Secrets of Speed: Today’s techniques for 4-stroke engine blueprinting & tuning

Chapter 1

Planning your project

Objectives

An internal combustion engine is a machine that converts the chemical energy contained in its fuel into mechanical power. Any unmodified engine can be altered to raise its power output and reliability. However, as greater rewards usually result from planning, it’s best to define your objectives rather than make random modifications – identify what is wanted, and how it will be achieved. For example, an engine may be from a classic vehicle that is being restored to ‘original’ specification. Alternatively, an ‘original appearance’ motor may be required.

In most cases, the original specification or ‘provenance’ of a production engine is easy to establish. However, building an original spec engine and keeping pattern internal parts can be at the expense of practicality. An original appearance engine can incorporate upgraded components – silicon-chromium steel valves can be replaced by harder-wearing nimonic alloy items, for example, or a cast or forged crankshaft may be replaced with one made from an alloy billet. Many older engines had general purpose cork gaskets; these can be replaced with modern gasket material, or dispensed with entirely by using face-to-face joints and a modern sealer – thus improving stiffness. Today, components can be assembled to much more rigorous standards and tolerances than was possible when a period engine was first manufactured. This, in turn, may result in improved performance and reliability.

Performance is an important consideration, as some classic vehicles are hard to drive in modern traffic, so modest increases in power or better cooling, for example, can improve drivability. Therefore, non-visible upgrading can broaden a vehicle’s practical use, arguably, without destroying provenance.

The original specification of racing machinery presents more of a problem. An engine may have been built to a certain pattern at the beginning of a racing season, but improved components may have been fitted, or modifications undertaken, as and when they were advantageous. What’s more, any advantage is unlikely to have been made public, so records or build sheets for period race engines may be unavailable. As a result, at times, the original specification of racing engines can be somewhat subjective: in other words, open to interpretation. In contrast, with regard to a modern production engine, originality may not be a concern, as the object might be merely to extract more power.

Generally speaking, an engine becomes less user-friendly the higher its level of performance. For instance, a 4-cylinder, 2000 cubic centimetre (cc) eight-valve motor from a road vehicle might develop 150 horsepower (hp). To raise its level of performance, the engine’s ports and valves could be enlarged, a high-lift cam fitted, and the compression increased. Such modifications would raise horsepower, but the point of maximum torque (low to medium speed pulling power) shifts further up the rev range. In addition, gas velocity through the ports would be low at slow engine speeds, as would vacuum. This could lead to reversion: a percentage of exhaust gas obstructs the inlet charge (see Chapter 3, ‘overlap’). As a result, such modifications may make the idle erratic, and upset the power delivery below 3000 revolutions-per-minute (rpm) due to poor combustion and the shifted torque point. Fuel consumption would probably be higher, and driving such a vehicle could be trying in an urban environment. Consider engine modifications carefully. What will be the common engine speed range? Will the vehicle be used for urban or track driving, or rallying?

Reliability is another concern as dependability is sometimes a trade off against performance. Nonetheless, powerful, reliable race engines exist. Although it’s easy to see the huge differences between an engine for a Top Fuel dragster, and one for a 200-mile race – there must be a balance between usable power and reliability. Aside from fitting tuning components like high-compression pistons or high-lift cams, several modifications or assembly techniques can be applied to existing components and castings to increase horsepower and reliability for a relatively modest cost. Nevertheless, if the primary purpose is to raise horsepower, it is well to consider the principal theory of the 4-stroke cycle that makes increases possible, namely:

• To reduce or remove needless friction

• To fill a cylinder with the utmost charge as often as possible in any given minute

• To convert as much of the heat released by combustion into energy

Engines can always be modified; just because something has been made or fits together in a certain way does not mean it’s best for the liberation of horsepower. The implication is not that most engines are badly designed. University-educated engine designers are hardly uninformed, but blank sheets of paper and unlimited budgets for designers to satisfy their creative talents rarely occur. Financial considerations, time constraints and metallurgical or machine shop limitations press on the professional just as they affect the amateur. As a result, the need for stronger parts, better materials or more thorough designs are sometimes secondary to other concerns.

Fig 1-1: The 4-stroke cycle.

Tuning

The word ‘tuning’ concerning internal combustion engines commonly refers to their level of performance; or how much power a given size of engine produces. This can be expressed as, horsepower-per-litre (hp/lt). When comparing power ratings, it’s important to note that some European manufacturers may measure in metric horsepower, rather than the more common, Society of Automotive Engineers (SAE). Rating power output in kilowatts (kW) is unambiguous, which is largely the reason for its adoption by commercial engine manufacturers.

