CBUEB1771 wrote:The inlet trumpets are interchangeable tuning parts on Webers just are are the chokes and jets. Lotus specified 1.5 inch (38 mm) long trumpets for the Twin Cam as installed in the Elan and +2 as seen in the work shop manual's data section. Replacement Webers may very well come with the wrong length trumpets. Trumpets in all the various lengths are available from all the usual suspects. If had the wrong ones I would swap out for 1.5" trumpets, assuming normal road use. Generally speaking, the shorter the inlet trumpet the higher the engine speed at which optimum cylinder filing occurs, meaning the peak torque point occurs at higher revs. This is due to the length of standing pressure waves in the inlet tract. Shorter trumpets lead to shorter standing waves and therefore higher frequencies at which the standing waves optimize cylinder filling. For Lotus the 1.5 inch trumpets were probably the best compromise between the desired torque curve and engine bay dimensional constraints.
I spent some time looking at this a while ago. I had assumed that a engine inlet system was a one-way mechanism - air flowed through the inlet into the engine. It is much more complex than that - the flow is bidirectional. The inlet tract / carb / trumpet forms a tuned pipe, in a similar manner to an organ pipe. The formation of standing waves at certain engine revs leads to the fuel being blown out of the carb air inlet and forming a stationary 'cloud' of air / fuel vapour (standoff) hovering over the trumpet allowing the standing wave to be visualised. These dyno videos clearly shows the 'standoff' as the engine approaches full revs.
https://www.youtube.com/watch?v=Sx_V9t48YaMhttps://www.youtube.com/watch?v=ssd9sLpYi0sI was never able to figure out the maths to predict the appropriate length of 'pipe' but it seems reasonable that changing the length of the pipe changes the resonant frequency and the point at which the standing waves develop. The aim is to have a high pressure node develop just behind the inlet valve at the desired revs - effectively giving a 'poor man's supercharging' effect.