CARBURATION
The term ‘carburettor’ stirs up a conversation about engines more than anything else in the modern world. Carburation remains a mystery to many, often forgotten about as if it were old technology within the pages of a history book — but that’s simply not true. With older Japanese and European cars that are still running carburettors from their era seeing a resurgence in popularity, people are chasing power on these carburetted engines, and, if a glance at what the boys in any historic race or rally series are doing is anything to go by, that power can definitely be found. And is there really any better sound than that of a Webers howling through the forest at 10,000rpm? To get the ins and outs of how carburettors work and where to start, we spun a yarn with Murray Johnson from Weber Specialties.
HOW DO I CHOOSE THE RIGHT SIZE CARBY?
“It’s easy to buy and sell a product without having any knowledge of how it works, and this is where the biggest mistake happens. Sizing is critical to your application. There’s a notion that electronic fuel injection (EFI) is by default better than carburettors, and that simply isn’t true. Both systems are just basic physics — I often say that this is where paying attention in physics class, instead of sitting down the back playing with yourself, actually pays off. Getting air into the cylinder is how you’re going to make more power — the more air you can introduce alongside the appropriate amount of fuel, the bigger the amount of power the cylinder can produce.
“For instance, with older style cars it can actually be cheaper to achieve the same results as EFI with carburation. You’ve got to bear in mind that original carbs, especially of the Japanese variety, were developed around economy and emissions standards, working on the principle of not a lot in and as a result not a lot out. The carbs were very small. As we don’t have emissions standards like that to restrict us, we can introduce more fuel and air by fitting a correctly sized carb and make nearly double the horsepower.
“On the other side of that, blow-through boosted applications actually reverse your thinking. A much smaller carby is required as the forced induction makes velocity through the venturi higher but the negative pressure is now greater so it can draw greater volumes of fuel — effectively turning a two-litre into a three-litre by shoving three litres of air in. The trick is allowing it still to function correctly off boost.”
WHAT COMMON MISTAKES DO PEOPLE MAKE?
“Well, the first is thinking that carburation is old technology and can’t stand up to EFI — which is incorrect. A well-sized and correctlyjetted carb can equal a well set-up injection system at a fraction of the cost in terms of power and drivability. It won’t have exactly the same fuel economy, but replacing a worn-out carb with an upgraded new example is always going to improve the economy regardless.
“The main issue I see on a day to day basis is incorrectly sized carbs. They are almost always too large for the application, particularly in V8s. Many of those who have previously dealt with them carry over the ‘if big is good, then bigger must be better’ way of thinking.”
HOW DO YOU CORRECTLY JET A CARB?
“Again, that’s generally down to previous experience. We’ve done it all before, but as there’s no magic formula for specific applications, because it will vary from engine to engine, we use an onboard gas analyser known as a LAMBDA. It is fitted to the tailpipe and measures the parts fuel to parts air ratio — take it for a drive and read the mixtures to determine what needs to be altered and how. We find that one engine may be happy on 12:1 whereas another of the same type will require 13:1, and another will misfire, requiring more fuel. Experience, driving, and testing allow you to find out what’s going on — the best part is that it doesn’t require a dyno session to do it correctly.”
HOW IMPORTANT IS THE MANIFOLD CHOICE IN RELATION TO THE CARB?
“The shape of the manifold is paramount for good cylinder filling. It is the manifolds that actually make the power, not the carb or injection per se. Unlike EFI, the air travelling through a carb manifold is wet, as the fuel has been atomized through the carb. You want the atomization to ensure the fuel particles are very, very fine, as liquid fuel doesn’t burn in a cylinder; it’s the vaporized fuel that burns. Getting the air and fuel into the cylinders with as little obstruction as possible and best flow is going to help you gain a lot out of your system.”
IS THERE ANYTHING ELSE TO CONSIDER?
“People often forget about the air going in — cold air coming into the system is powerful air. Take a 260z, for example; changing nothing else but the intake to a cold air induction system out to the front with an air filter can net you an easy 12bhp at the wheels. It’s also perfect for Porsches, where people often fit a K&N filter and lose 20hp because there is so much heat in the engine bay.”