Harley Head Flow Numbers: Bigger CFM Doesn't Automatically Mean More Power

“This head flows 380 CFM.”

Cool.

At what lift? On what bench? At what test pressure? And what engine are we putting it on?

Cylinder-head flow numbers have become one of those specs everybody loves to throw around when talking about Harley performance.

350 CFM.
375 CFM.
400 CFM.

Naturally, the bigger number sounds better.

But your engine doesn't know what number was printed on the flow sheet.

It only knows whether we actually filled the cylinder.

At TMF Cycles, we absolutely care about cylinder-head airflow. On a serious Milwaukee-Eight build, the heads are a major part of determining how much power the engine is capable of making.

But we don't choose a head because it has the biggest peak CFM number.

We choose it based on the displacement, camshaft, compression, intake, exhaust, RPM range and what we're actually trying to make the motorcycle do.

That's a much different conversation.

What Does CFM Actually Tell Us?

CFM stands for cubic feet per minute.

A flow bench creates a controlled pressure difference across the cylinder head and measures how much air can move through the port.

Normally you'll see the head tested at several valve lifts:

.100"

.200"

.300"

.400"

.500"

.600"

And possibly .650", .700" or higher depending on the head and intended application.

That's all useful information.

The problem starts when somebody ignores the entire sheet, scrolls down to the biggest number and decides that's the better cylinder head.

If one head peaks at 350 CFM and another peaks at 380 CFM, the 380 CFM head must make more horsepower.

Not necessarily.

Your Intake Valve Doesn't Live at Peak Lift

This is probably the easiest way to understand why peak CFM doesn't tell the entire story.

Your intake valve doesn't instantly open to .650", sit there while the cylinder fills, then instantly close.

It has to travel there.

The valve opens and passes through .100", .200", .300", .400", .500" and so on.

It reaches peak lift for a relatively short portion of the event and then travels through those same lift points again while closing.

So let's say we have two heads:

Valve Lift	Head A	Head B
.200"	160 CFM	175 CFM
.300"	220 CFM	240 CFM
.400"	275 CFM	295 CFM
.500"	320 CFM	330 CFM
.600"	355 CFM	345 CFM
.650"	365 CFM	348 CFM

If we're selling cylinder heads based strictly on the biggest number:

Head A wins.

365 CFM versus 348 CFM.

Easy decision, right?

Except Head B moved more air through most of the valve's travel.

So which one is actually better?

Now we need to know what engine we're putting it on.

That's the point.

The Camshaft Has a Say in This Too

Let's say you've got a cylinder head that puts up huge flow numbers at .650" and .700" lift.

Great.

Now put a .475" lift cam underneath it.

What are we doing with that .700" flow number?

Nothing.

The valve never gets there.

Even with a higher-lift cam, peak lift is only one portion of the valve event.

That's why we're interested in what the head does throughout the lift range the camshaft actually uses.

And lift isn't the only thing that matters.

Duration matters.

Opening and closing events matter.

Overlap matters.

Intake closing matters.

The cylinder only has so much time to fill.

We need the head and camshaft working together during that window.

A killer cylinder head with the wrong cam isn't automatically a killer combination.

Port Velocity Matters

This is where chasing the biggest possible CFM number can really get people sideways.

One way to increase the airflow capability of a port is to make it larger.

But there's a point where bigger isn't helping the engine we're building.

Increase the cross-sectional area enough and air velocity can decrease at a given airflow demand.

And the airflow demand is being created by the engine.

A relatively mild 107-inch Milwaukee-Eight turning 3,000 RPM doesn't have the same airflow requirements as a huge-displacement engine spinning 6,500 RPM.

So why would we automatically give them the same intake port?

We don't want the biggest hole we can possibly make.

We want the port sized for the engine.

A big port can be exactly what a big-inch, high-RPM engine needs.

Put that same head on an engine that can't take advantage of it and you may have bought airflow capability you'll never use.

Milwaukee-Eight Heads Are Already Pretty Damn Good

This is important when we're talking about M8s.

