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MG MGB GT V8 Factory Originals Technical - Differences in rover 3.5/4.6 specs

I am currently building a 64 mgb with a rover 3.5, rover 5 speed and rv8 headers. I have all the parts and it's pretty much togehter. I have been reading about rover 4.6 and even stroker rover motors but have not been able to find any info on how they differ dimensionally, and what would be required for a swap later on. Can anyone help?
Pete

Rover 4.0 and 4.6L motors use the same 94mm bore as the 3.9L motor, but have bigger main bearings for the crank. This equates to a lot more mass swinging around inside the block. They're great for torque, but not necessarily for reving up to high rpm's. 4.6's aren't necessarily bad, they're just different. I for one, like putting my motor up near 6000-7000 rpm's. It's fun. =) The pistons, crank, conrods, and I believe even the heads are slightly different. Late rover heads only use 10 bolts instead of 14 like your motor. The most preferrable motors from a Rover standpoint seem to be the 4.2L motors. You can make as much hp with that as you can a 4.6L, and let it rev pretty high too. 4.2L motors use the same size main bearings as the 3.5L and 3.9L motor. All the ancillaries will transfer to a 4.2 without a problem.

HTH,
Justin
Justin

Thanks Justin, I have heard the 4.2 could deliver more hp, but I have also heard that the 3.9 gives a better balance of torque and hp. What I am really interested in learning is if I want to swap out my 3.5 for a 4.6 or a variation on that block, what alterations will i need to make to the rest of the car?
Pete

You can make the 4.0/4.6L fit just like the earlier series RV8s, but you have to make a few changes. For one, the 4.6 has no distributor and therefore you will need to use the front cover off of the 3.5L. You will also have to switch out the cam shaft as the 4.6 has no place for the distributor drive gear. I am working on my second MGBV8 now and I am using a 2001 Disco II 4.0L and I am going to retain the distributor-less system but use the old hot wire EFI. It is very compact and very efficient. There is also no need for expensive adaptors for the oil filter. This is all well and good if you already have all the accessories, but would be expensive to piece mill and to have a custom flywheel made. Anyway these are just a few thoughts to ponder.
Evan Amaya

I have done 215 Buick, 402 Rover & 300 Buick swaps using the same headers, motor mounts, & engine bay location. You can literly remove one & bolt the other in, except that the 300 Buick may gine some hood clearance problems.

You can mix & match front covers, oil pans & lots of other parts. My 215 Buick has a 1992 Rover front cover with the gerator oil pump & reverse rotation water pump. My 4.2 had a GM front cover & oil pan. All have used a GM Delco distributer. Oil pan swaps may require 1 or 2 holes to be enlongated.

All engines are different, but all are great performers & lots of fun. You have to decide just what kind of fun you want, & how/where you are going to drive. I enjoy my 215 Buick with somewhere around 190-200 HP just as much as the BEAST with 325 HP from a 300 Buick.

My suggestion is to build what you have & start enjoying it. After the "new" wears off, if you decide you want more power, swapping in another engine will be pretty simple. You may decide to go for improved handling instead.

Jim Stuart
Jim Stuart

Thanks for the additional thoughts. Jim, I am planning on completing the 3.5 l conversion, but while i have the chance I thought i'd consider any prep that I might be able to do now- so that after a few years of fun, if I have an itch to make a change it would be an easy (relatively speaking) change. As usual it sounds like there are lots of options that create other options!
Pete

If you did a 4.2 from a 3.5, all you would really need to do is get bigger exhaust headers. the standard rv8 headers are too restrictive for the larger engines. Also make sure your radiator is in tip top shape. Otherwise, the trans, the intake, the timing cover, carb/injection, rear end, alternator, dizzy... it all converts over. The block is virtually the same from an external standpoint. Oh, and it won't take years for the "new" feeling to wear off. You'll want an upgrade sooner than later. :)

Justin
Justin

This thread was discussed between 20/04/2004 and 22/04/2004

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