Maximum Side-Draught Carburettor Potential

For many years now, the steel-formed inlet manifolds - produced by a well known steel tube manifold fabrication company in England - have been accepted as THE performance manifolds for side draught carbs. This was mainly due to the extremely poor quality and terrible basic design of the alloy cast manifolds on the market. There were one or two that were deemed on a par with the steel ones - and were supposedly multi-funcitonal to use SU's also - but were more expensive and not particularly readily available.

However, the ever-escalating costs of performance parts, my ever-enquiring mind, and Mini Spares policy of wanting to provide it's customers with the best available components at affordable prices, set me on yet another investigation. The prospects were not too daunting following the sucess ofthe Mini Spares SU manifolds, so a target of criteria were set.

The new manifolds would have to out perform those currently available off the shelf, ie. no further customer modifications would be needed. They would have to be cheaper, yet of superior quality, and include a simple, but effective throttle linkage. This last item was included as in many instances the cost of a decent throttle linkage added around £20 to the initial cost of the manifold, and in the case of in-line A series engine installations (such as Sprites and Midgets) or kit car made using current linkage kits impossible. Mainly due to the close proximity of the bonnet/boot line.

As always, close examination of those manifolds currently available started the procedure. A selection of randomly selected steel manifolds were flow-tested to set a data-base to work from. The results were some-what disappointing, as none of those tested reached the figures acclaimed by one of my esteemed colleagues - David Vizard. Not only was there a significant shortfall in the flow readings - but a marked inconsistency. This inconsistency was not only from one manifold to another, but also from one port to another on the same manifold. This would compound the power loss caused by port-biasing already suffered by the venerable 'A' series engine. From previous experience the generally available alloy manifolds were not tested. I had already found these to be much less effective unless extensively modified by an exponent of the art. Having said all that, the steel manifolds were still far superior than anything else that was available. The inconsistencies were not huge, but measurable - which means a measurable power loss at the end of the day.

With all the relevant data, and a set of criteria to meet, I decided on casting the manifolds in alloy. This solved the cost problem as it is far cheaper to cast in quantity than to fabricate in quantity. The expertise of the foundry, and repeatable accuracy of our machinists would provided the consistency of performance that I was after. The casting method also allowed me to incorporate the desired bosses needed for the throttle linkage, and also a boss for a servo take-off should one be required by the end-user. The only benefit of a steel manifold over an alloy one that I could not match "off the shelf" was its greatly reduced temperature conductivity. It has been claimed that this can add 2 or 3 hp to the engines output by reducing intake charge temperature. I myself have witnessed such an advantage. However, I have also witnessed the problem of icing up caused by too Iowa temperature. A problem suffered mainly by those using the steel manifolds on road-use cars, and by more than a few racers. This can be much more of a problem than the loss of a couple of horse-power, as with no fuel the engine won't go but will not suffer over-much by a very slight shortfall in output.

For those worried about this power loss, it is far better to use exhaust wrap to reduce intake temperature. Apart from anything else it is cheaper to use an alloy inlet manifold and 5 metres of exhaust wrap than to buy a steel inlet manifold. Chroming the inlet to maximise on this cool-charge benefit is even more expensive. I have recently been quoted between £20 and £25 to have an inlet properly chromed! And with the Mini Spares manifolds you do not then have the extra expense of the linkage kit. Next up was a visit to the people who produce a significant part of our ST stocks. They have many years experience in producing parts for the world of motorsport, and always produce ultimate quality components for the price required. The criteria required was explained, and using a similar approach to the way Mini Spares SU manifolds were developed, the initial drawings were produced employing a hi-tech cad-cam system which helps ensure ultimate accuracy with speed. These were checked and OK'd. The patterns made and first generation castings done. As soon as they were cool, they were checked for consistency against the drawings, found to be perfect, then whisked away to be flow tested.

Once again the flow testing was done by my good friend Mike Parry at Race Techniques. Those readers of David Vizard's "Tuning BL 'A' series Engine" will be familiar with Mike's name. Using his "standard" head (flows 124CFM) that he uses in all our trials, he tested the "straight cut out of the mould" alloy manifolds back to back against the best steel manifolds we had. The results? Well .... 3.75" Steel inlet manifold 3.75" Mini Spares inlet manifold 6.75" Steel inlet manifold 6.00" Mini Spares inlet manifold 116.2 CFM 116.8 CFM 116.6 CFM 117.4 CFM

And these were not finish machined. It was speculated that an extra .4 to .6 CFM would be picked up when finish machined. Mike barely tickled the 3.75" Mini Spares one to see what happened and gained an extra 1.0 CFM! But at the end of the day we had achieved all the criteria set. An off the shelf manifold that outperformed all others available at a considerably cheaper price. Sufficient material was put into the original pattern work so that the manifold would take 40, 45 or 48 side draughts. The linkage is a simple rose-jointed affair that keeps the linkage height lower than the float top. A by-product of this design of the 5.00" and 6.00" manifolds is a marked reduction in port-biasing of the fuel/air mixture. It is not possible to achieve on the short manifold due entirely to the lack of distance between carb and head.

C-AHT772 3.75" Manifold inc. linkage £58.75 C-AHT773 5.00" Manifold inc. linkage £58.75 C·-AHT77 4 6.00" Manifold inc. linkage £58.75 For comparison 3.75" steel manifold and linkage kit £82.92. All prices including VAT.

Keith Calver

All Prices are Historical