Torana ran the same engine as the Holden. Early LH 5.0L was the same spec engine as the HQ 308. When the HJ was released the compression ratio on the 308 was increased from 9.0:1 to 9.7:1. The LH got the same engine at the same time, and this carried over into the LX (pre HX circa 6/76). This is the 250hp engine low75 is talking about, and was the highest output L31 engine (possibly outdone by the manual 308 VK Police spec engines (V5H I think they were)). One of these engines in a post 10/74 LH-LX SL/R or LX SS is a pretty quick thing, will easily outsprint a post 6/76 LX (incl A9X), and mostly due to gearing will outdo an L34 (most L34 had 14" wheels and 2.78:1 gearing).
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All HJ get that same engine ie 250hp. Which in essence makes the HJ the most powerful Sandman but it is hardly a muscle car.
What constitutes a muscle car? Many will argue it is something like:
"Performance oriented model of a mid-sized vehicle series with a larger capacity engine installed from the bigger (full size) vehicle series, and the engine is unique to that model".
Not too many GMH vehicles fall into this category, probably only the HK GTS327 and LC XU-1. At a stretch you might include the LH SLR5000 however the 5.0L was also available (albeit limited) in the LH SL. I like the following qualification:
"To be a true muscle car, the vehicle must be 2 door, V8, 4 barrel, 4 speed."
This leaves only one Aussie car: the HK GTS327. You may be able to (on a technicality) squeeze in the A9X as while it was produced the 308 was not available outside A9X in LX Torana, although the existence of LX SLR5000 and SS with optional 308 do probably rule it out. This does not mean that the HK was the highest performance Aussie car, there were plenty of them that left it hanging (late HG GTS350, XW/XY GT-HO phase II/III and E38/49 charger to name a few). But it for its time was a true Aussie Muscle car.
So back to HJ, let's say the quickest Sandman, HJ Holden XX7 utility with 308 and M21. It is 2 door, V8, 4 barrel, 4speed. And having a 3.36 or optional 3.55 diff it would possibly out accelerate an LH SLR5000 (would weigh about the same). But the engine is not unique to the Sandman in HJ, and it doesn't come from a bigger vehicle.
So I wouldn't call it a muscle car. But it'd stick it in the same category as 4 door Holden engined HQ GTS or HJ-HZ GTS.
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