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Supa Charging Sigma

A show-winning Sigma that looks surprising stock from the outside - but packs the punch of a supercharged V6!

Words by Michael Knowling, Pix by Brian White

Click on pics to view larger images


A high performance Sigma is something of a rarity these days. Sure, there are a couple of hottie GH Sigma turbos still running around but unless the owners have ditched the factory draw-through turbo set-up, it's pretty hard for them to make serious power. Joe DeFaria of Perth, Western Australia started off walking down the forced induction Astron motor path, though today his Sigma now bears little resemblance to "the rest".

His show-winning beast is presently stuffed with, of all things, a 3.8 litre Buick V6; and, not only that, it's the positive displacement supercharged version that came fitted to US-market Pontiacs! Yep. We're talking full-blown (literally) big-cube grunt stuffed into a medium-sized Japanese/Australian shell.

Boosted 1.6

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First, though, let's rewind to more than 15 years ago when this poor unsuspecting Mitsu was just starting to flourish into a modified streeter. With barely two years of new car ownership behind him, Joe hit his '82 GJ Sigma's 1.6 litre donk with some hot-up gear - namely, a draw-through S82 Sprintex supercharger and 1?-inch Stromberg carby. Internally, the 1.6 motor was left stock - it was deemed tough enough to handle some light boost. Going the blower gave a significant hike in driving pleasure without much of a fuel consumption penalty. As per usual, though, it wasn't long before the horsepower was no longer enough to stir Joe's blood.

Bump Up the Capacity

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The logical progression was to move up a notch to the 2-litre version of the same Mitsubishi Astron four. This engine was installed into the bay together with an upsized exhaust system - 2?-inch. With this conversion complete, however, Joe still wasn't entirely happy with the result. Just like the ol' 1.6-litre motor, the 2.0 had quite a bit of lag - Joe says it was due to the extra-long path that the fuel had to take before arriving in the combustion chambers. It didn't help that the ignition timing had to be locked (retarded) to prevent up-top detonation.

Carb Conversion

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Out to seek outside advice, Joe was informed by a "guru" mechanic that he should swap over to a blow-through turbo system (as fitted to such cars as the G11R Daihatsu Charade turbo). It was an idea that seemed reasonable in theory. Willing to accept the grease monkey's advice and hand over a fair number of readies, Joe made the switch to a pressurised side-draft Weber.

This turned out to be a bad move.

B-i-g problems were had whenever the throttle valve was snapped shut under boost - the supercharger kept wanting to force in pressure. Not good. After a bit of pain-in-the-neck mucking around trying to fix the situation, Joe resorted back to sucking through the Weber. A familiar - but laggy - arrangement.

Show Me Yours

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With this mechanical action all taking place, Joe had got his Sigma fully resprayed in Impact Red two-pack by B&B Smash Repairs. This was a proper $5000-6000 exercise. A rear spoiler, 15-inch Simmons B45s and lowered suspension had also gone on to add to the subtle - but truly immaculate - appearance. Accordingly, Joe had now started to (very successfully) dabble the Sigma in the car show scene - where he was frequently being advised to go for EFI.

After hearing it so many times, he did.

Lethal Injection

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The initial injection system that Joe tried on the 2.0 motor was MoTeC 2-dimensional - an early banger. Tuned with some fairly large injectors being fed by a VL Commodore pump, the injection system got rid of the lag problem - but there weren't all that many horses on tap either. Three custom intake manifolds were tested but - as it turned out - it was actually the mufflers that were causing a lot of the flow restriction. These were swapped to straight-through MagnaFlows.

Still, it wasn't long before Joe reckoned it was time to upgrade capacity again.

Fatten It Out a Bit More

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This time, Joe went to the 'big block' 2.6-litre Mitsubishi Astron. Unlike the previous motors, the 2.6 was built up with a full balance, Ivan Tighe cam, ported head with bigger valves and gapless rings on stock-spec pistons. This package, not surprisingly, called for a bigger blower - a Sprintex S102. Boost was set to a substantial 15 psi, blowing through a front-mount intercooler. Oh, and the programmable management system was also upgraded to a MoTeC M4.

