Apollo231

I have a cuestion about how to find cfm for our motors. I know the equation to find cfm and how to convert it to lbs/min.

My questions are:

we would use 1.3 liters?
and what VE(volumetric effeicency)?

Apollo231

anybody?

mhwarner

Uh, oh: a displacement question. To make a long story short, if you work out the math for any given engine RPM, an NA 13b 2-rotor will induct the same volume of air per minute as a 2.6-liter piston engine will at that same RPM.

To calculate the actual amount of inducted air with boost, however, you need to know a lot more, including turbo output boost pressure and compressor efficiency.
-Mark in Tucson

Apollo231

Wrong on all counts. Can anybody else answer my question?

cardzrule

How do you know it is wrong on all counts if you don't know the answer either?

I found this in less than 3 minutes using Search:
https://www.rx7club.com/single-turbo-rx-7s-23/turbo-sweet-spots-20b-114193/

cardzrule

https://www.rx7club.com/showthread.p...hlight=cfm+lbs

https://www.rx7club.com/showthread.p...hlight=cfm+lbs

https://www.rx7club.com/showthread.p...hlight=cfm+lbs

cardzrule

https://www.rx7club.com/search.php

mhwarner

Wrong on all counts? Hmmm. If you don’t mind, please enlighten me with your reasons. I may well be incorrect, but I don’t think so until you show me. The math is pretty simple. Consider the following:

The volume of air drawn in by one of the three faces of a single rotor of a 13b engine is 654cc. This is just geometry. Not easy geometry, but geometry nonetheless. We’ll call this amount the face volume.

Now, each rotor has three faces that induct air as the engine spins. A 2-rotor engine therefore has six of these 654 cc “faces” drawing in air as the engine operates.

Engine speed is measured by the eccentric shaft rotation rate, not the rotor rate. The eccentric shaft rotates at a rate 3-times that of the rotors. At, say, 6000 engine RPM, the rotors are turning only at a rate of 2000 rpm.

To calculate the intake flow rate (that’s volume per unit time) of air being inducted into the engine, you simply multiply the face volume of a single rotor by the number of faces inside the engine by the rotor rotational rate:

654cc x 2 x 3 x 2000 rpm = 7,848,000 cc/minute

Or converting to cfm, we have 277 cfm at 6000 rpm.

Now for a normal piston engine, the standard method of calculating intake air flow rate is: displacement x rpm / 3456. The 3456, by the way, is simply equal to 1728 (which is the conversion factor from cubic inches to cubic feet) multiplied by 2 (which is required because a piston crankshaft has to rotate twice for a single intake-compression-power-exhaust cycle to complete).

Anyway, setting [Displacement x RPM / 3456] of the piston engine equal to the previously calculated 277 cfm value for the rotary, and solving for Displacement, leaves us with:

(277 x 3456) / 6000rpm = 159 cubic inches, or 2614 cc.

If you plug in any RPM value to these equatios, you will always end up with this same 2614cc. In other words, a 13b engine exhibits the same intake flow rate as a 2.6-liter piston engine. I’m not saying the 13b engine *is* a 2.6-liter engine. Instead, I’m saying that the amount of air drawn into a 13b at any engine speed is the same as a 2.6-liter piston engine at that same speed, assuming equal volumetric efficiencies.

Please show me where this calculation is incorrect.
Cheers,
-Mark in Tucson

Apollo231

Thanks cardzrule, you made me look through 7 pages of **** and I still dont know how to get the VE for our motors, your a big help.

mhwarner: thank you for the answer. by not showing the math in your first post i assumed you were pulling that number out of your ***. BUt your explenation looks pretty much right on, im sorry for my harsh comments, but your second paraghraph is still inncorrect. Sorry I really dont have the time to teach anybody how to read a compressor map.

Now can anybody give me the formula to find VE for our motors?

TT_Rex_7

You, my friend, are a complete tool. The first link told you the VE. Besides, if you can't even figure out the VE, then you need to contact a rotary shop to help decide what turbo to go with, because you have no clue what your doing.

Since i'm a nice guy... http://www.turbofast.com.au/freesoftware.html That should give you a pretty close estimate on finding out the VE. Use 2.6L (160 CI) If you have a 3rd gen., or a TII, then use N/A hp.

-Alex

Apollo231

Thats cool man. Im sorry i dont like reading through 7 pages of babble just to find a simple answer.


-tool