Chapter 2

Four-Stroke Performance Tuning big power. It is amazing how many people are trapped by this fallacy. Tables 2.1a and 2.1b indicate the valve sizes that you should be aiming for. Valves of this size in an engine in racing tune will yield maximum power at 7,000 to 8,000rpm. Some motors can, because of their over-square design, benefit from the bigger valves listed, and on these maximum power will be gained at 8,500 to 9,000rpm. Table 2.1a Cylinder volume (cc)

Inlet valve diameter (in) 1.16 1.25-1.31 1.31-1.37 1.37-1.45 1.45-1.53 1.56-1.60 1.62-1.68 1.70-1.75 1.75-1.85 2.00 2.10 2.25 2.40

125 200 250 275 325 375 400 450 500 600 700 800 900

Table 2.1b Cylinder volume (cc) 250 325 400 450 500 600 700 800

Valve sizes for two-valve motors Exhaust valve diameter (in) 1.0

1.06-1.12 1.16-1.20 1.20-1.25 1.25-1.32 1.32-1.37 1.37-1.42 1.42-1.50 1.50-1.56 1.65 1.75 1.85 1.93

Valve sizes for 4-valve motors Inlet valve diameter (in) 1.16 1.25 1.31 1.35 1.38 1.50 1.60 1.65

Exhaust valve diameter (in) 1.00 1.06 1.12 1.16 1.20 1.28 1.35 1.40

Using the following formula we can estimate at what speed maximum power will occur for a given inlet valve area:

GS is the mean gas speed in feet per second. From Table 2.2 you will be able to determine the approximate gas speed by estimating whether the camshaft is standard, sports, semi-race or full race. Keep in mind that many standard factory high14 performance road motors employ sports cams.

The Cylinder Head K is a constant, 5,900 for two-valve motors and 5,400 for three- and four-valve motors. Va is the valve area in square inches. This is found using the formula (PI)r2 where (pi) equals 3.1416 and r is half the valve diameter in inches. CV is the cylinder volume in cc. Table 2.2

Estimated gas speed at inlet valve (ft/sec)

Camshaft Profile Standard Sports Semi-race Full race

Combustion chamber type

Pent roof & hemi 200 215 235

45째 twisted 200 215 230

* Wedge 190 210 225

Bath-tub 175 210 220

260-280

245-260

240-255

230-240

*Note: motorcycles with pent roof and hemi chambers, but non-inclined inlet ports, flow very similarly to wedge chamber heads. If the engine just cannot physically accommodate valves of the size suggested by this formula, or if the rules preclude the use of oversize valves, you will have to take extraordinary measures to obtain the desired level of performance. Usually this involves the use of extra-long-duration camming and larger-than-recommended carburation. Top-end power will increase, but the mid-range could fall away completely. Restricted to 1.81in inlet valves in one class of competition, the 3.5-litre BMW straight six highlights the problem. It will work only in a narrow l,800rpm range - from 6,000 to 7,800rpm. Some engines, however, leave the factory or may be modified and fitted with valves much larger than the formula shows to be necessary. This usually does not create any problems; in fact, it can be of benefit by permitting the use of shorterduration cams. This has the desirable effect of broadening the power band, providing the inlet ports are kept deliberately small. If the ports are too big, low-speed performance will be poor. At times, though, big valves can suppress high rpm performance. This occurs when the valves are masked due to being too close to the edge of the combustion chamber or the cylinder wall. What Ford did in 1969 when it introduced its Trans-Am Championship challenger, the Boss 302, typifies the thinking of many when it comes to valve and port sizing. Ford engineers equated high flow rig readings with high horsepower, so they gave the 302 massive 2.23in inlet valves and inlet ports 2.5in tall by 1.75in wide, The engine just wouldn't work on the slower tracks, so the next year the inlet valve was reduced to 2.19in. These revised heads were used also on the Boss 351, but were still useless until the floor of the port was filled 3/4in to improve gas velocity. Interestingly, when Ford introduced its aluminium high-performance heads for the Boss 302 and 351 in 1983, they reduced the inlet port volume by 12%. However, they retained the very nice quick-burn combustion chamber of the early head. Having established the correct inlet valve size, the next area to look at is the inlet port. A round port will flow best and it should be 0.81 to 0.83 of the inlet valve diameter for best performance on road or track. I have found this figure to give the 15

