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MG MGB GT V8 Factory Originals Technical - carb for 3.9 V8 Conversion

Gday all,

Just wondering if anyone can help me out...i am currently contemplating throwing out the injection system on a 3.9 renge rover engine I have recently purchased for my v8 conversion...(after some advice from a relatively 'old school' gentleman who told me the EFI system wasnt half as good as a webber or holley!?!)

Just wondering if there is any advice out there regarding what model Holley I should use? I would like a 4 barrel but dont know too much more than that? Has anyone done this before?

I assume that my bonnet will need modifying...or can I have the carb machined to all fit (I am mindful that an air cleaner assembly will also need to sit on top of the carb)

any advice would be greatly appreciated!

Also I am considering a CAPA supercharger for the 3.9...visually it looks like it will fit, but not sure if anyone has tried this in practice?

Thanks again!

Look forward to your comments/advice


Hi Marcus,
A few questions there so where to start?

My suggestions only here so feel free to accept or ignore them as you feel.

Before you add on or heave out anything stat with a plan!
What are you going to do with the car? Daily driver , track days only , bit of both etc.
This will influence things like insurance and roadworthy / engineers certification.

To answer the questions you asked above:

If the EFI is intact and functioning correctly then fit it as it is. This will be quicker cheaper and have fewer hassle s than changing to a carb.

EFI , CORRECTLY SET UP is better than any carb arrangement. People stating otherwise are about thirty years out of touch.

A Holley 390 is a reasonable choice for a balance of power / economy / purchase costs and knowledge and spare parts availability.
That said there are a range of carbs suitable for the Rover 3.9.

The bonnet should be OK as is if the change over is done correctly.

The CAPA supercharger is an alternative BUT check your insurance and the legal side first.
The Rover 3.9 effectively doubles the power and torque of the original so you may want to try the non supercharger set up first (perhaps in another car) before you decide.

Also there are several books on this conversion which you might want to have a look at.
Try Pitstop Bookshop or Technical Book and Magazine online and search for books by Roger Williams.

Cheers , Pete.
Peter Thomas

Thanks Peter,

My plan for the car is for a "toy". I have a (sensible) work car provided to me, and would like something a little bit more exciting to drive and play in on the weekends.

My father and I planned to always do a conversion and I had a 1978 B as my first car...she was already to convert when my dad contracted cancer and passed I guess the project is a bit of a legacy too.

That being said I still want to roadworthy the car (ie not track use only) and would like to attain some form of insurance cover...i have done a bit of reasearch and Just Cars will insure me no probs (with or without supercharger!)

My plan with the build is to start her off naturally aspirated, and maybe force feed her if I feel the need for more poke down the track (Just as long as I can blow my mates imported Nissan off the line I will be happy!)

I notice you mentioned troubles changing over from this conversion present some issues?

Thanks again for your comments!

Marcus - there will probably be as many views on your options as there are contributors to this Board but those of us who watch (or who are forced to watch)fuel economy, will follow Pete's advice - the basic injected 3.9 set up will double the power of the standard B - cleaned up heads and a hotter cam will take BHP over 200 and still be good mannered in traffic. Then there are chip up-grades. As the archives will tell you, the plenum would need to be shaven to fit under a standard bonnet (and there is plenty of good advice about that) - the US brigade will tell you how to get a carb outfit under the bonnet should you decide to go that route.

No - the FI fit is not too difficult - you need an extra fuel line into the tank and a 40psi pump - the electrics are thought-provoking and easier if your loom has been labelled.

HTH Roger


In case you havenít already found it
plenty of photos to get the enthusiasm going
and I would also recommend the other Rogers books.
I Particularly liked the second one
(which can be got quite cheaply now that the new 3rd edition V8 conversion book has appeared on the scene.

