Harv's Repco HighPower crossflow head thread

[Errol62]

Yes, without joking, there are quite a few Jolly’s floating around the region.


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[Harv]

Some time back I read Phil Irving’s autobiography (he of the Repco head). It was heavily motorcycle-centric (as was his career), though there were a few pearls in there for the Repco head. I shared that info here as I figured it would be of interest.

I’ve recently finished reading another one of his books, “Automobile Engine Tuning. High Performance and How To Obtain It” (Temple Press Books Limited, 1965). It too is cycle-centric, but also has a few interesting bits for us, which I will also share here. What strikes me is that the info was written by someone who was actively engineering and racing the grey motor in it’s era, without the noise that comes from later red motor practices (for example the later availability of bolt-on parts). It represents how things could be done then (40-60’s, without all the bolt-on parts), and hence can be done now.

I'll post it in several threads, as I have tried half a dozen times to post it at one go and am struggling.

P66: “Tuning procedure for Zenith, Solex and Stromberg carburetors follows recognized textbook or instruction manual methods, as long as petrol is retained. A difficulty sometimes arises, however, when multiple mounting is used, because the intermittent flow conditions then present may make mixture regulation difficult, the usual effect being to cause the mixture to become very rich at or just before full throttle, and the engine will show a reluctance to run at a speed which should be well within its capabilities. This effect can usually be cured by increasing the air-correction jet size, though, in some Stromberg models, this jet is simply a drilled hole and is not normally alterable by fitting another jet, as in the Solex or Weber. For example, the air-bleed in the Holden Stromberg is a hole of 0.020 in. diameter drilled in a small bullet-shaped pressing near the outer edge of the venturi. When two are used to feed the six cylinders, this hole should be drilled out to 0.060 in. After this treatment, the carburetors will be almost impossible to tune correctly, if sold to some unsuspecting person who endeavors to use just one of them on a normal engine, unless the air-bleed hole is reduced back to its correct size.”

Phil is proposing here to open up the high speed bleeder holes (see image below), which are #70 drill (0.0280”). These should not be confused with the much larger (#52 drill or 0.0635”) idle air bleed holes. The high speed bleeder holes allow air to mix with the fuel coming out of the main jets, emulsifying it. By opening these holes up, the main metering circuit becomes leaner, with the fuel being more “bubbly”.

High speed bleeder.jpg (35.94 KiB) Viewed 86 times

[Harv]

P87: “The Coventry-Climax recommendation is to allow as much air space as there is oil, say 3 gallons of each in a six gallon tank…those who have disregarded this recommendation have subsequently regretted it, so it can be taken as excellent advice for this or any other dry-sump engine”.

I noted this one as it answers a question I have for the meth monster engine build – now that it is a dry sump, how full do I fill the dry sump tank?

P89: “In some engines, the rocker supply is metered by a small orifice in one of the oil connections; in others, there is a groove machined part of the way round one of the camshaft bearings, given an intermittent supply of rocker oil. Unfortunately, with the latter method, the higher the speed the less oil is supplied because the column of oil in the feed pipe objects to being suddenly started and stopped. It is therefore preferable to continue the groove right around the journal and then to restrict the flow, if necessary, by jetting down, or fitting a restrictor pin in one of the unions. A split-pin can often be employed for this purpose and makes an arrangement which is less likely to become blocked than a small hole of equivalent area”.

P116: (on how much crush (“nip”) to use on main bearing strongbacks)

bearing strongback crush.jpg (15.31 KiB) Viewed 84 times

[Harv]

P128: “The Repco “Highpower” head produced in Australia for “FE” Holden engines, shown in section in Fig. 12.3 can also be fitted straight to an existing engine, increasing the output of the 3-in bore x 3 1/8 in. stroke engine from its normal 64 b.h.p. at 4,400r.p.m. with one carburetor, to 88 b.h.p. with the same carburetor and 100 b.h.p. with two large ones. Further modifications produce still more power as shown in Fig. 12.4. these curves are representative of many readings taken during development of this head and shows the effect of successive modifications. The section (Fig. 12.3) shows the large 1 9/16 in. inlet valve with smoothly contoured port; the apparent interferences between it and the vertical, much smaller, exhaust valve is because they are offset in plan view and not straight across as usual. Even without this head, the Holden engine can be made to develop around 130 b.h.p. on petrol and 145 b.h.p on alcohol with three carburetors and can be run up to over 7,000rpm. The critical crankshaft speed is 6,300rpm, This head will not fir the 149 and 179 cub. In. engines.