The Milwaukee-Eight starts with a four-valve cylinder head that is already capable of moving a substantial amount of air.

We're not starting with some terrible factory casting that can't support power unless somebody attacks it with a grinder.

We've seen plenty of Milwaukee-Eight combinations make serious power without the biggest valves and biggest ports available.

As displacement, RPM and horsepower goals increase, additional cylinder-head capability becomes more important.

That's when port work, valve size and airflow capacity start earning their keep.

But the question still shouldn't be:

“How much CFM can we get out of this head?”

The question should be:

“How much airflow does this engine actually need, and where does it need it?”

Bigger Valves Don't Automatically Mean Better Heads

The same mentality gets applied to valve size.

Stock.

+1 mm.

+2 mm.

+4 mm.

People see +4 and assume it has to be better than +2.

Maybe it is.

For what engine?

A larger valve can provide additional curtain area and airflow potential, but the valve has to work with everything around it.

The valve seat.

The throat.

The port.

The combustion chamber.

The bore.

The piston.

And eventually we're dealing with other considerations:

Valve-to-valve clearance.

Valve-to-piston clearance.

Valve shrouding.

Valve weight.

Spring requirements.

RPM.

We're not just trying to stuff the largest valve that physically fits into the cylinder head.

We're trying to build a cylinder head that works.

The Combustion Chamber Is Part of the Head Too

This gets forgotten when everybody is staring at the intake CFM column.

A cylinder head isn't just an intake port.

It's also a major part of the combustion chamber.

Chamber volume affects compression ratio.

Chamber shape affects combustion.

Valve placement and chamber shape can affect shrouding.

The piston has to work with that chamber.

The cam has to work with the resulting compression.

If we change the cylinder head and chamber volume, we may have changed much more than airflow.

So a head that flows more air isn't automatically an upgrade if it moves the rest of the combination in the wrong direction.

Again:

It's a system.

Don't Forget the Exhaust Side

Everybody loves intake numbers.

That's the sexy number on the flow sheet.

But whatever we put into the cylinder eventually has to get back out.

The exhaust port, exhaust valve, cam timing and exhaust system all work together.

We're not interested in hitting some magical intake-to-exhaust flow percentage just because somebody on the internet says every engine needs it.

The actual engine combination matters.

The cam matters.

The exhaust matters.

The intended RPM range matters.

You can have an intake port with an impressive flow number and still have a combination that doesn't work very well.

The Head Doesn't Breathe by Itself

Here's another thing that gets overlooked.

When you see a cylinder-head flow number, you're looking at the airflow capability of the head under specific test conditions.

That's not how the engine breathes when it's assembled in the motorcycle.

Air has to travel through the entire intake tract:

Air cleaner → throttle body → manifold → cylinder head → valve → cylinder

Every one of those pieces can affect the final result.

Let's say we spend a pile of money on heads capable of moving significantly more air.

Then we put a restrictive throttle body and manifold in front of them.

What did we accomplish?

Same thing on the other side.

If the heads can move a ton of air but the exhaust doesn't support the combination, the heads can't magically fix it.

The entire airflow path has to support the power goal.

This Is Why Bigger Throttle Bodies Aren't Automatically Better Either

Cylinder heads and throttle bodies are actually a very similar conversation.

People look at:

55 mm.

58 mm.

62 mm.

64 mm.

70 mm.

And automatically assume the biggest one is the performance choice.

It depends.

What displacement?

What heads?

What cam?

What RPM?

How much horsepower are we trying to make?

There's no point putting together an induction system capable of feeding an engine far beyond what the rest of the combination can consume.

And there's no point building a cylinder head capable of moving massive amounts of air if we're choking it somewhere else.

Everything needs to match.

Be Careful Comparing Flow Sheets From Different Shops

This is another big one.

Let's say one company advertises:

350 CFM

Another advertises:

380 CFM

So the second head flows 30 CFM more.

Maybe.

But before we say that, we'd want to know how both heads were tested.

Same flow bench?

