StumpDrummer

Cant find any information on airflow for a 12-a engine throughout the entire RPM range for street ported engine any help with this will be appreciated

Needing help finding the right carb to modify for my turbo 12-a need the airflow info to help me decide Thnx

CobraStngSVT

From everything I have heard about 12a-T setups, it's fuel demand that is hard to achieve.

rx7lives

Redneck style? I expected to see crudely drilled out jets and lots of fire and of course, beer.

tom_sprecher

iTrader: (1)

Maximum NA air flow no matter how it is ported is as follows.

1200 cubic centimeter = 0.042377600066 cubic foot

0.042377600066 cubic foot X 8500 rpm = 360 cfm.

The type of porting will only restrict the air flow from there.

Aren't the RB Holleys something like 390 cfm?

Rotospeed

iTrader: (8)

For blow through you can use a nikki, a holley or a webber.

A racing beat holley won't work, it needs to by a double pumper if you go with a holley.

What are you going to use the car for? Each carb has it's pros and cons, cfm is not the first thing to think about for a blow through carb.
We need more info to be able to help you.

85rotarypower

This is actually a very basic look at the air flow requirements, and I think its less correct than you think. The RB holley is a 465, and IIRC, Sterling's modded Nikki flows just over 470cfm. From memory, the stock Nikki flows around 320cfm. By your logic about the cfm requirements, the RB holley and Sterling's Nikki would flow far too much for even a peripheral ported 12A capable of pulling in the full theoretical cfm of the engine, even though both these carbs would likely be too small for that kind of engine porting. Turns out around 450cfm is ideal for stock to streetported 12A engines.

As Rotospeed stated, cfm is just one part of the equation. Fuel delivery characteristics and how the air flows through the carb have a big effect as well. This is evident by the power going up, or down, by adding a spacer under a carb, even though total air flow through the carb has not changed.

There are some real old threads about boost prepped Nikki's somewhere in the forums, probably from back in 2003-2004, when I first joined here. I can't remember for sure anymore, but I think some even ran with the stock carb with just a few mods to keep it together under boost and of course change jet/emulsion tube/air bleed sizes. A blow through carb works relatively well for the most part, but from what I have read in the past, they are a real PITA to tune and can never fully reach the same potential as a well tuned FI setup. Probably a good guy to talk to would be Directfreak, I think I remember some years ago he ran a boost prepped Holley before going FI on his beast of an FB and he was pushing about 300hp from a 13B.

j9fd3s

iTrader: (3)

+1 there is a reason sterling says the engine flows around 330cfm, and goes on and on about how that is all the flow you need, and then sells a 465cfm carb because it makes more power.

the flow equation is correct, however it doesn't apply when the manifold under the carb is individual runner style. with IR setups its easier to look at sizes, but the flow number is basically the cfm number x 2, or more.

maybe think of it not like 1x1147cc but (4x (573/2))x 6. so its the rotor size, 573cc, divided by two runners, times 4 barrels in the carb, times 6 working chambers?

tom_sprecher

iTrader: (1)

I'm not an ME, but I did spend 5 years getting a BSEE.

The following is a educated wild *** guess.

My calculation, albeit not entirely accurate, is based on maximum cfm of the engine at the internal port openings. As you add port runners, intake runners and a carb that number decreases due to air flow restriction.

I would imagine a bigger carb cfm than the calculated value is desired to ensure it adds as little restriction to air flow as possible. Bigger ports attempt the same goal.

There are many other factors in effect as well like air velocity, port shape, runner shape, etc., but I will leave that up to the experts.

Turbos and super chargers basically overcome all this with brute force. I guess you would want a carb that can provide the cfm and AFR based on the turbo you plan to use.

Rotospeed

iTrader: (8)

carb cfm dont mean jack wen your in boost.

So whats the car for and what turbo are you planing to use that will push 20 psi?

we need more info to be able to help