One (1) metric horsepower = 0.736kW

One (1) SAE horsepower = 0.746kW

One (1) kW = 1000 watts

The origin of the word tuning, used in the context of engines, is unclear. Some people, versed in period history, credit it to George Dance, a highly successful British motorcycle racer in the inter-war years who was a development engineer for Sunbeam Motorcycles. The story goes that he was working on an engine in his shed-cum-garage when someone walked in and asked him what he was doing. In those days of magneto ignition, contact breaker points were tensioned by a series of two-inch long flat springs, formed in a gentle curve. Perhaps not wishing to disclose what he was really doing, he is supposed to have flicked a magneto spring so that it made a twang like a tuning fork and said, Just tuning it up. Competitive motorsport is very secretive!

The common understanding of tuning is described best by example. Imagine a particular engine that had a modest compression ratio, valve timing and port sizes; it would have reasonable torque for its displacement but meagre horsepower (low hp/lt). Overall, the engine would not be highly stressed and it would possess docile and dependable characteristics. The engine’s enhancement or level of tune could be increased by replacing components or making modifications to the engine castings. This would raise horsepower but lessen useable torque and at the same time, the engine castings and internal components would be subject to higher levels of stress. The engine could be further developed, producing more horsepower but less torque, until it became impractical or good for only one use. An example of such a motor is the V8 8.2-litre engine of a Top Fuel dragster. Fitted in a light projectile these engines put out a minimum 5000hp, yet the longevity of such an engine, when under load, is measurable in seconds before it will need to be overhauled. Such a 5000hp engine produces 609hp/lt but little torque. Compare this to the 36-cylinder, 127-litre, Lycoming XR-7755 engine. Its stated output is 5000hp, which works out to 40hp/lt. This engine was designed to have sufficient torque to power a large aircraft. When raising an engine’s state of tune, strike a balance between horsepower (horsepower-per-litre) and torque, for high horsepower low torque engines are impractical except for specialist uses. Tuning, then, is a generic term to indicate a particular engine’s level of performance enhancement.

Blueprinting

Blueprinting came about when factory teams competed in race series that restricted engines to standard parts. To make engines run better, they used only the best standard parts. So, blueprinting is a colloquial term that, in its original sense, meant ‘to assemble an engine with the best fitting original parts.’ It entails matching parts to the most favourable dimension stated on a designer’s blueprint. Yet without manufacturing tolerances, there would be no such thing as blueprinting. For example, if a designer wanted pistons made for an engine with a skirt diameter of 86.00 millimetres (mm) it is unlikely every piston would end up that size. There are a number of reasons: the limitations of machine tools (the inherent inaccuracy of shafts, bearings, chucks and feed screws); and worn grinding wheels and cutting tools. Wear-and-tear throughout machine life, and operator error compound the issue. As a result, there would be impossible wastage if every piston had to be exactly 86.00mm. This would increase manufacturing cost and make parts – in this case pistons – very expensive. Weighing these factors in mind, designers give components tolerances: optimum piston size is 86.00mm but the designer may also accept 0.0065mm (high) or 0.0127mm (low).

In that case the sizes become:

0.0065mm (+)

86.00mm optimum size

0.0127mm (-)

Adding the + and – values together, the total tolerance is 0.0192mm/0.00075in.

There would also be a tolerance on the engine’s cylinder bore size, and another assembly tolerance (the running clearance) when the piston is fitted in its bore. A difficulty might arise when attempting to blueprint an engine with a piston finished on a low (-) tolerance and a cylinder bore on a high (+) tolerance (a small piston and a big bore). In which case, a large running clearance would exist that may lead to piston-to-bore gas leakage. To obviate this problem, try to match piston size to bore size.

Any precision part has a tolerance, and matching a cylinder head that gives a higher compression, a camshaft with a higher lift, and pistons that limit gas leakage, can result in a smoother more powerful engine. However, blueprinting involves more than clearances. Blueprinting is also concerned with reducing frictional losses, whether they result from surface finish, or the misalignment of shafts or bearings. Aim for the finest surface finish on cylinder liners, journal shafts and bushes. Misalignment can be caused by worn machine tools or operator error. It’s wrong to assume that an engine’s crankshaft-to-cylinder-bore-axis is correct, or that the camshaft bores are in alignment. Dowels may locate castings, like the split crankcases of some motorcycle engines, but it doesn’t follow that main shaft bearing bores will be in precise alignment. It’s good to consider the original manufacturing conditions, and how much a designer may have had to compromise from the ideal.