Four-Stroke Performance Tuning

Valve sizes in two-valve engines Alfa Romeo 2.0 Austin Mini 1.3 BMWV12 5.0 K1000 Buick V6 Series II 3.8 Stage II heads Chevrolet Gen IIV6 2.8 small block V8 5.7 small block 18° heads Brodix 23° heads small block Gen III V8 5.7 big block 454/502 Dart Big Chief heads Chrysler 'A' block 340 W-2 heads Cosworth SCA race Ducati 600 TT race Ferrari Dino V6 Ford Crossflow 1.6 CVH1.6 Pinto 2.0 Pinto 2.3 Lotus Twin Cam 1.6 Windsor V8 5.0 SVON351 heads Cleveland V8 SOHC V8 4.6 429 Boss V8 Harley-Davidson XR750 race Kawasaki GPz 1100 Nissan 1.2/1.4 1.8/2.0 2.4-2.8 Opel Family II 1.8/2.0 Porsche 911 Carrera 2.0 2.2-2.8 RSR 2.8/3.0 928 4.5/4.7 Triumph 500 650/750 VW Golf 1.6 Golf Mk 2 1.8 Type IV dual carb 1.8 Yamaha FJ 1100

Cylinder volume (cc) 500 250-325 416 250 633 750 465 715 625-730 625-730 715 925/1025 1000-1350 700 625-730 250 300 400 325-425 400 500 575 325-440 625 625-730 625-865

575 875-1015 375 275 300-375 450/500 400-465 450/500 333 365-465 465/500 559/583 250 325/375 400 445 450 275

Inlet valve (in) 1.73 *1.48 1.65 1.34 1.8 *2.08 1.72 1.94 *2.12 *2.08 2.0 2.19 2.4 2.02 *2.05 1.45 1.61 1.67 *163 1.65 *1.8 *1.89 *1.69 1.85 *2.05 *2.19 1.75 *2.4 1.73 1.5 *1.5 *1.77 *1.72 1.65 1.77 1.81 1.93 1.77 1.53 1.6 *1.61 1.58 1.61 1.5

Exhaust valve (in) 1.58 •1.14 1.34 1.18 1.52 •1.65 1.42 1.5 *1.62 •1.62 1.55 1.88 1.9 1.6 *1.6 1.25 1.38 1.45 *1.35 1.34 *1.5 *1.59 *1.38 1.54 *1.6 *1.71 1.34 *1.9 1.38 1.28 *1.26 1.38 1.38 1.44 1.53 1.57 1.63 1.58 1.31 1.44 * 1.34 1.3 1.34 1.26

*Non-standard valve size

17

Four-Stroke Performance Tuning

Valve sizes in four-valve engines BMW M5 3.5 K1100 ChevZR-15.7 Weslake heads Cosworth DFY Formula 1 DFX Indy GAAGpA GAA F5000 BDB rally BDG BDH BDM Ducati 850 sports/race 748R Ford Cosworth BDA RS1600 RS 1800 YB Turbo Zetec 2.0 Cobra V8 4.6 Honda NS750 race VF1000R/VF1100 XR500/600 Kawasaki ZZR-1100 ZX-12R KLR600 Lancia Beta turbo rally Mercedes 2.3-16 500 V8 AMG heads Nissan SR20 Opel XE 2.0 Ecotec •1.8-2.2 Ecotec 1.6/1.8f Peugeot V10 Formula 1 Rover K 1.8/1.8VVC Suzuki TL1000 GS 1150 GS 1300 Triumph 750 Weslake race Vauxhall HS2300 Lotus head VW Golf 1.8 Weslake speedway Yamaha V-Max 1200 FJ 1100 TT 550/600 *Non-standard valve size

18

Cylinder volume (cc) 583 275 715 625-875 375 331 569 569 425 500

325 400 425 375 400 459 500 500 575 375 250/275 500/589 263 300 564 356 575 625 500 500 450-550 400/450 300 450

