Carbs & Throttle bodies
In this modern day and age there seems to be no end of ways to improve the power output of your engine, be it a turbo, supercharger, nitrous, cams, head work, chips, carbs or throttle bodies or more. Despite the choice though throttle body/carb conversions are still popular. This is probably due to many benefits and cost effectiveness. This article explains the fundamentals behind this tuning principle and the probable power gains as well as explaining the differences between carbs and throttle bodies.
How it works
The idea of slapping big carbs onto your engine isn't a new one. Going back to the 60's or probably before you'd find this being done and indeed it does seem a step backward in time replacing a modern day injection system with carbs but there are good reasons as i will explain.
Basically the factory injection engines these days are very good and generally very reliable. The fuelling is controlled by the ECU through the injectors and they run well for many miles reliably. When the injection system was created by the car manufacturers they had to take into account varying atmospheric conditions and fuel qualities throughout the car marketplace. Therefore the system is conservative and can be bettered for optimal performance. Most standard injection engines use 1 throttle body after the air-flow meter to supply air to the engine and then on into a 4 branch induction (inlet) manifold. The injectors then add the fuel before the mixture enters the cylinders. Now it stands to reason that if you want more power you need more air + more fuel so if you were to add 4 openings say of 40 or 45 mm one for each cylinder you can imagine the extra air for starters. Couple this with more fuel from injectors or carb jets and you are feeding the engine more. This means more power ... lots more !
Carbs
Basically there is a choice of makes here, commonly used are Weber and Dellorto. The problem you will have with other makes is finding a manifold to fit the carbs to your engine. Both these makes have readily available manifolds. The carb works by using jets to supply the fuel. The fuel flow delivered through these jets is basically determined by suction from the vacuum in the cylinders (Venturi effect). Different size jets will allow different amount of fuel supply. The Weber DCOE40/45 kits can be supplied with the correct jetting for a standard or modified engine for your car and the linkages to connect them up. Webcon (Weber) have released a series of kits for carb conversions on various cars. The cost is around £1000 including all you need which is both carbs, manifold, gaskets, fasteners, sensors, linkages and the alpha 3d programmable 3d ignition system. (more on that later)
Throttle bodies
A lot of people get confused and say throttle bodies when they mean carbs and vice versa and often think that they are the same thing. Essentially they are the same thing and do the same job but the main difference with throttle bodies is that they use injectors instead of jets to control the fuel delivery. This means that you can electronically control and program the fuel delivery from an ECU and therefore have more control. They are more expensive though and ultimately don't deliver a great deal more power than carbs but they do offer refinements An injected car and cold starting can be a bit spluttery with carbs whereas even a cammed up engine will start perfectly from cold on throttle bodies and idle smoothly as low down as 600 rpm in many cases. The throttle bodies do provide more torque lower down generally and ultimately a better spread of power. The Weber Alpha throttle body kit consists of all the same components as the above carb kit but with the throttle bodies instead of carbs, the kit is around £2000. The 3d ignition system allows programming of the ignition which bypasses the normal distributor and is therefore customisable.
Are they worth it?
At the end of the day these kits are a good way to go on a normally aspirated petrol engine. You can bolt the carb kit onto your standard engine and it will run well. It's a few quid but you could easily install it yourself if you are reasonably competent with the spanners and once you've driven it to a rolling road tuner you should expect around 15-20% more power usually. I would recommend the throttle bodies over the carbs for reliability and tune-ability but if budget doesn't allow then the carbs will do fine thanks ! They do sound the dogs bollocks as well which is a bonus ! You will need to go somewhere with a rolling road and knowledge of that system to be sure of a proper setup session but expect some real results.