When modified engine power reaches a certain level, the airflow meter needs
to be upgraded. The point at which that level is reached depends mostly on how
much extra power you’re making and how big the airflow meter was to start with.
So what are the common upgrades? An aftermarket or OE upsized airflow meter, or
a move to programmable management (which lets you ditch the airflow meter for a
MAP sensor).
But these approaches are expensive.
But now there is an alternative. You get to keep the original airflow meter,
but the system can flow far more air. You get to keep the original airflow
meter, but its output scaling is changed so that you don’t hit a voltage
ceiling. Want more advantages? You can map your mixtures at the same time, the
approach is cheaper than anything else we have seen, and it’s all pretty easy to
implement. And on the road it works superbly.
Magic, huh? Not really – it uses the Silicon Chip magazine’s Digital Fuel
Adjuster DIY interceptor and some simple mechanicals. (For more on the
breakthrough Digital Fuel Adjuster, go to The Digital Fuel Adjuster, Part 1.) And don’t think that the
results are sub-par – we used the technique on a Maxima V6 Turbo and got
excellent mixture accuracy (as measured by a top-of-the-line MoTeC air/fuel
ratio meter), driveability at least as good as factory – and halved the flow
restriction of the intake.
Yep, we did say that – halved....
(Oh yes. And we also passed a roadside emission test with the full system in place!)
Why You’d Want to Upgrade the Airflow
Meter
We briefly touched on this above but let’s get into a bit more detail.
Most airflow meters are hotwire designs called Mass Airflow meters, (often
abbreviated to MAF). We described them in Electronic Engine Management, Part 1,
but in brief, this is how they work:
The air rushing into the engine flows around a platinum wire that's been
heated by electricity passing through it. The airflow cools down the wire. The
ECU tries to keep the wire at the same hot temp at all times, and can work out
how much electricity it needs to pump through the wire to do this. The more
electricity it needs to use, the more air that must be being breathed by the
engine. Hot wire meters automatically compensate for intake air temperature
variations and use onboard electronics to allow them spit out an output in the
0-5V range.
We also said in that article that MAFs “cause little restriction to the
intake air”, but we’d no longer make such a blanket statement. Some MAF meters
actually cause quite a lot of restriction – exactly how much depends on:
- The
detail of their internal design
- Their
diameter
- Their
design flow versus the flow that’s actually being drawn by the modified
engine
Some MAFs have heatsink fins protruding into the airflow, some have very
small internal diameters, and some have close-knit wire screens at each end. And
all these things harm flow capability.
Measuring the amount of restriction being caused by an airflow meter is
dead-easy. It’s also an important step to take before replacing or modifying it,
because if the airflow meter has plenty of flow capacity for your engine, what
are you going to gain by spending the dollars? (Except if the meter’s reaching
max voltage output, which we’ll come to in a moment.)
The easiest way of measuring the actual restriction is to plumb a manometer
(just a sensitive pressure measuring instrument) to the intake system after the
airflow meter and take a full load measurement. Then do the same before the
airflow meter. The difference in the figures is the pressure drop across the
airflow meter – incontrovertible evidence of how well it flows. The lower the
pressure drop, the better its flow. This can be done at near zero cost – you can
make a manometer from a plastic drink bottle, a length of dowel and some plastic
hose. And its accuracy will be excellent.
In Modifying the VL Turbo Intake
we showed you how to do this and also took some measurements through the intake
system of a typical turbo car. In that case (it was a near-standard Holden
Commodore VL Turbo), the airflow meter alone had a restriction of 14.5 inches of water
– or nearly 50 per cent of the total intake system restriction. That’s pretty
major... A simple improvement can be had by removing the protective mesh screens,
but that still leaves lots of danglies projecting into the flow path inside the
airflow meter.
So let’s say that you make the measurement and find the airflow meter is
restrictive – for example, causing more than 5 inches of water pressure drop.
Looks like an upgrade is on the cards....
The other problem can come about if the airflow meter hits its output
ceiling. You’ve done some mods to your engine and you find that the meter is
hitting its max output (eg 4.8V) nowhere near full load. The meter is measuring
as much air as it can, and after that it’s all a straight line – even when the
load rises further. Mix that output with bigger injectors and once the ceiling
has been reached, the injector output is just the same straight line. Not good.
Again it’s time for an airflow meter upgrade.
Swaps
The most common approach to airflow meter upgrades is to swap in a larger
airflow meter. Some cars have aftermarket big-bore designs specifically made for
them – but always at a high price – while Original Equipment makers often have a
family of airflow meter sizes. For example, in the Nissan family you can look at
2-litre turbo SR20DET airflow meters being upgraded to RB25DET 2.5 litre turbo
meters or even VH45DE V8 meters.
