Induction - Carbs, Manifolds and Injection

What do we want from an induction system? How about this?

  1. A sufficient volume of air and fuel to allow the engine to develop its maximum potential power
  2. Fuel mixed thoroughly with the air, and in particle sizes uniformly small so they burn quickly and completely
  3. Uniform mixture distribution from cylinder to cylinder

Satisfying number one is easy; it's just a matter of making everything big enough . Number two is a bit harder, and seems to be particularly challenging for certain popular American carburetors. Number three depends mainly on manifolding, with some types it takes care of itself, others will take some work to get right. So how do we know when we have got it right? There are two key indicators, firstly the engine will make good power. And secondly it will have good fuel consumption relative to the power produced. I know this is supposed to be about high performance engines but it really is important to monitor fuel consumption as well; it's a sure sign of how efficiently the engine is running as a whole. Every now and again you might hear some young bloke tell you how he has built an engine so incredibly powerful he can barely back it out the driveway without refilling the tank. The correct response of course is to smile and nod, and to think to yourself: "You fuckwit. If you ever get that thing running properly it will make twice as much power and use a fraction of the fuel." Even a triple carbed, long-overlap cammed engine should give reasonably good mileage on the freeway, very similar to a stock engine with similar gearing if everything is set up right. So if your engine turns out to be thirsty in normal use, rest assured there is more power to be had by getting it to run efficiently. A good illustration of whats possible is the modern fuel injected engine; these are making much more power and torque than the engines of say 30 years ago, but at the same time using much less fuel and putting out less emissions. Burning every drop of fuel as completely as possible isn't just good for mileage, it's good for horsepower levels too.

Probably the biggest challenge related to induction systems is finding a well designed manifold - there are literally dozens of different types available but unfortunately the majority of them are extremely poorly designed. Don't be surprised to find you have to fabricate your manifold.

Injection or Carbs

So which is best? It depends. A well sorted injection set up will outperform poorly done carburettors, just as well tuned carbs will run rings around badly tuned injectors. What it really comes down to is where your knowledge and experience lie. If you've spent years tuning SU's for example you could probably have an SU equipped engine running optimally pretty quickly. Conversely, if you know EFI inside out and have the right gear then injection is the obvious choice. While injection theoretically has a slight power advantage and is easier to live with day to day (if it's correctly set up) I think it's much more important to pick whatever you can tune accurately in a reasonable timeframe. In other words it's irrelevant which system has the most potential, what matters is which system you can get the most from.

Types of Induction Systems

We'll come back to injection systems later... in the meantime we'll check out the two basic carburetor/manifold layouts: common plenum systems and individual runner (IR) systems. We'll look at the IR systems first..