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We Have a Record!

Reducing intake flow restriction to the absolute bare minimum

by Julian Edgar

Click on pics to view larger images

This article was first published in 2008.

OK, we have a new record. Now it might seem a rather obscure one but it refers to something that is vitally important in every performance car. And what’s this record? Achieving the minimum pressure drop, as measured after the airfilter in a modified system. Or to put it another way, developing the best flowing intake system ahead of the turbo or throttle body.

Measuring Flow Restriction

The best way of measuring what is happening in the intake system is on the road. No, not on a dyno. No, not on a flowbench. On the road – where you have real world engine airflow demands, real world outside airflows, and real world car configurations (like having the bonnet closed!). And best of all, measuring the intake air restriction is dead easy.

Click for larger image

If you connect a hose to the intake system and run the other end to a gauge, you can read off the pressure drop. The greater the pressure drop (vacuum if you like), the more restrictive is the intake. Simple, eh?

To do the measurement you can make your own accurate and zero cost instrument (a water gauge manometer) or use a Dwyer Magnehelic gauge. For both approaches, see Negative Boost Revisited, Part 2. We used a Magnehelic gauge but the self-built manometer is absolutely fine (you just need an assistant to read it and hold it).

Rather than measure the intake system at a variety of points, we chose to measure the total intake restriction of the snorkel, airbox and air filter. The car? A hybrid Honda Insight.

Measured in this way, the total pressure drop at peak revs in second gear (ie max power) was 4 inches of water. That is stunningly good. Here’s a comparison of the total intake system pressure drops (all measured in the same way at peak power) that I’ve recorded over the last 15 years.


Max Pressure Drop

Honda Insight 1-litre 3-cylinder


Toyota Prius 1.5-litre 4-cylinder


Holden Camira 1.8-litre 4-cylinder


Ford Falcon 4-litre 6-cylinder


Toyota Crown 2-litre 6-cylinder supercharger


Audi S4 2.2-litre 5-cylinder turbo


Subaru Liberty RS 2-litre 4-cylinder turbo


Click for larger image

This graph better shows the comparison. And yes, the Insight, Prius and Camira are the lowest-powered cars – but they also have the smallest intake systems!

Standard Intake

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So what does the standard intake system of the Honda look like? In a word, tiny. There‘s a little airbox, small filter and a long snorkel intake that passes across the front of the engine bay. The actual breathing opening is also positioned within the engine bay. The Honda is MAP-sensed and so doesn’t use an airflow meter.

I was initially prompted to make the measurement when someone on a discussion group said that the Insight’s performance could be noticeably improved by removing the airbox intake snorkel. This implies, of course, that the snorkel is restrictive to flow.

Now to put it mildly, the Honda Insight is an efficient car, so it seemed very unlikely that Honda’s engineers had made such a mistake. So I whipped out the trusty Magnehelic (actually, I have about six of them calibrated over different ranges), drilled a tiny side on the throttle side of the airbox (ie the measured restriction included the airfilter) and went for an around the block test drive. And when I saw the 4 inches of water figure, I realised that the guy was dreaming if he thought he’d be able to feel a performance gain by pulling off the snorkel. (Why? Well, even if the pressure drop decreased to zero, he’d still be breathing hot intake air from behind the radiator. Not good. In this case, the potential gain of reducing the restriction would not be enough to make up for the hot air. )

Trial Intake

Time passed then one day I thought of an improvement: why not connect the end of the snorkel to the front of the car? Inspection showed that if a plastic blanking plate was removed and a right-angled tube added to the original snorkel, the mouth of the tube would be pressurised by the forward motion of the car. However, would this upset the very finely optimised intake?

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I added a short length of snorkel salvaged from a Nissan Skyline and then used foam rubber to fill the gap around the opening. Foam rubber was also used to seal the gap between the original and new snorkels. It wasn’t pretty but it would be an interesting test.

The test I had in mind was fuel economy – would the open-road cruise economy improve?

However, ascertaining this was very difficult – when you get an open road economy of 3.0 – 3.3 litres/100km, you need to make a big improvement to see the change on the fuel consumption read-out!

However, my gut feel was that yes, with the trial snorkel there was a slight improvement. On some drives from the Gold Coast to Brisbane, the fuel economy was fractionally better than I expected in the conditions. That’s not very scientific, but it was good enough for me.

The next step was to make a proper intake.

New Intake

After trialling some different approaches I settled on some truck radiator hose that had the right shape bends. This original end of the Honda’s intake snorkel was cut off and the end heated and shaped until it was round (it’s usually an odd rectangular shape). A bellmouth was made from the middle of a plastic cake dish (see the end of and foam rubber used to seal off the gaps so that all air passing through the grille (really, just an opening between the bumper and bonnet) had to either pass through the radiator or go into the engine through the snorkel. Contact adhesive was used to hold the bits together.

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Here’s what it looked like unpainted...

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...and here’s how it appears after it has been disassembled, spray painted and then reassembled.


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Now what was that at the beginning of the article about something being a new record? Well, after the new snorkel had been installed, the maximum recorded pressure drop of the complete intake system – snorkel, airbox and filter – decreased to just 2 inches of water!

And, even better, in any constant throttle cruise over 40 km/h, there was in fact a positive pressure on the throttle side of the airfilter. This pressure was typically about half an inch of water.

To put this another way, in cruise conditions the intake system is posing less than zero restriction, and even at maximum flow the throttle is seeing 99.5 per cent of atmospheric pressure.

Downsides? Or, why didn’t Honda do it this way? In very dusty conditions the filter will need to be changed more often, and there’s now a just-audible induction note that can be heard at full throttle with the windows down.


Since your car is always pushing air out of the way, why don’t you use some of that effort to decrease the flow losses through your intake system? I’ll be watching the fuel economy of the Honda closely over the next few months but even without those figures being available, the better flow and decreased intake air temp are certainly going to improve throttle response – and even, a tiny bit, power.

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