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Frank's Exhaust, Part 2

Fitment and results

by Julian Edgar

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At a glance...

  • New extractors and exhaust
  • On-road results
  • Dyno results
  • Back-pressure (non)results
  • Part 2 of a 2-part series
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Last week we covered some of the aspects we considered before selecting the exhaust components. This week we fit the new exhaust system to the EF six cylinder Falcon and then dyno and road test the results.


Extractors are tuned length pipes that individually collect the exhaust gases from each cylinder. The Jim Mock Motorsport (JMM) design that we selected is called by the company their ‘Race’ series. Also available for the EA – AU Falcon six cylinder engines are the ‘Street Fighter’ series. The Race headers use longer pipes – so how long then?

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We measured the six primaries as being 50, 40, 37, 42, 36 and 31cm long. The 3 > 2 >1 design uses secondaries that are 55 and 69cm long. The primaries are formed from 1½ inch mandrel bent tube and the secondaries are in 2 inch mandrel bent tube. (All tube diameters in this story are outside diameters.) The flange is laser-cut and is in 10mm plate. The normal cost of the Race extractors is AUD$645 but the company chose to supply them to us free of charge (at the same time we paid for a cam kit and LSD centre that will covered later in this series.)

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With one exception, the extractors look to be well made. The mandrel bends are good, the welding neat with good penetration, and the port matching at the flange excellent. The exception is where the primaries collectors join the secondaries. Here the secondaries have been welded inside the collector outlets, rather that the secondaries having their pipes flared and then being welded over the top of the collectors. The result is a nasty lip against which the gases must hit. If you have small enough fingers, you can feel this lip by inserting a finger through the oxygen sensor hole.

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As we covered last week, you can’t just look at a set of extractors and make judgement as to how good they’ll be on the car. Therefore, a major part of the selection process must be based on the company’s reputation. In the case of JMM, we have always been very impressed with the company’s modified SOHC 4-litre Falcons that we have driven for AutoSpeed. One good example is their former workshop ute – see Unlocking Falcon Performance.


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Initially we were going to use a completely custom system, perhaps using lots of large-bodied straight-through mufflers/resonators from other cars. Our local muffler shop, Mac’s Mufflers, has available a wide range of secondhand mufflers. With plenty of OE designs now being stainless steel, such an approach has longevity as well as price on its side. But the Falcon is very tight for space underneath – simply, there aren’t many mufflers from other cars that’ll fit. That’s especially the case if you want to use the largest bodied mufflers possible.

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We then looked at an aftermarket straight-through muffler that would fit under the Falcon, started discussing using pre-formed mandrel bends for the tightest parts of the exhaust (eg over the rear axle) – and then realised it would be cheaper and a lot easier to just use an off-the-shelf 2½ inch mandrel bent exhaust with straight-through mufflers! Mercury make exactly that animal so that’s what we went for.


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Rather than go for a hi-flow aftermarket cat, we elected to keep the cost down and selected a secondhand Commodore V8 cat converter. JMM and others have suggested to us that this cat flows extremely well – and of course we’ll find that out when we back-pressure test the exhaust! The flanged cat, from an HSV VS Commodore V8, uses 2.4-inch inlets and outlets – well, as measured by our digital calipers, anyway.

On the Road

The first impressions were good and bad.

‘Bad’ included a body knock from the rear part of the exhaust (easily fixed on a subsequent exhaust shop visit) and a droning resonance at 2500 rpm.

‘Good’ included a sweet note away from 2500 rpm – and bloody revolutionized engine performance! Above 4000 rpm the engine was just so much more free-spinning it was incredible. We’ve got a 5000 rpm shift-alarm in place (see Frequency Switch, Part 2) and prior to the exhaust and extractors being fitted, it was rare to ever hit it. Simply, at those revs the engine sounded strained and unresponsive – why would you want to go there? But with the extractors and zorst in place, the engine’s top-end was transformed. It certainly didn’t make the Falcon six any sweeter at high revs, but at least now it felt like it was developing real power!

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Throttle response was also clearly improved and jeez, the bottom-end torque! We’ve always exclaimed about the Falc’s sheer torque/kg ratio but with the extractor and exhaust, this was clearly better. In fact, far better. Because this series of stories isn’t being done in the same time-frame in which it appears, we’ve had months to experience the exhaust. And oh boy, did it ever improve bottom-end torque. Like, how easy was it to tow a loaded 6x4 trailer up an incline – in fifth gear at 1100 rpm....

But that drone! We headed back to the exhaust shop and had a straight-through resonator fitted immediately after the cat, but that basically dropped the resonance to 1900 rpm rather than eliminating it. The ressy didn’t make any apparent difference to performances – but neither did it get ride of the drone. Incidentally, Bob of Mac’s Mufflers was surprised at how noisy the car was. He’d previously fitted plenty of Falcon extractor/Mercury exhaust combos (including to his own shop ute) but none had sounded like this. The difference was in the Mock extractors – not only did they change the resonant behaviour of the system but also, because of the length of the secondaries, pushed back the available space for mufflers. We emailed Mock Motorsport (“How do you shut up these extractors?”) and back came a quite reasonable answer – “Buy our complete exhaust!”

