I've been thinking about head flow numbers and had a couple questions... So, say you have a head that flows 250cfm on a flow bench. Since it's a V8 engine you will have two of these(for 500cfm total flow). Does that mean that the head will flow 500cfm before it starts becoming a restriction? 500cfm is only around 34lb/min(0.069 conversion) of air, so roughly 340hp. That seems really low... I have never taken any fluid classes so I was hoping someone could explain with a little more detail what the flow number tells you as far as HP support.

 

That is per cylinder so technically 2000cfm for a v8. Its enough to support about 450-500hp

 

First off you have a valid question but are a bit confused on the numbers and how they apply to a flow bench.

Obviously on a V-8 engine you have 8 intake ports. The engine makes 2 revolutions of the crankshaft in order to ignite the combustion mixture in all 8 cylinders.

Every intake port, if flowed on a flow bench, will be tested at different valve positions. The Superflow Flow Bench manual used a Lift/Diameter method. many other people just use testing points every .050" of lift up to maximum valve lift of the engine.

That being said, at maximum valve lift if you have a 250 cfm flow number that would be for that specific cylinders intake port.

A rule of thumb is that for every cfm measured in a specific intake port you should make double that number in horsepower so if all of
your intake ports AVERAGE out to 250 cfm flow then the engine can be expected to make about 500 horsepower if all of the parts are right.

So back to your question you have 8 intake ports not 2 intake ports. Each Intake port/combustion event helps propel the vehicle down the road.

So if you have a 250 cfm head and each cfm makes 2 hp you get 500 horsepower.

Typically 1 pound of air mass will make 10 horsepower so that means you need to have 50 pounds of air mass to make 500 horsepower.

If you have 50 pounds of air mass and an air/fuel ratio of 12.5 you would divide 50 by 12.5 and get 4 pounds of fuel per minute.

4 pounds of fuel per minute times 60 minutes = 240 pounds of fuel per hour.

240 pounds of fuel per hour divided by 8 injectors = 30 pounds of fuel per injector. You need a bit of a reserve to be around .85 duty cycle
so the number goes to 35 pounds per minute injectors. Right where your other number posted is.

The confusion is that you are using the rule of 1 = 10 for the fuel and it is actually for the air mass. 1 lb of air mass = 10 hp.

So the fuel injector size is correct. The cfm for 500 horsepower would be about 250 cfm per port.

This calculation is a very basic assumed calculation. Optimized engine specs will make another 10% more horsepower 550 hp.

Tom Vaught

I hope this answers your question/post.

 

Okay, I failed to realize that it was a rating for each intake port, and I didn't know at all the rule of thumb for head cfm to total engine power. I think you are confused about me being confused about fuel. While reading compressor maps, some are in cfm and some in lb/min. Injectors are lb/hr. I was attempting to use the compressor map conversion maths.

From turbobygarrett.com:

• Mass Flow Rate is the mass of air flowing through a compressor (and engine!) over a given period of time and is commonly expressed as lb/min (pounds per minute). Mass flow can be physically measured, but in many cases it is sufficient to estimate the mass flow for choosing the proper turbo.
• Many people use Volumetric Flow Rate (expressed in cubic feet per minute, CFM or ft[SUP]3[/SUP]/min)) instead of mass flow rate. Volumetric flow rate can be converted to mass flow by multiplying by the air density. Air density at sea level is 0.076lb/ft[SUP]3[/SUP]
• What is my mass flow rate? As a very general rule, turbocharged gasoline engines will generate 9.5-10.5 horsepower (as measured at the flywheel) for each lb/min of airflow. So, an engine with a target peak horsepower of 400 Hp will require 36-44 lb/min of airflow to achieve that target. This is just a rough first approximation to help narrow the turbo selection options.

I was using the conversion of cfm to lb/min of air with 0.069(some say 0.076). So I was saying 500cfm is roughly 500cfm*0.069lb/ft^3 = 34.5 lb/min. For every lb/min of air the rule of thumb is 10hp, so 345hp is what I gathered using my faulty logic of 250cfm per head. Why is this number I came up with so wrong, 34.5lb/min is significantly different from 50lb/min that should give 500hp. Or is it something to do with the 28 inches of water rating used on head flow benches?

So, now the second part of my question: Using your 500hp assumptions, this means the most the engine will flow at atmospheric pressure(naturally aspirated engine) correct? Once you add positive pressure, is there a direct correlation to boost pressure and flow? So say double atmospheric( ignoring temp increases) would give double the flow? Or is it not quite that simple? Or can you not say that the rating from the flow bench is a direct rating of naturally aspirated power potential?

I still think the measurement unit used for cylinder head flow and my thinking of it as an N/A test may be flawed.

 

The volume flow rate (cfm) never changes under boost or vaccume. The pressure changes and therefore the mass flow rate (lb/min) changes. This means that there is a volume flow limit for heads but no mass flow limit. Keep upping the boost and the power will keep going up.

Here is another fun way to think about it. If an engine consumes 500cfm at wide open throttle it also consumes 500cfm at idle. This is at the engine not at the air filter. There is some small amount of cfm at atmospheric pressure before the throttle blade that the throttle blade converts into 500cfm in the intake and heads at a very low pressure. 500cfm at a very low pressure means very low mass and very low power. Just enough to idle.

Yet another way to think of it. The piston moves down in the cylinder and there is some space there for air. This space could be measured in cubic feet. You can't change that. If you apply pressure you cannot get more cubic feet as the space is dictated by the engine. You can only make that air that goes into that space more dense.

