3.9/4.0 Differences (see below for Buick/3.5/4.0/4.6 differences)
by Lorne Goldman

What is the difference between a 3.9 or 4.0 liter engine? Both have a bore of 94.0 mm and a stroke of 71.1 mm.
There seems to be no difference in capacity at all!

The differences are:

- The 3.9 has the provisions like the 4.0/4.6 liter 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 liter.

- 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 (latest see Bosch 5.2.1 Motronic)

- 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.

If you know of other differences please send them in to keep the page current.



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-liter engine uses a rod 1 55.2 mm in length, and the 4.6-liter 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-liter engines. The capacity of the bowl in the piston crown varies slightly between the two engines (4.0-liter, 13.23 ccl4. 6-litrr, 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.

Engine Bore x Stroke Capacity
Buick 215/Rover 3.5 88.9mm x 71mm (3.5" x 2.8") 3500cc
Rover 3.9/Rover Crossbolted 4.0 94mm x 71mm (3.7" x 2.8") 3950cc
Rover 4.2 94mm x 77mm (3.7" x 3.0") 4278cc
Leyland Australia P76 4.4 88.9mm x 88.9mm (3.5" x 3.5") 4416cc
Rover Crossbolted 4.6 94mm x 82mm (3.7" x 3.2") 4554cc

Displacement  Bore Stroke Main Brg Rod Big End Brg Wrist Pin Rod C/L to C/L
 215CI BOP 3.50 2.8000 2.3000 2.0 .8750 5.660
 300CI Buick 3.75 3.3600 2.5000 2.0 .9390 5.960
 3.5L Rover
 3.9L Rover
3.70 1
2.8000 1
 4.2L Rover
3.0000 1,2
 4.4L P76
 4.0L Rover
 4.6L Rover

When using the Ford 255 Ford V8 or 2.8L Ford V6 (or similar bore size) pistons, you want 3 11/16" x 3/32" sleeves. For 305 Chevy pistons (dished LG4), you want 3.75" x 3/32" sleeves. For 2.3L Ford turbo pistons (3.78" bore), use the same sleeves bored 0.030 over.

A NAS 4.0 Anomoly (1998-2002 models)

Experience seems to show that, when in doubt, the US Plus 8s (4.0 1998-2002) NAS SAGEM fueling system ECU (computer) will revert to a default setting. Like most default setting, the car will be protected for a presumably a short period by running very rich. There will be a smell from the cats of rotten eggs and the car will react poorly with a bad idle and sluggish performance.

Many things can produce this state...a prolonged non-use of the car..a lack of power to the ECU or overfilling the petrol tank.

In such a case with these symptoms, the ECU must be reprogrammed to run properly again. TRhis is a simple matter if you know the setting and you have access to a LR dealer or specialsit with wither TESTBOOK (the LR diagnostic and prgramming serrvice unit) or Rovercom, an aftermarket item that does the same thing.

Unlike othe LR units, the Morgan is reporgrammable and you will have to tell the experienced mechanic with the diagnostic unit mentioned to use the disk for "1997 Range Rover GEMS 4.0L UK Manual Type 1 ID 9660". Once re-programmed to that specification, the car should run perfectly again.
The Story of the Morgan 4.6s February 2, 2018

I was just contacted by an UK mogger I like very much. Puzzled, he has asked a question that I haven't dealt with in years. The Morgan cogniescenti has largely withdrawn from view in the last decade and much has been forgotten.  People should know as much as is possible about their beloved cars.

His question arose from confusion on Morgan blocks numbers and the Morgan 4.6s.This is very understandable as for a period (1997-2000) Morgan simultaneously produced Plus 8s with two different generations of original LR V8 blocks/heads and fueling systems (the then already obsolete 3.9s with Hotwire fueling for the UK and the more advanced 4.0 blocks with GEMS fueling for overseas distribution). This ended in 2000 when Morgan went to GEMS for all Plus 8s. 

There were many reasons for this given by the company, few of them credible. For example, at the time the UK market was told that the later GEMS system or later the Land Rover Bosch Motronic system "would not fit under the Morgan bonnet!", which didn't make sense considering the hundreds of GEMS Plus 8s supplied overseas. The simple truth, I suspect, was that GEMS and MOTRONIC were more expensive for Morgan than the older Hitachi Hotwire systems and British law allowed the local sale of 3.9s until 2000. That being said, engine fiddlers like me and top tuners prefer the Hotwire for a number of reasons. It certainly allows for easier upgrades and owner intervention. 

But another problem appeared for Morgan.