500 284 325 375 575 575 450 500 300 275 550/600 •

Early model

Inlet valve (in) 1.46 1.04 1.54 1.5 1.44 1.33 1.46 *1.52 1.28 •1.36/1.4t 1.28 1.33 1.26/1.34 1.42 1.22 1.32 1.38 *1.34 1.46 1.28 1.18 1.42 1.10 1.31 1.5 1.34 1.5 1.46 1.34 *1.38 1.26 1.22 1.57 1.09/1.24 *1.57 1.10 1.3 1.15 1.4 *1.44 1.26 1.38 1.20 1.14 1.42

Exhaust valve (in) 1.26 .904 1.39 1.31 1.26 1.15 1.26 *1.29 1.08

•1.15/l,2 f 1.0 1.15 1.1/1.18 1.18 1.0 1.15 1.22 *1.18 1.18 1.12 1.04 1.22 .945 1.11

1.3 1.06 1.3 1.26 1.18 *1.18 1.14 1.08 1.37 .95/1.14 *1.3 .906 1.08 1.0 1.18 1.22 1.10 1.13 1.02 .984 1.18/1.22 f Later model

Four-Stroke Performance Tuning Correct.

Incorrect.

Combustion chamber w a l l .

Figure 2.1 A correctly modified inlet port promotes swirl. unshrouded (ie on the side adjacent to the exhaust valve) will flow more mixture. If you take a look at Figure 2.8, you will note that this extreme example of port offset is the reason why the siamese-port Austin Mini head flows so well. To improve flow and discourage charge robbing, the Mini inlet port is modified as in Figure 2.2. The port cannot be made much wider because of the location of the push rod holes, but this restriction has little effect on gas flow, provided that the peak between the two inlet pockets is cut back to open up the flow path. The peak deters charge robbing, therefore it should not be cut back further than the port wall alignment, as shown. Looking at Figure 2.3, you can see that there will be better flow on one side of the valve because of the bend in the inlet port. This is another aspect of flow that we must consider, as gas flow along the floor of the port, and around the short radius into the combustion chamber, can be as little as 25% of the flow on the other side of the valve. The majority of mixture flows along the roof of the port and around the more gentle radius into the engine. The initial flow into the cylinder at low valve lifts is along the floor of the port. As the inertia generated by this initial flow contributes much toward the total flow of the port at full lift, the lower side of the port must be very carefully modified to assist in cylinder filling. Obviously the mixture must be encouraged to flow better on the lower side of the inlet tract. Sure enough, the improperly modified port in Figure 2.3 will flow more air, but not with the valve fitted. The mixture flowing down the inside wall merely bangs into the valve and, having lost much of its energy, relies on the underhead shape of the valve to direct it into the combustion chamber. It is an interesting phenomenon, but an improperly modified port will usually 20

Pushrod holes.

Figure 2.2 This modified Siamese inlet port improves gas flow but deters charge robbing. flow more air with the valve removed, while a correctly shaped port actually flows slightly less air when the valve is removed. This is assuming that a valve with the correct underhead shape is fitted. If a valve of the wrong shape is used, air flow will always increase when the valve is removed completely out of the head. The correct thing to do is to remove metal in such a way that a progressive radius is retained on the inside wall. This will cause the air mixture to turn progressively as it remains attached to the port wall. Little energy will be lost as the mixture spills into the cylinder. This contributes to more mixture being introduced. Figure 2.3 The inlet port should be modified to improve gas flow around the back of the valve.

