Does anyone know how cfm is computed? How do you figure out what cfm a carb or TBI is flowing? Is it simply the size of the holes and how many you have or is there more scientific krap to it like whether or not you have an open air filter or a stock one? Never have understood this really.

 

well the way I understand it is this:


youll see carb or tbi ratings in two forms, at 3" of vaccuum or 1.5"
1.5" is the standard for four barrel carbs
3" is the standard for two barrel carbs and it seems that most 2 barrel throttle bodies are measured at three aswell


basically there is a suction placed on the tbi unit or carb that is enough to generate 3" of vaccuum (for a two barrel) the air that flows through the tbi at 3" of vaccum is then measured in cubic feet (of air) per minute or CFM for short.

As you could guess a 3" measurement yields a higher CFM rating since there is more draw on the carb or throttle body. The ratings holley gives on its big block sized two barrel is 670 cfm at 3" of vaccum. There is a multiple (that i cant remember right now ) to roughly convert that rating to what it would flow at 1.5" of draw and i believe it works out to around 475 cfm under the same standard that a 4 barrel throttle body or carb is measured which is pretty pathetic actually but nothing that cant be accounted for with proper planning and parts selection.

In a running engine you never want to see as much as 3" of vaccum at WOT. I think ideally you want something that couldnt be measured by your standard vacuum gauge. So in actuality a TB that flows 670 cfm is flowing much less on an engine that is not seeing any vaccum at WOT (mine for instance sees no measurable vacuum at wot) so its not ingesting even near what is measured at 1.5".

This is where you get all those arguments about small blocks needing much less carb then they typically get
you get some tool saying a 350 at 6000 rpm with a very high efficiency rating only needs 650 cfm (random number) so that means it only needs a 650 carb...
Yeah sure, if you want it to be pulling 1.5" of vacuum at 6000 rpm WOT with that carb and lose power that would make sense (sarcasm).



Pablo

 

Man, that was pretty good. So it is fairly difficult to tell just what your particular engine needs for cfm then huh. There just seem to be a million different factors in deciding the proper cfm. I bought a (590 cfm) throttle body from turbo city and I have it on a 350 crate motor and I am wondering if I should try something different for more cfm. A friend of mine has a 350 with a 750 cfm carb on his and it runs good. .

 

Compare the two. My 670cfm holley street avenger vs. a holley 670cfm TBI. The carbs cfm is measured at 1.5 vaccum combared to the TBIs 3. does this mean the carb flows twice as much?

Not to be biased but im sure the carb does flow much more air and fuel since it has two extra butterfly valves and jets(would be the same as injectors sort of)

I liked TBI it was alright but right now im running carb and I like it more. Believe it or not the carb starts in the cold easier, idles better, and runs better IMO. and yes it is cold here....~30 degrees or lower in the mornings.

 

To convert from one flow rating to another:

1.5"Hg flow = 3"Hg / 1.414

So a 670CFM 2bbl = 670 / 1.414 = 474CFM @ 1.5"Hg

This is from the HP books Holley Carb manual.

RBob.

 

yep 670tbi is equal to a 474cfm 4bbl carb. Yet no one will believe me when i say they are airflow limited. Everyone keeps saying it's fuel. Yet I don't see anyone on this board running a 474cfm carb and making big power.

 

This brings up a good question then. Can you put 2 TBI units on a 350? Hmmmmm. Do you think that will be too much? That would be two @ 417cfm. (My TBI is rated at 590cfm). Look at this. Yeah baby does that look good or what?

 

I dont disagree that its flow limiting


Im starting to think more and more that it is one of the biggest issues concerning power production what with all the nitroused TBI cars running so well. Theres less airflow through the throttle body or carb when you run nitrous


as for the twin tbi thing, thats been hashed out a few times on this board and i think on the diy prom board you might want to do a search. Basically youll need to come up with some extra injector drivers at the minimum and work out something for the IAC motor If I remember correctly. Id also look into one of the 4 barrel holley TBI units.

 

they should sell a 4 barrel TBI that will bolt onto a stock 4 barrel manifold...that would be nice.

So what would one of those 950cfm throttle bodies flow? it still wouldnt outflow a 750 carb would it.

 

They do make 4bbl TBI's, both Holley and Accel have them. The kicker is the cost along with requiring two additional PnH drivers.

RBob.

 

The four barrel TBI's are flowed at 1.5", just like a four barrel carb. So it's a direct comparison between the four barrel TBI's and the four barrel carbs. Their four barrel TBI's will bolt onto a stock four barrel intake.

 

how much do those cost??

 

Edelbrock has a 4 barrel TBI setup for $1949.00 Holley has one for $2299.95 both include intake and computer. These figures come from Summit.

 

There's this kit from Holley that you can get from Jegs.

Click here ----->

It's About $1400 for that Commander 950 system flowing 900 cfm (not including accesories needed).

That's A LOT of flow for A LOT of HP.

 

When I get lockers hooked up I'll measure the drop in manifold pressure with much greater accuracy. From what I've measured before I'm only loosing about 3kpa at WOT so it looks like the TB isn't an issue for me yet (might be though). BTW, I know of this motor they call the 502 ram-jet that makes gobs of power with a stock TPI TB unit. Makes you wonder why they did that.
OSU is getting a flow bench so you can bet your **** I'm going to be testing the 670 with holley injector pod, then with GM injector pod, then without. I'll be sure to compare it to a few dry flow TBs and a couple carbs.

 

The ram jet 502 TB only flows air and its still limiting for that motor.


J, I think the TB is flow limiting for you especially on that dual plane. Just got done reading "Carburetors and Intake manifolds" by Dave Vizard (you oughta check it out, great book). Its further convinced me that a single plane and TBI is the only way to go, the tbi appears 20% larger on a single plane. You oughta get ahold of a 650 carb and run it on your setup I bet youll pick up mph. I also think youll get it back with the TBI with a single plane and an open spacer. If I were near my car id have done this allready.

Also if you havent allready, shave down the throttle shafts on the TBI unit thats an easy way to pick up id guess like 30 cfm