Calculating optimum CFM for rotaries

[dr.occa]

I've been going over some math 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 140cc[rounded up from 139.866421]) / 3456) X .85 = 275.46CFM


((8000 x 210cc[rounded up from 209.7996322158]) / 3456) X .85 = 413.2CFM

Which should it be?

[730RWHP12A]

do you want the answer to your math question or do you want to know what carb will work best for your application.....

[dr.occa]

Originally Posted by 730RWHP12A

do you want the answer to your math question or do you want to know what carb will work best for your application.....

I'd like the answer to the math question which would then explain the carbs that currently are known to work best and point to other carbs that could also fit the bill.

[730RWHP12A]

Originally Posted by dr.occa

I'd like the answer to the math question which would then explain the carbs that currently are known to work best and point to other carbs that could also fit the bill.

i cant answer your math question but i can tell you what carbs i have had the best luck with

[dr.occa]

I'm running a 48 IDA with 38 chokes currently on my 12A. I'm looking to run a pair of Mikuni TM40(HS40) flatslide carbs with their smallest diameter section being 40mm (at the intake side mounting point).

I was planning on taking a spare factory intake I have laying around and routing out the primaries into the secondaries (giving me 2 oval intake ports) and then having a pair of runners fabbed up into a mating flange on top of it for the flat slides. It would look not too dissimilar to a dcoe intake.

[scarsofcarma]

Originally Posted by dr.occa

I've been going over some math 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 140cc[rounded up from 139.866421]) / 3456) X .85 = 275.46CFM


((8000 x 210cc[rounded up from 209.7996322158]) / 3456) X .85 = 413.2CFM

Which should it be?

Your latter number sounds better, but I think it should still be a bit higher... You were right to question the effective degrees of rotation because the rotary motor breathes more like a 2-stroke than a 4-stroke. I did some rough calculations before but I don't have the numbers in front of me as I write this. I remember getting something between 600 and 700 maximum CFM's for a 13B, but that number will change depending on the redline value you plug into your calculation, and wether or not it is ported.

[chibikougan]

Here is a Quote from Data Crazed Individual see if he knows who he is...

BDC volume is 654cc or there abouts per chamber for a 13B and it is very possible to achieve Ve's in the region of %120 or greater. (And yes this is pure VE, not boost added as some people do!)

There are very many variables to calculation the flow rate through any given rotary engine or ANY engine for that matter.........good luck to anyone trying :wink:

The variables you need to account for are as follows.

Capacity
RPM
VE
Barometric pressure
Air temperature
Boost pressure (If not NA)
IC eff (If not NA)
Compressor eff (If not NA)

All of the above will let you work out the actuall flowrate through the engine BUT you will need to know the effect of exhaust restriction on VE and what base line VE's apply to certain RPM ranges for certain engine combinations........ Things like porting type, size, End plates used (if side ported), Intake manifold design, throttle body size, the list goes on and ALL can have very huge effects on the VE and hence change the flow calculation massive amounts.

Every engine is different, and no generalizations can be made as variations by certain builders make huge differences to VE @ certain RPM ranges.

Once you figure all this out

Then if you know the BSFC (again cant use generalisations here) varies based on porting type, engine health, compression ratio, Engine/turbo mechanical efficiency, Injector location and function (fuel mixture) & the a/f ratio you can figure out how much power that engine will make.

BSFC
A/F ratio

You will need to make a "look up table" of some sort to relating BSFC to A/F ratio which will give you the relationship needed to give a real accurate power calculation.

Have fun with the formulas. Good luck finding the data

[RICE RACING]

Originally Posted by dr.occa

I've been going over some math 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 140cc[rounded up from 139.866421]) / 3456) X .85 = 275.46CFM


((8000 x 210cc[rounded up from 209.7996322158]) / 3456) X .85 = 413.2CFM

Which should it be?

Thanks Cheebs (previous post).

This is how you do it.

Capacity (per rotor chamber in Liters) x 2 (number of rotors) x Ve (1.15 = 115%) x (rpm)
0.573 x 2 x 1.15 x 9000 = 11861.1 L/m
Liters to m^3 / 1000
11.861L/m = 418.867 CFM

So that is the bullshit maths after I consulted with Howard Coleman......

The VARIABLES are the Ve.
and RPM

A PP will do over 125% if done right and run up to 10,000rpm
A RICE RACING STREET PORT will do 115% Ve and up to 8,500rpm
Stock port will do around 100% Ve and up to 7,000rpm

ALL of this great variability means you have to rely on experienced c-u-n-t-s who have done allot of this to recommend a carb for you that they have trialed and worked out for you

Racing Beat did allot of this already for the shit house 'double pumpers'

IF you want to do it on your own go ahead just keep in mind the way holley rate the CFM is at a certain amount of pressure drop in the rating test.

Ask rotaryshack nice and I am sure he will accommodate you

[chibikougan]

Haha shouldn't have said Don made it a little harder..


Edit: Oh i did and still got it Ooooooh.