Engine Air Consumption – Intake Air Speed – Calculations

Do you want to know how fast the intake air is travelling through your intake?

Want to know how fast the air passes through your air filter?

Want to know how fast the air is moving through your throttle body?

Want to know how much air your engine consumes per minute?

Read on!

So we are doing some consulting work which involves improving a product that is already available on the market. Without divulging too much information, the product is used on the intake side of the engine.

Before we got into the bells and whistles we needed to understand what we were dealing with. By this I mean how fast is the air entering the engine at something like full throttle on a normally aspirated engine, we used a 2 litre engine as example for the sake of simplicity.

So we did a bit of Googling and found this http://www.f1technical.net/forum/viewtopic.php?f=6&t=5729

It says a 2.4 litre engine at 6000rpm consumes around 253CFM of air.

So using a rough approximation we figured a 2 litre engine at 7000rpm is consuming around 220 CFM per minute. For the sake of argument and to come up with a worse case scenario lets assume 250CFM.

Converting this to cubic metres we get a figure of 7.079 cubic metres of air per minute.

Assuming the intake tract is 3 inches in diameter the area of the tract will be π x r x r which equals 45.58  sq/cm or 0.00456 sq/m.

Engine air consumption information. Intake air speed and volume. Image shows K&N intake air speed tests on Silverado

So to flow 7.079 cubic metres of air per minute through a 0.00456 sq/m pipe the air needs to move at 1552 meters per minute (7.079/0.00456)

Or 25.87 metres/sec (1552/60)

Use this to convert m/sec into various other speeds

Highlight figures are 90 kph or 55 mph.

Assuming the area of a typical panel filter is around 10 times larger than the intake tract, in the case 3 inches we can make an approximate guess that air is passing through the filter at around 9kph or 5.5mph (10 times slower)

We were a bit sceptical of the original figure of 250 cfm so we found this video where K&N did similar analysis of intake air speeds (but for a completely different reason). We did this to see how our calculations stacked up with their findings, granted their findings were on a much larger engine but it would give is a feeling for the accuracy of our calculations.

httpv://youtu.be/_SPd10AswCo

They came up with a max figure of 12 mph through the filter which equated to a max speed of 90 mph across the throttle body.

Give that the Silverado at a minimum has a 5.3 litre engine and probably has a throttle body of around 3 inches but only revs to 6000 rpm or so, our figure of 5.5mph for an engine 63% smaller but with a 15% higher rev limit, looks, for lack of a technical term “bang on”.

Intake air speeds and engine air consumption – Summary

A 2 litre engine, with a 7000 rpm rev limit consumes around 250 cfm of air at red line.

Air speed through a 3 inch intake/throttle body at redline is around 55 mph

Air speed through the panel filter with an area 10 times as large as a 3 inch the intake pipe is 5.5mph.


This page was last modified May 21, 2018 @ 1:26 pm

Richard @ StrikeEngine

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