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Awakening the Aussie Ford Six - Part Two

We speak with Jim Mock Motorsport about modification of the Ford 3.9/4.0 straight six...

Words by Michael Knowling, Pix by Julian Edgar

Click on pics to view larger images

At a glance...

  • Last of a two-part series on SOHC Falcon sixes
  • Modification guidance from Ford tuning experts
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In Part One of this series we looked at the differences in engines used from the EA to AU Falcon. Jim and Brendan Mock from J.M.M. (Jim Mock Motorsport) suggested the best platforms for tuning are the EF and EL Falcon as well as the Tickford XR6. However, since all of the good bits are interchangeable, it doesn’t matter too much if you’re starting with a different model.

Now let’s look at approaches to modification...

Exhaust and Headers

Jim Mock says the best place to start modification on a Falcon six is the exhaust system.

"The standard exhaust system on the Falcon ranges between about 1¾ to 2 inches diameter for most of its length," says Jim. The XR6 exhaust system flows better than a normal Falcon exhaust but it should still be the first area of aftermarket development.

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J.M.M. offers a number of off-the-shelf Falcon upgrades (which we’ll come to) and all use a collector-back 2½ inch mandrel bent pipe. Jim suggests there’s no need to go bigger than 2½ inch for a street/strip application.

"On a car making 170kW at the wheels our full-length exhaust causes only a 2kW restriction compared to having no exhaust at all," says Jim. Depending on the components used, a cat-back system generally gives about 8 or 10kW at the wheels.

Note that the Falcon is also very sensitive to muffler selection - resonances and excessive noise are common in custom exhaust fabrications. J.M.M.’s exhaust systems are developed to avoid this.

And what about the exhaust manifold, you ask?

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"When doing an exhaust upgrade you should replace the factory exhaust manifold at the same time," says Jim. "The standard 2-piece manifold is pretty basic and a good set of scavenging headers will make a tremendous improvement.

"We recently did a DEV 1 upgrade (comprising an exhaust, headers, spark plugs and a retune) on an EF Futura and power went from 95 to 133kW at the wheels on our Dyno Dynamics dyno. But that was exceptional – most cars make about 125kW."

Air Intake

Once the exhaust system has been upgraded, the air intake is probably the next modification on most people’s list.

"The standard intake is not very restrictive near the factory power level but it’s an easy and cost-effective area to modify," says Jim. J.M.M.’s kits involve converting to a large pod filter mounted inside the factory airbox. We’re told that a drop-in aftermarket panel filter is unlikely to increase power.

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Interestingly, Ford also increased the length of the bell-mouthed snout inside the airbox lid of late model Falcons. This helps reduce induction noise but, since the diameter of the snout tapers along its length, there’s more restriction than earlier ‘short snout’ lids.

EB era air boxes are quite different to those on later models. Induction air enters via the top of the airbox and Brendan Mock says you’ll get a guaranteed power increase by removing the lid. The only downside is engine bay heat.

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Contrary to popular opinion, J.M.M. says the EF Falcon’s divided passage induction pipe between the airbox and throttle gives more power than the larger, single-bore pipe used on the EL and AU. A fabricated 3 inch mandrel pipe is necessary only for engines making more than 140kW at the rear wheels. Gains also depend on the type of headers and exhaust system used.

The factory throttle body – which remained the same across from EA to AU – doesn’t cause enough restriction to justify modification.

Intake Manifold

The non-variable intake manifold fitted to the MPFI EA to ED Falcon is decent, but the dual-stage manifold introduced in the EF is a quantum leap.

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If you own a pre-EF Falcon, the dual-stage intake manifold will bolt onto the head along with a few other incidental changes. The pre-throttle intake set-up must also be revised. The biggest issue is obtaining the appropriate ECU that’s mapped to suit the new manifold and has an output to control the switch-over valve.

Jim says you need an EL Falcon computer because it’s the only model combining the dual-stage manifold with a distributor-type ignition – the EF and AU computers are designed to run a coil-pack ignition system. We’re told the new computer is a straight plug-in but there is a small amount of wiring work required.

