When supplies of the 2138cc Triu mph four-cylinder engine dried up in the late 60s, Morgan were left without a high-performance engine for their flagship Plus 4 model. Help was on the horizon, however, in the form of Rover's all-alloy 3.5-litre V-eight derived from a discarded Buick design of the early 60s and recently introduced in the big P5B saloon and coupe.
so, is it the 3.5, 3.9 or 4.0?
The real differences are:
- The 3.9 has the provisions like the 4.0/4.6 litre for the crossbolts but they have not been drilled.
- The 3.9 has the smaller main journals from the 3.5 while the 4.0 version has the same journals as the 4.6 litre.
- The 4.0 crank has a longer nose.
- The 3.9 has the concentric oil pump but coupled with standard distributor the 4.0 has a distributorless system and thus no hole for a distributor.
- The 4.0 uses longer rods, lighter and shorter pistons.
- The 3.9 had the 14CUX 'hotwire' fuel injection. The 4.0 has the Lucas 'GEMS' engine management system
- Because of the way the fueling and ignition is set up, the 4.0 has knock sensors and the 3.9 does not.
- the 4.0 puts out 186.4 bhp and the 3.9 puts out 190.4 bhp.
differences between 3.9 and 4.0/4.6
1. The 4.0/4.6 crankshafts are also longer to drive the new oil pump
2. A new connecting rod design was used for the 4.0/4.6. They are made from forged steel and now have balance pads on both the small and big ends; the screw-in bolts retaining the caps are of a more robust design and are manufactured to extremely fine tolerances. The 4.0-litre engine uses a rod 1 55.2 mm in length, and the 4.6-litre is 149.7 mm with a55.5 mm big end bearing diameter, the older version having a 50.8 mm big end bearing. The increase in length was done to reduce the angularity of the rods in the engine, thus reducing vibration. The small end size has been increased from 22.2 mm to 24mm.
3. The pistons are also new and are a common design between the 4.0 and 4.6-litre engines. The capacity of the bowl in the piston crown varies slightly between the two engines (4.0-litre, 13.23 ccl4.
6-litre, 22.29 cc) to determine the compression ratio. To maintain equality of material thickness on the piston crown they are different castings. Having said that, the compression ratios are the same for both engines --9.35:1 although lower 8.2:1 compression pistons are available. The original 3.5/3.9-litre engines have pistons 80.9 mm long overall, with a compression height (or crown height) of 49.5 mm, while the longer stroke 4.2-litre engine uses a piston 72.85 mm long with a compression height of 45 mm.
4. The 4.0/4.6-litre engines have a shorter piston of 66.6 mm, with a compression height of only 35.9 mm.
5. The 4.04.6 gudgeon (or wrist) pin diameters have been increased and they also have a gudgeon pin offset. The 4.2-litre engine was the first production Rover V8 to use this feature, but on the 4.0/4 6
litre engines it has been increased from the 0.55 mm of the 4.2-litre to 0.60 mm. The reason for this offset is to produce a slight side loading on the piston at TDC, thus eliminating piston slap all part of the infinite attention to design detail by the Rover engineers in their quest for refinement.
6. The 4.0/4.6 cylinder blocks have been made significantly stronger by the addition of stiffening ribs (taken from the Sd1 Vitesse) in key areas (along the block sides near the main bearing webs, for instance) and, more importantly, both 4.0 and 4.6 litre blocks now have cross-bolted main bearing caps. Of course Rover have produced cross-bolted blocks before, but they were special items and expensive, one of the reasons being that the main caps had to be individually ground to match the block. The new blocks have this feature productionised, although they are now a press fit in the block. The cap material has also been upgraded from grey to SG iron.
7. The interior profile of the 4.0/4.6 block has been altered to make room for the redesigned crankshaft with its bigger bearings and bigger counterweights. The cylinder liners are 5 mm shorter and the oil
pick-up from the sump no longer fits into the block casting, from where the oil made its way to the pump via a drilled gallery; the pick-up on the new engine.
8. There are two new crankshafts, one of 71 mm stroke for the 4.0-litre and one of 82 mm for the 4.6-litre version.
9. The main bearing size has gone up from 58.4 mm to 63.5 mm and the counterweights are larger, although of the same number as before. These new crankshafts will not fit into older engines because there is insufficient room for the bigger counterweight to rotate inside the block. The stroke of the 4.0-litre crank is identical to the old 3.9-litre, as is the cylinder bore of 94 mm, so the capacity of
these two engines is the same.
the b usually describes the fuel system on these engines although i'm not sure on the morgan lumps!
have that ya bastid!!!
