(INTAKE) Carb Sizing Question

 
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Old 05-02-06, 03:54 PM
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(INTAKE) Carb Sizing Question

I've been talking with a few first gen owners, and they are talking about putting squarebore carbs that are as big as 600cc on a 12A.
That's fine and all by me, but a 12A doesn't flow that much air, so why does everyone overcarb?

a N/A rotary runs at between 60 and 70 percent volumetric efficiency, and using airflow calculations for it as a 2300cc motor revving to 8000 rpm at 100% ve, the airflow through the motor doesn't even reach 400cfm.

It might just be me, but does it not make sense to put a smaller carb (like a 390cfm 4bbl) onto a 12A vs. overcarbing it?

Mainly, can anyone explain the airflow physics behind why everyone bolts big carbs onto their tiny 12A's?
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Old 05-02-06, 04:04 PM
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Been discussing this very topic elsewhere. My theory is as follows.
The intake ports are *almost* never totally shut off, which keep the velocity of the intake reasonably high.
The rotary is not as capable of pulling air in, compared to a piston engine, and therefore, never can utilise more than about 70-80% of a carbs *rated* CFM. Think about how much worse a partly clogged air filter affects a rotary compared to a piston engine.
While a 12a mathmatically cannot exceed the CFM rating of the stock Nikki (300-330 CFM, IIRC) it still loses the ability to pull much over 6K. A higher CFM carb will allow the same engine to pull harder, at a higher RPM.
This is all based on my theory, which I have neither the tools or funds to check for accuracy..
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Old 05-02-06, 04:10 PM
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hahaha, paul this is greg.. you're one of the 'first gen guys'

since you mentioned pulling air in, what kind of vacuum do 12A's pull?
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Old 05-02-06, 04:19 PM
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Here ya go. Thanks to Sterling!

Tranquil is correct.
I suggest that a Sterling Nikki can be tuned to out perform a Weber or Dellorto, or just about anything else, but I don't have enough compiled data.
I base it on physics for now; -A theory to be tested.

The carb flows 465 cfm. As Tranquil stated, that's the "yippee-skippy, my speedo goes to 200 MPH!!!" number. In other words, the number that sells the carb to people that like bigger numbers, but don't realize that bigger numbers don't always mean everything!
If you do the math for a 13B 80 cid engine, multiplied by 10,000 RPMs, at 100%Volumetric Efficiency, you find that it can't possibly eat more than 463 cfm!

Now there are plenty of other factors involved, including pulsing harmonics, how well the air flows in due to exhaust gas expansion back pressure, the flowability of the manifold and the intake port...But these all detract from the VE number.
There are rare, and very specific situations where the intake pulse rate, engine RPM, exhaust pulse rate, temperature, and perfect alignment of the planets can cause such a perfect harmony in an NA engine that the VE actually exceeds 100%, but it's only for a brief second, and considered a rare anomaly.

But flow alone doesn't make the carburetor. Not at all.
In fact, a carb too small for the rotary will often out perform a carb that's too big on the street, because the big carb only has a very narrow RPM band, way up high, when the velocity is actually fast enough to create a strong, and therefore tunable, signal to the main circuit where the fuel delivery can be properly metered and atomized.

I love the Nikki, and here's why;
VELOCITY is key. It's the single most important part of having a tunable carburetor.
The Holley carbs have 4 venturis that are the same diameter. That's V8 stuff!
The rotary engine has such a terrifically wider RPM range, that it really needs two carbs; a tiny one that will provide high intake velocity for the lower part of the band, and a much bigger one for the higher end of the band for when the little high velocity carb gets maxed out.

A 4 bbl carb does exactly that. The smaller the primary carb is, the lower in the engine RPMs that high velocity air flow will effect the carb. That's good, because you want that strong signal to the main fuel circuit to pull up fuel into the air. But too small, and it will max out the air it can flow too soon. Then the secondary carb has to come on.
...Well, that's not really too much of a problem. Even uncut Nikki primary venturis will suffice as far as air flow for a performance primary carb's duties. But then the secondaries would have to be cut more than we do.
The Nikki secondaries don't have enough material to remove that much without starting to disrupt the critical inlet & outlet angles that are so crucial to making a venturi have such high velocity at it's apex, so we have no choice but to do most of the removal to the primaries. Still, when they are done they flow far less overall than the Holley primaries, which translates into higher velocity by comparison. MUCH higher. This combined with the fact that Holley venturi geometry leaves much to be desired has me scratching my head at Holley, saying "WTF?"

