Figuring CFM max's (factoring volumetric efficiency)

A long time ago I heard you needed to figure 85% as a max VE, and the formula ran:

1/2 MAX RPM X cubic inches X 0.85 divided by 1728 = Cubic Feet per minute.

Is the VE factor still true, or is it a variable set by nitrous use, blower pounds, turbo PSI, and so on?

Has manifold and head porting come farther than about the late eighties when I picked up that figure?

(Nitrous also gets into pounds of fuel -vs- nitrous shot, etc...)

PS: I won't always have a computer handy, so I could give a rats *** about anything but the straight math (IE: Leave off the links to programs that figure it for ya)

 

Blower motor (CID X max RPM)Div. by 3456 X(boost div by 14.7) +1=CFM don't forget blower CID blower motors can go 110%

 

For a four cycle IC engine, you need to divide by two because two of the strokes are compression and ignition.

Don't mean to stick a thorn in, but how does it run now?

Or was that factored in already?

Never mind, I ran the numbers.

1728 doubled is 3456.

****, I better explain my method.

1728 is a factor for converting cubic inches per minute into cubic feet per minute (12X12X12=1728)

Double that and you have 3456 (which gets rid of the divide RPM in half stage)

 

the optimum carb size doesn't really correlate to the formula. in many cases. The 85% number is ok for a mild engine, but the race engines would use a higher number. And for a street car, most 4bbls have some type of air valve in the secondary, so you can use a 750 on an engine that only "needs" a 500-600 cfm carb and it will work very well.

As suggested by the equation, with a boosted engine you figure airflow using the boost number. Again, it depends on what you're working with, some blower setups don't mind having a lot more carb than the formula says.

 

I ran into that earlier today. The suggested CFM for my buds setup is 750 or more.


It's a 351W let out to 408. The numbers came out dead on 700 CFM, plus some pocket change.

There's an 850 possibly in the background that we might borrow, and none of the race quads use power valves. They run them WFO, or shut down to idle.

NON-aspirated, I wonder what the best match is...

 

VE can easily exceed 100% in certain NA race applications.


85% is a good compromise number for daily driver econo boxes.

 

heh...the converter or launch rpm might have a lot to do with which carb to use.

 

This really isn't a jack*** response!

Call Holley. Talk to thier techs. Tell them what you have and what you want to do with it and they'll tell you which of thier carbs will work best for you. It'll work better than any equation, honest. Even if you don't plan to use thier carbs, you'll still get a good idea of what your engine is looking for.


Ed!

 

A.Graham Bell wrote in his book "Performance tuning in theory & practice" (ISBN 0 85429 275 6) :

" At maximum torque rpm the volumetric efficiency (VE) would be 90-100% in a racing engine, and this would fall, relative to the engine's torque curve, at higher and lower rpm."

So not only does the Max efficency change depending on how good the engine is, but it changes for a given engine depending on the RPM it runs at, and its torque curve.

Another good book to read is "Scientific design of exhaust & intake systems" by Philip H. Smith & John C. Morrison. ( ISBN 0-8376-0309-9)

 

Not a snide comment, but I'm wondering if you ever plopped a carb on an engine just to find out how it does run?



I've run a 600 Holley, dual 500 Carters and a single 750 Carter on my 462" Buick engine.

They all ran well.

The Holley was a little lacking due to it's set-up and not the cfm capacity.

The dual quads ran the best - straight linkage - 600 rpm idle with a big cam, good throttle response and smooth tip-in as each carb's secondary air valves opened at slightly different times.

The 750 Carter as well as a 750 Edelbrock I'm now running on the engine work well too.

A little experience goes a long way as compared to numbers scribbled on paper....

 

I think that's what I was trying to say

 

That probably would be the best way to find the right size carb.

To try and calculate it, but guess on the main number in the formula doesnt seem very precise to me...

But the 2 books I named are really interesting if you want to read about it some more.

I never calculated anything to do with the Intake side.
Because I have never tried to build my own intake...

I just bought one or I was stuck with whatever came with the engine.
And I can follow how the shock wave tuning would work on individual runners ( the priciple of it ), but how that works on siamesed ports or intake is way too complicated for me to understand...

I bought those books to be able to calculate the Exhaust side.

I think there is more to gain there, in efficiency...

On the Carb size, I'd just guess, or run a couple of Dyno tests ( depending on how serious you are...)

 

for me the calculations are good to know

but for a quick geusstimate I take the cubic inch times two minus 15 percent or so for cfm

say you got a good breather 350,
multiply that times 2 to get 700 cfm,
mutiply that times 0.15 (15 percent) to get 105,
subtract that from the 700, to get 595 cfm.
round that to the nearest practicle carb size, and there you go.

your warmed over 350 should be fine with a 600 cfm carb.

a warmed over 454 should like a 750 cfm carb and so on.

 

That sounds like a lot of unnecessary math...I just remember:

 

Reading and learning be good.

If nothing else, the carb tables will get a noob off to a good start, but the carb tables don't take into account the ability of a particular carb to flow more cfm . . . or less in some cases.

An overly large Holley can be regulated to an extent by judicious selection of the secondary vacuum spring.
As well as jetting, but jetting is a given that the owner has to iron out for him/her self so we won't worry about it here.

The Carter/Edelbrocks (C/E) are self regulating.
Stick a 600 cfm C/E on an engine that will only pull 500 cfm due to internal engine factors and perhaps rpm limitations and 500 cfm will be all that is pulled through the carb due to the self regulating effect of the air valve.
Venturi flow at that cfm level will pull the proper amount of fuel so the engine runs well.

