A completely new generation body design emerged with the HQ series in July 1971, including the new Monaro 'LS' (commonly believed to mean "Luxury Sports") model. There were no longer any six-cylinder versions of the Monaro GTS, just 253 or optional 308 V8s or the top level GTS350 coupe. The base model Monaro standard engine was enlarged to 173 cu in (2,830 cc) whilst the Monaro LS had a broad spectrum of engine options from a 202 cu in (3,310 cc) six to the 350 cu in (5,700 cc) V8. The new coupe design had a much larger rear window and a squarer rear quarter window; it was somehow seen as not as sporty looking compared to the earlier HK-HT-HG series, but is often now considered one of the best looking body designs to come from an Australian producer.
Up until 1973, the HQ Monaro GTS did not wear any body stripe ornamentation and the 350 cubic inch (5.74 L) Chevrolet Small-Block V8 engine was a little less potent than in previous HT/HG versions, especially with the optional Turbo-hydramatic 3-speed automatic transmission. This, and the fact that the same 350 engine was also available as on option in the large Statesman luxury sedan, probably contributed to a downgrade of the Monaro GTS range in muscular image terms, as did the replacement of the bigger coupes with the six-cylinder Holden Torana GTR XU-1 as the chosen GM car for Australian touring car racing. The introduction of bonnet and bootlid paint-outs in 1973 coincided with the release of the HQ Monaro GTS in four-door-sedan configuration. It is generally considered that Holden created the bold contrasting paint-outs in order that the new Monaro GTS sedan would not be mistaken for the humble Kingswood sedan upon which it was based.
The continued erosion of the GTS350 cachet was compounded by the deletion of specific '350' decals on the post-1973 cars, with all Monaro GTS coupes and sedans now being externally labelled with the generic HQ series 'V8' bootlid badge. In the final year of HQ production, i.e. 1974, the manual transmission version of the GTS350 was discontinued and sales of the automatic version were minimal prior to the engine option being quietly and unceremoniously deleted.
A factory 350 HQ GTS Monaro is very valuable today, with a 350 sedan fetching as much as $50,000, and close to $100,000 for a 350 GTS Coupe.
ADR27 emission control standard was mandated for vehicles produced from 1/1/1974. Holden began introducing required ADR27 measures on vehicles from around Septembr 1973 although cars did not have ADR27 on their compliance plate.
These measures mainly focussed on reducing emissions as idle through caps on carburettor idle mixture needles and with the addition of a pre-heat tube taking hot air from around the exhaust manifold up into air cleaners. The second type HQ 350 engine also had a thermal controlled vacuum switch which denied vacuum advance to the distributor while the engine was cold.