Jamie’s HG Monaro GTS: shakin’ the streets with 598cid and 900bhp!
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Jamie’s HG Monaro GTS: shakin’ the streets with 598cid and 900bhp!

By MarkOastler - 25 December 2023

This big-block Monaro Pro-Streeter is packed with more cubes than a freezer full of ice-trays. We’re talking 598 cubic inches, or an implausible 9.8 litres. We’re also talking 900bhp or 671kW! Such staggering numbers validate Carroll Shelby’s old-school decree that there’s no substitute for cubic inches and Shannons Club member Jamie Claffey feels it every time he turns the key.

“Owning this car is one of those childhood dreams,” he says. “I’ve always been a huge Holden fan so it’s kind of like the Holy Grail to me. I always go for high performance over standard and that’s something that really attracted me to this car, given how much had been done to it.

“I’ve spent a lot of money building up some tough Holden 308s and I’ve owned a few toys over the years, mainly Commodores, but I just love this car because everything’s been done right.”

From any angle this tungsten-tough Monaro looks intimidating.
Image: Jamie Claffey/Jordan Leist Photography

Perth-based Jamie bought the Monaro from a mate, Brad Durtanovich, who steered its transformation from a tough but tired streeter to the steroid-stacked show-winner you see here.

Brad purchased the 1970 HG Monaro GTS from Karratha, about 1500km north of WA’s capital, more than a decade ago. It was originally a white six-cylinder GTS from Melbourne, which after finding its way to the west coast had already launched down the Pro-Street path with a small-block V8 and a pair of mini-tubs to run steamroller 15 x 12-inch rear rubber.

After arriving at its new home in Perth, Brad commissioned Jeff Clarke to build him a mild big-block, which effortlessly powered the Monaro for years until signs of wear-and-tear prompted him to take the plunge in 2020 and commit to a full ground-up rebuild.

Chevy big-block muscle looks right at home in its pristine engine bay. Check out those billet bonnet hinges!
Image: Jamie Claffey/Jordan Leist Photography

Neil Moneypenny at Xclusivefx was entrusted with the panels and paint, which required new front guards and a bonnet from Rare Parts, plus perfecting the rear-quarters. Neil’s flawless skin of gloss black provided an ideal backdrop for the naturally-aspirated Chevy big-block residing in a surgically clean engine bay.

The mountain of muscle that pokes through the bonnet’s enormous power bulge is the work of renowned WA engine builder Derek Paulik, who used the finest components to ensure it had the go to match the show.

Derek started with a bored and stroked Dart Big M block loaded with a Scat steel-billet crank, Scat H-beam conrods and Diamond high-compression forged pistons. A Crane solid roller-camshaft was paired with big-valve AFR Magnum 357 heads. MSD ignition was also given the nod, along with a large-capacity Brown’s aluminium radiator to keep a lid on temperatures.

Who needs a blower when you've got almost 600 cubic inches to play with.
Image: Jamie Claffey/Jordan Leist Photography

The big block’s thirst for high-octane E85 was quenched by a 100-litre fuel cell hand-crafted by Nelg’s Ali Mods. This was connected to an Aeromotive fuel system, feeding a pair of huge QFT 1050cfm Dominator E85 four-barrels on a towering Edelbrock tunnel-ram, polished to perfection by Wheelgleam. Big 2.0-inch Castle Headers and booming Mufflers-R-Us 3.5-inch full-length pipes completed the package.

Its staggering output of more than 900 horses was proven on C&R Motorsport Development’s engine dyno. On a rolling-road version, for its final tune-up at Blown Motorsports, that translated to an explosive 748hp at the rear tyres!

Since these shots were taken, Jamie has engaged Derek to revise the engine’s tuning, from its ferocious quarter-mile-focused peak power at high rpm to a more ‘street-able’ specification, with lower-compression pistons, hydraulic roller-cam and 750cfm carbs that runs on premium 98.

Wilwood discs at each corner provide ample stopping power.
Image: Jamie Claffey/Jordan Leist Photography

As a result, Jamie has happily sacrificed a fraction of its stupendous power for a big increase in torque, making it less peaky and nicer to drive on the street, which he does most weekends.

The driveline is just as tough as the monster it serves, comprising a Reid-case Powerglide with ratchet shifter and Dominator torque converter with 4500rpm stall. A robust Final Drive tailshaft connects with a Strange Engineering nine-inch diff centre loaded with 35-spline axles, Truetrac LSD and 3.5:1 gears.

Handling was also enhanced with a complete Southern Chassis Works front-end assembly, incorporating Viking coil-overs and the sharper response of rack and pinion steering.

Three-piece Weld rims allow the tailor-made offsets required to accommodate 12-inch-wide rear boots.
Image: Jamie Claffey/Jordan Leist Photography

You might be surprised that the Monaro has not been fitted with a typical Pro-Street four-link rear-end, having retained its original leaf-spring live axle. There are not even traction bars to control spring-wind-up. This is surprising perhaps, given the engine’s enormous output, but Jamie is adamant it’s more than capable of coping with it.

“You would not believe how straight this car runs with leaf springs,” he says. “I drive it around on street slicks and even with the 4500rpm stall speed there’s no axle tramp at all.”

There’s also a Wilwood braking system with a quartet of cross-drilled and slotted rotors. The 15-inch Weld Alumastar 2.0 wheels are fitted with Mickey Thompson ET rubber, which creates the classic Pro-Street stance and gets that fearful grunt to ground through 295/55s bulging under the tail.

The Monaro’s pillarless styling creates a bright and breezy interior that embraces its 1970 origins.
Image: Jamie Claffey/Jordan Leist Photography

Dicker’s Speed Shop is credited with pulling these elements together into the flawless finished product, which included a full rewire. And, with more help from Rare Parts, the supply of a complete GTS interior in contrasting Sandalwood, superbly installed by Nic from ANF Automotive Interiors.

The Monaro was completed just in time for Motorvation 35 in 2021, Perth’s premier annual modified car show, in which it secured a place in the prestigious Top 10 and won the Judges’ Choice award among other accolades. Jamie also earned a Top 10 place when the Monaro made another appearance at the 2023 event.

Not that we’re surprised by any of this, because the Monaro is like a magnet for onlookers whenever Jamie turns up at a car show or other public outing. He says he must allow a lot of extra time to answer a multitude of questions each time he pops the bonnet.

The Holden Monaro is an Aussie muscle car benchmark. This one takes it to another level.
Image: Jamie Claffey/Jordan Leist Photography

Which isn’t a bad thing, given Jamie’s recently opened his own shop under the banner of Pro Performance Parts. The Monaro is obviously a great way to promote PPP, particularly given that his new business was previously known as Dicker’s Speed Shop – yep, the same enterprise that played a key role in the car’s creation years ago! 

So, what goes around comes around. And in terms of the metamorphosis of this amazing Monaro, Shelby’s age-old claim about the importance of cubic inches has never been better demonstrated.

View Jamie's Shannons Club Garage and Connect with ClaffJamieClaffey