The revival of forced induction

A somewhat pompous heading such as this can he readily transscribed into the polite exhortation “blow it!”. In 1972 we can understand readers wondering why we should bother to dig up that age-old aid to extra engine power—the supercharger, and its kissing cousin, the turbocharger. However, there are strong indications that supercharging and turbocharging have a lot of life left in them as fresh sources of extra performance from the much-abused internal combustion engine. This is particularly true of turbocharging, for the principle of deriving power from exhaust-driven turbine and compressor is attractive to manufacturers, who are faced by public demand for both high performance and even cleaner exhaust emission levels. In the UK turbocharging seems doomed to the diesel truck and the race track, but in Germany and the USA there are firms selling turbocharged saloons for normal road use, with the backing of their manufacturers. Notable examples are the Turbo-May kits for Fords in Germany, and the Chevrolet Vega in the United States, where both the maker and Don Yenko have shown the results of their research, though only Yenko is offering his equipment to the public at the time of writing.

The Turbo-May Capri kit has been offered to the German Press for road tests when based on the 2600GT model, and their independent fifth wheel timing shows 0-60-m.p.h. times of under 8 sec., 0-100 m.p.h. in just over 19 sec., coupled to a top speed of 125 m.p.h. or so. Although these are astonishing results in terms of improving performance on a basic motor car, the end result and price is little different to a production line Capri RS2600, despite May’s company claiming 207 b.h.p. whilst Ford claim 150 b.h.p.! A German Mercedes race preparation specialist called AMG is currently developing the o.h.c. 6.3-litre V8 for Group 7 sports-car racing. Utilising an Eberspaecher turbocharger for each bank of cylinders, the Germans expect well over 700 b.h.p.

However, there are strong theoretical and governmental reasons for pursuing the turbocharger’s benefits earnestly, so what does it do, and how? Briefly the turbocharger consists of a turbine vane placed in the path of the outgoing exhaust gases connected by a shaft to a second compressor vane. As the vanes turn in response to the pressure building up from the primary (or header) exhaust pipes, the shaft turns ever faster (in excess of 100,000 r.p.m. on a good modern unit) and the centrifugal compressor supplies pressure to an outlet pipe, which in turn is connected to the engine’s induction equipment, be it a carburetter or fuel-injection system. Alternatively the equipment can be laid out so that the carburetter is supplying unpressurised air-fuel mixture to the turbocharger, where it will be pressurised and returned to the inlet manifold.

Naturally there are snags that complicate the typical turbocharger installation. Primarily one has to prevent pressure rising too high inside the unit, for a surplus of pressure will lead the engine to such a frenzied excitement that it will destroy itself. So a pressure release valve is incorporated to regulate pressure to a predetermined level— somewhere around the 10-lb. mark would probably be more than enough for a normal production engine. Another problem, which the Holset and Schwitzer’s compact units seem to overcome pretty successfully, is that of throttle response. Whereas a supercharger can supply boost right from the start, because it is mechanically driven by a belt from a special double-run pulley on the crankshaft, the turbocharger’s vanes have to build up running speed as the exhaust gas velocity picks up. In practice this can mean that, on a road-going application, the engine feels just as it normally does until 3,000-4,000 r.p.m. is exceeded, when it will smoothly start to supply extra pressure that gives the impression of a magician suddenly doubling the size of one’s power unit!

Thus for road use it pays to have a turbocharger with lightweight vanes, so that they speed up and slow down responsively. We are informed that the turbochargers used for the Offenhauser-powered USAC single-seaters in America are of rather more robust truck-like construction, offering larger pressure capacities, as in American bowl racing consistently high r.p.m. can be used, and there’s little need to decelerate, then need instant maximum power. In Europe a couple of seasons ago the Group 5 Touring car BMW 2002s used Eberspaecher EBO turbocharging with dramatically effective results to close the performance gap that existed between the Porsche 911S types and the Bavarian machines. The works found that the most they could reliably wring out of the 2-litre single overhead camshaft engine with a conventionally race-prepared unit, using fuel injection, was nearly 210 b.h.p. at 6,500 r.p.m. In contrast, their turbocharged motors gave a much older spread of usable power, culminating in close to 280 b.h.p. at closer to 6,000 r.p.m. I was allowed to ride in one of these cars whilst Dieter Quester conducted it around Brands Hatch. The engine’s docile manners soon converted me to this way of gaining extra power, though it wasn’t until 1971 that I was able to find out what it was like to drive a turbocharged car.