Blueprinting, like engine building in general, involves a modest amount of number crunching, and a builder’s job is to check and verify specific numbers. For example, a 2000cc eight-valve single overhead camshaft engine has, amongst other components, 4 pistons; 4 wrist pins; 8 pin circlips; 4 ring sets; 4 con rods; 4 wrist pin bushes; 8 big end bolts, etc. Each individual component of a specific group – a wrist pin bush for instance – needs checking for alignment and fit in its con rod and mating wrist pin. This operation must be replicated for each bush. Measure individual components and consider their relationship to other components. To avoid confusion, split complex jobs into smaller operations – be methodical; check one set of components at a time before moving on to the next. For example, check the wrist pin bush-to-con rod fit in all the rods of a set before moving on to check the wrist pin clearance, rather than performing both operations on one rod at a time. Adopting such a method helps to minimise errors as it enables the mind to do complex tasks without having to retain large amounts of information: repetition simplifies work. It’ll help if you use the inspection and build sheets presented in the glossary.

Nowadays blueprinting has come to refer to selective assembly with standard or non-standard parts. A ‘grey area’ exists when using non-standard parts – such as modern pistons made of high silicon alloy in an older engine whose pistons were a different composition, or altering castings or re-directing coolant flow – as the original blueprint specifications are invalid. This blurs the distinction between blueprinting and tuning.

The reason behind a so called ‘dummy build’ or ‘mock-up’ – temporarily building an engine prior to the final build – is to check specific measurements that are only verifiable on an assembled engine. Namely, the fit of certain engine castings, how machined surfaces contact each other, the interaction of gaskets of various thicknesses, and the movement of internal components. All such concerns need addressing so the result is an engine where internal components and castings fit together to best effect, thereby minimising stress and friction. This liberates horsepower and improves reliability. Best fit depends on application; two examples are:

1) A qualifying engine built for maximum power yet minimal use.

2) An endurance engine that has to produce good power for many racing hours.

Overall, the implication of checking and verifying is accuracy: leave nothing to chance. An unchecked or unmeasured component is an unknown; and is, therefore, a potential problem. Consequently, blueprinting could be defined as: selective assembly that increases horsepower and reliability.

Dynamic compression

Do not fit different cams or raise compression without linking cam profile to dynamic compression. Matching cam profile increases engine harmony and optimises gas speed and combustion heat. A cylinder’s dynamic compression ratio is the association between its combined, clearance and swept volumes, at the point of inlet valve closure, divided by the clearance volume. To put it another way, it is the compression existing in a running engine. The point of inlet valve closure largely determines dynamic compression. The inlet valve closes after bottom dead centre (ABDC), yet the piston has already begun to move up its cylinder. Effectively, this shortens the stroke as part of the charge has escaped out of the open inlet valve pushed out by the rising piston; in theory, reducing compression.

Dynamic compression is expressed as:

DCR = Dynamic compression ratio

SVI = Swept volume, at point of inlet valve closure

CV = Clearance volume

Dynamic compression is not the figure quoted by manufacturers; they quote static compression (see Chapter 6, ‘static compression’). Static compression, for purposes of calculation, uses the whole of an engine’s stroke: piston movement from bottom dead centre to top dead centre. Problems may arise when fitting a ‘sports’ profile camshaft without altering the compression. Sports cam profiles have greater duration (see Chapter 3, ‘duration’). Duration, properly sequenced, optimises gas wave inertia (see Chapter 9, ‘extract and ram effect’) at higher engine speed, making engines more responsive. But, duration lowers dynamic compression, as the intake valve closes later. If compression is not raised to compensate, the result is an under-compressed charge and power-loss. On the other hand, retaining standard cams but raising compression leads to over-compression and possibly, detonation. The answer is to raise power output by increasing compression matched to cam design, specifically the point of inlet closing. This maintains or increases dynamic compression from standard levels, but always linking it to cam profile. To determine an engine’s dynamic compression, first compute the effective stroke by converting degrees of inlet valve closing to linear measurement.

Fig 1-2: Convert degrees of inlet valve closing to linear measurement.

Imagine a single cylinder engine whose dimensions were:

Bore = 87.00mm

Stroke = 75.70mm

Con rod length, centre-to-centre = 120.00mm

Clearance volume = 55.50cc

The stock cam is 272 degrees duration (inlet closing at 65 degrees ABDC), but a sports cam is available at 298 degrees duration (inlet closing at 75 degrees ABDC). Convert the closing degrees to linear measurement for both lobes and work out the two swept volumes (see Chapter 6, ‘swept volume’). With this information, use the formula listed above to compute the engine’s dynamic compression with each cam. The diagrams show that fitting the 298 degrees sports cam actually reduces compression. Therefore, to optimise high-speed performance, fit the 298 degrees cam but restore compression to the stock level by reducing the clearance volume 6.74cc. In most cases, this would be done by skimming the head or block, or by fitting a domed piston. On naturally-aspirated motors, for the sake of reliability, do not exceed a dynamic compression ratio of 8.0:1 for 2-valve engines, or 8.5:1 for 4-valve engines.