The Nissan won't be a problem, just work out the relative power to weight ratios. You are going to love driving this thing once done. The other car will tend to be neglected I promise.
That reminds me, because the MGB is so light relative to the rangerover you will need to either get half an inch machined off the rangerover flywheel, or buy one already done or buy a manual rover flywheel (which are quite rare).
can sell you a flywheel, but they do use a coarse thread in the bolt holes so you will need to align carefully.
Be interested to hear how the supercharger goes, In my lighter headed moments I consider doing this since I have a the low comp' engine common to range rovers. You will likely have to change the diff' if you boost the power that much. You will also have to put on antitramp bars. Or fit a trailing arm set up as I am about to.
P.N. Sherman

Had a look over on the capa site
obviously you are not short of a dollar if you are looking at this sort of gear.

That kit will sit up too high at the front, although I dare say the brackets could be adapted to sit it down lower. There is certainly room on that side of the motor. Neal at Plus Four over in Dingly st Dandenong(03) 9791 8025 recently did a vortec supercharger instalation on an MGB V8. This is a neat instalation that fits under the standard bonnet and is street legal. Produced huge amounts of horsepower aparently.
P.N. Sherman

ps-this is a fuel injected car
P.N. Sherman

Changing over from EFI you will need to source the relevant carb , manifold , linkages , lines and what not. Not a major problem but one which can easily become long winded and entail a lot of time.

I am speaking from experience on this one having been the unwilling helper (and ended up doing the bulk of the project for the owner)on converting a Renault 16 TS to Gordini spec. with twin DCOE Webers in place of the standard carb and a set of factory extractors.
What should have been a weekend with the spanners turned into a six weekend nightmare involving fabricating brackets , exhaust hangers , throttle assemblies , extra hose extensions and a lot of other things besides.
And that was using the factory "kit" such as it was.

Not an experience I would wish on my worst enemy!

In most EFI setups as well the ECU handles both the duties for the spark and fuel so some provision will need to be made for the spark if the EFI is taken out.

I am not trying to put you off here but it is so , so easy to get in the deep end very quickly.

Is there an MG car Club somewhere near you? Most likely there will be at least one member with the conversion you are thinking about doing already done.

Good luck , Pete.
Peter Thomas

(Just as long as I can blow my mates imported Nissan off the line I will be happy!)

Plan for some type of limited slip differential (Quaife or otherwise) and other rear end traction mods if you want to do this. The MGB is light and doesn't hook up to the road well from a standing start.
Edd Weninger

Marcus, there was a guy in Mudgeeraba who dabbled in MG conversions & other things MG. I don't know if he's still around though. :- Brian Darke
Maintaining the Breed
07 5530 2089
I use Shannons for insurance. I would recommend that you stick with the EFI. A new manifold & carby will set you back about $800 at least. A lot more if you go down the Edelbrook road. Check out the archives for efI info, there's lots of good advise there. Barrie E
Barrie Egerton


Tell the old fart to go back to school and try, this time, to get past first grade.

At the age of 63, I junked the carb, 500 CFM Webber, and switched th he Rover fuel injection, with more than a little help from this list. No more carbs for me, after alost 2 years of driving.

another old fart

Jim Stuart
Jim Stuart

Marcus, just saw your email. You could contact David who used to build V8s for Rod Hiley (Abingdon Motors). He operates solo out of one of the northern suburbs near samford or brendale I think and can be found in yellow pages. Suggest you also call 32534852 and talk to the Motor Vehicle inspectors before going to radical. As an ex-motor vehicle modifications officer in Qld I can tell you that whilst anyone will sell you any bits you want and any mechanic will fit them somehow, when you crash into someone like me who has a good idea of what is legal and not then you will pay with everything you own when I sue you for driving what may be legislated as an uninsured (as in no third party, not comprehensive) motor vehicle...naturally all your comprehensive insurers will also wipe their hands of you as well...Bonnet scoop heights are one thing that come to mind if a supercharger is fitted
mark (MGA with V8)

This thread was discussed between 29/11/2006 and 01/12/2006

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