Actual cfm used vs carb size

[banjo]

You're looking at it backwards. At a given operating condition, say 7,000 RPM WOT, the engine will draw into the cylinders a fixed volume of air*. The more restrictive the carb, the greater the pressure drop into the manifold and the lower the resulting all-important mass of air/fuel ingested. *The air meter measures said volume at atmospheric pressure, so a greater carb restriction will draw in less mass and thus show lower atmospheric pressure CFM.

The pressure drop across the valve varies hugely with crank angle and valve lift but can be anywhere from a few inches of H2O to 100" or more. In fact good software programs like Dynomation show inflow continuing as cylinder pressure rises above port pressure as the intake valve closing event is approached, as charge inertia can for a short time overwhelm the rising negative pressure drop.

But the thing that hangs me up, for a given pressure differential across an orfice, you can calculate the airflow in cfm.

The higher the pressure differential, the faster the air is moving, thus more cfm for a given orfice. It takes more work though(pumping lose)

To me from a theoretical standpoint, addition vacuum(pressure differential) would be beneficial up until the point that it meets the cfm demand of the engine because like you said, the engine is pulling a fixed volume of air. It seems to me that the pumping loses are from the engine speeding up the air more.

How can the carb flow more air with a lower pressure differential? This is the thing I am stuck on. From a theoretical standpoint, unless there is some other force at work here, a 1050 dom will not flow enough air to keep up with the demand of my engine at .7 inches

I get that the addition pressure differential create the pumping loss, because it has to do more work to get the air in the engine, but it comes with the gain of faster air movement and cfm. But like I said, it seems this would only be beneficial up to the point of meeting engine demand.

Again I am trying to understand this stuff and I may be out in left field.

[F-BIRD'88]

It does not matter how fast the air goes in. What matters is how much air goes in (Mass)(how much stuff). Manifold vacuum is a lack of air in the manifold relative to the ambient air above the carb.
This lack of air is vacuum. A higher manifold vacuum means less air got in.
Let more air in (bigger carb) = loweer manifold vacuum.
More air in (and out the ex) makes more power.
Bigger carb = less engine pumping loss. More net power at the crank.
Yes the air does move thru the carb venturi faster if smaller. But at a cost .Less air flow because of higher restriction. Less air= less power. Lack of stuff (mass air)
More engine work to pull air thru a smaller hole.
Pumping loss.

[learner1]

An orifice measures airflow by creating a restriction to airflow, that's how you get the pressure differential.
The higher the pressure differential through the carb the greater the restriction. Flow bench and running engine are different here.

[randy331]

So would everyone agree that for any given carb, it flows more air at 1.5 inches of vacuum versus .75?

And that the closer it gets to zero, the less it flows?

If yes, could we agree, that there needs to be a certain amount of vacuum(pressure differential) to meet the demands of the engine?

If no, please explain? What else is in play here?

What kind of vacuum is run in a high end fuel injection engine?

From a theoretical standpoint, my carb doesnt flow enough air at .7 inches of vacuum to support max performance.

I think there are diminishing gains the closer you get to the actual demand of the engine.

Any ideas what the pressure differential would be at the intake valve?

This all see counterintuitive to me.

I am just trying to better understand what is happening inside the engine, so please dont take anything I say as telling anyone they are wrong. But I love these discussions getting down to the physics of things.

If you have a 850 carb on your engine and it pulls 1.5" hg at a given rpm, and you replace the 850 with a 1050, the engine will only need to pul 1" hg now to still be pulling 850 cfm through the engine. But,... the manifold plenum will now be at a higher pressure. (closer to atmospheric)

Most of the improved power potential will be from the higher pressure, but without a way to know what the carb change will do to the BSFC#s there's no way to know if it will result in a power improvement without trying it. There are lots of things that affect power besides cfm.
I've put bigger carbs on engines before and dropped the vacuum, but were unable to net a power improvement. One recently, it was pulling 1.4-1.5 inches vacuum, went to a bigger carb and it dropped vacuum to .7-.8 but no power gain. It was a nice new 4 circuit Braswell so we could do anything we wanted to the fuel delivery curve, but still no gain. Power was the same.

You can try some cfm/size on this calculator, it's in " h2o so you'll need to convert your intake readings to "h20. The convertion is 13.5951.

http://www.wallaceracing.com/calcdchg.php

Randy

[F-BIRD'88]

This is wrong. The engine will inhale more than 850 cfm with a bigger 1050 cfm rated carb when swapped from a 850 cfm rated carb moving 850 cfm @1.5". The manifold vacuum at that wot point will be lower because more air got in.
And the engine will use less of its power to work to pump the air in. There is a bit more stuff and the engine eorked a bit less hard to do it. more air supports more fuel combustion = more power.
The working pressure stays the same (the ambient air pressure) .Thst working pressure is what pushes the air in thru the carb. When the carb is made bigger its is less of a restriction and thus net air flow goes up. The result is lower manifold vacuum @ that wot point.
Cause more stuff got in. Eg: now 875 cfm@1" thru the bigger 1050 cfm rated carb.
The net airflow in does not stay the same. It increases a bit.

