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SU HS4 Carburettor Air Intake

When I was studying for my Associate Diploma in Mechanical Engineering I learnt that if there is a square end on pipes for intake air, turbulence and vortices occur from the air flow which has a large effect on the efficiency of the air inlet pipe.

Adding a bell mouth to the inlet smooths out the air flow.

Adding a tangent plate to the bell mouth does not effect it’s performance.

When I looked at the air intake system on my MGB, I noticed that was exactly what I had on my SU HS4 carburettor intakes under the “Ram Flo” Air filters. I also investigated the original Cooper air filters and found that they have an aluminium plate, to which the air filter bodies attach, with a radiused inlet (bell mouth) to smooth the air flow into the carburettors as illustrated in sketch 2.

It is also worth noting that if a stub stack sticks out (as most of them do) the flow becomes what is called re-entrant, which can create more losses. This is due to the big sweep that the incoming air does, causing vorticies (swirling, energy eating air) depicted by the red ovals on the third picture in the inlet. The negative effect that this has is almost exactly equal to the positive effect of a good bell mouth, which is possibly why many people experience no gains from installing them.

With a bell mouth radius of ( r ) and a Carburettor intake choke of diameter (D) a determination for the radius for bell mouth can be calculated from r = Ratio * D
A Ratio (r/D) of 0.1 is OK but you wouldn’t want to go much smaller, a Ratio of 0.2 is twice as good again but the effect tapers off after about Ratio of 0.25.

I decided that I needed to emulate the original as I probably could not afford to replace the “Ram Flo” filters with the “Coopers”. I came up with an idea to use “MDF” board to make my own bell mouth plate induction See the picture below.

I wanted to maximise the radius on the bell mouth “r” compared to the inlet diameter “D”. The inlet choke on the SU HS4 carburettor is 35.3mm diameter so the optimum “r” was 0.2*35.3 = 7mm radius. However I only had a 6mm radius router bit so compromised.

The finish of the internal diameter and bell mouth was unsatisfactory so I painted the surfaces with a mixture of epoxy resin & hardener. On curing I then polished the bore and bell mouth surfaces with fine abrasive paper to get a good joint with the carburettor inlet choke bore.

I then painted the finished product with silver heat paint.

The base plate inside diameter of the “Ram Flo” filters was enlarged to clear the bell mouth radiused area and filed with a taper to minimise the material thickness edge. A minimum of 38 mm is also recommended between the filter medium and the bell mouth to minimize air flow deviation effect. The “Ram Flo” filters just meet this criteria.

Graham Haywood

February 2021

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