Northstar Air Consumption

[Ranger]

The PCM being cooled by intake air has always made me wonder what kind of air flow it has. So I pose this question to you math wizes. What is the CFM of intake air flow of the Northstar at 2000 RPM (70 MPH on an LD8)?

Here is what I came up with.

280 CI = 35 CI per cylinder.

Intake on every other rotation would mean 35x1000=35000 CI. per cylinder X 8 cylinders = 28000 CI converted to CF = 16.2 cubic feet per min.

Did I do that right?

[JasonA]

You can calculate a theoretical maximum airflow assuming 100% VE (Volumetric Efficiency) with the following equation:

CFM = CI x RPM / 3456

So it follows that:

CFM = 281 x 2000 / 3456; or CFM = 162.1 (you are off a decimal)

That said, an engine's true VE rarely approaches 100%. Most modern engines typically achieve a VE of about 90%, give or take, but this also varies with RPM. It's generally said that an engine system (including intake and exhaust) achieve the best VE near the torque peak (so, 4000 or 4400 RPM depending on Northstar version). So the true CFM will be somewhere south of the theoretical maximum.

This shows why carburetors are often WAY over-specified for hot-rodded engines. Consider even a big block Chevy, at 5000 RPM.

CFM = 454 x 5000 / 3456; or CFM = 656

But most guys will slap on a 1000 CFM Demon or Holley carb and wonder if they even have enough for their engine. Those old 500 CFM Carters were plenty for the engines on which they were installed.

A Northstar should/would use about 530 CFM at full song (6500 RPM), assuming 100% VE (which it won't have).

[Ranger]

Thanks Jason. 162 CFM is a lot more cooling than 16.2 CFM. Makes me feel a lot better.

[JasonA]

Regarding the cooling, I don't find the PCM to really be "in" the airflow on a 98+ Seville chassis. It's in the airbox, but forward of both the airbox inlet (which opens downward) and the airbox outlet (which points rearward, to the engine). The only face of the PCM that would see real airflow (assuming a relatively straight path from inlet to outlet) would be the rear face of the PCM...and not much at that. I know the G-chassis DeVille's airbox is set up a little different.

[Ranger]

I suspect that was probably intentional so as not to disrupt the intake air flow, while the PCM still sits in a cooler environment and thus sheds heat more rapidly.

[JimD]

Forced air flow is nice to have but is not in any sense considered critical. If air flow was the key to cooling the PCM (or fill-in-the-blank component), the concept would fail in stop-go traffic in the California desert in August. Think of all the modern cop car Chevy Impalas/Ford Victorias that sit at idle for hours at road hazards/crime scenes.

Ninety nine percent of solid state component temperature control depends on heat sink surface area and mass. You can cook eggs on most heat sinks! With no problem for the electronics or the eggs (except for the taste part).

[Cadillac Jim]

Looking at the airflow as [displacement/4] X [RPM] X [conversion factors] is assuming zero vacuum as well as zero resistance through the intake; this is a crude WOT estimate. If you are pulling 15 inches of vacuum, you can divide the WOT air flow by two.

Back in the day when I had a 427 Corvette with 850 cfm Holley 4160 with vacuum secondaries, I found that it would cruise at top speed on the primaries, pulling lots of vacuum. The secondaries wouldn't open with the engine below about 3500 rpm. I had an acquaintance ask me to drive his 350 hp 327 Corvette to tell him whether his secondaries were opening. I had already done the math and doubted that they should (this car had the same carburetor), except at very high RPM at WOT. I couldn't get them to open with what I could do legally and told him that I didn't know.

One factor to consider when doing high-performance engines is the pressure drop in the intake. The CFI rating of a carburetor is done by measuring the flow for a given pressure drop. If you want less pressure drop than that figure, you need to have a higher CFI rating than the air flow that the engine will pull at WOT/redline. Thus the 850 cfm Holley on a 327 cid engine.

Another factor is that just about all modern engines including all Northstars have resonances in the intake that provide negative vacuum -- pressure -- at the intake during part of the intake stroke, and resonances at the exhaust that provide positive vacuum at the exhaust during part of the exhaust stroke. These resonances effectively provide a significant supercharger effect at several narrow RPM ranges that are put by design between the torque and horsepower rpms and thus increase airflow over the simple air pump equation, and can defeat pressure drops. You can read the MAP sensor through the OBD interface with a Tech II or other reader and it won't show this effect except as increased vacuum.

A DFI such as used on all Northstars will have a very low pressure drop at any air flow rate seen in the engine. The pressure drop is lower than that seen in any remotely appropriate carburetor for the engine size and is probably equivalent to over 1000 cfm in a carburetor or Accel fuel injection. Don't let anyone fool you about the MAF sensor, either -- the pressure drop across it is minuscule and the MAF reading is important in keeping the air-to-fuel ratio responsive to quickly-changing throttle position and RPM increases.

As far as the PCM is concerned, I think the real reason that it's in the intake air flow is because the under hood temperatures of Northstar applications is very high, and putting it there keeps the PCM out of harm's way and its temperature within bounds while driving across the desert or down the Autobahn at 120 mph (195 kph). It doesn't need to be in the main air flow and it doesn't need to contribute to pressure drop.