[randy331]

more air supports more fuel combustion = more power.

Sorry fbird,..but it isn't that simple in the real world. Increasing air flow is only part of the battle. It needs to be distributed evenly and in a form that will burn efficiently. Your missing the BSFC part of it. it plays a very big role in making power.


i just did a big block that pulls 10 gallon an hour more fuel through it than my 421 does and at the afr, yet it makes the same power. WHY ??
The BSFC#s are wrose.

Some pumping losses are a necessary evil we are stuck with. if that wasn't true there would be no end to the size things could be on an engine, and there clearly is.

Randy

[F-BIRD'88]

More fuel goes in also. More air + fuel in = more power out.

[randy331]

More fuel goes in also. More air + fuel in = more power out.

Wish it was that simple in my world. ( The real one )
But it ain't.
But I'm ok with you thinking it is.

Randy

[F-BIRD'88]

Use 2 carbs instead of just 1 carb and fuel mix quality and overall distribution will improve and the bsfc and power will increase. The 2 carbs with 8 venturi instesd of 1 carb with just 4 big venturi will be better.

Fuel vapourization in the manifold on the way to the cylinders has to absorb heat. No heat to absorb stops fuel vapourization. Poor vapourization equals poor bsfc and power. (And lousy acceleration)
Not all 100% of the fuel has to vapourize in the manifold.
Spme can and does enter as liquid. But if not enough fuel does get vapourized in the manifold then you have to make up for yhat by adding more extra fuel. That makes bsfc go down. Look for a lack of heat.
Yes emullsion quality of the air fuel leaving the csrb matters.

[F-BIRD'88]

Heat vapourization conection and bsfc.
Look at the tempersture of each of the 8 intake runners.
If one messures a lot diffetent temperature yiu can bet the vapourization going on in that runner is DIFFRRENT.
That cylinder eill now need extra fuel to compensate or it will loose power. Bsfc drops.

[F-BIRD'88]

Yoyr bbc example probabily is a lot diffetent than the 421 sbc engine in many ways that matter.
Exhaust reversion also is a factor and will kill bsfc.
The intake manifold may need work.
Too much smooth surfaces inside. Not enough shesr points to breakup liquid fuel and kick it back into the air stream or vapourize. Lack of heat. 1 ot more runners.
Poor basic disttibution. Try 2 carbs on a adapter.
Now you got 8 venturi to mix and introduce the fuel into the plenum in a better more emulsified state overall.
Vapourizes better. Just realize the other side of that coin.
Too much vapourization eill use up all the available heat.
Runner (s) too cold 1 or more. Now vapourization decreases. Liquid fuel increases. Bsfc dtops.
This is why you cannot make a good intake manifold with just a flow bench.

Look at the exhaust side of that bbc.
The lower bsfc is the symptom not the cause.
But too much fuel as liquid and or too much exhaust gas present at the moment of ignition in the chamber is the cause.

[Tom Walker]

Just because you have a greater quantity of fuel and air trapped in the cylinder does not always equate to increased power. Quality of the air fuel mixture may have decreased, and by quality, I mean what ever type of atomization, temperature, swirl, tumble or whatever type of air fuel mixture that particular engine responds to best at that particular load and R.P.M. I believe it to be very different with many variables at play. What does that type of chamber want for air fuel preparation for the most efficient combustion event? Very dependent on what that very engine wants. Many of the reasons for more mass not always equating to more power were explained earlier in this thread in a very informative way. Much appreciated.

[Steve.k]

So does a bigger cc intake runner produce more power with this above theory all other things equal?

[GRTfast]

Just because you have a greater quantity of fuel and air trapped in the cylinder does not always equate to increased power. Quality of the air fuel mixture may have decreased, and by quality, I mean what ever type of atomization, temperature, swirl, tumble or whatever type of air fuel mixture that particular engine responds to best at that particular load and R.P.M. I believe it to be very different with many variables at play. What does that type of chamber want for air fuel preparation for the most efficient combustion event? Very dependent on what that very engine wants. Many of the reasons for more mass not always equating to more power were explained earlier in this thread in a very informative way. Much appreciated.

^^^this

[F-BIRD'88]

J"ust because you have a greater quantity of fuel and air trapped in the cylinder"
Ya it usually is. Most of the time.
More stuff in and out is usually better.