As you might imagine, this set-up gave quite a bit more right foot enjoyment.

While the 2.6 was being done, the rest of the Sigma was also becoming an increasingly impressive show car. The engine bay copped extensive colour coding, polished stainless steel and chrome accessories. Indoors, the standard front and rear seats, door trims and hoodlining were all reupholstered, and an American "lush pile" carpet covered the floors. This was the work of Coastal Tint and Trim. Complementing this is a chrome-centre SAAS wooden steering wheel and a butt-kicking sound system. Joe pushes music through his ears using dual Kenwood 10-inch subs, two Kenwood amps, Kenwood full-range speakers and a Pioneer 12 stacker up the front.

This - plus a whole lot of other detailing work - earned Joe a large swag of trophies at shows.

Unfortunately, though, the seriously hyped-up 2.6 turned out to be a pretty temporary engine. The 15 psi boosted motor lasted only four months (with "minimal punishment") before it broke a ring land.

Hang It - Go the Supercharged V6!

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Faced with the option of a rebuild - which wasn't going to be cheap if it was done right - Joe went well-away from the Astron tinkering that he'd grown so accustomed to. His choice to grab a supercharged Buick 3.8 is very understandable given the fact that he could get one new for only $3750. According to Joe, it's very similar to the blown engine that's inserted into the local VT-series Commodore.

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The marriage between the V6 and the nose of the Sigma was a surprisingly simple one. Joe made up some custom engine mounts (which went onto the unmodified Sigma cross member) and relocated the heater outlet by 80mm. Probably the most challenging part was modifying the front swaybar for clearance against the harmonic balancer. Cooling for the big motor required a VS Commodore radiator teamed with a large Davies Craig electric fan. The engine management side of things was taken care of by a Holden Delco system, which has been remapped to suit the forced induction application. A standard Holden coil pack has been a-fitted along with a Holden airbox to pick up induction air.

Then - with the mechanical engine bay work done - Joe set about applying some of his now trademark attention to detail. He tidied up the wiring, put in just the right balance of colour coding, braided lines and fabricated some elegant stainless steel trim pieces - like a radiator cover and coolant overflow reservoir. Also note his work-of-art stainless tower bar.

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On the exhaust side, a pair of bolt-on Walker extractors needed slight modification to suit the Sigma chassis, and they then flow into a 2?-inch stainless system with a MagnaFlow resonator and muffler.

Behind the blown six, Joe elected to install an ex-Holden VN Commodore T5 manual and its standard clutch. Together with custom mounts, a hydraulic actuation system also found its way in. A modified Sigma tailshaft passes torque into a Ford Borg Warner 3.5:1 LSD. Interestingly, the Sigma shaft was fatter than the Ford item. To help compensate for the extra go of the Buick big banger, the standard rear drums have recently been replaced by Sigma discs. The rest of the braking system remains unchanged.

In all, Joe's pretty much over the moon with how well the V6 conversion has turned out. "It looks like a near-factory fit and it's great to drive," he smiles. Not surprisingly, torque is the keyword. On a Dyno Dynamics chassis dyno, the V6 has put out an easy 233 horsepower at the rear Kelly Chargers. And that was on only 5.5 psi boost!

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Despite still being driven every day, Joe somehow manages to keep his Sigma immaculately detailed inside and out and regularly enters it in car shows; and it's been well worth it. The car won an incredible six trophies at the 2000 Motorvation show (with the V6 j-u-s-t fitted in time) and went on to take out Top Engine Bay and Sedan at this year's event. Not bad considering the huge number of cars that were entered; Joe says the success of his Sigma really got up the nose of some competing Holden and Ford V8 owners!

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For now, future plans include increasing the boost to around 10 psi; but, after that, Joe wants to call it quits. No more spending money. Quite rightly, he just wants to get on with enjoying the car as-is. Certainly, not too many people can claim to have spent around $40,000 building up a Sigma...

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