The fuel signal primary half of a Holley, just like any Weber or other 2 bbl, could be air jet regulated a bit on the delicate side so that it has a high signal low in the RPMs, at relatively low velocity. But at low air velocity the fuel won't atomize very well and will be more like it's dribbling in, compared to the good atomization that comes from high velocity. Also, by the time the primary circuit is maxed out in it's air flow, the velocity is sucking too hard on the fuel circuit for that calibration, and you'll have to limit the fuel by the primary jets. This can cause a flat spot that pops up on you seemingly at random, when in fact it's occurring when you're under load. (The flat spot should ordinarily be hard to notice except that it will come on directly after a point where the air velocity is very high, and the fuel mixture is just perfect for that tiny window of the RPM band. -That happens in all carbs and is what you're always working to tune out by finding the perfect circuit transition...whole 'nother chapter...)

All this can be applied to the secondary portion of any carb, too, though there is almost always enough air flow velocity by the time the secondaries kick in, mechanical or vacuum, that the fuel is from that fuel circuit gets well atomized.

So to find the best carb set up for a rotary, do this;

Calculate the maximum flow of your engine.
Choose an appropriate carb based on this number.
You want one that will give the maximum air flow velocity possible...the smallest venturis you can get that will deliver your air, but at the same time you don't want one you're going to max out at your high RPM.
Then divide that number up between two carbs, -One big, and one little.
Combine them into a single two stage 4 bbl.
Make the two circuits completely tunable so that you can control maximum fuel they get, bleed off the signal to the main circuit with air bleeds, and control when the second one begins to work.
Now add an accelerator pump that will squirt fuel not only to get you going from an idling dead stop, but will also squirt fuel as the second carb is opening. ...And make that tunable.

Now you have a Sterling Nikki.

Nevermind what it looks like, and nevermind what brand name it sports.
Just undertand that it was engineered by Nikki, and modded by me & Carl based on physics.
-And make no mistake about it, this is NOT a "bolt-on & go" performance part.
Those folks who have done so and have had good results either have been extremelt lucky, or most likely, are not running their Sterlings @ full performance potential.
Everyone's application is different due to environment, altitude, and driving style.
It really does need to be regarded as a fully tunable carb in a "driver's application customisable" way.
Check the archive's they do tend to have good info.
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Old 05-02-06, 04:21 PM
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pinknuggit, you hit the nail on the head

unported or lightly street ported 12a's should be running 465cfm carbs (or around there)

Large street port and bridge port motors should run 600s

Jon
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Old 05-02-06, 04:45 PM
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Do the large port motors need the extra cfm's because of a higher VE and higher airflow through the engine, or the fact that they will turn a higher rpm?
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Old 05-02-06, 04:59 PM
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Ports do not affect RPMS, but yes, they would effect VE/flow.

At higher RPMS VE drops because not enough air can get into the engine in the tiny slice of time the port is open. Bigger port, open longer, cut for good flow = better VE throughout RPMS.

As my meager mind understands it.

Jon
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Old 05-02-06, 05:07 PM
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Large port engines usually get a bigger carb because the goal is high-end power.
The power band is generally moved up a thousand revs or so. (I believe.)
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Old 05-02-06, 06:29 PM
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so porting a motor really raises its ve that much to need a 600cfm carb?
That's way up in 175% VE + @8krpm territority with the turbo motors.
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Old 05-02-06, 06:43 PM
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Square bore carbs aren't really that good on a rotary. Like the quote from sterling says. I think the secondaries are actually flowing much higher than the primaries, due to the intake port runners. These are larger on a 12a, and even moreso on a 6p 13b. However, there aren't that many spreadbore options, so they tend to use a larger carb than should be needed. A vacuum secondary squarebore is even worse than a mechanical sec, due to the primaries never really getting maxed out.
Now, if there were some quality spreadbores rated around 475-525CFM, I bet they would outperform a 600CFM squarebore. However, their just isn't much of a market for this type of carb, other than for the old-school rotary crowd. About the best option we have, is a Sterling carb........
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Old 05-02-06, 06:45 PM
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And one helluvan option it is!



