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Airflow Meter Bypass, Part 1

You've heard of heart bypasses, now try an airflow meter bypass.

by Julian Edgar

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At a glance...

  • Forget swapping the airflow meter for a bigger one...
  • Instead, give any airflow meter more flow capacity
  • Also gives you the ability to tune the air/fuel ratio
  • Part 1 of a 2-part series
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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.

  • Flow
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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.)

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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....

  • Voltage

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.


Click for larger image

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?

Click for larger image

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.


Click for larger image

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.


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.

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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?

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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.)

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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...

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...or ditching the factory airbox and using two new filters. In short, there’s plenty of flexibility.

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.

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!)

Click for larger image

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.

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.

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 [] 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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