The opportunity came via a Mr. Robert Henderson, who runs MPG and H Engineering at Minnow House, Lochgilphead, Argyllshire. Formerly based in the south Henderson moved his business, best known for the manufacture of Minnow-Fish carburetters, to Scotland, where he started to develop an Avenger which he had already converted by conventional means to outperform the GT version. Henderson installed an obsolete CAV turbocharger unit on some freshly fabricated tubular steel exhaust manifolding, lowered the compression ratio to a vintage 6.5:1 with the aid of a slightly thicker head gasket and scalloped combustion chambers. Two bits of extra piping are needed on the Henderson set-up, or any carburetter-pressurised layout that is to give reliable turbocharged service, a high output electrical fuel pump and a fuel return pipe from carburetter to petrol tank to dispose of any surplus. On the Henderson layout the CAV unit was arranged so that it drew its own fresh air supply and then fed pressurised air to the carburetter. Be warned that there are some carburetters that do not take kindly to increased air pressure; they tend to fight back by admitting air, flooding and eating gaskets. The answer is to re-arrange the system so that the carburetter draws mixture in the normal way, which means that it will be even more thoroughly atomised by the time it is fed on through the turbocharger, and then returned to the intakes under pressure.

The car was tested in pouring rain along the exciting roads that abound in the Scottish Highlands. The extra fuel offered by the electrical pump can be dispensed with via a cockpit switch if the engine is flooded, but we were always rewarded with instant starts without recourse to this measure. Henderson declined to reveal the boost pressure blow-off point on this unit, though another authority on turbocharging pointed out that the deliberate loss of compression on Henderson’s machine might well lead to a loss in low-speed performance, compensated for by higher boost at over 4,000 r.p.m. What he did say was that power had been increased from 85 to 125 b.h.p. with the addition of the £200 (fitted) turbocharger kit. We had no facilities to accurately check acceleration figures, nor was the weather in our favour, but on the speedometer from 0-60 m.p.h. took 9 1/2 sec.

The dramatic performance increase certainly occurred at the top end of the r.p.m. range. The driver could sense the engine’s eagerness to outperform the production GT at anything over 4,000 r.p.m. From that point to 6,500 r.p.m. the Avenger just gathered speed at a terrific rate, all with very little noise. Although the car was 46,000 miles old it willingly pulled 6,500 r.p.m. in 4th, which corresponds to 112 m.p.h. or so! Perhaps more surprising is that it felt as though it would put at least another 10 miles into every hour, if we were prepared to ignore the owner’s request for nothing more than 6,500 r.p.m.

The author’s main criticism of the Henderson layout was that it felt as though the car was an off-tune Avenger 1500GT below 4,000 r.p.m., but we were briskly informed that this was his imagination. Be that as it may, we left Argyllshire with the conviction that the Henderson car had a lot to offer in straight performance, while the consumption of four-star fuel at 22.6 m.p.g. didn’t seem an exhorbitant penalty to pay for so much fun. As an incidental it ought to be noted that the handling and braking of this Avenger was quite exemplary, and Scottish readers might well find that the trip to Minnow House was worthwhile on this score alone.

On our return to London, as if by one of those quirks that are the breath of life to fiction writers, we found that a corner of our desk was taken up with a book on turbocharging. Perhaps it would be fairer to call it the definitive work on practical turbocharging, for the author, Hugh MacInnes, has worked actively on turbocharger design since 1952 and did a lot of work on the Chevrolet Corvairs that used this aid to performance in the early sixties. He now owns Rayjay, a Californian company that specialises in this sort of work for competition and road use. Luckily for his readers this means that MacInnes understands both the practical and theoretical problems that can arise— especially when it comes to matching up a particular turbocharger to a specific engine. Machines called his $5 book “How to Select and Install Turbochargers”, and it is available from HP Books, PO Box AU, Los Altos, California 94022. According to our Secretary the post and packing charges on this 144-page publication could be calculated in £s rather than new pence, but anyone who wants to increase his knowledge further than the very sketchy outline I had to give earlier on (for which I make no apology as we still have to tackle supercharging later on!) will find this book a worthwhile investment. If I were writing this for an American magazine I would be forced to say that MacInnes’ enthusiasm for his own products ought to be borne in mind when reading the book, but in Britain his equipment isn’t yet generally available, so one can just absorb the information.

The source of turbocharging equipment in the UK tends to be Holset Engineering Co. Ltd., PO Box A9, Turnbridge, Huddersfield, HD1 6RD. This company have seen their units, which were designed primarily for diesel engines but incorporated the compactness that makes the Schwitzer such a useful unit, used for all sorts of cars. The best known demonstration of Holset effectiveness was Alec Poole’s Complan Mini T/C, which performed very successfully in Ireland during 1971.

