karism

Most of you guys know that i have a 12A that is Cheese ported ,the Bridgeport consisting of holes,and not a open port.


Firstly ,thanks to those who kept faith, in the year it took to complete.Thanks for all the constructive critism,advise and oppinions.

At the current stage,i`am running a heavilly modded NIKKI,the estimated 400cfm,isnt enough.

I would like to hear suggestions,of what fuel system i can use. IDA webber comming to mind,as well as 650 Holley. Any other carbs that could be used ,or should i consider a stand alone engine management system ?

If a management system,what size throttle would be advised ?


This is not my first ported Wankel ,however ,this time i`am doing everything differently.

I appreciate all input.

Karis

Sterling

The rotary 12a can only ingest 340 CFM @ 8400 RPM @ 100% VE.
A 650 will be too big for you to benifit from.
Your Nikki is'nt modded well enough if you're only expecting 400 CFM.
Ours flows 429 so far, and we're continuing to search for more ways to improve.
But unless you have air bleed interchangeability in your emulsion tubes, you'll never get that thing to give you the power in the part of the band you need it most.
Simply slapping on a bigger carb is not the answer.

You need a "bigger than stock" carb, but one that's still small enough to flow a high velocity at high RPMs.

Velocity is what it's all about.

karism

Your right saying that velocity is the answer.
I`am not quite sure that i will achieve enough flow,for my port config.

Do you think that your 429 cfm nikki will supply enough for my bridge ? If so,i will be more than happy to prove to the world what a bridge can make with a nikki.

(yes,i do have faith in your work)

Rx7carl

I think he might need the supersize 470 Nikki.

Hey where did you get the 340CFM number from?

inittab

Does that come with fries?

purple82

Sterling,

When you bench test a carb to measure airflow, what pressure drop do you use?

Sterling

Carl does the testing.
I do metalwork.

Carl, 12a is 70 cid, times 8400 RPM max redline = 588,000 cubic inches per minute @ 8.4K RPM.

1 cubic foot = 12 inches x 12 inches x 12 inches = 1728 cubic inches.

588,000 cubic inches divided by 1728 cubic inches per cubic foot = ...

...340.27 cubic feet.
-But again, that's at 100% Volumetric Efficiency.

According to Yaw, the stockport 12a gets 88% @ 8000 RPM with his ported manifold and modded street Nikki.

-That's 299.44 CFM required. He says his carb is flowing 429 CFMs, so this is using it to 69.8% capacity.

Rx7carl

Or onion rings.

Rx7carl

1.5" hg. Standard for a 4bbl carb. Also that number is artificially low cause Im not using a bell mouth inlet (havent made it yet). So look for the numbers to go up.

Sterling

Uhhhmmm...I dunno!

The_7

umm.......are the holley carbs named by flow? like does teh holley 600 flow 600cfm? if so, then why do people use that carb on bridgeported 12As when it would take 14800 rpm to flow 600cfm? why not use a smaller one?

FB II

most people dont take too much in to consideration when getting a carb. the usual assumption is "bigger = better".

Sterling

My feeling on carb size selection- and I could be wrong here, but I don't think so- is that you should mate your engine with a carb that will flow not much more than your engines needs. If your engine needs 600 cfm, you should get just one step above that so you're not "maxing out" the flow capacity of your carb.
-Reason for that is because they may say it flows 600, but that might be at the absolute optimum conditions, an perhaps even an overinflated rating. (The stock Nikki Carl tested flows 313 CFMs, and the one I just finished flows 429.
You see how they don't land on nice, neat, perfect numbers like 425, 600, ect!)

Remember our previous thread regarding velocity and the tube (carb throat) diameter?
The engine only will take in a certain amount of air. Make the carb throat as small as possible but still able to deliver that much, and the vacuum signal to the fuel main circuit will be stronger.

A super strong signal is key for tuning. You can always curtail the signal by bleeding it off, but you can't amplify a weak one.

purple82

So when someone quotes the CFM of a carb, they are quoting at 1.5" of mercury pressure drop across the carb? Then how do you equate this to some rpm of an engine? It's going to be different for every engine and intake system.

Sterling

I don't know about the testing basis and calibration for flowbenches.
As to the other question, you simply do the math to find out the engines volumetric needs for a given range of RPM.
-It's gonna take that much in no matter what. It's how rich you allow the mixture to be at specific RPM bands that will decide the engines performance. The midrange should be made richer than the top end or the low end because that's where you'll do the most accelerating to "power out".
(Keep in mind the rotary only starts to make power after about 4.5K anyway. Making the mixture too rich before that will be counter productive.)

karism

Mmmm,lots of good info to make notes of.Thanks!

I`ll definately consider flowing my nikki some more :-)

Rx7carl

Yes, 1.5"hg is the standard test pressure. Its an arbitrary figure, totally un related to any real world usage, its just a way to keep comparisons of different carb equal.

[/B]It doesnt. Yes it would be different for every engine.

purple82

So how do you know your Nikki flows 429 CFM at 8000 rpm?

Rx7carl

I can tell you but then I'd have to kill you. I have ways of telling flow at any given rpm.

purple82

OK, you're just measuring your carb on a real engine then, which is still specific to the intake and engine setup.

Rx7carl

Nope, I have a flowbench, a BIG flowbench.

Cloud

So if the Nikki can easily be modded to flow that much...isn't the question this: How much can the intake be modded to flow?

Do you guys have numbers for stock intakes? Modded intakes?

I personally like the RB Holley intake design about a bajillion times better than the stock intake, but since I'm sticking with a modded Nikki on my stockport 82, I've set out with the porting of the stock intake.