Stromberg 97 vs. Rochester 2G on a 3x2

"Is all this written in stone? No...but it is a way to see what you are really looking at.

Marty McF."


This formula is a general rule of thumb, not everybody has the availability of a flow bench, a dyno, or a lifetime to engulf themselves into picking the correct induction system for their particular motor. It gets you close, a place to start. Most times it is not correct but it is more correct than the cfm estimate you could pull out of you ass, right?

Marty McF.

 

So according to your theory, how many cfm's would be needed for 292 CID with the 3x2, 180 degree manifold? Say a max rpm of 6500.

Would the early 3x2 for the 97's be considered a 180 degree manifold?

 

PANIC,
WRONG is too strong of a word I believe, SLOPPY fits it better .

As stated on you website:
"In summation, don’t rely on a formula to determine your carburetor choice - it’s only by accident that they’re not too far off."

Hmmm...not too far off... huh?
Yes it is by accident, but you have to begin somewhere.

Not that I don't agree with what you posted, I just think it is too harsh for no reason.



Yes considering manifold design is important to calculate flow, but generally what works and has worked for many before is more important. Go with what you know works. The combinations that I posted earlier work great. If the pursuit is for for absolute maximum horsepower and throttle response, then real life testing on a dyno would need to be done on the specific combination.


Marty McF.

 

Heres a way of getting close, then fine tune..1.5 CFMs per CI..Stock or mild build....2CFMs per CI..greater then a mild built engine..
This is my way..
Duane..

 

The only time it gets you into trouble is where is predicts that X CFM is good but you choose a square-bore carb with big primaries for use on an single plane manifold with a big plenum (Victor).
It also gives you a bit of a power drop when predicting for a dual-plane such as Performer, because most engines can use another 100 CFM here, especially with vacuum secondaries.
Yes, there is such a thing as "too much carburetor" (venturi area is a bit better, but venturi sizes can only be used for comparison between carbs of the same design - not Holley vs. Carter or dog-leg vs. annular), but it's actually limited by the point where vacuum at the venturi is so low that the fuel curve goes to hell. This is almost always true by .5" Hg. for common 4 bbls. (a Weber DCOE will run well at this level), generally true by 1.0" Hg. This is a big advantage to more sensitive boosters (annular) and to fuel injection - which doesn't need any vacuum at all, and can be very, very large for the engine size (BBC injection 8 × 2.90" butterflies, far larger than 2 Dominators).
But the "CFM rating" of 4 bbls. is at 1.5" Hg.
What does this mean?
A carb enough larger to drop the vacuum from 1.5 to 1.0 adds some power.
How much?
Perhaps as much as 8% if there are no other restrictions.

 

Do you need more than 750 cfm? How much do you need ?

 

I think he probably has this solved by now. It was 17 years ago....