Incorrect

21

Four-Stroke Performance Tuning The blend from the valve throat to the seat and from the seat into the combustion chamber would ideally be a continuous radius, as on the right half of Figure 2.4. A safe inlet seat width is from 0.035 to 0.050in. The valve seat itself requires a radius of 10-12% of the valve head diameter. The port throat is 0.9 of the valve diameter and blends into an enlarged throat section 0.95-0.97 of the valve diameter. The inlet port in turn blends into the enlarged throat. These changes in the inlet port cross-section are intended to produce a venturi effect to aid gas flow. For longer valve seat life in endurance racing and road use, or where very heavy valve spring pressures are used, a multi-angle seat is more practical. Air flow is almost as good as with a radiused seat, and the seat will last longer, to provide a good seal right to the end of the race (see the left half of Figure 2.4). When a multi-angle seat is used, the actual seat angle should be 45°, and 0.050in wide. The outside diameter of the seat is cut 0.015-0.025in less than the valve head diameter. The top cut to blend the seat into the combustion chamber has traditionally been at a 30° angle, but flow bench tests often show a 38-39° cut is superior, and a 60° throat cut blends the seat into the inlet port. In some applications, an additional 15° hand-blended chamber cut, and a further 80° hand-blended throat cut, may improve flow. The exhaust valve seat is the exhaust valve's main point of contact with the engine coolant. In this instance the seat should be cut at 45° and be of 0.070in width. The exhaust valve reaches a temperature of 1,500-1,800°F each ignition cycle, so adequate cooling must be maintained through the valve seat and valve guide. The overall diameter of the seat should be the same as or up to O.OlOin less than the valve head diameter. (The heat will normally cause the exhaust valve to grow 0.010-0.020in). To finish the seat, a 38° chamber cut should be used, and a 55° throat cut may be beneficial. The inlet valves in many motors are tulip-shaped; this is the preferred valve shape in motors with symmetrical port flow. However, very few motors lend Figure 2.4 Inlet valve seat profiles.

22

Figure 2.5 Modified tulip valve for a hemi chamber. themselves to this type of flow. The few that do are purposely designed highperformance units such as the Ford Lotus Twin Cam and Cosworth four-valve. These are in the happy position of having the right relationship between port, valve and combustion chamber angle, to eliminate any sharp bend by the port into the combustion chamber. This promotes good gas flow around the entire face of the inlet valve. As shown in Figure 2.5, the underhead radius is 0.24-0.26 of the valve diameter. The majority of engines have the valve/port arrangement shown in Figure 2.3, which lends itself to a valve shape with an underhead radius of 0.19-0.21 of the valve diameter. Flow on the lower side of the port is improved with this valve shape and this is what we are aiming for. Comparison of the two valves in Figure 2.6 shows the improved flow path. You will notice with all inlet valve designs the improvement brought about by re-contouring the valve seating area to eliminate any projections. The inlet valve face seat should be cut at 45째 and be 0.075in wide. Often a 25-35째 back-cut is used to reduce the seat width to the required 0.075in. The back-cut Figure 2.6 Valve modified for a flat-roof chamber.

23

Horsepower by the numbers During the late 1970s air-flow test benches became available from the Superflow company to race engine workshops. Along with the popular and economical Superflow 110 flow bench came a user's instruction manual, which suggested that an engine's horsepower potential could be estimated from the intake port air-flow numbers. The formula for a welldeveloped race engine was: hp = 0.43 x number of cylinders x air flow (cubic feet per minute (cfm) at l0in water). Thus, if a four-cylinder engine flowed lOOcfm at 0.420in valve lift and it was fitted with a cam lifting the valves at least 0.420in, the potential power output would be 172hp. This idea got many tuners into thinking that air flow was all important when it came to making power; more air flow means more hp. True, an engine will not make good power if the head does not flow well, but a good deal more is involved, such as the quality of the fuel/air mix entering the combustion chamber and the quality of the burn within the combustion chamber during the ignition process. To illustrate, take an average 2-litre Ford Pinto race engine. It will make only about 200hp with a wild race cam lifting the 1.8in inlet valves in the vicinity of 0.500in, with a compression ratio of 12:1. The inlet flow will be about 117cfm, with the best heads flowing 1.20cfm. On the other hand an Opel Ecotec 2-litre will also make 200hp, but it will make this power on a completely standard head with a pair of tiny 1.26in inlet valves and a 10.8:1 compression ratio, running very mild hydraulic lifter road cams with only 0.410in valve lift and a duration of 232째 at 0.050in lift. Inlet port flow at 0.410in valve lift is 113cfm. Why does the Opel make similar power with slightly less air flow, less compression and baby road cams? Put simply, the fuel/air mixture entering the

Four-Stroke Performance Tuning

Wall cut back to improve gas flow, A Chamber wall radius equal to valve lift.

