The Holden Grey motor is often referred to as the Holden side plate. The engine earned its name as they were painted grey from the factory. They came in 2 sizes, 138ci and 132ci. More engineering and after market modifications have gone into this engine than any other engine in Australian automotive history.

April 27, 2020

Dunstan rotary valve engine

SEACRAFT, September, 1957 ( Pages 50-52 )


This is the engine which may revolutionise speedboat racing


The engine used in the record-breaking skiff Southern Maid is fitted with a "Holden" engine adapted to "Dunstan" rotary valve and fuel injection equipment for which patents have been applied.

Advantages of a rotary valve are: Almost unlimited breathing capacity, freedom of reciprocating motion, absence of hot spots, compact com­bustion chamber, any desired control of turbulence. The results are: High power outputs (limited only by bal­ance and strength of crankshaft, con rods, bearings, etc.), high anti­knock rating and high efficiency. The existing engine at 11-1 compres­sion ratio will run on any fuel from second grade petrol about 75 octane) to pure Methanol and running changes can be made by manual con­trol of fuel pump setting, as no criti­cal jet sizes are involved.


This shows a Holden engine conversion on the dynamometer but with the rotary valve head removed. The engine was later dry-sum ped by driving the pumps off the camshaft gear. Swashplate fuel injection pump is shown (driven at half speed). This has a distributor to ensure even supply to high or low cylinders whets starting but has no useful purpose under normal running conditions when the jets could be
sprayed continuously and simultaneously.
This is the first Dunstan rotary valve applied to a full sized engine. This version had six separate floating cylinder heads. The fuel
injection is by Dunstan swash plate type pump. Mixture control by swash plate angle is not shown in the view


Most of the running has been with a standard Holden 6 volt coil ignition system with 12 volt applied. However, between raising the 135 cubic inches restricted hull class record from the previous holder's 53 m.p.h. to 60.624 and our latest 63.015, we fitted a Lucas magneto which is undoubtedly preferable to coil ignition at speeds over 6000 r.p.m.

Most of the racing is done on stan­dard or super petrol (about 80 octane) with addition of A pint 0/two-stroke oil per gallon for valve lubrication;

1 pint for touring. Methanol fuel, because of its high latent head and rich mixture, increases output about 12 per cent, over petrol and this fuel is used for special occasions. About

2 per cent. of castor oil is added to the Methanol to lubricate the valve which incidentally never shows any sign of wear or scuffing. A few foreign bodies such as jet tubes have in fact been through the valve and cylinders without affecting seal or perform­ance. This is of course not recom­mended operation procedure.



It is possible of course to fit a metering lubrication system of low consumption to eliminate the need for adding oil to the fuel. Although with self-mixing oils now available petrol/oil is no bother.

A cleaned up version of this design is being put into production for the Holden engine and this will be sold to speedboat, road and track racing car enthusiasts or private car owners on the basis of valve head only, or head plus enclosed front end drive for valve and accessories, plus fuel system if required, or complete dynamometer tested engines suitable for oars or boats. Many orders and interested inquiries have already been received and efforts are being made to expedite completion, testing and manufacture of the new model.

Such a valve could be fitted to any type of engine, using one valve for each bank of cylinder in the case of a Vee engine. However, all present efforts are being concentrated on "Holden" which is readily available at low cost and fits into competition classes very well and also stands a remarkable amount of "hotting-up" for a production car engine.

The valve is of grey cast iron and runs the full length of cylinder block and is hollow for water cooling. It runs at a quarter of engine speed and has two opposed pockets at each cylinder station. These pockets work on succeeding cycles. The original engine has six separate lower heads in floating contact with the valve. The new version has one aluminium head, split on centreline of valve, with six floating inserts which main­tain valve contact and seal. The new head could be fitted in lieu of the ordinary overhead valve head in about an hour. Arranging a drive for the valve is, however, more in­volved, especially if a front end drive is required to leave flywheel and clutch available for normal use.

Valve diameter is approximately the same as the cylinder bore (3") as this gives reasonable proportions of ports over a range of useful tim­ing diagrams and also gives a satis­factory balance of pressure areas to ensure valve seal without scuffing.

The heads have main ports about 1" ;" wide by 21" long and 1" or more deep depending on compression required. These ports are also the combustion chambers. Compression ratios of up to 12 to 1 can be achieved with standard flat top pistons. We might mention that the original set of standard pistons are still good after 12 months frequent racing, although the cylinders were recently linered as the bores were tapered and ridged and were breaking rings. The spark plug is 10 mm. and because of space problem in the original engine the points are i" down, a 5/16" diameter hole, but fire perfectly. Because of the shielding of plugs, plus cooling of the alumi­nium heads most of the racing has been with K.L.G. ten L30 plugs which are quite "soft" for such an engine. Recently we have used K.L.G. ten L80. Plug trouble is unknown. The latest engine used K.L.G. 10 L100 for racing at 7000-7500 r.p.m.

The inlet and exhaust ports are approximately horizontal and on op­posite sides of the valve and are about t" x 2,i". The valve pockets connect these with the main central port or combustion chamber accord­ing to the timing desired. The edges of the combustion chamber port and the edges of the valve ports control the exhaust opening and inlet clos­ing points and the lower edges of the exhaust and inlet ports control ex­haust closing and inlet opening or the overlap timing. Valve timing can be adjusted in detail by machining or filing port edges and the timing can be moved bodily by means of slotted setscrew holes in the main chain wheel.

Valve timing of present engine is approximately: exhaust opens 70 degs. before bottom d.c., closes 30 degs. after top d.c.; inlet opens 20 degs. before top and closes 60 degs. after bottom. This is not necessarily ideal as this is about how the original cast­ings cleaned up and no changes, apart from bodily varying timing (which is not very critical) have yet been made.

The motor (132.5 cubic inches stan­dard Holden bore and stroke) has de­veloped 200 b.h.p. at 7500 r.p,m.

The rotary valve will, of course, not give greater torque at any given capacity and compression ratio than poppet valves at moderate speeds. It does, however, enable higher compression ratios to be used with ordinary petrol and this raises torque and efficiency. It also can be made to give breathing capacity adequate for any desired speed such as 7000 r.p.m. and to 10,000 up to even 15,000 r.p.m. if the crankshaft, rods, bear­ings, pistons, etc., were designed for such speeds. I have a design on which I am working, of a 139 cubic inches engine weighing 225-250 lbs., which is much more compact than a Holden and which would run safely at 12,000 r.p.m. with 15,000 r.p.m. over-runs. This is eight cylinders 111 two banks with two rotary valves. The stroke is quite short and there-foie piston speeds are low, as are the inertia loads.

In racing conditions, such an engine would (IrnsuperchargerVi de­velop 300-400 b.h.p. The design allows for a moderate change of capacity between say 2,000 c.c. to 2,500 c.c. by varying stroke or bore.

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