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Negative Boost Revisited, Part 5

Fixing the intake system

by Julian Edgar

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At a glance...

  • Last in a 5-part series
  • New throttle body duct
  • New airbox exit duct
  • New intake snorkel
  • New inlet duct to snorkel
  • The results!
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Last week in Part 4 (see Negative Boost Revisited, Part 4) we showed you exactly where negative boosts (ie flow restrictions) existed in the EF Falcon’s intake system. And now it’s time to get rid of as many pesky negative boost critters as possible.

But first a brief recap.

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We found that the chief flow restriction is the airbox exit – over the few centimetres between the inside of the airbox and the outlet duct, there’s a pressure drop of 6 inches of water - that’s nearly 38 per cent of the total intake restriction! Next up on the smelly list is the flow restriction to the airbox intake snorkel. The air reaches the intake snorkel through the gap between the bonnet locking platform and the leading edge of the bonnet and this intake has a pressure drop of 3.2 inches of water. Not far behind in terms of restriction are the dual ducts that connect the airbox to the throttle.

In fact, just about the only things that can stay unchanged are the airbox and filter. The above graph shows that swapping the standard air filter element for another better flowing one would achieve almost nothing in terms of improvement. After all, of the total 16 inches of water pressure drop caused by the intake system, just 1 inch was being caused by the filter...

The Modifications

So where to begin? We started by working backwards from the throttle body.

  • Throttle Duct

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Gone was this dual duct, replaced with....

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...the intake duct from a BA Falcon. The BA intake uses a single, much larger diameter tube, with a resonant chamber tee’d off it. The connection to the throttle body is normally maintained by a spiral spring that slips around the ruber trunking, but to improve this seal we used one of the original hose-clamps from the EF plumbing. The BA Falcon duct cost $35 from a wrecker.

Time: 5 minutes Cost: $35

  • Airbox Exit Duct

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The BA intake stopped about 75mm short of the original airbox, but that was no problem because the original inside airbox pick-up was replaced with...

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...this much larger duct. The original was unbolted and a new duct made from 75mm plastic stormwater pipe. The end was heated over a hot plate and then forced down over a small upturned bowl, flaring it into a good looking bellmouth. Using a sanding roll held in an electric drill, the hole through the airbox was enlarged slightly until the new pipe was a tight push-fit. The pipe was cut to length to provide a good match with the BA Falcon intake duct and then the PVC pipe was sanded smooth with wet-and-dry and painted black with a spray can. Cheap and easy!

Time: 60 minutes Cost: $8 (for the paint – already had the pipe)

  • Airbox Intake Snorkel

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This standard intake snorkel to the airbox was replaced with...

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... another factory Ford part, this snorkel being from a Tickford EL GT (Ford part number EFTVE 9A624A). We bought this part on eBay for $45 plus freight. The intake mouth is much bigger than stock and the body of the snorkel also is a little larger. The factory rubber bonnet seal fits nicely around the new snorkel which screws straight into place.

Time: 5 minutes Cost: $55

  • Bonnet Feed Duct

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But what about this small bonnet gap that feeds the airbox intake snorkel – one that we figure is located in an aerodynamically low pressure area? Well, the easiest way of overcoming that is to connect the airbox intake snorkel to a high pressure area – like the one existing in front of the radiator.

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This new flow path was easily achieved by installing a new 75mm diameter plastic pipe from adjacent the mouth of the intake snorkel down through two plastic panels to the area in front of the radiator. Heating and bending the plastic pipe gave a nice finish – easily achieved with a heat gun or electric stove and some simple hand tools. To clear some of the obstructions on the way, the pipe was also dented slightly – again by using a heat gun and a tool to form the correct shape.

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A forward-facing elbow was placed at the base of this duct, with the mouth flared with the heat-and-bend technique. The elbow is optional – just connecting the pipe to the area in front of the radiator will allow it to pick-up high pressure air (see Eliminating Negative Boost - Part 5 for how this approach was followed on an Audi S4.)

Time: 90 minutes Cost: $8


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Previously, we’d recorded an intake system pressure drop of 16 inches of water – that included the snorkel, airbox, air filter and filter-to-throttle duct. As was stated earlier in this series, 16 inches of water pressure drop at full power is a pretty good result for a standard intake system, in turn making it harder to create improvements.

But with the modified intake system, that full power pressure drop was reduced to 9 inches of water – an improvement of 44 per cent!

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And we’re talking only about $100 and under 3 hours for all the intake mods. We also haven’t diminished the factory quality of air filtration and the intake air temp has not been increased – in fact, it’s now almost certainly cooler than previously. For those who don't know Falcon engine bays, the underbonnet view is also close to stock.

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Because the reduced intake restriction depends to some extent on the fact that the car is moving through air, we were uncertain as to whether to bother doing a dyno run. But since we needed a baseline for the later mods, we put the Falcon on ChipTorque’s dyno anyway. And even in the dyno room (which no way has the characteristics of on-road airflow) we still saw a distinctive and repeatable gain of 3kW at the rear wheels.

On the road the engine now revs out more freely above 4000 rpm and the throttle response at all revs is clearly sharper.

For under 3 hours of time and $110, success!

The dyno run was performed courtesy of ChipTorque.

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