Thread: 3.9V8 to 4.6V8

A 4.0/4.6 has bigger main bearings than a 3.5/3.9. So the 4.6 crank will not fit into a 3.9 block The reason you need different rods/pistons is that with extra stroke the old ones would stick out of the bore at TDC. You need to get a whole 4.6 short motor. The only long stroke conversion that will fit the 3.5 is the rare 4.2 crank/rod/piston set. I don't know how it would go with the 3.9 piston bore size.

It might be worth visiting "RPi Engineering - Specialised Rover Engines" for lost of info on Rover V8s, conversions etc.

Originally Posted by bee utey

So the 4.6 crank will not fit into a 3.9 block

I understood the 3.9 and the early 4.6 out of the P38 were the same block - the higher graded blocks became 4.6s and the lower graded blocks became 3.9s.

Garry

Not 3.9 but 4.0.

Name changed reputedly to match Jeep in advertising but a 4.0 has the same bottom end layout as a 4.6 but shorter stroke.

Crank, rods, pistons, are different to a 3.9
Regards Philip A

Ok - so I have misunderstood.

So the original basic engine design line ended with the 3.9. A new block design (well heavily modified) was introduced with the 4.6 in about 95 which was produced concurrent with the different 3.9 blocks until late 98 when all V8 engines had the same same blocks with different strokes resulting in the new 4.0 (same actual capacity as the 3.9 but different block/induction) and the old 4.6.

Thanks for clearing that up as I might have looked at getting a 4.6 crank etc only to find it would not go into my 3.9. I assume 4.2 cranks etc are rare and as expensive as hens teeth.

How does the P76 4.4 compare to both the 4.2 and the 4.6?

Thanks

Garry

There is a complete p76 motor for sale on ebay

4.4 V8 Range Rover Land Rover, P76 Leyland Engine Motor - eBay Engine Parts, Engines, Components, Car Parts, Accessories, Cars, Bikes, Boats. (end time 15-Aug-10 14:37:24 AEST)

The P76 motor is physically wider (about 25mm) as the Australian engineers used longer rods to maintain smoothness. Crankshaft main bearings and flywheel mounting flange are different. Inlet manifold is a 2 barrel downdraft type. The inlet manifold off a Rover can be used with spacer plates but is prone to vacuum leaks around the spacers. Plenty of P76 motors have been fitted to early Range Rovers etc but they are getting harder to get parts for. My engine shop can no longer source main bearings for them. ACL made them last in 2004 but I suspect that is the end of them. The P76 heads lack the mountings used on the Rovers for alternators and a/c brackets. Using the Rover heads involves using hollow pushrods as the P76 does not run pressure fed rocker shafts.
Really the most reliable conversion is a proper 4.6 rebuilt to best practice etc, then just bolt your bits on to it.

I know of a complete P76 with auto gearbox for sale in Melbourne.