Figure 2.7 Bath-tub chamber modifications. mixture around the edges of the combustion chamber. This assists in maintaining a steady (not violent) burn rate, and offsets any tendency for high-speed detonation or pre-ignition to occur. Under certain conditions it is possible for the combustion flame to pre-heat the fuel charge directly in front of the flame to the point of self-ignition. When this flame collides with the spark-ignited combustion flame, explosion-like combustion results (detonation). The quench area normally prevents this by removing excess heat from the outer gases. With the temperature lowered, the end gases do not self-ignite to initiate a dangerous detonation condition. The quench area also lowers the piston crown temperature by momentarily restricting the combustion flame to just the area of the combustion chamber. This increases piston and ring life, and helps prevent the piston from becoming a hot spot, able to pre-ignite the fuel/air charge. The exhaust valve requires very little unshrouding at all. I generally work to a figure of 60% of valve lift for the radius between the valve head and chamber wall. It is a mistake to exceed this figure as the exhaust valve flows very well partially shrouded; in fact, it seems to enjoy being shrouded. I well remember an occasion where I was able to pick up 6hp on a VW 1600 by shrouding the exhaust valve a little. Power went up from 163 to 169hp, which represented a 4% gain. The heart-shaped chamber of the Mini (Figure 2.8) is an easy one to modify. It yields high power due to the inlet port offset producing a good swirl effect. The valves should be unshrouded as with a bath-tub chamber. A later development of the bath-tub chamber, which also borrows from the 26

The Cylinder Head /Cylinder outline. Port angle gives good flow into chamber.

Figure 2.8 Austin Mini chamber modifications. Austin Mini in that the inlet port enters the chamber at a similarly favourable angle, is what is called the 'twisted bath-tub' chamber. This chamber is found in the Isuzudesigned Opel/Holden/Vauxhall Family I and Family II OHC engines. As you will notice from the photograph, the combustion chamber is turned, or 'twisted', through 45째, producing a head with inlet ports on one side and exhaust ports down the other side. With the chamber twisted like this a much greater area around the periphery of both the inlet and exhaust valves is unmasked, so flows into and out of the engine are improved. Also there is a good squish area opposite the spark plug, which is itself located deep into the chamber, giving a good quick burn. For even better inlet flow some of the squish area near the spark plug can be cut back toward the cylinder outline, but take care not to overdo this. Remember that we were talking before about squish and its importance in the combustion process. The primary or major squish area should be somewhere about opposite the spark plug, to squish the bulk of the fuel/air mixture over into the combustion area. Now behind the spark plug we have a secondary squish area. Its Twisting the combustion chambers opens up the inlet and exhaust flow path while maintaining generous squish areas.

27

Four-Stroke Performance Tuning effect is also to squish and create a pressure wave. As the spark plug ignites the mixture, this wave assists the flame front to travel through and ignite the mixture that the major squish area has pushed towards the spark plug. If the combustion chamber is cut back right to the bore wall, what is the result? Gas flow will improve, but remember that I said right at the outset that gas flow has to be balanced against good combustion. With no secondary squish to assist the flame front and impede the mixture rushing across from the major squish area, you must lose power. This is what happens: the mixture being purged across to the spark plug bangs into the chamber wall and spark plug and devaporises, wetting the chamber wall and possibly the spark plug too, depending on its temperature. Try holding your hand in front of an aerosol spray pack when you are spraying and you will see what I am getting at. At the very least, a head modified in this way will waste fuel and dilute your engine oil. Some of that lovely fuel mixture you have managed to cram into the cylinder will end up going out of the exhaust or, at low revs and/or with a cold motor, it will end up in the sump, wrecking your super-good oil. One problem with many of these Opel heads is that the inlet ports are way oversize, so keep the grinder away from them or these engines become even more sluggish at lower rpm. Fortunately a number of models have good-size valve seat inserts and the valve spacing is such that larger valves can be easily fitted. Bigger valves will provide a good hp increase at higher rpm without taking anything away from the bottom end. The 2- and 2.3-litre Ford Pinto head appears to be the same as the twisted bathtub but it is in fact slightly different in that the inlet and exhaust valves are not vertical. As you can see from the photograph, both valves are tilted, or 'canted', a few degrees. While this gives rise to all sorts of valve gear problems, as anyone who competes with these engines is only too well aware, splaying or canting the valves does have several benefits. First, as the valve opens it moves away from the combustion chamber wall, Splaying or canting the valves improves flow because the head of the opening valve moves away from the wall of the combustion chamber and provides a wider flow path.