 

Ah, okay. That is where my understanding was breaking down. Since the throttle plate is closed or close to it at idle, and the engine is trying to consume that much air, it produces the intake vacuum due to lack of flow of new air to replace the consumed air. While reading through some of the turbo documents I guess I never really grasped the differences in flow rates and charge density. I think I'll go back through and read again haha

 

Also there is port velocity and efficencys to overcome as well. I'm a big velocity fan over intake flow. To much flow and it can hurt velocity to a point. Many like over all flow vs max flow. As your only at max flow for a blip of the cam lobe. Velocity is the speed the air moves into the cylinder port.

 

Something else to keep in mind is that cam, intake and exhaust selection are critical for how much air an engine can flow. Red line is also critical for flow over a given period of time as well as flow for a single cycle (more on that below.)

First off, some things you have to remember
-You don't get full flow all the time from a head
-Air has mass.
-As a subset of this point, tuning is critical.
-Fuel has more mass than air does.
-No matter how much flow you have, if you don't have the red line for it you're never going to use it. The opposite is true as well, if you don't have enough flow you're going to run out of RPMs you make power at.

Starting with the first point, most of a cam's life is spent at points below peak lift. This means that the quicker you get the valve open (durability ignored) in general the more air you're going to flow with less power to make it flow and in turn the more power you'll make. If you look at ramp rates for Comp's XE lobes versus their conventional ones (adv [email protected]) you'll notice that the comps ramp up significantly quicker. This is why they make more power. It comes at a price however, as longevity of parts can be affected.

Something people tend to forget is that air has mass. This means that when the intake valve opens and the piston starts going down, you have to get the air moving again. While this may seem like a down side, it's actually a quite useful concept when worked with properly (see below.) It also means that in general, you're never going to realize the total air flow potential of the port except for maybe a small RPM band.

Tuning the length of runners in the intake and exhaust tract makes more power for a reason. First off, if you have a properly tuned exhaust (long tube headers) versus logs, you'll end up increasing scavenging. Basically where the cam's intake and exhaust overlap, the outgoing exhaust gas moving through the tube of the header creates negative pressure in the cylinder so that incoming air is more efficient. The other thing is that intake manifolds do something similar in the fact that there is a pressure cycle. When the valve closes it creates a wave and if the circumstances are ideal, the wave reverberates and hits the valve back as it is opening again for the intake cycle.

Fuel has more mass than air. Seems sort of captain obvious to say, but it's relevant in non-race vehicles. The reason why over intaking or over carbing a vehicle has bad affects on drivability is it affects the ability of the fuel to get to the cylinder properly. Due to low velocity it tends to puddle on the outside walls of the intake manifold. All wet fuel intakes do this (carb, TBI) and port EFI does it to a lesser extent as it injects the fuel pretty much into the head port. This is why port injection systems are far less sensitive to having large flow rates. Even my 190hp 305 TPI came with a throttle body that flowed around 750 CFM. You'd probably pay throwing a 750 Holley on a stock 305 that way at least on some level. Q-jets don't count in my opinion, as there's more going on there that affects raw air flow through them.

The last point I'll make is that too many people get stuck on the idea of air flow. It is an important metric typically, but it can be critical to recognize if you don't turn the RPM to make use of this flow you've mismatched your parts.

An example I always got a bit of a chuckle out of was Hot Rod did a 302 Chevy with a Vortech blower that made 550hp at 10 PSI and something like 330hp N/A at 5000-5500 RPM. Seems pretty decent, right? It does until you consider that the heads they used were AFR 190s. A set of box stock ones will flow something like 260CFM, which by our rule of thumb is enough for over 500hp. What happened? Due to the engine being under cammed for the amount of head/CID they had they are not truly taking advantage of the flow offered. They could have used a set of aluminum LT-1 heads or L98 heads and probably gotten very similar numbers if not slightly better average HP with the red line they had. And likely better mpg since it was a factor on their build.

The inverse of this is true, the classic example that comes to mind is the TPI. It's got small cross sectional area for the intake runners, so it tends to hit a wall below 5000 RPMs on the stock 350. No matter how high you rev, it's not worth it due to the power dropping like a rock. You may as well stay within the confines of the intake or swap it out. On the inverse side, it does exactly what GM wanted it to; it makes gobs of torque at lower RPMs due to high velocity. This was a compromise that GM willingly made, because it made them "feel" quicker especially with anemic rear gear ratios.

 

I think it should be a rule for the advanced section that the phrase 'air flow' alone is not allowed to be used. We need to stick with 'mass air flow' or 'volume air flow'. This will add clarity to our discussions.

 

I can live with that. Since mass is different than volume of air.

 

So...when choosing a cam do you target lift to match peak flow numbers of the heads you plan to use? Or do you focus more on mid lift numbers and intended powerand?

 

I think establishment of the definitions of "volume" versus "mass" via sticky would be clearer personally, and easier to write. I also believe that "mass air flow" is an off term as mass doesn't have volume, so how do you measure how it flows? I definitely agree that there is a tendency to use the two interchangeably.

Just my opinion for what it is.

One last thing to keep in mind, is that this all boils down to the ideal gas law. No matter what, Pressure*Volume=# of moles*The ideal gas constant*Temperature or PV=NRT for short. Molecules of air=Moles*avagadro's constant or 6.0221413e+23. That means if you have twice the pressure in the same cylinder volume at the same temperature you're going to have twice the oxygen molecules. Naturally you don't end up with the same temperature etc. so there is variance to this. Damn my head hurts now from remembering all that chemistry I haven't done in years.

 

I would also agree with that suggestion.

Tom Vaught