Calculating optimum CFM for rotaries
 

I've been going over some path in determining various carb options for the rotary motors. I'm very familiar with the tried and true IDAs (running one on my 12A), DCOEs & 44PHH (running a pair on my 4AG) and the 4 barrels: what I'm scratching my head about is do I use the cc displacement at the taxed 720° rotation or the actual full 1080° rotation in calculating optimum CFM?

Here are the answers for my 12A using both displacement numbers (in cubic inches):

((8000[rpm] x 140[rounded up from 139.866421]) / 3456) X .85 = 275.46CFM


((8000 x 210) / 3456) X .85 = 413.2CFM

Which should it be - the higher one?

doh! uh, path should be math. :/

Maby this helps:

https://www.rx7club.com/general-rota...length-199788/

Unless you're using a plenum manifold, ignore CFM and just go by the traditional throttle plate/venturi size method.

Like, a "traditional" single 45DCOE with 36mm chokes drives pretty much just like a stock Nikki, so for performance use you will want larger than that. Two of them is just about right... for a street port :)

My bridge port has what looks like a huge carb on it - Holley 750 - but in reality it's pretty small, the throttle plates are only 42mm in diameter. Difference is independent runners vs. having a plenum.

I was planning on taking a factory intake and opening it up from the 4 independent ports that the nikki carb sat right on top of to a single open port. Then fab up a flange with 2 runners to a pair of 40mm flatslide carbs hovering over the motor: similar to how a single dcoe style side draught would mount.

so if you've picked the hardware, why bother with the maths?

i think if you measured the air going into the engine somewhere, like the air cleaner snorkel, or something, then you'd see something close to the 280 cfm number.

however the carb needs to be able to handle demands at the runner/plenum, and then carbs are rated at a different pressure drop as well, so even with a plenum, a 280cfm carb has a different pressure drop than a 280cfm engine.

this is why a going from a 330cfm stock carb to a 465cfm stock carb makes more power even when the engine only flows 280cfm.

if you like PJ's idea, the factory MFR p port engine runs a 43mm venturi, and the runner is also 43mm, coincidence?

What I'd like to know is what displacement number should be used to determine if the carbs I'd like to use will meet the demands.

I'm looking at a pair of Mikuni TM40(HS40) flatslide carbs. The inner venturi measures at 40mm.

the displacement is fine, the part you're missing is that the flow thru the carb isn't constant, especially if its individual runner.

and secondly they rate carbs at a different pressure drop than you'd come up with in the formula, that is why a 465cfm carb isn't big enough for an engine that flows 280cfm.

those carbs are meant to be individual runner? if yes, i think they should be about right, maybe a little big

If it's worth anything, 2x40 is the throttle plate diameters of the secondaries on a GSL-SE.

That's on a plenum, and they're way too small even for that.

Well if you plan on getting rid of your 44phh, let me know. I only need 1 though.

Hey I appreciate it y'all. I'm going to go ahead with the plans and also check in with some Cycle techs regarding a ballpark jetting for the carbs.

Again, much appreciated.