The HQ vs HZ thread.

[HK1837]

why a hj engine ,gearbox and diff aren't they all the same?

HJ are the high performance 308. HT-HQ are around 9:1 compression. HJ are 9.7:1 and no pollution gear, especially 1974 HJ which don't have a canister. HX and HZ 308 saw compression drop back to I think 9.2:1, cam was retarded and the inlet manifold on these choked them badly, HT-HJ manifold flows essentially the same as an Edelbrock Performer, HX one is shocking. Rated hp difference between HQ and HJ was only about 10hp but on the twin exhaust cars it was closer to 25-30hp. The much more powerful HJ engine was one of the main reasons HJ 308 got the much more expensive M41 transmission, the HQ trimatic couldn't handle it. A manual HJ GTS 5.0L is essentially a performance match for a manual HQ GTS350, auto is a bit slower as the TH400 kills it a bit more than it does to the torquier 350. Another little piece of evidence if you can find it is media tests of the first of the LX XU2 (SLR5000) or SS 5.0L. One of these will show an L34 how to get down the quarter. Most tests you see are on early LH which had the HQ engine and an undersize exhaust which also carried into the later LH with the HJ engine. It wasn't until LX when a proper twin 2" system was used.
So forgetting the auto 308 Sandmans as standard gearbox and diffs varied between HQ-HZ, but stick one of each series Sandman in 308 manual (all will be L31 M21 GV4) all in the same bodystyle on the dragstrip. HJ will win easily, followed by HQ. By HZ the doggy ADR27A engine was sorted a little better so the HZ will probably run 3rd. Give them all a twin exhaust and HJ will probably widen the gap slightly.
Longwinded answer, but this is why I said HJ engine. Auto box is just my personal preference, plus I like to see expensive factory options! And I also like virtually maintenance free transmissions. If HJ 308 had copped a T10 4spd which it really should have(Muncie was no longer used in US cars at that stage as there were no big hp cars anymore, so they used the BW T10 as it was good enough for lower hp cars) i'd probably have that instead.