an extra 2.2lbs/min of air (which can then be converted over to CFM, LFM, KG/CM3 or ML/CM or whatever you like) you roughly get a 27.5 hp which is pretty damn close to a 40% increase in power if go from 75HP at 1800 rpm to about 102.5HP: So what it is about a 37 percent increase in power? And it only required an addition 2.2lbs/min of air mass to reach the cylinders.

Until you know exactly how much air/mass is reaching the cylinders for a given boost pressure will not tell you anything of importance. Because guess what moving more air at any pressure means the potential for more air-molecules to be found in that specific volume of gas. Damn that air density.

Also on a side not I think you are misunderstanding Pressure Ratio on the Turbo chart. It doesn't measure the ratio of boost produced for a given volume of exhaust- it measures the ratio of compressione from absolute pressure at the inlet of the turbo to the outlet of the turbo. So lets say you have a 1.25:1 Pressure Ratio at 55,000rpm (impeller speed) means that you out pressure is 1.25 times that of the absolute pressure reading of the inlet side. Most turbos never get much more than 3.0 to 3.5 to 1 ratios. But that is pretty damn impressive. That's why staging turbos can get really insane pressures rapidly.