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Blown LSX-powered Holden HQ Monaro GTS - MRGM8

By Iain Kelly | Photos: Chris Thorogood, 07 Apr 2020 Features

Blown LSX-powered Holden HQ Monaro GTS - MRGM8

Dave Guilfoyle’s genuine Purr-Pull HQ GTS Monaro swapped a 6/71-blown 502ci Chev for a Magnuson-blown 427ci LSX, and we can’t get enough

DAVE Guilfoyle’s HQ Monaro has been around for a couple of years, and it’d be surprising if you haven’t seen it before. You see, it scored plenty of attention after winning tinware at Summernats 28, it was featured on the cover of Street Machine’s December 2015 issue, and even landed a nomination for Street Machine Of The Year in 2016.

This article was first published in Street Machine's LSX Tuner magazine #6, 2017

Holden HQ Monaro

However, those who recognise the car and have a keen eye might notice something different about the genuine Purr-Pull GTS two door (reportedly one of only 10 built in the amazing colour). Where a brilliantly polished BDS 6/71 blower and bug-catcher hat used to sit is a slim reverse cowl hiding a modern four-rotor TVS-style Magnuson Heartbeat 2300 supercharger, while 427ci of LSX replaces 502ci of Gen VI big-block.

Read more: Dave's HQ when it was running a 6/71-blown big-block

Holden HQ Monaro sideWhile the styling had some Pontiac inspiration courtesy of General Motors, the HQ coupe is one of Australia’s greatest designs. The pillarless doors, fat rear haunches and aggressive nosecone are only enhanced by the eye-bashing colours Holden made available 

So, after a five-year build why would Dave pull it apart to change to a smaller motor?

“I have a VL Calais that I put an LS in it and I had a ball in it,” explains Dave. “It is an LY6 with Flowcraft heads and a 2.9-litre Whipple blower, and the last time it was on the dyno it made 570kW at the wheels. So, while the big block was good, after driving the LS in the VL I decided I needed to get on board.”

Holden HQ MonaroColours can make or break a muscle car’s value, both here and in the USA. While the HQ was red when he bought it, Dave took it back to the factory Purr-Pull magenta with Glacier White stripes. Of 30 HQ Monaros painted this colour it is believed 10 were coupes, and only four survive today 

Replacing a blown 502 rat motor means the LS needed to rock a tough combo, and Dave didn’t disappoint in this regard, though he actually lucked onto it thanks to a fortuitous purchase by a mate!

Read next: 1000hp twin-turbo LSX-powered HQ

Holden HQ grille

“A mate bought an HQ that had this 427-cube LSX in it and brought it to me to do some work on it. I didn’t want to give it back, so he told me I could buy it if I paid what it owed him. I pulled the motor down just to check it out, changed a few things over and sold that HQ off as well as the big block parts which meant the change-over cost was pretty much negated.”

Read next: Elite-level LSX454-powered 1972 HQ Monaro

Holden HQ wheel

The 15x3.5in and 15x9in Weld AlumaStar wheels and Kingswood disc/drum brakes have been replaced by 17x4.5in and 15x10in Weld V-Series wheels and Wilwood discs all ‘round. The two-speed Powerglide ‘box and Pro9 9inch diff have also made way for a three-speed TH400 and Chris’ Diffs-built Strange 9inch

Inside the iron 427-cube LSX block is a Texas Speed & Performance rotating assembly featuring a Callies crank and SRP pistons, with a BTR solid roller cam moving the T&D shaft-mount rockers in the cool Higgins HRD six-bolt heads. Those mods support the Magnuson four-lobe Eaton-style 2.3-litre blower perched under that new reverse cowl bonnet, while Dave took some tips from the Americans before installing the whine-maker. “We modified the blower snout, porting it out like people do in America so it comes onto boost a bit better.”

Holden HQ boot

Dave whipped up a couple of fresh headers, but reused much of the twin 3.5-inch exhaust already under the Monaro, with a couple of extra mufflers out back. That wasn’t the only change made to get the late model block into the GTS, though.

Read next: Daniel Szabolic's twin-turbo big-block HQ Monaro

Holden HQ engine bay

The HQ’s engine bay used to feature a highly polished 502ci traditional big block, but fitting the 427ci LSX is still no easy feat thanks to having to fit the 9.5-litre ASR sump on the bottom of the new-gen motor. The four-lobe Magnuson blower sits at a much lower height than the old school BDS 6/71, meaning Dave can get away with a small Alfa Motorsport cowl bonnet

“It had an aftermarket sump on it and my engine bloke said to throw it in the bin. We put an ASR 9.5L sump on it due to the cubes, and we did a couple of mods to the crossmember to get the motor sitting a bit better in the bay.”

