When tuning starts to get serious, no matter what you do with regard to induction kits, there will still be some kind of restriction elsewhere on the inlet side. Here’s our guide to stage 2 induction upgrades.
Your induction system includes every part of the car up to the cylinder head itself, and while we’re not going to talk about turbos and superchargers (as they’re subjects in their own right), your standard inlet manifold and throttle setup is all part of it. And these may be the parts holding your engine’s performance back.
Uprated throttle body
Standard throttle bodies are generally designed with standard power in mind and not a single horsepower more. So, increasing power and increasing throttle body diameter often goes hand in hand; particularly on naturally aspirated engines where the air isn’t being forced in by a turbo or supercharger.
With most production cars designed with cost, as much as performance, in mind, size isn’t the only problem. Often general flow is poor as the expense required to make super-smooth flowing parts outweighs the performance improvements in the eyes of most manufacturers.
As you’ve probably guessed, uprated throttle bodies tend to have larger diameter throttle plates and a smoother flow than the factory versions. Larger diameter throttle plates allow much more airflow than standard, but you have to be careful not to fit an overly large unit, as going too big will just reduce throttle control; sometimes meaning no more power is gained from about three-quarter throttle, to full throttle. A smooth flowing throttle body is an easily forgotten upgrade as it’s not visible when fitted. But reducing any steps, bumps, or sudden changes in size within the throttle body can make very noticeable improvements in performance, if the standard setup is a poor design.
Performance inlet manifold
Factory inlet manifolds are usually made with maximum drivability at the standard power figure in mind, rather than for power increasing modifications. So, on many engines, they seem to have been designed as an afterthought with little performance consideration at all. Because of this, on certain engines, aftermarket inlet manifolds can give great gains.
While the long, thin, inlet runners and tiny plenum of a standard inlet may give the best throttle response and low down torque, on most engines, it’s usually at the expense of high-rpm performance. A larger, freer-flowing inlet manifold can, potentially, give a big performance increase, especially when combined with other engine mods, such as uprated camshafts.
Individual throttle bodies
This is the ultimate induction setup, and the vast majority of race cars, as well as many highly-tuned road cars, utilise this design. Individual throttle bodies, often known as ITBs, are exactly as they sound like. They replace the standard single throttle with one throttle for each of the engine’s cylinders. Having one throttle per cylinder not only tends to give the highest airflow and least restriction, but it allows the throttles to be placed really close to the inlet valves, giving the engine lightning throttle response.
Another advantage of ITBs is they allow an engine to run wild cams while retaining drivability, as big cams often struggle to run well with a single throttle inlet design. An ITB conversion may be the ultimate in performance induction setups, but it’s not cheap or easy. Not only do you need to buy the ITBs themselves, but they require aftermarket engine management, an inlet manifold to bolt them to, a new air filter setup, not to mention loads of other bits and pieces.
While it’s not a performance consideration, the final bonus of running ITBs is a big draw for many, and that’s the fact that not only do they give a huge induction roar, but they transform an engine bay – every engine looks awesome with ITBs fitted. Fact.
Swapping factory fuel injection for carbs is rare these days. Modern emissions laws mean it’s hard to get a carb equipped car to pass an MoT, and ITBs (which give similar maximum performance but much better drivability), are far less expensive than they used to be. But carbs are still a viable option if you want them; especially on an older car.
For most inline four or six-cylinder engines, a twin or triple-carb conversion looks just like an ITB setup, and gives similar peak performance gains too. Many V8s simply run one giant carb in the centre of the Vee, which gives a big performance increase, without the expense of fitting eight separate throttles for an ITB setup. This eliminates the need to fit aftermarket engine management.