Admission cones for Volkswagen Combi Split

What a horn actually does

At the inlet of a carburettor body without a horn, air enters from the sides and has to flow round a sharp edge. It lifts off, forming a constricted flow that is narrower than the duct, and the flow rate is reduced as a result. The bell replaces this edge with a trumpet-shaped radius: the air follows the wall, the airflow occupies the entire cross-section, and the effective flow rate increases for the same throttle opening.

The gain is not merely theoretical and is all the more pronounced when the engine is fed by short, independent carburettor bodies, as is the case with horizontal or vertical twin-barrel carburettor set-ups. This is also why a poorly chosen throat – one that is too short or incorrectly fitted – impairs performance rather than improving it.

The length tunes the engine

Every time a valve closes, a pressure wave travels up the intake manifold, is reflected at the open end and travels back down. If it returns just as the valve reopens, it aids the filling process. This timing depends on the total length of the manifold, including the intake pipe.

• A long header extends the intake system and shifts this effect to the low and mid-range revs. The engine gains in smoothness and acceleration, which is generally what one is looking for on the road.
• A short bell shortens the intake manifold and shifts this effect towards high revs. Low-end torque is reduced, whilst maximum power increases.

There is therefore no single ‘correct’ length in absolute terms; rather, there is a length suited to the intended use. Changing the length alters the air-fuel mixture, and thus the richness: a jet adjustment should be carried out after testing, rather than as a cost-saving measure.

Diameter, connection and fit

• The diameter of the carburettorbody is the primary criterion: the intake tube must fit exactly, without any steps or internal protrusions. A step at the junction negates much of the aerodynamic benefit.
• The method of attachment varies: some intake manifolds are screwed on, others clip into place using a retaining clip, whilst others are fitted together with the air filter. Check what your specific setup requires.
• The available space under the bonnet, which often limits the length before the intake pipe. An intake pipe that touches the bonnet panel is unusable, and short versions with an integrated filter are available for this purpose.
• Finally,the material – aluminium or stainless steel – which does not affect how it works but does influence the finish and how well it holds up over time.

Filtering, even on an open-type fitting

A bare intake manifold looks good but leaves the system wide open. Anything that passes the inlet ends up in the cylinder: road dust, sand and debris. The resulting wear isn’t immediately visible; it becomes apparent later when measuring compression.

Filters – made of foam or fabric – can be fitted directly onto the air intakes, including very compact versions where space is limited. They cost a fraction of the price of a top-end engine overhaul. On a vehicle that is only driven in good weather and on clean roads, not fitting a filter remains a justifiable choice; on a car used regularly, it is a saving that ends up costing you twice over.