Same test pressure?

Same bore fixture?

Same valve size?

Same intake radius?

Same manifold setup?

Same exhaust fixture?

Same testing procedure?

If the answer is no, we're not necessarily making an apples-to-apples comparison.

That doesn't make the numbers useless.

It means they need context.

A flow bench is incredibly valuable when we're making controlled comparisons.

Take a head.

Test it.

Change the port.

Test it again.

Change the valve job.

Test it.

Change the short-side radius.

Test it.

Now we can see exactly where we gained or lost airflow.

That's useful information.

Looking at two peak numbers from completely different tests and declaring a winner is something else.

Flow Bench Numbers Aren't Horsepower Numbers

This is probably the biggest takeaway.

A flow bench is a development tool.

It can show us how much air a cylinder head moves under controlled conditions and how that airflow changes throughout the lift range.

That's extremely useful.

But it doesn't tell us:

“This cylinder head will make 180 horsepower.”

The head isn't the engine.

Eventually that head has to be bolted onto an actual engine with an actual:

Camshaft.

Compression ratio.

Piston.

Throttle body.

Manifold.

Injector.

Exhaust.

And tune.

Then we find out what the combination actually does.

That's where the dyno comes in.

Flow Bench + Dyno

We look at these as two completely different tools answering different questions.

The flow bench helps develop and evaluate the cylinder head.

The dyno tells us what the complete engine did with it.

A head porter can make a change and see exactly what happened to airflow.

Maybe it gained 15 CFM at peak lift.

Great.

Did it lose airflow somewhere else?

Did the port get substantially larger to achieve it?

What happened to velocity?

Then put the complete engine together.

Did torque improve?

Did horsepower improve?

Where did it improve?

Where did it lose?

That's the information we're actually interested in.

More Head Than You Need Can Just Be More Money

Sometimes an oversized cylinder head isn't going to ruin the engine.

Sometimes you simply paid for capability you're never going to use.

If we're building a street 117 or 128 and have a realistic horsepower and RPM goal, we can select a cylinder head around that.

If we're building a huge-inch engine intended to make considerably more horsepower and operate at higher RPM, we're going to make a different choice.

Neither cylinder head is automatically better.

They're built for different jobs.

This is why we don't start an engine build by asking:

“What's the biggest head we can buy?”

We start with:

“What are we trying to build?”

So Is More CFM Bad?

Absolutely not.

There comes a point where an engine simply needs more airflow.

Increase displacement and the engine consumes more air.

Increase RPM and the airflow requirement goes up again.

Put the right camshaft in it and additional high-lift airflow can become extremely valuable.

Feed it with the right induction system.

Get it out with the right exhaust.

Now a serious cylinder head starts earning its money.

If we're trying to make 200 horsepower, we're obviously going to need more airflow capability than we need for a 130-horsepower street motor.

That's common sense.

The mistake is treating more CFM as automatically more horsepower regardless of the application.

It isn't.

The Highest-Flowing Head Isn't Always the Best Head

If we're building a 128-inch Milwaukee-Eight street engine that needs to hit hard in the midrange and carry power through the RPM range where it's actually ridden, we're going to select components around that goal.

If we're building a giant race engine that spends its life at high RPM, we're going to make different choices.

The race head isn't automatically the better cylinder head.

It's the better cylinder head for that engine.

That's the difference.

Build the Combination, Not the Flow Sheet

At TMF Cycles, we're not interested in putting the biggest possible number next to every component in an engine.

Biggest cam.

Biggest throttle body.

Biggest injector.

Biggest valve.

Biggest port.

Highest CFM.

That isn't how we approach engine building.

We want all of those pieces working together.

Cylinder-head airflow absolutely matters.

But so does:

Air velocity.

Port size.

Valve size.

Cam timing.

Valve lift.

Displacement.

Compression.

RPM.

Induction.

Exhaust.

And most importantly:

What are we trying to make the motorcycle do?

Because we're not trying to build a cylinder head that wins on a flow bench.

We're trying to build an engine that works.