What exactly are we talking about here, Volumetric efficiency??? i ask because i am using this "hot rod math" site to see what my CFM needed for my 520 cubic inch 385 series im building.

 

I googled Volumetric efficiency and found that my brain is not very efficient.
Here's a pretty good explanation.
http://www.epi-eng.com/ET-VolEff.htm

 

Some of the dyno programs will display volumetric efficiency at various RPMs.

 

Volumetric Efficiency???

It is at the point at which maximum engine torque occurs. Just think of it as trying to fill the cylinder with air and fuel at different engine speeds. There is one point that all the variables come together...that the most air and fuel actually get into the cylinder for the next power stroke. It depends on a whole host of things: anticipated speed range of the engine (including whether a broad range of torque is needed -- I.E. road racing--or just top end power -- I.E.drags), cam profile and valve train design, carb design, manifold design, charge temperature/humidity (density), port size, port angle, port shape, bowl shape, valve guide shape, valve size, valve angle (canted), valve shape, valve position relative to the spark plug, the spark plug design (projected nose, etc.) combustion chamber design, piston pin offset, piston head design, ring land placement, compression ratio, rod length/offset and the entire exhaust system design.

I've probably left some things out. Anyway, there is typically one engine RPM setting that allows for the best combustion (cylinder filling) possible for your engine.

Enough of this garbage. )

If you have a single cylinder ( 4 stroke) engine, running at 4000 RPM with perfect cylinder filling taking place (100% V.E.), you can calculate the approximate amount of air required by first cutting the RPM value in half. (For every two revolutions of the crankshaft, there is only one intake stroke....that is...only one cylinder filling taking place. So, an engine turning 4000 RPM is only experiencing 2000 cylinder fillings per minute.) If your cylinder displaces 10 cubic inches, then 20,000 cubic inches of air (at 100% V.E.) will flow through your motor at 4,000 RPM. Doing some quick and dirty math ( just getting an approx. answer) that converts to about 12 cubic ft. per minute. ( You have to convert from cubic inches to cubic ft...I rounded things off and did it in my head.) This means that if your lawn mower engine is operating at 100% volumetric efficiency (and it is for sure NOT !) and you planned to operate it at 4,000 RPMs, your carb choice would have to be at the very least a 12 CFM unit....however carbs don't flow at 100% of their designed capacity, so in reality, you would need a carb that was larger.

This might sound like a load of crap, but it is not. Expereince is the best teacher, however. If the manufacturer suggests a certain model 14 CFM carb for your application, it is because testing has proven it to be the one that works best for your application.

In your case (385 series Hemi), If you are planning a race-prepped 520 cubic inch motor, I would ask the carb manufacturer for his recommendation. Now that you understand more about volumetric efficiency, you will have a better appreciation of some of the variables that have to be considered before making your carb recommendation.

I hope that some of what I have tried to say is of help.

 

Tiremine; that's some good reading material you posted, thanks
for sharing.

 

volumetric efficency a measurement of how well the heads flow, and how well the cylinders seal. This number can be over 100% in a forced induction (ie..blown/turbo'ed ) applications. Most street engines will see 70-80 % If anyone off the street tells you that 90-95% is no problem, run the other way...quickly. For your application, figure 85-90% should get you in the ballpark.

 

Volumetric efficiency is just a ratio of the volume of air that enters the cylinder on the induction stroke over the swept volume of the cylinder. It can tell you how well the heads/cam/etc are working, but it isn't a direct measurement of any of those things. Volumetric efficiency is also variable through the rev range and through the throttle sweep from high to low, and varies from things like EGR, exhaust gas residual (from the previous combustion stroke), among other things.

 

The numbers CAN be over 100% on a NA motor without turbo or a huffer.
A well thought out daily driven motor can be in the 90% plus range, also different dyno programs came up with the same numbers.

 

Thank you everyone for their input on this, I understand whats rollin now.