Ok guys, I know people have talked about cfm a lot in the past but I need something more.

Does anyone know where I can find a chart for cfm across the rev range of a s4 N/A13b??

I've searched already and all I can find are people saying oh it flows 350 cfm or something at WOT. But I'm wondering if anyone has numbers for across the range.

Thanks.

 

No help on this one eh?

 

got flowbench?


I don't think this has ever been published(afaik)

 

why would you need a chart? for initial fuel mapping? sounds like some good info to have.


-mike

 

Try these numbers. The calcs are based on the rotors turning one complete rev for every two turns of the e-shaft.

Rob

1000 22.95 cfm
2000 45.91
3000 68.86
4000 91.82
5000 114.77
6000 137.73
7000 160.68

 

isn't it a 1:3 ratio?

 

so is that one rotor and full rotor rotation, 1 chamber and full rotor rotation, 1 chamber and 1 e-shaft rotation, or 1 rotor and 1 e-shaft rotation??

Not trying to be mean, just a little vauge on your description of the numbers.

The reason I'd like to know the cfm through the range is for matching cfm for a supercharger of sorts. I'd like to be able to see the flow through the range of both.

 

Well from what i have read one rotation of the rotor will rotate the e-shaft thrice. Not sure if his calcs are off or if i'm wrong.

 

yeah, 3 e-shaft turns for 1 rotor turn, but I seem to recall somewhere reading that max flow from a 13b at 8000 is 35X cfm or something so maybe that's 1 rotor full rotation.

 

The flow of a 13b would be for both chambers imo i could very well be wrong though.

 

what I mean is maybe he calculated it for one rotor and for the engine it's twice what he said in that list.

Hopefully while I study for my final tomorrow morning someone will help us out

 

Each rotation of the rotors (1300cc) equals .04591 ft^3. So if the rotors were turning at 1000 rpm, the rotors would be displacing 45.91 cfm. And if the rotors were turning at 7000 rpm you would have 321 cfm. The key word here is Rotors. So the question is - how fast are the rotors turning in relation to the e-shaft? If rpms are measured by e-shaft rotation and the ratio is 3:1 then you can take my numbers in the table above, multiply by 2 and divide by 3 to get the proper answers. or 107 cfm @7000 rpm.

Rob

 

are you telling me the 13b flows 45 cfm @ 3k showing on the tack, and 107 at 7k? That just sounds a lot lower than what I've heard.

 

You guys are really going to confuse yourselves if you harp on the eccentric shaft and rotor configuration.

It's actually very easy... The 13B engine displaces 1.308 L per revolution of the flywheel. In other words, at 1 rpm (tachometer reading) it displaces 1.308 L per minute. Now, this is assuming 100% volumetric efficiency, but the base equation is very simple.

Therefore, your theoretical flow rate (assuming 100% volumetric efficiency) is:

rpm /cfm/ lbs per min @ ISA conditions
1000 /46/ 4
2000 /93/ 7
3000 /139/ 11
4000 /185/ 14
5000 /231/ 18
6000 /278/ 21
7000 /324/ 25
8000 /370/ 28

Notes:
1) Non-ported engines will consume less cfm because their volumetric efficiency will be less than the optimal 100% used in these calculations.
2) Volumetric efficiency is not a constant, and varies with rpm and other factors, depending on the configuration of the engine system. It is only used as a constant here to simplify things.
3) These cfm numbers are NOT compatible with carb cfm ratings which are based on the vacuum signal of a piston engine rather than the calculated flow rate of the engine.