To gain an insight into supercharging in this country, there’s really only one man to talk to, Mr. Allan Allard. Despite the complexities imposed on him by the family Allard business (one of the biggest Ford dealerships in the country) and associate companies Allard has remained an enthusiast. He alone has shown sufficient faith in supercharging to ensure that it is still readily available in this country. Initially the Allard Performance Centre at 51 Upper Richmond Road, London, SW15, were distributors for the Shorrock vane-principle superchargers, but a couple of years ago Allard became disenchanted with the way that the units were being assembled, and arranged to take over both manufacture and distribution. This gave him a range of blowers that covered from 500 to 1,300 c.c. (the C75B with kit prices beginning at £105) and from 1,300 to 2,000 c.c., using the larger capacity C142B with retail charges beginning at £127. Some months ago Allard signed an agreement with Wade Engineering in Brighton, Sussex, for the world distribution rights to these twin-rotor Roots-principle superchargers, thus extending his engine capacity coverage to 5,000 c.c. Supplies of the bigger Wade RO34 for engines from 3,000 to 5,000 c.c. are just trickling through, but Allard has already created considerable interest with the smaller RO20, which is utilised on a Capri 3-litre kit costing £248. However, it would be unrealistic to think that this sum of money really covers a safe car as the performance figures of such a Capri are right in the same bracket as the Jaguar V12; viz 0-60 m.p.h. in 6.0 sec., 0-100 m.p.h. in 16.2 sec., and a top speed that’s only limited to under 130 m.p.h. because of the desire to stay within the manufacturer’s recommendation on crankshaft revolutions. So for safety and reliability reasons Allard is also offering a complete new Wade-equipped Capri with stiffer and lowered suspension, heavy-duty tramp arms to locate the axle, low-compression cylinder heads, free-flow exhaust system, larger front bonnet bulge (minimal change if the sidedraught 2-in. choke SU carburetter is fitted instead of the production downdraught Weber) and a large air dam under the front bumper. Assuming the SU carburetter is fitted, the complete package deal will sell at £1,998. The boost on this particular installation is just 5.5 p.s.i., giving a static compression ratio of 8.7 to 1, equivalent to 10.4. to 1 at 5,500 r.p.m., when full effective boost is applied.

Turning to the subject of turbo-charging Allard surprised us by saying that he is currently occupied in some practical turbocharging research, but before moving over to the pros and cons of super- versus turbocharging, he had a few general points to make. Allard began: “I am still tremendously keen on the idea of extra performance gained with what amounts to an air pump, whatever its principles. First I think it is important to say that the term pressure charging should be used when employing boost pressures up to 8 p.s.i., beyond which I use the term supercharging. There has been, and still is, a certain amount of prejudice against pressure charging, based primarily on ignorance of the true facts. It is entirely unreasonable to believe that the average bolt-on kit represents the best that can be achieved by pressure charging; we’re only just scratching the surface of what could be achieved by a motor manufacturer with proper facilities. Obviously a bolt-on kit is designed as such and built to a price, bearing in mind what the customer is prepared to pay. Bolt-on kits should be judged for what they are, good value for money in terms of performance gain, but for all that they represent just the first stage in the development of a supercharged engine—an engine in its own right. The principle disadvantage of pressure charging has been the increased heat developed as a result of increased volumetric efficiency. However, this is only a disadvantage if we think only in terms of simple bolt-on blowers. If the engine is designed and built as a blown engine, then it will have the correct heat flow characteristics inbuilt: improved water jacketing, improved valves with timing set to suit the blown engine, suitable ignition timing characteristics, redesigned cylinder head, plus new induction and exhaust systems.”

“Bearing in mind all these points it should be possible to make a production engine capable of a volumetric efficiency of 150%, with good thermal efficiency and specific fuel consumption. Power output could well be around the 120 b.h.p, per litre with maximum r.p.m. limited to 6,500. In competition at least 200 b.h.p. per litre should be realised, without the use of special fuels”, Allard concluded.

He places the theoretical advantages of pressure charging over a conventional engine as follows: (1) Substantially increased volumetric efficiency. (2) Increased BMEP and torque. (3) Increased power output over a wide rev.-range. (4) Smoother combustion. (5) More complete combustion of mixture, and therefore less dangerous exhaust fumes. (6) Better thermal efficiency. (7) Large increase in power with no increase in r.p.m., therefore giving no extra frictional losses and loadings that can occur with a conventionally tuned engine. (8) High power, low weight.

Allard lists some further points in favour of turbocharging over the belt-driven pressure or supercharger thus: (1) Specific fuel consumption decreased. (2) Less exhaust noise. (3) No power loss with altitude (a belt-driven pump is naturally dependent on an engine which loses power at altitude). (4) Still better thermal efficiency. (5) No mechanical power losses. (6) Even better exhaust emission levels, which may mean that this is the only way for a manufacturer to keep performance standards up in the heavily-legislated future.

From Allard’s point of view the knowledge that exists in Britain is virtually useless in helping to market a turbocharger. He says, “The only way we can work is to actually try it out in the light of any theoretical knowledge we can acquire—and most of the experts seem in the dark when it comes to talking about car engines.” He added that the twin Holset 31D turbocharger installation on the Ford team’s powerboat racers was a shining example of what could be achieved by engineers with proper facilities. “They have strengthened and redesigned every possible stressed component”, he told us, “and that’s what ought to be happening in the car field as well.”

Looking at what he can offer the public now, Mr. Allard was certain that the Shorrock units offered the finest value for money, so long as the customer didn’t arrange the pulley gearing to allow the internal vanes to exceed 6,500 r.p.m. Both the Shorrock and Wade installations give dramatic power increases in the range that the road driver uses, “but wouldn’t it be nice to have a supercharger providing low-speed boost, then disengage to allow the turbocharger to take over?” he mused!—J. W.