Holden HQ engine bay

While the Magnuson Heartbeat 2300 already runs through a water-to-air intercooler mounted up front with the radiator, Dave is looking for even more efficiency by potentially adding an Interchiller. They work off a car's air-conditioning system to help give an extra layer of cooling to the intake charge, and have helped VF GTS HSVs run into the nines!

On top of getting it planted nicely in the HQ Dave switched up the radiator for a big alloy unit that located the Magnuson’s water-to-air heat exchanger in a nicer, more integrated fashion. “I had to modify the radiator support so it all sat as one piece, but it works well as the temp logs show the Maggie blower is definitely more efficient than the Whipple on the VL. I want to put Vintage Air on it so I can run an Interchiller!”

Holden HQ engine bay

While on face value 430rwkW might not blow many skirts up, the reality is that Dave is only pushing 6psi through a 2300 Heartbeat blower and it could take double that. The limiting factor is in the high-duration of the cam, which gives Dave the choppy idle he wants, but doesn’t fill the cylinders as well as a cam with a wider lobe separation angle would

The fuel system is much the same as it was in the HQ, though the injectors are Siemens 1500cc units and the car now drinks a strict diet of boost-friendly E85. Another change is the Microtech ECU has been replaced by a VY Commodore PCM, which is flash-tuned through the communications port. With only 6psi up its guts the purple tyre-smasher threw down 430kW at the treads, which Dave admits comes partly down to his cam choice.

Holden HQ engine bay

“That big BTR solid roller limits us with boost. Thanks to the duration it means it’s more of an NA cam, but I wanted the car to still sound tough, and it does!”

While the five-oh-two only needed a two-speed Powerglide the LSX puts the hurt on a reverse-pattern three-speed TH400 with a trans-brake, fronted by a 4200rpm torque convertor by The Convertor Shop. The old Ford 9-inch diff has been upgraded to a Strange-cased 9-inch built by Chris at Chris’ Differentials, filled with billet 31-spline axles, a Truetrac LSD centre and 3.7 gears.

Holden HQ interior

Like the rest of the car, the cabin of the GTS treads a fine line between stock and modified, with the GTS gauges, steering wheel and seats, and even an original GTS Air Chief radio in the dash! Going by the tags, Dave’s car actually came with straight black vinyl interior, but he says he couldn’t go past the Houndstooth trim for a full 70s feel

So, what has been the biggest change having moved from a blown big block – a combo that many street machiners would have considered was the ultimate for a tough, ground-pounding street car in years past?

Holden HQ rear seats

“We’re using it more now, and that’s the main thing,” Dave explains. “I can jump in it and just drive it, and there are no problems or hassles. The blown big block was cool but I got over it.

“The HQ actually makes more power now and I can drive it anywhere: I can pull up next to the cops and have no worries, it never gets hot when cruising, I have power steering on it now so that’s a lot nicer to drive. All up it makes more power but is also far friendlier to cruise in – it’s no comparison.”

Holden HQ gauges

With better driveability and more on-road performance it sounds like Dave now has the perfect recipe to enjoy his dream car whenever he wants. And driving our cars is really what it’s all about, right?

Holden HQ onroad

DAVE GUILFOYLE
1973 HQ GTS MONARO

PAINT
Purr-Pull Magenta, Glacier White stripes

ENGINE
Type: Chevrolet Performance LSX
Block: Iron 427ci
Blower: Magnuson Hearbeat 2300
Crank: Callies
Heads: Higgins HRD 6-bolt
Injectors: 1500cc Siemens
ECU: VY PCM

TRANSMISSION
Gearbox: Three-speed TH400, reverse pattern, transbrake
Converter: 4200rpm
Tailshaft: Motive 3.5in, Strange 1350 yokes, Spicer unis
Diff: Strange 9in, Tru-trac LSD, 3.7 gears, 31-spline billet axles

SUSPENSION & BRAKES
Springs: Pedders HQ springs (f), Pedders 3.5in lowered springs (r)
Shocks: Pedders 90/10 (f), Pedders 50/50 (r)
Rear-end: Pro9 adjustable upper & lower arms (r), Pro9 adjustable swaybar, mini-tubbed wheel-arches
Brakes: Wilwood discs & calipers (f & r)

WHEELS & TYRES
Wheels: Weld V-series 17x4.5 (f), 15x10 (r)
Tyres: M/T 165/80 (f), M/T ET Radial 295/55 (r)

THANKS
My supportive wife Jodie; my right-hand man and brother Shane; Allan from Blitz Custom Exhausts; Fraser; ’Shelle and Hayden from Alfa Motorsport Fibreglass; Matty from Pro-Addict Paint; Adam from Tyrepower Melton; Eugene from Flowcraft Race Engines; Billy at Garage Worx; Chris from Chris’ Differentials; John at Race Parts Melbourne; Jason from Tuff Mounts; my uncle Rod; my cousin Simon; Brad, Jamie and Deano

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