Fig 1-3: A sports cam can lower the dynamic compression. With the stock cam, the dynamic compression in an 87.00mm x 75.70mm cylinder is 7.48:1 (left). Fitting a sports cam (right) lowers it to 6.69:1.

Component stress

When an engine is under stress – when cylinder pressure and piston speed increase – the chance of parts failing also increases. Wholly rigid structures do not exist. All components flex to some degree, yet in well-constructed assemblies this is a manageable amount.

Included in the appraisal of an engine (its suitability for a particular purpose) must be an assessment of its general robustness and strength. Will it be strong enough to do the job? It may have potential weaknesses – for example, a heavy flywheel that could shake loose at high rpm; gears, sprockets or rotors secured by narrow section nuts; or poor quantity fasteners that impart flexibility into bolted joints – such parts may fracture. Designers sometimes use a 10-to-1 safety factor above yield strength when deciding component design.

A part’s material (an alloy’s constituents) largely governs its strength when in service. When a part fractures the fatigue fracture usually emanates from a surface crack. Exceptions are due to conditions like slag particles, porosity or brittleness. In such cases a crack can start under a component’s surface. However, all cracks, whether starting internally or externally, develop at a stressed area: ‘a stress concentration’ that spreads during cyclic loading until reaching critical size, whereupon fracture takes place. Without a stress concentration, there would not be a fatigue failure. A stress concentration can result from normal use over a long period (old worn out parts), stress beyond design limits (overstressed components) or from a defect in design or surface damage. The internal surface of a tube or hollow component, like a wrist pin, is also a potential point of failure. Fatigue cracks can develop due to problems with heat treatment. Most engine building is concerned with modification rather than redesigning components. From this perspective, investigate two causes of stress when assembling a performance engine:

1) Surface finish-asperity, marks, scratches or burrs.

2) Loose-fitting component parts that should fit tightly together.

It’s best to examine these two causes separately.

Surface finish

Highly-stressed components usually have a high-quality surface finish. ‘Stress raisers’ cause stress concentrations. These may be surface inclusions that cause sudden contour changes like burrs, sharp edges or tool marks or scratches. Gouges or dents, like a centre punch indentation, or score marks left by electric engravers, key ways, sharp-edged 90 degrees shoulders, and splines abruptly terminating at root diameter; these all interrupt surface flow and are potential stress raisers. There is a limit to what can be done without redesigning components. Nonetheless, remove inclusions and sharp edges; all the while aiming for the smoothest scratch-free finish on all components as this lessens the chance of failure. Depending on the part and resources, use a sharp triangular-pointed scraper, a fine file, fine abrasive paper, a fine flop wheel or polishing mop or a carborundum stone. In the same way, remove any sharp edges on components left after casting or machining like casting flash, sharp edges adjacent to mating surfaces or burrs around holes.

To help avoid failure, handle parts carefully to avoid bruising or nicking. Where a mark cannot be removed, on a valve spring, for instance then the only remedy is replacement. A mark on any stressed component becomes more threatening when at a right angle to the component’s route of stress. That means marks on a valve stem or a con rod that run along their lengths are less dangerous than those going across the beam. Removing marks on a con rod is not lightening, but stress relieving. Therefore, take off the minimum material to achieve a smooth finish. Forged ‘I’ section con rods are notorious for marks travelling across their beams. On un-dressed rods, a distinctive line around 5.00mm wide is usually visible along the length. This excess material is ‘flash,’ residue from the forging process. Remove marks on the flash line using a flap wheel or by draw filing along the length. Follow this by light sanding to produce a scratch-free surface (equivalent to that of 800-grit abrasive paper). Polishing to a mirror finish used to be routine, and it certainly does no harm, but now quality rods are shot blasted (see Chapter 6, ‘shot blasting’).

Fig 1-4: Surface finish and changes of contour.

Plate 1-1: Stress raisers. The marks on the beam of the left con rod, and stamp indentations on the right, interrupt surface flow. Both marks can lead to a stress raiser.

Plate 1-2: The removal of stress raisers. A smoothed, shot blasted con rod beam. Blasting compresses the surface so there’s less chance of a stress raiser.

Loose-fitting components

Another source of stress