You see, by 1996, news had come of the production of the anticipated 4.6 version of the venerable Buick/ Rover/LR blocks. Every fan of Land Rover and the other marques using the block were excited. The 4.2, never used in a Factory Plus 8,  had largely been a disappointment and the 4.0 and 4.6 went much further, with cross-bolting and other features. And this was before the infamous plague of Land Rover V8 block cracking and slipped liners had begun in earnest

So Morgan had a goodly number of 4.6s on order in anticipation, at a premium price. Even Peter Morgan put his order in for one to be his personal car, AB16! It was also to have the newer "wide body" with the new Superform wings (much wider than anything contemplated in prior Plus 8 history). Problem is, everyone wanted the 4.6 in their truck when it appeared and Land Rover couldn't deliver to Morgan. I believem (unconfirmed) that the problem was the scarcity of the new GEMS fueling system, not the block.

But Morgan came up with a solution to save those sales. They went to John Eales. John should need no introduction to Plus 8ers. He deals in race preparation and supply of LR engines. Morgan asked Eales to supply new 4.6 blocks, modified and fit with 3.9 Hotwire top ends and fueling systems. (With a bit of modification parts, all blocks from 1976 can be retro-fit with earlier or later systems.) So now they had bigger 4.6 blocks fit with earlier fueling systems made for 3.9s! 

Here is the Morgan announcement that was sent to me. It is confusing and inaccurate, indicating the state of the company management technological understanding at the time. I ask the reader to overlook these errors.  

Morgan Motor Company: June 1997 Dealer Notice:

"As it has not been possible to obtain the current Range Rover 4.6 litre engine, due to supply problems with Rover and because of the modifications required for installation, we have arranged for a special engine to be prepared.

This uses the current 3.9 litre engine, but built using the standard 4.6 litre block. This involves stripping a new engine, removing the block, specially modifying a 4.6 litre block to accept the front cover, and rebuilding the engine.

This produces a special unit using largely standard Rover parts. The distributor set-up is largely the same as 3.9, as is the camshaft and ECU. The block is cross bolted for strength. The crank, pistons and block are standard Range Rover 4.6".

Of course, logic dictates that  one cannot get much more horsepower by providing the same amount of fuel to a larger bore engine, EVERYTHING ELSE being equal. In fact, the combination MUST run lean and hot as you are feeding a larger combustion chamber with the same amount of fuel! In fact, if you examine the figures given by Morgan, they show a mere 1.8% increase in bhp over the 3.9L block of the day albeit the 4.6 had a 2500 GBP price premium. Almost 20 years ago, John Eales told me he tried hard to get Morgan to allow him to create or provide them with a more appropriate chip/eprom (aka fuel map) which would release the extra power eveyone assumed would result from the bigger block and keep the motor problem free. Land Rover, with merely their stock 4.6 fueling and a truck 4x4 cam produces 30 more bhp (almost 20%!) with the same engine. However, a more appropriate chip was refused by Morgan, either for reasons of economy or perhaps worries about a need to re-test the engine for emissions. In any event, it was never done. Sadly, this also came at time when the fueling was already as lean as the fuel map of the 3.9 could be made to provide. This was why LR was transitioning to the GEMS at the time. Additionally, owners would soon compound the lean fuel mixture with K&N air filters and performance exhaust systems. 

WATCHPOINT:  I have always advised the Hotwire 4.6 owners who contact me to fit a Tornado chip and they are universally thrilled, not simply with the added power and torque but with the new experience of running cool. Sadly, by the time they get to me, they have all often spent a fortune trying to improve the cooling systems. I am am a fan (pun!) of better cooling for Plus 8 but trying to fix a fueling issue with better cooling is analogous to using a huge bandaid in hope of curing pneumonia.  

So the UK Morgan 4.6s (aside from some overseas GEMS units) overheat. Aside from the few of them that have addressed the issues, the owners adapt to the new they-all-do-that attitude. 

I have a number of Buick, Rover and Land Rover V8s in spares. I enjoy them all, they all give the cars a different personality. But after more tha 200,000 miles, two continents and more than 20 years, I unabashedly admit to loving the 4.6 variants more than all others in Plus 8 history. They are infinitely owner friendly, with the best Morgan touring engine ever as they are super flexible. Roadside mechanics are familar with them on all continents. The mistakes Morgan made with their UK 4.6s are tragic. However, they are easy to sort. But the current community is no longer as mechanically adept as it was and who can recognize inadequate fueling without an exhaust analysis and a basis for comparison? Owners are now unaware of the issue and we all unconciously adapt their driving to what they got. The increased capacity indicates that the 4.6 MUST be more powerful so everyone assumes it. Yet it is merely another problem Morgan waiting for the right owner to ask the right questions and act upon the right answers. It would be a shock for these owners to see what that engine can really do..assuming it is undamaged after a decade of incorrect fueling. 

These cars are easy enough to recognize. UK 4.6s. And their block numbers do not indicate a made-for-Morgan origin. John (Eales) would have their source and numbers but it would be unkind to place him in a conflict that is not of his making. He is a good guy.