Offshoreonly.com - Carb opinions 540's 700 hp...

Quote:

Originally Posted by MILD THUNDER (Post 4438775)

I don't necessarily think CFM rating, is the only thing that matters when choosing a carb. While you need to be able to move enough air, there is more too it.

This is from prosystems website.

But really cfm is not the main level of importance. Fuel shear, atomization properties and fuel curve are your main areas of concern.

To size a carb for the application, you're looking to achieve minimal restriction at the finish-line yet have enough signal at launch as to be sure that the booster is atomizing the fuel and supplying the proper air to fuel ratio.

Horsepower equals air flow (of course). Launch rpm/trap rpm equals a reference of the range of the air flow.

If the carb is too big or signal/curve is too poor at the launch rpms created airflow, the fuel does not properly atomize and plates out (turns back to raw fuel) on the intake. Losses of 10-12 percent of available torque at launch can easily be recognized without a lean cutout or backfire. Then as rpms increase, the plated fuel is picked up and alters the air to fuel ratio down-track as it is cleaned out of the intake. More loss of power. So you jet it down to compensate for the plated fuel being picked up and the launch gets even worse. See the dilemma.

The wider the range of rpm you're going to subject the design to, the more you need to look at the range of airflow and available options.

I'm sure you remember this old formula:

CID x RPM x V.E. / 3456 = CFM

Well that formula is still being quoted by magazines and companies etc...but times have changed and carburetors are operating on almost immeasurable amounts of vacuum. 10 years ago a carburetor would require 10 inches of water to pull signal and shear fuel. Now they can can pull and shear fuel at only 3. Remember 20.4 inches of water (wet) is the cfm rating guide with reputable designers so we aren't looking to match cfm requirements with cfm ratings.

20.4 = 1.5 hg.

You can see that going from 10 inches of water as a requirement at launch to only 3 inches as a requirement really allows a serious increase in cfm size. This removal of restriction really pays off in cylinder head flow numbers and hp of course. Imagine altering this upstream restrictor when flowing your heads.

Because, most of you have specific application designs, a custom shop/unit is typically the plan.

In the future, use this calculation as a general rule on a modified carburetor:

CID x RPM x V.E. / 2820 = CFM
555 x 6000 x .9 / 2820 = 1062 CFM

Now you'll be a little closer.

A .9 Volumetric Efficiency (V.E.) number represents a pretty good combination and a 1.1 V.E. number represents an all out assault on the engine blocks stress handling capabilities.

How true this is. Funny thing is the last engine I was involved with on the dyno was a 502 with the 741 cam and we started with the 800/9022 Holley as a baseline. Others were a 850 Speed Demon - an old school 750 Barry grant - lastly a Holley hp 950.

Well guess what the 850 speed demon came in with the most hp and torque with the nicest curve. Funny thing is they were within like 20 hp from highest to lowest. If we would have tuned accordingly we probably could have narrowed it down to less than 10

Given more patients and time we could certainly have precisely calculated what engine needed but threw that chit in the trash and started doing pulls. Results were quick and I have to admit not what I expected. .