28

The Cylinder Head thus opening up the flow path. This also opens up the way to fit larger valves, which would otherwise be severely masked if arranged vertically. Additionally, as we will discuss in depth later, reducing the port-to-valve angle improves flow around the valve head into the combustion chamber. As with the Opel, these heads also have overly generous inlet ports relative to the standard-size inlet valves. Only when larger valves are fitted should the ports be touched. The wedge chamber in its most basic form is in reality just a bath-tub chamber inclined, usually at around 15째. This is the type of chamber found in almost every American V8 and it presents the serious tuner with many problems, particularly when the chamber is tilted steeply, as it is in the small-block Chev, at 23째. Why use the wedge chamber then, you may ask? Figure 2.9 illustrates the most obvious benefit from this design, namely the reduction of the bend in the inlet port, allowing the valve to flow better, particularly on the lower side. A straight (noninclined) port flowing into a bath-tub chamber flows only about 20% of the mixture on the lower or 'dead' side of the valve, but the wedge chamber can flow 25-30% of the mixture on this side. The other obvious benefit is the concentration of fuel/air mixture close to the spark plug in the deepest part of the chamber. This effectively shortens the flame travel, allowing for complete combustion if a flat-top piston is used. However, and this again highlights the importance of balancing good air flow with good combustion, in performance and competition applications we soon run into a situation where with many wedge chambers we just cannot get better than a 9.5:1 compression ratio without resorting to high-top pistons. Any piston becomes in effect the 'floor' of the combustion chamber, so the shape of the crown can either promote or Figure 2.9 A wedge chamber decreases the port entry angle.

Wedge chamber with valve inclined 15째.

Bath-tub chamber non -inclined valve. 29

Four-Stroke Performance Tuning

retard mixture homogenisation and combustion flame propagation. A high-top piston obviously masks the spark plug and disrupts flame travel across the combustion chamber. Also of course there is a mechanical penalty; high-top pistons weigh more, so loads on bearings and connecting rods increase. This could mean that heavier, more expensive rods and crank, or a lower rpm limit, are necessary. This has led to a serious rethink in Detroit. First in 1989 Chevrolet introduced its 18째 Bow Tie range of heads for competition engines. These heads feature smaller, shallower combustion chambers, which in themselves promote a better burn and less detonation trouble. This has meant less piston crown protruding into the combustion chamber while allowing the compression ratio to climb to 15:1 on 112 octane race fuel. Then at the end of 1996 Chev introduced their revised small-block engine, dubbed the Gen III. While still retaining wedge chambers, the valve inclination is further reduced to 15째 to produce a more compact combustion chamber and, like the L31 Vortec '906' heads mentioned previously, the combustion chamber is now heartshaped. Ford SVO has done something similar with their N351 cast iron competition heads. In these the valves are inclined at only 10째 as opposed to the standard 22째. This effectively makes the combustion chamber more compact and means that only a small Figure 2. JO Big-block Chev head design.

30

Chevy ZL-1 427 chamber.

Ford Boss 3O2/351 chamber.

Four-Stroke Performance Tuning

32

Old port.

33

Four-Stroke Performance Tuning

Poor squish area.

Figure 2.14 Early Jaguar head design. efficiency. Also, because of the large chamber area, the ignition flame must travel an extreme distance. Consequently two spark plugs were used in racing engines to aid combustion. Even today, after years of development, the Chrysler hemi and the aircooled flat-six Porsche still work better with two plugs. Remember our friend squish? Well, he is also lacking with this design. In Figure 2.14 you will notice that some squish does occur due to the radius of the combustion chamber being smaller than that of the piston dome, but this isn't enough to promote good combustion. Later, the piston dome design was changed to that shown in Figure 2.15. This modification was adopted by both Chrysler and Jaguar. The piston dome in this design is supplemented by a flat section running right round the crown, which comes into close contact with the lower reaches of the combustion chamber. Squish is improved, bringing an improvement in combustion. This has resulted in higher power outputs and a reduction in spark advance. Figure 2.15 The later Chrysler Hemi has improved squish control.

34

Cylinder outline

Semi-hemi chamber squish area.

