my77project

Hey all, I have a question. I've been rather intrigued to see a lot of guys claim that you need a 750 cfm carb for a 347 or you'll be choking it to death, or such things. Or even a 650 for a 302. Cause if you actually crunch the numbers:

cfm=Displacement*RPM/2*VE

for a 347 spinning at 7000 RPM that works out to roughly 630 cfm. And for a 302 spinning at the same speed, 550 cfm. (both of these assuming 90% VE which is pretty generous in most cases)

So am I missing something, or are we just obsessed with big carbs? :-P

projectresto83

Theres nothing wrong with going a little bigger than needed on a carb.

Portmaster

Just remember that just beacause a carb is rated at 750 cfm doesnt mean it will flow that. Most places rate the carb just through the venturies. You have to get the air past the boosters, air horn, choke blade as well. Until you move into a Holley HP, BG Mighty Demon, and modified versions take that CFM rating with a grain of salt. Also some carbs are rated in dry cfm but the real players are rated in wet CFM. There is a huge differance in the flow of a Holley 750 HP and an Edelbrock 1407 750 CFM carb.

Joel5.0

Using that formula as the one and only guideline to base a carburetor size selection is really outdated. It does not take into consideration other factors that are easily proven in real life (ie. performance and driveability). I've had a 700 cfm DP Holley on my 302 for years, had a 289 with a 600 cfm Holley and it performed and drove a lot better with an 850 cfm DP Holley..... I've had import setups (Toyota's and Wankel's) with 2V 500 cfm Holley, 600/650/750 cfm Holley's, single and dual side draft Weber carburetors....... and they, even though are/were viewed as "over carbed" setups...... drove and performed better than with a formulae imposed limited selection.

Same goes for EFI TB/injectors sizing .

my77project

Well, let me rephrase my question, though I think part of it was answered.

What would make a car run better with a larger carb when all the air it can possibly use could be provided by a smaller carb.

Part of the answer apparently is that the shelf rating, and the actual rating are not necessarily the same. but what I'm wondering is, most of an engines life it doesn't spend at WOT, so you essentially always have "too much" carb. At cruising speed you're mostly running on the idle circuit. So in that case perhaps a larger carb has an idle circuit that better matches your car than the smaller does? just thinkin out loud. I'm sure there must be other factors.

lol, no offense, but the equation isn't "outdated". it still does exactly what it's supposed to. Tell you how much air your engine needs under a certain RPM. But I do agree it doesn't cover all the angles, which is why I asked the question in the first place.

mjr46

the carb is the restriction in your sytem, go ahead put a 650 on a 347 , BTDT.....SEE WHAT HAPPENS..LOL........tHE EQUATION YOU are married to is
somewhat effective for building stock motors but not so well for HP street/strip or race motors, it does not account for a modified runner, size or length as well as cross section in a head, without going to technical it suffices to say all heads and intakes are not created equal in the volumes of air they can move at any given time while working in conjunction effectively with the valve timing events.

mjr46

here read this, It'll answer what you need to know, and pay particular attention to one "Troy Patterson's" post
http://sbftech.com/index.php/topic,8087.15.html

Joel5.0

LOL... none taken, but why use a formula that is usually on the lacking side for its application. Why does a 302 with untouched GT40 heads and a B-303 "likes" a 750 cfm Holley over a 600 cfm? Driveability was the same with both but acceleration and top end were favored by the 750 cfm.

my77project

lol, well I'm not an idiot believe it or not, and I'm not married to the equation. :-P It's simply a scientific fact that that equation predicts the ammount of air the engine will need at a given rpm. lol, I would love technical. I am well aware that different heads are capable of moving different ammounts of air due to supeior flow characteristics, but this is taken into account by the engines volumetric efficiency, and very N/A 302's have greater than 100% VE. if you have heads that don't flow as well, you essentially don't get as much air as is possible into the cylinder.

The runner size and length I'll gladly admit I'm not as clear on the effect it has on the engines performance.

I'm certainly not interested in starting an argument, just trying to sort out what the apparent discrepancy is, cause the engine certainly does use more air than that, so there must be other nonexplicit advantages to having a larger carb if they run better. (and I'm not doubting that they do) My point is I'm looking for an explaination of what those reasons/advantages are, not just "it runs better". No offense.

I will certainly read that page once they approve my membership. :-)

mjr46

let me post the one post I was refering too for your tech liking .