615ci Pro Street Holden HK Monaro – PRO HK

A Holden HK GTS Monaro that kicks butt on the dyno, in Summernats judging and at Drag Challenge

Photographers: Chris Thorogood

BRETT Armstrong is well known to Street Machine, with his HK Monaro earning a swag of tinware at Summernats 32, followed by a strong showing at Drag Challenge Weekend 2019. Brett bought the car just six months before Summernats, taking the reins from previous owner and builder John Mawhinney (whose 1000hp blown ’57 Chev we featured in September 2015). John had just run a sub-nine pass in the HK despite suffering a misfire, but the bonnet had flipped open, damaging the engine bay and roof (see more below).

This article was first published in the November 2019 issue of Street Machine

Brett wasted no time getting the damage sorted. “I got a steel bonnet for it and fixed the paint,” he says. “But the miss was still there. We weren’t 100 per cent sure if the engine was hurt. I was just hoping it was an electrical fault or something.” About six weeks out from Summernats, a leakdown test saw two cylinders a fair way down on compression, so Brett ordered eight pistons from the US and madly scrambled to have them installed before the Christmas knock-off.

Astoundingly, Brett didn’t even have time to dyno the rebuilt motor before heading off to Canberra: “We just did a few laps and didn’t give it a hard time.” The hard time was instead saved for the Summernats Horsepower Heroes dyno comp. “We were the last car to qualify on Friday and rolled 939hp; a bit more than I was expecting. We went out that night and celebrated pretty heavy,” Brett laughs.

The next day, the Monaro yielded a stonking 962 horses at the hubs, around 150 more than the next-best competitor in the Eight-Cylinder Naturally Aspirated class. Even better, the HK was also crowned Tuff Street Champion!

With his successful Summernats campaign over, Brett pressed PROHK into drag racing duties after the ‘quick freshen-up’ on his Nova went overtime. “I’ve been racing the Kenda Drag Radial Series for a long time in various cars, so I thought: ‘Let’s race it and see what it does,’” he says. PROHK ran pretty sweet over the eighth-mile, delivering a 5.62 first time out.

Not one to shy away from a lark, Brett decided to level-up and attack Street Machine Drag Challenge Weekend, making the call only a month out from the event. “My mate Scott Cortina had to make a custom towbar to clear the exhaust, and also put his hand up to be my wingman,” says Brett. Jai Schluter modified a trailer to suit the purpose and tagged along, with Scott’s brother Paul completing PROHK’s support team.

“Before we left, we pulled therocker gear off to re-tension the cylinder heads and found that it had a broken spring,” Brett says. “We only had two weeks to get that sorted; we ordered a set on the Saturday, they arrived Tuesday week, then we fitted them up and the following weekend we were racing.”

The team’s first DCW pass at Willowbank netted an 8.82@155mph, a PB for the Monaro’s quarter-mile terminal speed. “A new diff centre from Western Suburbs Differential probably helped,” Brett says. “Going from a 3.7:1 to 3.5 doesn’t sound like much, but it makes the difference over the quarter.”

The PB was cause for celebration, but Brett had concerns about the ongoing issue of the misfire. “Jai and Scotty were checking over the car, revving me up to go again as the rain was looming,” he says. “We did an 8.78@150mph, this time PBing the quarter-mile.”

Despite that success, the Monaro had a habit of melting plugs on big runs, an issue connected to the misfire and not one they’d been able to resolve. “It was only later I saw that I’d melted a plug on the first pass,” Brett says. “Jai and Scotty had pulled the sparkplugs and replaced them with fresh ones without telling me.” At that point, Brett got a bit emotional. “I didn’t drive 400 kays to hurt the motor.”

Brett hit up carb guru Ray Edwards, who was spannering for Al Vella, and explained his issue. “It was no problem for Ray; he came over and changed a few things in the carby, giving us about 15 per cent more fuel volume. I’d been about ready to throw in the towel and trailer it home.”

But while there were some issues on the track, the Monaro’s road manners were exemplary. “I’d never driven it more than 60km/h outside of a race track,” Brett admits. “It was really bloody well-mannered, ran cool and didn’t give any trouble.” Despite the stress of getting the car ready and racing, Brett grew to love the Drag Challenge concept. “It’s just a great experience – likeminded people cruising around, filling Eskys, talking shit and going racing.”

However, Brett’s on-track woes were about to return in bum-puckering fashion. While prepping the car for the Day Two eighth-mile pass at Warwick Dragway, the team forgot to empty the oil breather vacuum pump overflow. “I hit the parachute, then the brakes and the catch can pissed all the oil and water out onto the front wheel,” Brett says. “The left-front locked up, dragging me into the left lane. I got back off it, back on it, locked up again, but kept it off the walls. I overshot the return road and had to back up the drag strip. We had considered not fitting the parachute; lucky we did.”