In fact, with the Nissan Maxima that was our guinea pig, we first looked at
doing just this sort of swap. Trouble is, unless you want to shell out big bucks
and have a lot of aggravation, it’s not nearly as easy as it looks.
First up, take the cost. Really big airflow meters are rare, so straightaway
you’re looking for something that lots of other people also want. And that means
only one thing – they’re expensive. Or maybe you don’t worry about going for a
‘name’ airflow meter, but just head off to the wreckers to see what big orphan
meter you can nab. Sounds fine until you’ve made your diameter measurements,
inspected the internals carefully, shelled out the cash, and then got it home.
Hmmmm, I wonder what all these pins are for?
Airflow meters use a variety of pin-outs and without (a), knowing exactly
what model the airflow meter is from and (b), having access to a decent workshop
manual, you’re not going to be able to easily find out. (And many workshop
manuals don’t even show the pin-outs of the airflow meter – they just tell you
the colour codes and then the wires disappear into the ECU. Which is signal,
which is power, which is earth, which is hot-wire burn-off....? It’s often quite
hard to work it all out – we’ve been there and tried that!) Plus you’ll often
also need to grab part of the loom if you’re to get the right matching plug –
and try getting a wrecker to cut a pristine loom...
In short, this option isn’t nearly as great as it’s cracked up to be. And
even when you’ve done all of the above, you’ll still usually need to have your
ECU remapped to suit the new airflow meter.
Modifications
Some people polish and modify the insides of the airflow meter, removing the
protective screens and smoothing the end result. And that’s fine if you want to
do a lot of work and don’t really want to know what the end result voltage
outputs really are. Certainly, if you’re going to modify an airflow meter,
you’ll want to check the air/fuel ratios before and after the mods – although it
must be said, we’ve had very good results from simple screen deletions. But if
you need to take the modifications further, we don’t think it’s a great
idea.
Bypasses
But what about a completely different approach? One that keeps the standard
factory airflow meter – complete with screens if you want – and so avoids all
those sourcing and wiring problems.
Think about it for a moment – the reasons that you would want to change an
airflow meter all comes back to one thing: too much air is being forced to flow
through the meter. You don’t want to reduce the airflow to the engine (cos then
you’ll also decrease power) so why not simply allow some of the engine’s airflow to come
from another source?
Does all the engine’s airflow
have to come through the airflow meter? The answer to that is simple: nope. In
fact, most hotwire airflow designs don’t measure all the air that’s flowing
through them. Instead they measure a proportion of that air and then output
a signal that takes into account the rest that’s also passing. For example, some
airflow meters use a tiny ‘side passage’ to actually sense the airflow and are
then calibrated on the basis that if this much air is passing down this
side passage, then this much air must be passing
through the meter as a whole.
So what actually is a bypass, then? Let’s start with an unmodified
system.
This diagram shows the basics of an airflow metered engine. Air passes
through the filter box, through the airflow meter and then into the engine.
Simple, huh?
Now we’ve added a bypass – a way in which filtered air can get to the engine
without passing through the MAF. Some air will flow through the MAF airflow
meter, and some will flow through the bypass. The result is that less air flows
through the meter (and remember, that was the problem – the meter was too small
for the amount of air that was flowing through it.)
How much air flows through the meter and how much flows through the bypass
depends on a bunch of factors including:
- How
restrictive the meter is at different loads
- How
big the bypass is
- The
way the plumbing is arranged (eg bends)
But (and this is a really important point): in all cases, the more air
flowing into the engine, the more air
that will always flow through the meter! So all that we need to do is to
recalibrate the meter so that the flow through the meter accurately reflects the
flow through the whole system – ie the total flow through both the bypass and the meter. (As you
can see, this system is quite like those airflow meters that internally measure
flow through one passage and then work it out for the total flow.)
But before we leave the plumbing diagrams, when taking the bypass approach
there’s nothing to stop you simply adding a new input tube with its own
filter...
...or ditching the factory airbox and using two new filters. In short, there’s
plenty of flexibility.
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If you decide to use two filters it’s important that both get dirty at the
same rate. If one filter is blocked faster than the other, the air/fuel ratio
will alter as the relationship between bypass flow and airflow meter flow
changes. In practice this means that two filters should be mounted within the
one fabricated airbox – or at least, close to one another. Filters should also
always be replaced in pairs.
However, if absolute mixture accuracy is required, it is best to use a single filter to feed
both the bypass and the airflow meter.