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Old 05-02-06, 06:55 PM
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3 2 barrels FTW. Jk although they do flow better than 2 4 barrels in most cases
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Old 05-02-06, 07:09 PM
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I'm one of the other first gen guys. I just bought a Holley 600 Double Pumper off ebay for $45 to replace my vac secondary carb. Mine's a 13b 6-port motor, so it out flows the 12a a bit. Anyhow, my answer is uhm... much less 'serious' in tone.

My tire guy says rotaries run on 'magic'. Well, beings anything going into the ports (without vacuum leak) has to go thru the carb, the CFM of the carb must also account for the 'magic'. It may only flow 463CFM of AIR into the motor, but that other 137CFM is to allow it to pull in the 'magic' as well.

Anyhow, I got the 600cfm Double Pumper because there are others running the same carb with good results, better than my non-rb 600cfm vac secondary. Also, if I ever want to blow thru turbo the holley, vac secondary is a no-no. This won't be for a *long* while, but someday. When that day comes, I'll be well prepared.
http://cgi.ebay.com/ebaymotors/ws/eB...tem=8059801846
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Old 05-02-06, 07:12 PM
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well i have a '79 12a and i keep running into problems with my stock carb. mainly it keeps overflooding. I am looking to increase the power by using a new carb but i am wondering what to go with. Any advice?
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Old 05-02-06, 08:13 PM
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Another thing I was thinking (on a more serious note).

I remember reading somewhere that part of the reason RB had to 'modify' holleys to work on the rotary was because of the pulses of air going back through the manifold. The same stuff that the second gen's used to get that supercharging effect for a few more HP. Since carburetors are generally NOT exposed to this type of thing their CFM ratings are also based on no static pressure coming back at them. The same type of thing is used to rate fan CFM. Flowbenches don't necessarily take static pressure into account. Sure, a fan can flow 1000000 CFM with no static pressure, but throw it behind say a radiator and that 1000000 CFM is NOT all making it through the rad.

Perhaps another reason to oversize your carb on a rotary is to make up for CFM lost to the pulses heading back at the carb? I don't imagine they kill flow a lot, but it's feasible that it has something to do with the Nikki's choking out above 6k? This may also explain why a properly sized carb does NOT run properly on the rotary.
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Old 05-02-06, 08:46 PM
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Are your volumetric efficiency calculations based on the fact that it's a 1.1L engine?

You realize you can't compare rotary displacement to piston displacement on a 1:1 basis. Someone will be able to offer a better explanation but from what I can tell, rotaries make power 270 degrees of every rotation and piston engines make power 180 degrees out of every other rotation.... or something like that. So that's why 1.1L rotaries race in classes with 2.6L piston engines.

I'd assume that messes up the volumetric efficiency calculations a bit eh?

Jon
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Old 05-02-06, 08:49 PM
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pulses going back to the carb? I don't think it's air that travels back up the manifold.
All engines do that, it's why the hondas scream so loud through their intake and why any motor will sound throaty when you take the intake off.. it's just resonating sound from air getting sucked into the chamber, not airflow.

No, I'm doing the calculations based on the 12A being 2200cc, and the 13B being 2600cc.
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Old 05-02-06, 08:50 PM
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Found These

http://www.rx7.com/techarticles_displacement.html

http://www.digi-go.biz/articles/volu...-miguel-p.html

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Old 05-03-06, 07:01 AM
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It's very simple math. No need to overthink it.
The 12a has two 35 cubic inch chambers that are compressed, or fired, or exhausted during each revolution.
That's simply intaking 70 cubic inches per revolution.

I use an unusually high Revolution Per Minute of 10,000, both for ease of multiplication as well as to nip any arguements right at the bud regarding the .05% of rotary owners who actually have an engine built to handle those RPMs.
This equals 700,000 cubic inches @ 10K RPM.
Multiply 12 cubic inches by 12 cubic inches by 12 cubic inches, and you get 1728 cubic inches in one cubic foot.
Divide the 700,000 cubic inches @ 10K RPM by 1728, and you get 405 cubic feet per minute.
Now, for arguement's sake, I like to go ahead and give kudos to the bestest porters in the world by multiplying that 405 CFM @ 10K RPM time a Volumetric Efficiency of 110%.
The result is...
445 cfm!

