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
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.
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,
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
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
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!
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. )
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?
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.
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
blog.autospeed.com) 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.
Here’s what it looked like unpainted...
...and here’s how it appears after it has been
disassembled, spray painted and then reassembled.
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
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.