We ran a stop-watched 0-100 km/h and found a best time of 7.5 seconds – about 0.5 seconds faster than standard and very much matching our seat-of-the-pants impression. Exhaust drone aside, we love this upgrade – our more specifically, we love these extractors. With the Mock exhaust (which we’ve previously experienced to be quiet), this is a must-have upgrade.

Ah, Exhaust Back-Pressure...

Exhaust back-pressure – hmmmmm. Here we ran into a problem. Last week we measured total exhaust backpressure at the oxygen sensor point on the manifold. We unscrewed the oxy sensor, screwed in a pressure tap and ran the hose to the pressure gauge. And read off a peak of 8.7 psi – not good but certainly not bad for a standard system. But with the fitment of the extractors, the oxy sensor is no longer placed at the outlet of all six pipes – instead it monitors only one of the secondaries (ie 3 pipes). But what’s the problem in that – after all, it’s what’s going on after the tapping point which will determine backpressure.

Or is it?

Last week we said: “ doesn’t make sense to put a pressure measuring fitting on one pipe in a set of extractors and measure back-pressure. (If you were going to measure anything, it would be the size and timing of the pressure pulses running up and down the pipe – and for that you need to use an electronic pressure transducer and high speed data logging.)” And that’s exactly what a pressure reading taken from the oxy sensor port on one of the secondaries showed – a wildly fluctuating pressure as the pulses ran up and down the pipe. By heavily damping the pressure reading we were able to measure a peak of 5.8 psi, but is this exhaust backpressure? Not really – it’s a damped average of the pressures pulses rushing around!

So why not put the exhaust tapping further down the exhaust pipe? Sure, but where? The tuned length system extends as far as the cat, so it would make sense to measure backpressure immediately after the cat. But then the pressure reading doesn’t take into account cat flow – and the cat is likely to be the most restrictive part of the exhaust! So what about a pressure reading just in front of the cat? Well, again we’d expect to see a helluva lot of pressure variations as the high pressure pulses arrive and are then reflected. Perhaps if you installed a big damping chamber in front of the cat you’d get an accurate back-pressure reading – but to be honest, we just don’t know.

On the Dyno

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So the on-road performance results were exemplary – but what would a rear-wheel power figure show on ChipTorque’s dyno? Well, interestingly enough, not as much as we’d expected. Peak power went from the 134kW we’d got with the cold-air intake to 142kW with the extractors and exhaust – just a 6 per cent gain. (Incidentally, note that the better shape of the power curve at about 2500 rpm is an artefact of the changed dyno control system.)

But how could just 6 per cent make such a big on-road difference? We discussed this with ChipTorque’s Lachlan Riddel and he made a very good point. We’d been telling him the story of the torque increase (as clearly evidenced by the loaded 6x4 trailer and 5th gear at 1100 rpm) and he pointed out that the dyno doesn’t show the part-throttle torque gain – and in fact, that testing for this on a dyno is very difficult. We’d suggest that the Falcon’s part-throttle low-rpm torque is significantly increased, even though the dyno doesn’t show this.


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Without making lots of comparisons of different extractor designs, it’s impossible to say that the Mock design is the best. But with their small diameter runners and long primary and secondary lengths, they appear to work extremely well on the road. However, equally clearly these extractors demand a lot in noise suppression in the rest of the exhaust. Cruise at 100 km/h with the windows up and the combination of Mock extractors and Mercury exhaust is pretty much unacceptable. (Lower a window and much of the cabin resonance disappears.) We’ve got a plan for solving the resonance but in the mean time, we’re just enjoying that combination of bottom-end torque and top-end power....

Engine Management?

Lachlan Riddel made an interesting point about the combination of the Falcon modifications (ie cold air intake, extractors, exhaust) and the standard engine management.

“With the car running on normal unleaded,” he said, “I couldn’t get any more power out of it with a chip.”

In other words, the current air/fuel ratio (with the mods, back from about 11:1 to 12.2:1 at high loads) and the factory ignition timing are optimally suited to 91 RON fuel. This is an important thing to know if you’re doing a Falcon like this one and intend running on the cheapest fuel – don’t bother getting engine management changes at this stage of the process.

However, when we slot in the new cam and run on 98 RON, then there’ll be a need for engine management changes!

The extractors were supplied courtesy of Jim Mock Motorsport

The dyno run was courtesy of ChipTorque

The Mercury exhaust and exhaust fitting were by Mac’s Mufflers at normal commercial rates


Mac’s Mufflers – Helensvale, Gold Coast - (07) 5529 4399

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