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MG MGB GT V8 Factory Originals Technical - 3.5 to 4.6

I have an old 3.5 block with new ancillories.
If I purchase a std 4.6 block (inc pistons) would I have any problems fillting my std SD1 heads, Webber 500 carb with dual plane manifold? I have SS ceramic block hugger headers, with twin SS system.
Would I get a worth while increase in power, assuming similar pistons as my existing 9.75:1 ones?
I would graetly appreciate any help?
Regards
David Tetlow
D M Tetlow

David
Yes all the parts from the 3.5 will fit the 4.6 short motor(not crank,rods,pistons). I would suggest that you have the heads skimmed enough so that composite head gaskets can be used without lowering the CR. To get the best from the capacity you should have the heads flowed etc.
Better still have 4.0L pistons fitted to the 4.6 to up the CR.
I have gone from 3.5 to 4.0 and now 4.6
I can guarantee you will have a huge grin every time you take it out for a spin.

Mark



Mark

Mark
Well I am interested. I have a currently have a Rover3.5 and would like to do another motor. The 4.6 looks interesting.
I have done a quick search on the Internet and see there are at least 2 different type heads available for the 4.6. One with 16 valves and one with 32 valves.
I guess the question is, which is the best Rover 4.6 to go for? And which vehicle does it come from?

Thanks

Bruce
Bruce Mills

Bruce
The Rover V8 only has 16 valves. You have probably been looking at the BMW V8 which is fitted to the latest generation of Range rovers.
The 4.6/4.0L heads are different in that the outer row of head bolt holes are missing and they have a 28cc combustion chamber to allow for the thicker and superior composite head gaskets. The rover v8 blocks 3.5, 3.9, 4.0, 4.2 and 4.6 are all virtually external dimensionally the same.

Cheers

Mark
Mark

Thanks for your help Mark.
Regards
Dave
D M Tetlow

Hi Mark
Do you have an original MGB gearbox or a 5 speed? I am thinking of fitting a LT77 to my new engine ( I have an adaptor plate and the original 4 syncro box on my 3.5)but am unsure which bellhousing would put the gear lever in the correct place for the MGB.
Regards
Dave
D M Tetlow

Dave
The original gearbox was pretty weak for the 3.5 so no way would it cope with the massive torque of a 4.6.
I fitted the R380 box which is similar to the LT77.
To get the gearlever in the right place I cut an inch out of the ally remote and had it welded, then shortened the shaft by same amount.
You would also benefit from Anti tramps which are quite easy to make.
I also went for the quafe LSD but its quite expensive and not essential. ( just means you can leave 2 black lines on the road instead of 1 ).

Mark
Mark

Hi

As I research the 4.9 I see that the bore is the same (94mm) as the 4.0 and the stroke is increased 10.9mm in the 4.6.
Mark you mentioned "Better still have 4.0L pistons fitted to the 4.6 to up the CR."
There must be some difference in the pistons?

Bruce
Bruce Mills

I have a couple of other questions as well

Does the 4.6 have to have the oil journal reamed out to allow for more oil flow?
Does the 4.6 need an up-rated oil pump?
And what kind of distributor comes with it. Would my pertronix
Bruce Mills

Bruce
Yes the 4.0L pistons have a smaller dish.
No modifications are needed to the oilways, but there are 2 options.
1/ Use the classic SD1 timing cover with cam driven oil pump.
2/ Use the intermediate front cover which has a crank driven oil pump and a serpentine drive belt for water pump and alternator.

Im using the classic timing cover with mallory unilite distributor.

If you have top hat liners fitted then a 96mm bore is possible which can take the capacity beyond 5.0L with the right crank.

Mark
Mark

Dave,

I suspect that you may have a setup similar to mine, an old ex-Costello installation. This uses an adapter plate between the Rover engine, a P5B one in my case, and an MGB gearbox. However, for reasons that I do not know, Costello decided to cut away part of the Rover block and mount an MGB starter motor direcly onto the adaper plate.

If you want to replace your 3.5 Rover engine with a later one, you may have to consider anoth way of starting it!

I'm looking at an ex-Landrover Discovery 3.9 engine and looking to see whether I can get an LT77 5-speed gearbox, a R380 5-speed gearbox or wheter to use the much more readily available ZF22 4-speed autobox.

Peter
P L Hills

Hi Peter
That is exactly what I have. I have fitted a High Torque low current starter (MGB model of course).
Do you think the 3.9 is a better bet than the 4.6?
What I want is a bog standard motor smooth as silk with about 200bhp so I can run decent speeds with low stress on the engine.
I am a Auto fan, but have assumed that old auto's are inefficient and clunky!!
Regards
Dave
D M Tetlow

Dave

There are a few 4.6 engines coming up for sale on e.bay, two at the moment one with only 40.000 Mls apparently, but you do have to be careful with the 4.6 as it seems more prone to the liners moving and causing major problems, you can of course have top hat liners installed which solve the problem by sandwiching the top of the liner between the heads and the block, but not cheap to do especially if you add a set of 4.0 pistons to get a decent compression ratio. If you only want about 200BHP a 3.9 in good condition with injection produces 194bhp in standard form (the RV8 engine) and you can easily add another 40bhp with stage 1 heads and a mild cam and probably an RV8 style exhaust system which is worth 10-12bhp on its own, without spoiling the flexibility of the engine.

Re the gearbox, the Rover LT77 5 speed is the minimum requirement, but an auto is not a bad idea. The ZF HP22 is a nice 4 speed box, I had a couple of BMW's with this gearbox and found it very smooth changing and quite responsive to part throttle kick down, you can also change manually if you wish especially if you want to hold a gear for a corner or climbing a steep hill, much better than the clunky old BW35.

I've got an LT77 in my V8 conversion GT and it can be quite tiring in traffic, I'm sixty years old and haven't enjoyed the best of health for the last couple of years though hopefully things are now improving. I may consider fitting the ZF box it's an easy swap as it is fitted to the range rover albeit with a transfer box for the 4 wheel drive bolted on the back, does anyone know if this can just be removed to return to two wheel drive, presumably it will need s different rear cover plate but that shouldn't be difficuly to source. Also i haven't checked the dimensions of the ZF yet but it doesn't look to be much bigger tham the LT77 and theres no clutch release mechanism to worry about, perhaps someone has done it already if so perhaps you could
let us know.

Kevin.

Kevin Jackson

Kevin,
the other problem with using the Land Rover/ Range Rover auto box is they use a very long bellhousing/ input shaft which would position the box too far back in a mgb. Have a word with the Ian or Dave at http://www.ashcroft-transmissions.co.uk/ they know all there is to know about land rover gearboxs and will know if other bellhousing are available and how the box could be used in 2wd form.
mike
mike

Mike,

Thanks for your input I have only seen poor quality pics of the ZF attached to the Rover on e.bay, but did think the bell housing looked a little on the long side, I'll have a word with Ashcroft as suggested and report back with my findings.

Kevin.

Kevin Jackson

This thread was discussed between 19/10/2006 and 03/11/2006

MG MGB GT V8 Factory Originals Technical index

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