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Recalibrating the Airflow Meter
Output
Those of you who have ever left a hoseclamp loose between the airflow meter
and the engine will know how badly the engine then ran – so what’s he talking
about when he says you can have a big bypass feeding air around the airflow
meter?
The trick is in the recalibration – the airflow meter will have a lower
output at all loads, one that needs to be boosted until it matches what the ECU
expects to see (or at least, one that results in a suitable air/fuel ratio).
However, that’s not all: the airflow meter voltage needs to be increased
disproportionately at higher loads. This is because as the flow through the
system increases, more and more air will travel through the low restriction
bypass rather than through the airflow meter.
(It’s like two roads, one that has lots of speed humps and the other that is
smooth and flat. As the traffic increases, more and more traffic will head
through the smooth road as traffic bunches up over those speed humps.)
We’ve already covered the Silicon
Chip Digital Fuel Adjuster
and so here all we’ll say
is that the device allows the detailed and accurate adjustment of airflow meter
output voltages, in small increments both across the output range and also in
the up/down adjustments. (In this case, where the airflow meter output needs to
be boosted, the adjustments will all be in the ‘up’ direction.)
(If you haven’t read the background on the Digital Fuel Adjuster, the
following probably won’t make much sense. Spend the time and go look at the The Digital Fuel Adjuster, Part 1
article!)
This graph shows the actual recalibration data for the Maxima airflow meter
when equipped with a large diameter bypass (we’ll cover these mods in detail in
Part 2).
The range of the Digital Fuel Adjuster is set for 0-5V, which corresponds to
Load Sites 0-128. But as you can see on the bottom axis, we use only Load Sites
25 to 61 which in this car, correspond to the outputs from idle to full power.
Near idle, only minor adjustments are needed, but as load increases, so does the
need for more and more compensation. This is no big deal; the ECU is interested
only in the voltage coming in from the airflow meter and from its perspective,
everything is fine.
In fact even with the Digital Fuel Adjust in its shown coarse mode of up/down
adjustments, there is still enough resolution that the air/fuel ratio can be set
to mid-fourteens at idle and low loads, mid-thirteens at moderate acceleration,
mid-twelves at high loads, and high-elevens at full loads. (The car was run
without its oxygen sensor during this testing – using the closed loop control of
the oxy sensor would have made things even easier.)
The turbo Maxima starts, idles, accelerates and decelerates just like the
factory car. In fact, as we said, it even passed a Queensland roadside emissions testing
station when configured in this way.Yet when the intake restriction was measured
with and without the bypass operating, it was found that the total intake system
restriction (total, not just the component introduced by the airflow meter!) had
decreased from a full-load 20 inches of water to just 10 inches of water...
How we did it in detail – next
week...
The 350hp Airflow Meter
To put it another way: when equipped with
the big bypass, the Maxima’s 70mm airflow meter is now capable of accurately measuring a total airflow good for
probably 350hp. And this is the cheapest, accurate, 350hp airflow meter you ever
saw.
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Vane Airflow Meters?
We’ve discussed hotwire airflow meters throughout this article because that’s
the type that we fitted the bypass to in the prototype installation. However, we
can’t think of any reason why the technique wouldn’t also work with vane airflow
meters - it’s just that we haven’t tried it.
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The Background Story
So how did the Digital Fuel Adjuster come about? It’s worth briefly
backgrounding its genesis, if only so that you know what has gone into it.
The Digital Fuel Adjuster was developed and designed by Silicon Chip
[www.siliconchip.com.au]
electronics
magazine. The kit, along with many others, is covered in the Silicon Chip
publication – Performance Electronics for Cars – which is available from
newsagents in Australia and
New Zealand, or
online through the AutoSpeed shop. The book is a must-have for DIY modifiers.
The kit for the Digital Fuel Adjuster is available from Jaycar Electronics or through the AutoSpeed
shop.)
The electronics design and development of the Digital Fuel Adjuster was
carried out by the brilliant and modest electronics engineer John Clarke, while
I came up with the concept and did all the on-car development. (During this
period I wore a different hat to an AutoSpeed contributor, working for Silicon
Chip Publications as a freelance contributor to Performance Electronics for
Cars.)
The unusual mixture of ingredients – development funding from an electronics
magazine, design and development by an electronics guru with no interest in
modifying cars, and concept development and project management by me – has
resulted in something of which I am very proud. It’s a unique product that works
superbly and undercuts by a huge amount similar commercial products... none of
which, AFAIK, can do all that this one can, anyway!
So by no means should the Digital Fuel Adjuster be seen as an
AutoSpeed-developed project, but at the same time I am happy that AutoSpeed
endorses it and promotes it.
Julian Edgar
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