Yes, a 12a at an incredible VE of 110%, running at an incredible 10,000 RPM, needs a flow of FOUR HUNDRED, FORTY-FIVE CFM.
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Old 05-03-06, 07:33 AM
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I usually dont post over here but I have an idea on why you might want to use a bigger carb. I could be wrong though.

Assuming the max the carb can flow is 465 cfm and the max the motor can ingest is 445 cfm as sterling said above. As the airflow of the motor approaches the max flow of the carb wouldnt the restriction the carb provides increase. This would result in a power loss. By using a carb with a CFM rating slightly higher the the CFM of the motor the resriction is reduced and it frees up a little power. Going to large will of course be bad but a little larger seems to be beneficial. Am I wrong?
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Old 05-03-06, 11:14 AM
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Even if what Rx7Roadster said made sense to me (which , sorry... it doesn't really) that still means that the 465 carb is more than enough.

Think about it. 465 is more than 110% of what our engine can do at 10krpm! How much more headroom do you need?

Well, let's figure this out for Jon's "reality engine"

Jon's engine does also have a displacement of 70CI

Jon shifts at the redline buzzer. For the sake of simplicity we'll say 7,000rpm

Jon's engine may have a Sterling carb and a ported manifold, but it's a stock port. We'll give it a very generous volumetric efficiency of 90%


70CI x 7,000rpm = 490,000CI/m

490,000CI/m / 1728(CI in a CF) = 283.56cfm

284cfm * 90% = 255cfm

So Jon's "reality engine" tops out at 255cfm, leaving a whopping 210cfm of headroom.

And I would need more than a Sterling carb, why?

Jon
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Old 05-03-06, 11:19 AM
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Originally Posted by Sterling
The 12a has two 35 cubic inch chambers that are compressed, or fired, or exhausted during each revolution.
That's simply intaking 70 cubic inches per revolution.

And *this* is why everyone can ignore my posts about displacement vs. piston engine.

IT makes so much sense!

The rotor makes one full rotation for 3 rotations of the e-shaft.

The RPM on your tach = RPM of your e-shaft.

There are three faces of the rotor. So 3 "dynamic chambers" working at the same time. Each one does a full rotation every 3 rotations of the eshaft

So 3/3 = 1 . For every rotation of th e-shaft, ONE of those chambers does its intake part of the cycle. The e-shaft rotates 3 times, all 3 faces get an intake cycle and the rotor has now turned 360*.

Jon
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Old 05-03-06, 11:20 AM
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Originally Posted by vipernicus42
And I would need more than a Sterling carb, why?

Jon
You don't need it, send it to me, LOL.....
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Old 05-03-06, 11:24 AM
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Originally Posted by vipernicus42
And *this* is why everyone can ignore my posts about displacement vs. piston engine.

IT makes so much sense!

The rotor makes one full rotation for 3 rotations of the e-shaft.

The RPM on your tach = RPM of your e-shaft.

There are three faces of the rotor. So 3 "dynamic chambers" working at the same time. Each one does a full rotation every 3 rotations of the eshaft

So 3/3 = 1 . For every rotation of th e-shaft, ONE of those chambers does its intake part of the cycle. The e-shaft rotates 3 times, all 3 faces get an intake cycle and the rotor has now turned 360*.

Jon
A piston engine displaces the same amount of volume every rotation of the crankshaft, but only uses 1/2 of it's displacement to make power. Unless it's a 2 stroke.
That's the arguement I use to counter the 1.1/2.3 and 1.3/2.6 debate.
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Old 05-03-06, 12:40 PM
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Thumbs up

I had no idea rotarys couldn't pull a decent amount of air in like a piston motor?

Probably one of the reasons rotarys respond so well to boost and N20.

My little 12A picked up a ridiclious amount of power from meth injection and N20. I'm giving it all the air and fuel it needs right down it's little throat with the N20.

I wondered why my car felt so good with a little 35 shot. The difference a 35 shot makes on a rotary compared to a honda or similar small displacement piston car is tremendous! Now I just need like 4 10lb bottles so I can go whoop some butt at the local SCCA meets.


Awesome thread! I actually learned something...
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