When is a Brabham Viva not a real Brabham Viva?
This is a very touchy subject, which neither the relevant car clubs nor the Classic Car Magazine World will embrace or even discuss. This is my take on the subject as I feel people have a right to know what they might or might not be paying a fortune to purchase. I will start by outlining a few facts.
1. To start with a mint Brabham Viva is 'supposedly' worth in the region of £1800- £3500, while a standard SL90 HB Viva is £1100 Max.
2. The Brabham kit was introduced in 1967, a year after the production started on the HB series Viva. The Brabham kit itself was sold by Vauxhall Dealers, who would then fit it to your car. They would also sell the kit to you over the counter. The kit did not invalidate a new cars warranty.

So now let's explain what is used to make a Brabham Viva. There are many accounts of this floating around in books so here is my version based on all books and my experience.

The base car MUST be a "90" HB (Deluxe, Sl, Standard). In fact it wouldn't matter if the car was a 4 door, 2 door, Estate or even Auto. (The latter is rather a hopeless case as the auto-gearbox saps all the engine power).
The SL90 is a high Performance version of the standard 1159cc engine. This means it has a High compression head (9.0:1 rather than 8.5:1), a higher lift camshaft and normally runs a single 150CD Stromberg Carburettor. The exhaust was a 4 into 2 into 1 manifold and with a slightly larger overall diameter bore for the rest of the system.

The one thing people forget or do not know is that for this high performance engine to spin freely, revving nicely and to give that so called punch (Well, it was in the Sixties!), Vauxhall's lowered the rear axle ratio. The SL90 uses the 4.125:1 ratio rather than the standard 3.89:1 ratio. This doesn't sound much, but on a small engine car it definitely does give it the ability to pull away quicker and feel more alive. Estates and auto transmission cars had this lower rear axle to start with, regardless of the type of 1159cc engine.

If you decide to use a standard 1159 HB car instead, it would just use more petrol as the engine isn't designed for it and the high back axle ratio just kills it. It might look good, but it doesn't go much better than standard.