Repco head power output.jpg (39.04 KiB) Viewed 84 times

Cross section of Repco head.jpg (33.91 KiB) Viewed 84 times

[Harv]

P151 (on Straight Six-cylinder Engines): “When fitted with only one carburetor, six cylinder engines are often even more restricted in choke sizing than comparable four-cylinder models, in order perhaps to take advantage of their inherently better low-speed flexibility, but, as distribution is quire a problem, intensive hot-spotting is also fitted. Consequently, many sixes will benefit as far as power is concerned, simply by enlarging the choke and eliminating or reducing the hot-spotting, but the manifold still exerts a limitation on power. There are several porting arrangements, necessitating different thinking to arrive ta the best alternative manifolding with regard to cost and gain in performance. Three-port Manifolds. Fig. 14.3, A shows the conventional scheme of a single manifold feeding three “siamesed” ports A better method, B, is to bolt or weld on an adaptor to carry a Weber dual-choke instrument, with the chokes positioned as shown; adding spacers between the manifold and the head, about 2 in. long, will give a little “ram” effect and assist in maintaining top-end power. Scheme C, the “straight rake” type, is quite effective. The off-takes, 3 to 4 in. long, spring from a gallery about 25 per cent. larger in diameter, with buffer ends extending past the off-takes. Two carburetors are located almost midway between the ports, or with a slight bias towards the outer ones. It matters little whether they are vertical, horizontal or inclined, as the gas has to turn 90º to enter the gallery in any case. The original carburetor plus an additional similar one on this type of manifold will give a power increase of up to 15 per cent., mainly in the middle and upper speed ranges; but, in addition, it can be used with “longer” valve timing than the elementary form will accept. The best high-speed performance with this porting is given by three carburetors (scheme D) mounted on stubs joined by balance-pipes of smaller diameter. The suction intervals on the center carburetor are equally spaced, but on the outer two they are spaced at 120º and 240º and are consequently the mixture delivered to the cylinders of each end pair varies in strength. The provision of balance pipes removes this effect to some extent and the final result is quite satisfactory. However the bore of the balance pipe is fairly critical: if too large, the result is simply an engine with excessive choke-area and very poor low-speed characteristics. In the experimental stage, two lengths of radiator hose of say 1-in. bore can be used, and the effective bore altered by means of washers, with holes of varying size, placed inside. As with any other “pulsing flow” system, the lengths of the inlet-pipes is also important to obtain the correct ram effect. (See page 158.).

inlet manifold development.jpg (24.3 KiB) Viewed 84 times

Note that by using the term “hot-spotting”, Phil is referring to inlet manifold heating.

[Harv]

P158 (on Amals) “With these, or any other carburetors with sliding throttle valves, a balance-pipe is neither necessary nor desirable, because at low speeds the manifold vacuum becomes almost constant instead of pulsating, and the slides will be reluctant to close fully”.

I noted this one, then wondered if Pete had put balance pipes on the triple Amal setup our mate from Newcastle made for me (with thanks): (https://www.fbekholden.com/forum/viewto ... al#p247144) – this time I won, no balance pipe.

P168 (on superchargers): “Because of the clearances, gas leakages becomes excessive at pressures above 10 lb. per sq. in., whereas the vane types can be used up to 35 lb. per sq. in. Pressure of this nature would only be considered for extreme conditions and, in fact, there is nothing to be gained by using pressures in excess of 38 lb. per sq. in.; beyond that, any increase in power is more than offset by the power lost in compressing the charge”.

I had wondered where the theretical limit was for vane superchargers, and here is an alternate view - it is about double the boost where Eldred felt that you met the point of limited return. Interestingly, the blower on the FED was last on a Toy Yoda engine at 30psi (at least according to the seller).

P173 (on superchargers) “”Swept” manifolds with nice easy curves are, however, unnecessary, and with alcohol fuels sometimes lead to an excess of fuel in some cylinders, even to the extent of wetting the plugs, while some of the others run dangerously weak. On racing models, which are not run at low throttle openings, it appears to be better, and simpler, to use manifolds of large diameter with square corners at the junctions. The blow-off valve is best placed close to the block and in direct line of the pressure wave. The moving part must be as light as possible otherwise its inertia will prevent it from opening quick enough and, even if the blower if not damaged, any hose connections may burst and possibly catch fire.”

Looks at the FED blower manifolds, with their boxy corners and lightweight relief valve, and smiles

P179: “The overall length of the induction system from the valve to atmosphere affects the shape of the torque curve as well as the maximum power. With each carburetor feeding one or a pair of cylinders, a length of about 14 in. form the manifold face to the end of the air intake is a good starting-off point if peak power is required between 6,000 and 7,000 r.p.m. Experiments can quickly be made by slipping various lengths of radiator hose on to the inlets, and observing the result; it may be necessary to adjust the main-jet size at each length variation in order to reap the maximum benefit. As an actual example, a Holden engine with Repco head and two 1 ¾-in. S.U. carburetors gave 124 b.h.p at 4,000 r.p.m and 136 b.h.p. at 5,800 r.p.m with inlet extensions 7 in. long, but 114 b.h.p. at 4,000l r.p.m and 150 at 5,800 r.p.m with extensions 1 in. long; the long extensions were used on short twisty tracks and the short ones for fast courses.”

P190 (on brakes): “Finned or ribbed drums may be purchased as replacements, these being beneficial partly because of their improved cooling and partly because they are stiffer and this give a firmer “feel” to the pedal and less lost travel. Alternatively, the outside of the drums can be machined and ribbed aluminium muffs turned up and fitted thereto. An interference fit of 0.003 in. per in. should be heated to 200ºC (392ºF). Any existing ribs near the mouth the drum should be retained, as these will provide more stiffness than the comparatively flexible aluminium would replace.”

Cheers,
Harv

[Errol62]

Great reading. Interesting about the balance pipe diameter effect.


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