And are there any gains in altering the manifold’s change-over point? No. Ford got this spot-on with a switch-over point at around 3800 rpm.

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Brendan Mock says the dual-stage intake manifold is good for more than 170kW at the wheels before it should be ported. "There’s a lot of turbulence inside the manifold that can be heard when it’s on a flow bench," he says. "Once it has been ported you can hardly hear anything – airflow is much smoother."


Once you’ve done the easy bolt-ons your next move should be a new camshaft. This applies even if you have a XR6, which already has a more aggressive cam than the everyday Falcon.

Jim says the standard ECU (which uses a MAP sensor) doesn’t like being used with long duration cam profiles. "For the best result you need the greatest area under lift that physics allow," he says.

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A more aggressive camshaft will improve high rpm breathing but the effective rev range is limited by the flow limit of the SOHC head. "The EF Falcon’s 5700 rpm limiter corresponds to where the head flow drops off," says Jim.

Porting helps this situation but, really, the short-revving nature of the engine cannot be overcome. Not with the SOHC head, anyway...

Depending on the specs of the camshaft, it’s likely that a firmer set of valve springs will be required to avoid high rpm valve bounce.

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Note that the XR6 camshaft can be slotted into a conventional Falcon engine. And, although it’s claimed the XR6 engine uses firmer valve springs, J.M.M. says there’s no measurable difference in spring tension.

Don’t bother trying to alter the cam timing while retaining the factory camshaft – you won’t pick up any more power.

Cylinder Head

As a generalisation, the flow capacity of the Ford cylinder head gradually improved with each model. The exceptions to this are the ED (which appears to use the same head as the EBII) and the AU (which use relatively small ports).

Jim says the EA head should be avoided - "there are weaknesses in the casting, design faults and flow problems." The EB was slightly improved and the EBII was better again.

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The EF/EL head uses nice large ports and runners and improved combustion chamber design. Be aware, however, some ELs were released with hybrid engines using an AU-spec small-port head.

By far, the best head is that fitted to the EF/early EL. "The EF head makes good power in standard form and, when you decide to go further, they’re easier to port," says Jim.

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Valve sizes went unchanged except the EF XR6 and AU received 2mm larger exhaust valves. Inlet valve size cannot be increased due to space limitations. Regardless, Jim says there’s no gain in fitting bigger valves unless the engine is already modified in a big way.

Compression Ratio

Since the EA was introduced with an 8.8:1 compression ratio, the Falcon has received more compression in almost every model.

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But Jim says there’s a big difference between the claimed compression ratio and the actual (measured) compression ratio. For example, the AU engine is supposed to have a 9.6:1 compression ratio but J.M.M.’s measurements reveal its closer to 9.35:1...

Either way, the AU’s factory compression ratio is very high for design of the engine.

Jim says their in-house performance-built engines usually go out the door with a compression ratio of less than 9.55:1 – and that’s an engine built for premium unleaded only! For use with normal unleaded, Jim prefers to keep to around 9.2:1. This can vary depending on ignition timing strategies and the other modifications made to the engine.


The bottom-end of the Ford 3.9/4.0 is plenty strong.

From EF onward, the 4.0 uses hypereutectic pistons, which enables tighter clearances and improved durability. AU Falcon pistons use a shorter skirt and are lighter. The 3.9 EA and EB use cast aluminium pistons which aren’t as strong as those used in later models, but they are nevertheless tough.

Rods are durable, the crankshafts are not prone to failure and the blocks are rigid enough to make in excess of 300kW at the back wheels (so long as there is no detonation present).

Engine Management

EEC-IV engine management systems were used on the Ford six until the release of the EF Falcon. We’re told EF-onward Falcons use an EEC-V system featuring a knock sensor for more efficient running.

Interestingly, the knock sensor strategy varies across different models.

