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Into the Intake - Part 3

The complete guide to modifying intakes.

By Michael Knowling

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In last week's Part 2 of Into the Intake we showed you how to select and install an aftermarket pod filter, create a revised airbox and site a cold air pick-up. This week, in Part 3, we'll travel a bit further downstream to the airflow meter and through the induction pipe...

Reducing Airflow Meter Restriction

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Many EFI systems have their engine load signal derived from an intake airflow meter. There are several types of airflow meters - vane, Karman Vortex and hot-wire. Without making complete replacements, Vane and Karman Vortex airflow meters are virtually impossible to modify for less restriction, but hot-wire meters - the most common - are easily enhanced.

As the name implies, a hot-wire airflow meter contains a very fine heated wire strung across the intake path. Immediately up and downstream of the hot-wire is a wire mesh screen, which serves to protect the hot-wire during servicing. From a flow point of view, these screens are a major hindrance. Depending on the diameter of the meter and the engine's power output, they can be responsible for about a quarter of the restriction through a factory intake; the screens typically cause half the restriction across the meter, and the meter itself is responsible for around half the overall intake restriction.

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Removing the airflow meter screens is usually simple: just prise out the circlip retainers and the screens can be plucked out - this will reduce the flow restriction through the meter. However, you need to realise that you are tampering with the device that measures intake air mass, so the meter's output may be altered a little. It's therefore a good idea to check mixtures before and after removing the screen(s).

You can remove only one screen if you don't want to take the full plunge.

Other meters can have fins polished or removed, but this level of modification is very likely to alter mixtures - though that's something that is often actually wanted as well.

No Airflow Meter?

Those EFI engines not equipped with an airflow meter derive their load signal from a MAP sensor. A MAP sensor is an air pressure sensor that's connected by a small-bore hose to the intake manifold after the throttle body.

MAP sensor type intakes pose zero airflow restriction and, therefore, cannot be improved upon in the flow stakes.

Induction Pipe to Throttle

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On naturally aspirated engines, the induction pipe that leads from the airbox to the throttle can be the cause of considerable flow restriction.

There are two common approaches to this problem:

  • Discard the induction pipe and relocate a pod filter near to the throttle
  • Revise the induction pipe

As you can imagine, removing the induction pipe and whacking a pod filter immediately close to the throttle gives the maximum possible airflow - there's pretty well nothing left to cause restriction. There are, however, some significant downsides to this route...

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Depending on you vehicle's under-bonnet layout, relocating the filter closer to the throttle can make heat shielding and cold-air ducting much more difficult. And, as we keep stressing, heat shielding and a cold air supply are vital for any pod filter installation.

Furthermore, you might find that the engine doesn't 'like' having the induction pipe removed; often, the length of the induction pipe comes factory-tuned to give optimal intake volume and velocity. Discarding or drastically shortening the induction pipe can reduce torque at various points of the rev range.

The aftermarket filter mounted to the throttle of pictured 1.5-litre F2 Daihatsu Charade, for example, generated less mid-range and top-end torque compared to using a longer induction pipe. In other words, the pressure drop caused by the longer induction pipe was more than compensated for by the affect of the tuned-length intake.

Look out - intake tuning isn't always as simple as it seems!

In a typical street application, revising the induction pipe (as opposed to putting the filter on the throttle body) makes a lot of sense. Have a good look at your existing induction pipe, paying particular attention to diameter and number of bends, sections of convolute, fittings with in-pipe roughness, and resonant chambers. If there are obvious restrictions - remove them!

Resonant chambers that are tee'd into the intake system usually cause little flow restriction - and have been placed there for the noise reduction and/or induction tuning benefits that they provide. Most times it makes sense to retain them.

However, tuning volumes where all of the induction air passes through the box often cause flow restriction at high power outputs, as there is a pressure drop across them. If a length of pipe can be easily substituted for the resonant box and the results quickly measured, it makes sense to take this path. We'd be wary of automatically replacing all resonant chambers without 'before' and 'after' testing.

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If the factory induction pipe diameter is inadequate, you may need to fabricate a replica using fatter tube. The neatest way to do this is to follow the design of the original item, but using steel pipe and mandrel bends. Weld sections of mandrel bends onto straight lengths of pipe as required, paying attention that the weld doesn't penetrate inside the pipe too much - any in-pipe roughness causes airflow restriction. A fitting for the engine idle air by-pass may also be required; simply drill a 20-25mm hole into the pipe wall and weld a short hose fitting into place.

(Don't be tempted to use plastic PVC pipe and fittings inside the engine bay - the temps are just too high, especially from heat-soak when the hot engine is turned off.)

Finding the Right Length...

When you're fabricating a completely new induction pipe it's advisable to undertake a bit of testing. To find the optimal induction pipe length - where there's the best balance of low, mid and high-rpm torque - you really need to use a chassis dyno or do some careful on-road stopwatch work.

Taping different sections of pipe end-to-end can be done to easily vary the length of the induction tube, allowing the running of back-to-back comparison tests. This is the only way to see where you're going with mods. You may find that an extra-long induction pipe works best - don't disbelieve the dyno or stopwatch if it says so. In these instances, you may need to insert a bend or two so the induction pipe can route its way around the engine bay.

Do whatever it takes.

More than likely, however, you'll find the factory induction pipe has adequate diameter and layout; in these instances, you can chop and change to remove the other sources of restriction.

Sections of convolute (which cause in-pipe turbulence and restriction) can be replaced with smooth rubber sections - but be careful. The purpose of these convolutes is to allow some flex between the engine and components fixed to the body (such as the airbox). Do not make all the intake plumbing rigid or you may break an airflow meter or your airbox.

Induction Pipe to the Turbo

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The induction pipe into the compressor of a turbo engine can be tackled the same way as the intake pipe to the throttle on a naturally aspirated engine, however resonant pulse tuning effects can be safely ignored. All that you need to do is to get the maximum amount of air to the mouth of the turbo with as little pressure drop and temp increase as possible.

You can discard the factory induction pipe and place an aftermarket pod filter at the compressor inlet (so long as you can keep the induction temp down!) or you can revise the existing hardware. Keep in mind that as we have mentioned previously, the performance of the filter at stopping the passage into the engine of small rocks and pebbles and birds is even more important when you have the fragile blades of a turbocharger ready and willing to meet them...

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In Part 4 of Into the Intake - the final article - we delve into post-turbo intake plumbing and the throttle body. Stay tuned...

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