Improving Induction Efficiency

Commander Charles Featherstone, R.N., describes a successful experiment with a “12/50” Alvis.

For part of the war I was driving a 1931 “12/50” Alvis over the length and breadth of England and have nothing but praise for its unfailing reliability and ability to cover the ground quickly. Good though the performance was, I always felt, however, that the standard inlet manifold caused a considerable loss of efficiency and power. For the benefit of those who are not familiar with the standard Alvis induction system, the carburetter, a Solex updraught 30 MOV, is fitted low down, with air-inlet masked to some extent by the starter motor. There is then a vertical riser of about 8-in., during which heating is introduced from the exhaust manifold. The riser connects at the top with a horizontal tee-piece running fore and aft which has sharp right-angle bends to the forward and aft pairs of cylinders. The objections to this manifold are: —
1. The gas has to make three very sharp ninety degree turns before it enters the cylinders.
2. The manifold is a casting and therefore rough inside, impeding the passage of the mixture.
3. Some unequal distribution of gas is suggested by the uneven exhaust note at speed.
4. Masking of the air-inlet prevents maximum power.

Now, with any given engine, other things being equal, the power developed is dependent upon the amount of air that enters the cylinders through the induction system. With normally aspirated engines, therefore, the easier and straighter the path of the air, the more will enter the cylinders and the better will be the distribution.

The “12/50” Alvis has an overhead valve engine with ports so arranged that the charge enters horizontally — also 1 and 2 cylinders and 3 and 4 cylinders each share a common port. This, therefore, appeared to call for a horizontal induction pipe and horizontal carburetter to give the easiest passage of gas. (See 2 and 3 of Appendix.)

The order of firing is 1, 2, 4, 3, and it seemed that considerable improvement should be possible by making up a Y-type manifold with fairly long branches so as to give the gas an easy alteration of direction from a centrally-mounted carburetter to each pair of cylinders. At the same time, the amount of available room under the bonnet must be considered and in this case was a limiting factor.

The first step was to find a suitable carburetter and, by a piece of good fortune, I was able to pick up a 30-mm. S.U. complete. It should be noted that the float chamber should be ahead of the choke tube so as to give a slight enrichment of the mixture when hill climbing, which helps to cool the engine.

The inlet manifold was then designed. The points to note are: —
1. The internal diameter (1 3/16-in.) is critical, as obviously it should be the same as that of the carburetter bore and inlet port on the engine. These are 80-mm., but 1 3/16-in. is as near as makes no practical difference. This is an unusual internal diameter of pipe but can be obtained. In my case I was able to obtain a length of solid-drawn, mild-steel boiler tube.
2. The material can be mild steel, copper or brass, depending upon what is available.
3. The manifold is sufficiently strong to support itself and the carburetter without additional staying.
4. If a welded construct is adopted, all internal roughness should be smoothed out with riffler files or other suitable devices.

It will be noted that no heating is incorporated. There were two reasons for this, viz.: –
(a) Any heating increases the specific volume of the gases and therefore reduces the weight of gas induced per stroke and thus the power developed.
(b) To introduce a hot spot calls for a casting, with its attendant high cost for “one off” and rough interior finish. (Note. — Welding can be employed but is apt to introduce distortion.) (See also 4 of the Appendix.)

The manifold was made up for a very reasonable sum by Messrs. D. Bilton and Sons, of North Shields, who have intimated that they are willing to make up others if required.

As regards the standard manifold, there are two alternatives available: —
1. If of the pattern prior to about 1928 with inlet and exhaust manifolds cast in one piece, the inlet branches and induction pipe from carburetter must be sawn off or otherwise removed.
2. If of the later pattern, the top of the inlet manifold is detachable and should be removed.

Some difficulty will undoubtedly be encountered in fitting up the accelerator control. It was finally arranged by a bracket 5 in. high secured to the crankcase, and at the top of the bracket was mounted a bell-crank with 2-in, arms, which changed the near-horizontal of the rod from the accelerator pedal to a vertical motion suitable for the carburetter throttle. Various control rods, ball-and-socket joints, etc., were obtained from a local car breaker.

It only remains to add that when finally fitted up and the carburetter tuned, the results far exceeded my expectations, as detailed under.
1. Starting. — Very easy indeed. The S.U. seems to be the best starting carburetter I have yet met.
2. Acceleration. – Improved out of all recognition.
3. Maximum speed. — Considerably improved. No opportunity has yet arisen to try it out, but 60 m.p.h. can be reached and held with ease, even with a heavy saloon body.
4. Hill climbing and top gear performance. – Both considerably improved. Hills which could barely be climbed on top before are now climbed with comparative ease.
5. Exhaust note. — Much more even and regular.
6. Fuel consumption. — About 28 m.p.g.
7. Generally, the engine runs much more sweetly and smoothly with much more power available and also tends to run cooler. The difference has to be experienced to be believed. and the car handles more like a 15-h.p. car than a 12–h.p.