Is upgrading your air intake manifold worth it?

Some 6.7 engine owners believe that the heads are the limiting factor when it comes to airflow and that the engine can only put out so much CFM. So, any improvement in the air intake elbow is worthless, in their opinion. But I couldn't agree more with one of the experts that the limiting factor is their head, not the Cummins engine.

Forum Insights on Intake Elbows

Here is an opinion from the forum on intake bends:

User 1: "It serves no purpose. The head is the main limiting factor."

Then, user 2, who seems to be bucking user 1's leg

User 2: "I guess people don't get it."

User 1 replies

"The original intake horn bleeds out of the original headers." How do these guys know this?"

User 3 says

"It probably does help. Is there really a limit? Not really.

Then he goes on to say

"The original speakers will put out more power than the original head unit. You can put out a lot of power with stock speakers. It's just personal preference."

The last one was a user with theoretical knowledge who said

"Cummins's heads flow even less than 250 CFM. stock horns flow well over that. So even if you change the increased CFM, it doesn't make any sense. There is no way the head is flowing more than the stock horn allows. You can look it up or ask a mechanic".

To the above point, first of all, a stock 6.7 Cummins engine has a displacement or pump flow of 331 CFM at 2800 rpm, which is peak horsepower. What matters here is the density of air per cubic foot entering the intake.

Historical Context: Diesel Engine Evolution

Back 40 years ago, the diesels used in American cars and pickup trucks didn't have turbocharging. They were naturally aspirated four-stroke engines. (All four-stroke engines change cylinder volume with every two revolutions of the crankshaft. With each revolution, half of the cylinders are intake, and half are ignition.

Therefore, it takes two revolutions to intake and ignite all cylinders.)

People who play with performance might turn the engine faster, around 3,400 rpm, which means the engine exhausts about 400 cubic feet per minute.

So, if the above point is about CFM, then there is a problem.

Understanding Air Density and Mass Flow

You're feeding 1,200 CFM into the intake, and what's flowing to the engine is 400 CFM. What's the difference?

Someone said, "You have the cold air system flowing at 1,200 CFM, 1,000 CFM or higher. But the engine is only flowing 400 CFM. What's going on?" It's actually because the air density has changed.

From the air intake, which is ambient air density (about 72 pounds per thousand cubic feet), to the engine, you've compressed the air (3:1, for example), and now the density of the air going into the intake manifold drops to 400 CFM. The difference is air density.

About 60 years ago, many experts started using engine mass flow instead of CFM.

Then, the MAF sensor was introduced, and today, all cars (most) use MAF sensors to measure mass airflow.

Here's why: 1200 CFM into the compressor of the turbocharger.

A turbine drives the compressor on the turbocharger. There is a common axis between the two.

The turbine is driven by exhaust gas pressure and temperature.

In other words, the exhaust gas energy (in the form of exhaust gas temperature and exhaust gas pressure) generated by the piston rising during the exhaust stroke drives the turbine.

The horsepower needed to push the exhaust gases out of the cylinder comes from the crankshaft.

It's produced on the combustion stroke.

The horsepower produced by the turbine, which is probably here around 40 to 50 horsepower...

This thing, the turbocharger, is a density machine. It increases the air density.

How does it do that?

It compresses the air.

When you're compressing the air, you're putting the same air mass that entered the compressor into fewer cubic feet of air.

Now each cubic foot weighs more than it did entering here.

The constant here is the air mass flow—how many pounds per minute are we flowing through this stock engine?

When the 6.7 came out, the airflow was mid-40s, like 45 lbs per minute, through the system and into the cylinders.

Now, the later models are like 48 to 52.

An expert has data logged the 2020 HO, which collects 52.4 pounds per minute at 400 horsepower. This is a constant value throughout the system. The compressor on the turbocharger increases the pressure but heats the air. If it were cooling the air, the air density would increase.

We want to maximize the density of the air entering the engine intake manifold.

So, we cool the air at the intake manifold.

The CFM reduces once again, but the air mass is the same.

Out of this very low-restriction boost tube into the factory throttle.

This system is emissions-compliant in all 50 states.

Product Recommendation

Based on the opinions of some of the experts above, as well as real-world examples, SPELAB has designed 6.7 Cummins air intake manifolds, such as the Performance option for the 6.7 Cummins engine, to optimize airflow and enhance air density variations to ensure maximum efficiency is achieved. The following diagram illustrates in great detail the principle of SPELAB intake manifold operation: