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MGB V8 4.6 Power Upgrade.

A Classic MG Roadster, Traveled in from Germany

After so many weeks of deciding an exact power upgrade for this 4.2 MG Roadster, It would seem our better judgment prevailed once again.
Before it came over, it only had 132 BHP (4.2Ltr) No wonder the owner was not impressed. So I new we could do loads to help, but It was not until it arrived and we got deeper into its actual status, that I really new what.

MGB 4.6 ltr. V8 stage-3

Castello special chassis-No Piston bores are contaminated, The raised area of the engine numbers is required to be removed to facilitate the fitting of approprate Headers.
CC test on a 3.9 Castello engine
Careful Measuring the bores, however reveals that the stroke of this engine is that of a standard 3.5 or 3.9 so with the 94mm Pistons we now know it is only a 3.9.
V8 Hydrotaps
The contamination on the original engine's surface is also very bad for such a low mileage engine.
This is a very unhappy engine,
Hands on, The customer finds out the worst for himself,
This M.G. Roadster 4.2 , as you can see, turned out to be only a 3.9, also the plenum mods carried out by, Its no wonder this engine produced no more than 135bhp.
Before the engine had been seen, the original plan (plan A) was to improve the injection system (as with all stock EFi V8's and tweak the 4.2 with new stage 3 heads and Piper 285 cam. (obviously this was not going to happen, so nothing left for it but to resort to plan B)

Plan B. Take it all out, fit a stage 3 4.6 with A Weber 500 and Mallory ignition.>

We soon realized that it would not cost much more to put a whole new 4.6 stage 3 in, Run it on the Weber Carb. system ( Plan B) and there it was, up to 270bhp engine and a very very happy customer. The above & following pictures, show some of the problems with the original 3.9/4.2?. engine. we have many more pictures that should appear soon.

By this time he had nearly had enough!!.
Customer is working
The owner, Hands on.
Remooving the engine
Very Weak Mixtures.
work is ready
Now its better
The results of preceding mods to the injection system, show clearly how weak this engine has been running, The Valves in the Heads show it all. And close up shows center Cyl's are weaker still, it's lucky the unequal combustion of this engine has not already caused a complete failure. at best it has been running very inefficiently. Anyway once the Rpi 4.6 engine is in then nothing else matters, and all things before it are just another learning experience, not only for the customer but for ourselves too., Who knows what will turn up next?.
discusion on open bonnet Now its all over, The engine you can see here Is a complete replacement RPi 4.6 Stage 3 With Weber 500 and Mallory ignition, and considering what the customer has been through with his previous engine. He is now a very very happy Man.
With at least an extra 100Bhp on tap, and improved fuel economy, It just shows that a happy engine is an efficient engine.

" The Owner, Looking Very pleased, as you would expect".

Happy Custumer Happy Now ?? he certainly looks it.,

You bet. There is nothing to match the character of a 60's sports Car (MGB Roadster) with the immense torque and drive-ability of a 4.6 V8,
It's so unstressed an engine this car could be used as shopping trolly In Tesco's, or a "Street racer" to rival anything on the road.
But the ultimate is That "Big Big" sound of a Large V8. coming ever closer.


Prices do not include local EU.Tax.(VAT).
Prices & stock are subject to change without notice. Information and advice, as always, is free.

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