Stick that same 600 on an engine capable of pulling 900 cfm and you'll find the 600 carb will flow considerably more than it's designation indicates.
Seems like I saw a figure noting that it would pull 750 cfm or so, but don't hold me to that.
Fuel mixture is the same as the low flow noted above except reversed . . . the higher venturi flow pulls more fuel due to the stronger vacuum signal.

Just like cams, street runners should err on the conservative side when selecting carbs.

A lot of it has to do with how we drive our cars.
Most times they're cruising under 3000 rpm and not flowing enough air/fuel mix to even warrant having headers on the engine.
A free flowing exhaust manifold - like the good looking Rams Horn - will flow sufficient air for that rpm level as well as quite away above 3000 rpm.

When I say headers, I'm speaking of free flowing ones and not the looks-to-be-overly-restricted-convenient-to-use-blockhuggers.
I'm pretty sure most of those don't flow as much as a mildly cleaned up pair of Rams Horns.
I'd love to see a dyno test on this, but given the lock-step march the magazines and advertisers engage in, we'll never see it.
Entirely understandable, but too bad.

Regardless of header style most of them don't do much until rpms exceed 3000 rpm.

Getting back to the driving our cars bit, how often do you exceed 4000-4500 rpm in yours?
Not often I'll bet.
All of which makes a lot of the speed equipment we put on our cars unnecessary except for the eye-wash factor.

Light car + mild cam + modest carburetion + reasonable exhaust flow capabilities + reasonable gearing = A dependable and strong running car that's a whole lotta fun to drive.


Formula's?
Formula's?
We don't need no stinking formula's....

 

good stuff!

also, carbs are rated for CFM at a specific vacuum level (what is it, 1.5"?), so at higher vacuum it'll flow more CFM, at less vacuum it will flow less. This is ***uming WIDE OPEN THROTTLE operation! so the vacuum level and air flow will indeed depend on the size and efficiency of the engine and the rpm it's turning.

 

Something I've been wondering about...

Does the optimum Carb size change with elevation?

 

Ok . . . I can't figure out whether this is a statement of fact or . . . what?

Looking from here, I don't think you have any problems with 'lectric stuff, but you never know....

 

another hamber has a sig that says "i hate wiring", so I figured it'd be a good thing to show that not everyone is in that position.

I don't really LOVE wiring, but I do understand it.

 

I would say no.
Jetting etc., you know changes are required there, but for the al***udes we run our cars at - sea level to 6000' being most common, no real reason for a cfm change.

Perhaps true for aircraft, but I'm fairly sure they don't have variable venturi carb's.

Some unsupercharged civilian aircraft can reach about 16,000' before they hit the limits of physics and horsepower at the same time.
As a fwiw, FAA rules call for pilot oxygen above 10,000'.

If I remember right, unsupercharged engine horsepower levels at 16,000' are down to about 75% of what they would be at sea level.

Keep in mind as well that lift from the wings and thrust from the prop (an aerodynamic lift device as well, cept it generates lift in a horizontal mode most times) are lessened at higher al***udes.
Drag is lesser at al***ude as well.

Flight is achieved when lift + thrust = drag + gravity.

Whoops.
That was a formula wasn't it?

Damn . . . I hate it when I do that....

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Short, but cool airplane story.

Pilot was flying a DC-3 load of Army troops who'd just come off a three day p*** and more than a few had a snootful.

They were cruising at 10,000' and the troops were a little rowdy.
Pilot and co-pilot donned oxy masks and took the DC-3 up to a little over 12,000'
Wasn't long until the troops were asleep.
When the plane descended to 10,000' the troops remained asleep and no more problems....

 

Got it.
Thanks.


I like wiring.
Specially after I did my first full car wiring job and set things up so trouble shooting would be easy.

Learning that made other wiring trouble shooting jobs easier and in fact led me to a career in the electric utility industry where I got to see lots of wires.
Didn't work on them - unless I was extra that day and sometimes I'd give the techs or electricians a hand.

My job was monitoring the system and cranking switches.
Fun stuff and different every day.
Sometimes, during storms it was so much fun we almost couldn't stand it.
Then . . . there was the paperwork....

 

Unless you are schooled in thermodynamics and fluid dynamics, it would be best to dispense with any equations and just bolt on something in the range of 600-850 nominal CFM.



Carburetors are ***igned nominal CFM ratings based on certain parameters. In general, these numbers are found by measuring the volume of air moving through the orifice at a given differential static pressure. One thing you have to know is that the motor doesn’t operate at a fixed differential pressure. A nominally rated 850CFM carb on a 283 will only flow half the rated CFM, because the differential pressure will never be close to what they measured it at during testing. Same with a 500CFM carb on a 500 cube stroker motor. The differential pressure will be far above the measured parameter from testing, and the carb will flow over twice the rated volume.



As far as volumetric efficiency, poorly designed motors will be around 70-80%. Good motors are around 80-100%. Specialized race motors will be well over 100%, sometimes approaching 130%. A blown motor with 10 pounds of boost is operating around 160-170%.



There are way too many factors involved in carb selection to ***ign a linear algebraic equation to do the job. Testing is the only way to get it right.

David

 

http://moparmusclemagazine.com/techarticles/155_0307_exhaust/

http://chevyhiperformance.com/techarticles/49178/




Maybe what you're looking for?


Ed!