My two cents on Chieftain:
The only figure I've seen for the V8's power was some 700hp, which would be quite a bit of power/L or power/cylinder but doesn't sound unachievable when one considers that the AVDS-1790 was effectively a diesel conversion of a carburetted gasoline engine from the late 40s, which is unlikely to be that efficient. The MB 838 V10 that went into the Leopard 1 was bulky, but IIRC was not purpose-made for tanks and Germany had to kinda rush things with the Leo 1. They tested a 1000hp version of it in 1960 and they could probably have made it reliable with 1960's technology if the will had been there.
Therefore, I think 700hp out of the brand new RR V8 is doable.
Also, the Chieftain with the V8 was supposed to be a foot shorter. The L60 installation suffered from vibrations which mandated modifications that moved components to the outside of the vehicle. It is doubtful that this layout was ideal from a reliability, powerpack-change and weight standpoint. The notoriously high oil consumption also led to a large oil tank. It is said that getting to the L60 increased weight by 1 ton, but I do not know if it included only the increase in hull size, or included the engine weight difference, the extra oil and other changes too. In the latter case, the total weight growth might have been up to 2-3 tons, but more likely 2.
The L60 also was not very fuel efficient IIRC, so the V8 might have increased range with the same amount of fuel, maybe 500km instead of 400 out of the 850+ liters the Chieftain carried. IIRC torque is also not that great on opposed piston engines of that generation and it's certainly the case with that one, so the increase in performance may be higher than the horsepower rating suggests. As someone said earlier, outside of possibly being available and reliably doing its intended power output, the RR V8 might have revealed flaws with the rest of the automotive components sooner. In that regard, it is absolutely possible that the Chieftain would have been ready sooner on that front, maybe allowing designers to work on the rest of the tank or introducing it sooner. The bean-countering argument likely became moot with the constant upgrades and reliability issues of the L60. From a commercial standpoint, the RR V8 would also likely have been more successful than the L60 (the Vickers Medium gets a net mobility increase from the start, Centurions can be upgraded with it...).
IMO, the Chieftain has always been held back by bean-countering moves and occasionally questionnable design choices/bad timing:
- I know that many myths surround the Horstmann suspension, but IMO it is very overrated and should have remained a WW2/Centurion thing (even the Centurion was behind the curve in suspension design). It's suspension characteristics are poor and restrict high speed performance and the ability to fire on the move. It is also very heavy for what it does. Going to torsion bar would save over 2 tons at least, likely more, and IMO the height increase argument is overrated as the Chieftain already had features that made it rather high, TBs can fit in dead spaces and you can also reduce the space they take by stamping ribs they fit through like on Soviet MBTs. And you can get more mobile.
- the 120 bagged charge concept with APDS was a logical evolution at the time it was proposed, although the lack of British interest in smoothbore guns and APFSDS would be problematic later on. Bagged charges were logical when semi-combustible ammo was not yet ready. Nonetheless one cannot ignore that the execution of the L11 was a bit botched: the steel used was excessively elastic which mandated the use of a shroud to prevent it from warping too much (even beyond thermal effects). The canvas shroud itself was pretty much obsolete, far inferior to the aluminium shroud used on the AMX 30's 105 in 1967 or the fibreglass one on Soviet, German and then US guns. The problem is that canvas rots with humidity. The raw power is also a bit underwhelming, 1370 m/s with APDS out of a 120 L55, slower than 105 APDS even.
- keeping the steel roadwheels of the Centurion instead of using more modern steel or aluminium ones guaranteed a needless increase in weight... The steel tracks (rubber addons were an afterthought) were rather heavy too.
- the secondary choice of ammo is questionnable: HESH was considerably inferior to HEAT against armored targets (also very slow), and useless against composite ones. A well-made shaped charge in the 60s would have made even early Soviet composite armor vulnerable (note that NATO 120mm HEAT with weak compound B explosives and a small diameter warhead due to high velocity could already penetrate 480mm, Soviet requirements were 450 for their armor at first). HESH is also poor for various reasons against infantry relative to HEAT-FRAG or HE-FRAG. Good against concrete yes, but you have AVREs for demolition.
- the armor was made with the 100mm D-10T firing full bore rounds in mind, which is good except that this threat was nearly 2 decades old at this point. It was mostly inadequate against the 115mm of the 1962 T-62, and even against NATO 105mm APDS. I can't really blame the British since even the early Leopard 2 and MBT-70 merely requested protection against 105 APDS from 800m, but the weight of armor required to achieve that objective was high and it is surprising that mere cast steel was used and that the British did not put more effort into more modern armor arrays until Chobham. The late entry into service meant that Chieftain was deployed when such protection could have been achieved with lighter spaced arrays. The Leopard 2 prototypes weighed some 42-45 tons with huge turrets and a really large engine in 1969, yet could get the protection of a 56 tons Chieftain...
- I think an optical rangefinder had plenty of reasons to be there even with the 12.7mm RMG.
- the reclined driver's position didn't really decrease weight, it just improved comfort. The weird hull layout with slightly sloped side walls and floor aren't conducive to a low layout because of reduced space along the width of the tank and on the floor (and you can't always efficiently make use of this weird space to fit things, which are sometimes square). IMO that shape was too much complexity for next to no gains. They could have got Abrams-level low. If you want mine protection, do it like on the Leopard 2 and DON'T USE HORSTMANN.
(Look towards the roadhweel hubs, it's sloped)
- the wet stowage increased weight and complexity relative to armored ammo bins and was of questionnable value per American and Soviet testing. Maybe it worked specifically well with bagged charges, but it's weird that the Brits did it on the Chieftain but not on the WW2 Centurion?
- the ruby laser rangefinder they added in the 70s was actually worse than optical rangefinders due to false returns. According to tankers, Marconi did a much better FCS/laser rangefinder for export than the botched IFCS the Chieftains ended up being upgraded with. Bean countering.
- No passive night sights that do not require swapping with day sights in the 70s. Bean countering. The loss of No.21 cupola was unfortunate.
- no heater until the 80's. SHAMELESS BEAN COUNTERING.
- IIRC the Vickers MBT MK.1 got better stabs. Bean countering.
IMO, getting under 50 tons with plenty of other improvements was perfectly doable if the Treasury wasn't so penny wise and pound foolish. Paying for all the unreliable components to replace and losing many export contracts (Canada before Trudeau arrived namely) ended up being more costly. The torsion bar technology used by Vickers since the late 1940s should have been applied. That and the RR V8 are the easiest and most bang for the buck "assumptions" you can use in this thread.
Sidenote: After the Chieftain, the coherent option in this thread would be to avoid the FMBT program with Germany in 1972, just start with the MBT-80 concept this early so you get the next tank in the late 70s/early 80s instead of the late 80s, avoiding the shenanigans caused by the Iranian revolution.
