General: Velocity stacks, or ram pipes, are necessary on SU carburettors for a variety of reasons. Running an SU devoid of anything to regulate air flow around the carburettor mouth creates a constriction. This is caused by air being forced around a 90 degree angled edge immediately prior to the carb mouth, effectively reducing the flow throughout the induction system.
To maximize air flow potential from any SU carburettor, a radiused entry of some sort is essential, especially on single carb applications. While dyno testing vintage racing engines, various styles of radiused entries were used. Although many different lengths and shapes were tried, the features that influenced flow potential most were: radius size, overall shape and bore taper. Using this data we can now supply a range of velocity stacks for 1.25" and 1.5" SU carbs.
Overly long stacks would severly restrict air filter fitment; the loss of 0.5-1 hp due to a shorter velocity stack is far outweighed by the power loss seen after an amazingly short time from running with no filter! The difference may be even less when using a good filter, such as the K&N, which stabilizes the air flow around the carburettor mouth.
The short velocity stacks are for engines putting out power at higher RPM levels (over 7,000) or where space for deep filters is a problem. The long stacks produce better mid-range performance and are generally better for engines up to 7,000 RPM, but a deep (around 3.5") filter is needed. One thing is certain; either stack is a vast improvement over nothing at all or a badly shaped item.