Finishing Day Two in fifth outright, Brett’s focus was on chasing Al Vella’s Capri. “We were only about a 15th of a second behind him at that point. I think we were the first lot to really put some pressure on him in the Pacemaker Radial Aspirated class.”

Heading back to Willowback later that day, Brett’s easy run on the road stages came to an end. “The road through the back of Stanthorpe was completely shit. I was dodging potholes and Skippys both alive and dead,” he explains, deadpan. “Coming out of Inglewood, the engine made a critical noise.”

The mandrel had spun out from the crank, hitting on the radiator. “We got her fixed up, but only got 20 metres before the front suspension started making a racket.” Diagnosing that a bolt had loosened off and was catching on the back of the hub, Jai got up under there and sorted it out. “I blame that bloody road in Stanthorpe,” Brett laughs.

Despite the car practically shaking itself apart on crummy roads and Brett almost collecting the mother of all kangaroos just outside Willowbank, the team’s woes were finally over. “We ran an 8.70, then took the exhaust off and ran an 8.63,” Brett says. “We checked everything and it was fine, but with the brakes and suspension issues, I was happy to pack up at that point. We’d had a good weekend and I was mindful not to bin it chasing Al.” Brett had one more run in him though, PBing again with another 8.63 pass and placing himself in sixth Outright and second behind Al in Pacemaker Radial Aspirated.

Brett and the Monaro had a great time together, but unlike John who originally built it, Brett has other automotive desires. “There’s a lot of money sitting there if I want to chase my own dreams. I want to finish my twin-turbo LSX HQ Monaro, plus I’ve got an LSA-powered HG wagon, a restored HG Monaro 350 and my Nova,” he says. “There’s always something rolling.”

It seems that Brett may consider moving the Monaro on. “I’m sure John’s happy that the car has really got out there and didn’t become a garage ornament,” he says. “But if I do keep it, I need to tune it up better and have another crack at old Al Vella!”


BRETT is quick to deflect credit for the amazing quality of PROHK’s build; that honour went to his mate John Mawhinney, who was more than happy to fill us in on his journey to Munro-dom. “I actually spotted it at Willowbank years ago and fell head-over-heels in love right there,” he says of his dream ride.

Bought as a roller, John wasted no time installing a built 434-cuber pumping out 830hp, but with the wins and losses stacking up in equal measure, the 434 was sold to make way for a monster Brodix 615ci Chev V8.

Concurrently, John orchestrated a very clean rebuild, which included a trick Gazzard Brothers rear end, Reid-cased ’Glide and a new rollcage.

When the Monaro ran an 8.8sec pass in full street trim on its maiden run, John was elated. “It was one of the proudest achievements in my life,” he says. An 8.79sec pass followed, but a minor disaster struck; not only was the engine presenting a faint miss, the bonnet opened during the run, damaging the engine bay and denting the roof.

For John, the disaster was compounded when a change in work circumstances meant he couldn’t give the car the attention it deserved. Calling Brett Armstrong, John offered him first option – after all, if you have to sell the car of your dreams, who better to buy it than your mate?

The handover was a bittersweet affair, with the boys enjoying a couple of beers together while John said goodbye. There and then, Brett made the promise to get the damage fixed and take the car to Summernats with John riding shotgun.


Paint: Glasurit Silver Mink

Type: 10.2in Brodix alloy big-block
Capacity: 615ci
Intake: Edelbrock Super Victor II
Induction: Mark Sullens Holley Dominator four-barrel
Heads: Brodix SR20
Pistons: CP custom SR20
Rings: Total Seal file-fit moly
Crank: Callies Magnum 4.635
Rods: Carrillo bullet 6.750
Cam: Big custom-grind
Pushrods: Manton custom 7/16in
Lifters: 0.903 Isky
Valve springs: PAC Racing triples
Valves: Ferrea stainless
Oil pump: Moroso billet high-volume/high-pressure
Ignition: ICE
Fuel pump: MagnaFuel
Exhaust: Custom 2.5in extractors into twin 4in stainless pipes, MagnaFlow mufflers

Transmission: Protrans Powerglide
Converter: TCE 6200rpm stall

Front end: Afco Race springs, Santhuff double-adjustable shocks
Rear end: Gazzard Brothers split mono-leaf springs with sliders, Santhuff double-adjustable shocks
Diff: Sheet-metal 9in with Wavetrac 3.5:1 gears
Tailshaft: Carbonfibre
Brakes: Wilwood (f & r)

Rims: Weld AlumaStar; 15×3.5 (f), 15×10 (r)
Rubber: Moroso DS2 (f), Mickey Thompson 275/60 Radial Pro (r)

My family: Tanya, Sophia, Lachlan and Harry; John Mawhinney; Scott & Paul Cortina; Jai Schluter; Ray Honfay at Straight Line Performance; Brett Miller; Shayne Curd