12/14/2023 0 Comments Su velocity stacksThe key parameter, for me, to analyse the performance between 2 and 3 carb is the ability (on the configuration with 2 carb) to maintain a continuous and bette flow on each of the 2 carb. On my side based on the same understanding, I am surprised when I see 3 carb. PeterI like the way you present it and the associated arguments. In that peculiar situation a further rise in air flow above that needed to fully lift the piston will benefit from a bellmouth. The piston pressure drop in effect negates any gain from the bellmouth, except in the highly unusual tunes that remove the spring and air flows are so high as to lift the piston fully. Very easy to measure with kitchen scales and a ruler. If the piston had been incoporporated the its pressure drop is : (mass + spring downforce)/ lifting area. ![]() The bellmouth at this air flow drops it to about 98000 ( thats 2%) and the bridge to 96,000 ( another 2%) ( ingnore the local effects of the needle). He doesn't show the pressure dropped across the piston at part lift, which will be normal for an SU even at full power.ġ00,000 Pas is atmospheric pressure 14.7psi What looks to allow more mixture flow - whether its an extra SU or bellmouths - may not match the reality of fluid mechanics.ĬFD false colour depiction of pressure drops across the bellmouth and across the bridge: The manifold can be symmetrical with triples, so far easier to split the flows equally.Īnd then there's pressure losses which are set by the piston downforce and its area. The tricky bit is in the manifold splitting one flow into three equal flows of air and fuel droplets and fuel films. Ie equal spacing, which promotes equal mixtures. Very difficult to get ideal mixtures on all six, and the middle needle is often made leaner ( I think, not sure) than the outer two. The longer gap means that 1 and 2 see different mixtures due to inertia of fuel in the jet etc. But a longer gap is found between 1 and 2. ![]() ![]() So the middle carb sees equal times between iv openings - which is good. Putting gaps are when an iv is not open, so for each of the three carbs: However, if the piston runs fully lifted normally - which is unlikely (except in circuit racing) -then adding a bellmouth could lower the pressure drop, a bit.Įxpanding Andy's comment about firing order: Which is why SU stock fittings often never bothered. The pressure drop overall, with or without a bell-mouth, stays much the same as it is set by the piston downforce and area. Air particle velocity will rise a bit, which might help atomisation. The entry loss depression will help to lift the piston, so eliminating it with a bell-mouth will result in less piston lift, because less pressure has to be dropped across the psiton itself. They look mighty impressive - how do they affect performance/ where to use.Ĭompared with a sharp-edged, flat mouth to an SU they will reduce the loss of pressure at the entry.
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