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In EF and EL Falcons, the knock sensor is used primarily as a failsafe for when you encounter bad fuel. The knock sensor is relatively inactive and, when necessary, ignition timing is progressively retarded.

According to J.M.M., the knock sensor is much more active in the AU Falcon. The ECU is constantly adding and removing ignition timing under direction of the knock sensor. Unfortunately, the ECU is quite slow and the knock sensor is prone to picking up engine noise.

"The AU computer also takes out 10 degrees of timing in one hit. The AU is prone to detonate with its high compression ratio – especially with a bad tank of fuel – and you can feel the knock sensor pulling out big chunks of power," says Brendan Mock.

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Note that in Falcons with a distributor (all models except EF and AU), it is easy to rotate the dizzy for a couple of extra degrees of advance – so long as you run the engine on high-octane unleaded. Early models with low compression will tolerate considerable advance, but EL Falcons (at about 9.3:1) are relatively limited.

J.M.M.’s in-house Falcon kits incorporate an adjustable fuel pressure regulator in conjunction with an unaltered management system. Their approach has been to make mechanical alterations that are well suited to the management system – not to modify the management system to suit.

An interceptor (such as a UniChip) can be fitted to the Falcon and Jim says it’s possible to pick up improvements in power and response. The amount gained depends on the factory program - for example, the AU XR6 VCT program is apparently pretty bad so it’s possible to make sizeable gains with an interceptor.

Driveline Strength?

Once you’ve modified a Falcon to go hard you’re unlikely to encounter any driveline problems.

The factory automatic and 5 speed manual gearbox is very strong. Even the standard clutch can cope with an engine making around 160kW at the wheels. The tailshaft, axles, universal joints and bearings are all up to the task.

Improved suspension, brakes and performance wheels and tyres are recommended.

Power and Performance

So how much power can a modified Falcon make in comparison to a stocker?

Well, as a guide, standard EA MPFI 3.9 makes about 88kW at the wheels and you can expect 95 – 98kW ATW in an EF to AU Falcon (including XR6s). The AU VCT XR6 makes about 115 – 118kW.

J.M.M.’s entry-level upgrade (DEV 1) provides more than 112kW at the wheels using a 2 ½ inch exhaust, headers, new spark plugs and tune. This retails for AUD$1190.

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DEV 2 adds an air intake, new leads, cam upgrade with adjustable gear and valvetrain mods. This delivers 125kW ATW at a cost of AUD$2230.

DEV 3 – at AUD$2450 - comprises a more aggressive camshaft for in excess of 134kW ATW.

A high lift version of the DEV 3 (DEV 3HL) takes the cam about as far as you’d want to go without performing head work. At AUD$2750 this gives more than 146kW at the treads.

DEV 4 involves cylinder head reconditioning and porting together with more compression. The price steps up to AUD$4900 but you get more than 160kW ATW.

DEV 5 adds more porting and compression to achieve 170kW ATW at a cost of AUD$5500. For slightly more power – 175kW at the wheels – you can buy the AUD$6700 DEV 5a kit which includes special valve springs.

The ultimate naturally aspirated development – DEV 6 – is a custom rebuild with price on application.

Note that ultra-efficient Race Series headers and other options are also available – call J.M.M. for details or visit their website (listed below).

In terms of straight-line performance, J.M.M. has a customer car with a DEV 5 kit that runs 14.6 second ETs – with an auto trans and full sound system. Another vehicle with the same kit has run 14.04 seconds thanks to a manual gearbox, short ratio LSD and grippy tyres.

So they’re certainly not slow!

When to Go Forced Induction?

In this series we have focussed on the naturally aspirated approach to performance. But when should you opt for a blower?

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Brendan Mock (who owns a supercharged EL XR6) suggests a blower is your best bet if you want more than about 220kW at the wheels. A supercharger kit along with a suitably rebuilt engine is the best way to go if you need all-out power.

Jim recommends avoiding superchargers mainly due to legal reasons. There are also servicing and engine life implications.


J.M.M. (Jim Mock Motorsport) +61 3 9399 4401

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