Figure 2.16 Lotus/Ford Twin Cam head arrangement. In 1963 the Lotus/Ford Twin Cam was introduced. Not a true hemi but a semihemi, this motor set the pace in head design, in the car world at any rate, for many years. In 1.6-litre form this motor will produce 210hp on carburettors, and will run quite happily with just a 30-34째 spark lead. In fact, the motor I ran on the road in my Ford Anglia for many years operated with only 25째 total advance. It was no slouch either, producing 156hp at 6,800rpm. The semi-hemi combustion chamber (Figure 2.16) has two distinct squish areas that drive the fuel mixture towards the spark plug. The valve angle has been closed up (54째 on Lotus/Ford Twin Cam), allowing the use of a flat-top piston. Consequently this design enjoys a more compact combustion chamber than the true hemi, and this feature, combined with good squish control, makes for high power outputs while still allowing a large valve area and a relatively straight inlet port. You will also note that with this design, as with the hemi, the inlet valve, and also generally the exhaust valve, are completely unshrouded. This allows for fairly even mixture flow right round the valve head. Instead of the valve flowing 80% on one side and 20% on the other, as with the bath-tub chamber, it will probably be flowing something more like 57% and 43% in this instance. Therefore the available area is being used more effectively so that air flow and horsepower go up. The next development of this type of chamber is what we call the pent-roof chamber (Figure 2.17). This design is immediately associated with Cosworth because of the pioneering done by Keith Duckworth in developing the 1.6-litre 225hp Ford FVA, and later the 3-litre 470hp Ford DFV Grand Prix engine, proving the advantages of the four-valve pent-roof chamber. From the outset the advantages of the design were not so 35

Four-Stroke Performance Tuning

Cylinder outline. Pent-roof chamber squish area.

Figure 2.17 Cosworth Chevy Vega four-valve head. obvious. Both Coventry Climax and BRM had earlier tried this layout but without much improvement over the more conventional semi-hemi. BRM, in its typical enthusiasm, even tried a four-valve with diametrically opposed inlet and exhaust valves. Can you imagine designing the intake and exhaust system for a V12 so equipped? It was the motorcycle world that brought us this design. Back in the early 1960s Honda were using the four-valve principle on their Grand Prix bikes. At that time they were running a five-cylinder 125, a six-cylinder 250 and 297 and a four-cylinder 500, all with four-valve pent-roof chambers. The 250 produced 60hp at 17,000rpm. Honda then carried the design on to the car Grand Prix circuit with their 160hp 1-litre Formula 2 engine. The two-valve 140hp Cosworth SCA was no match, so when the Formula 2 capacity limit was lifted to 1.6 litres, Keith Duckworth designed his new motor, the Ford FVA, around the four-valve pent-roof chamber. He got his maths right, for the first time the FVA rode the dyno it pushed out 202hp at 9,000rpm. Initially Honda used a fairly wide valve angle of 63째, which required the use of high-top pistons. Cosworth, however, closed the valve angle up to 40째 (32째 on DFV, DFX, DFL; 221/2o on later DFY Formula 1), producing a very shallow combustion chamber that allowed the use of flat-top pistons. Squish was good, and from opposite sides of a single, central, spark plug. Now virtually every designer utilising the pent36 roof four-valve arrangement follows the design refined by Keith Duckworth.

Four-Stroke Performance Tuning

Top of port raised & radius increased.

Four-Stroke Performance Tuning

The Cylinder Head It should be noted at this stage that quoted compression ratio figures are always theoretical compression ratios and are expressed by the following formula:

CV is the cylinder volume, and is easily found by dividing the engine capacity by the number of cylinders. Therefore a 2,000cc four-cylinder motor has a cylinder volume (or swept volume) of 500cc. CCV is the combustion chamber volume, and is not so easy to calculate. It is made up of the combustion chamber volume, plus the volume that remains above the piston when the piston is at TDC, plus the volume caused by the thickness of the head gasket, plus the volume of the dish if dished pistons are used, or minus the amount displaced if high-top pistons are used. Assuming that the pistons are flat tops, the formula for finding the volume of the head gasket or the volume above the piston is:

where n = 3.1416, D = the diameter of the bore in mm, and H = the compressed thickness of the head gasket or the clearance between the piston crown and the block deck in mm. The volume of the combustion chamber is measured as shown in Figure 2.19, using a burette filled with liquid paraffin. If the head has been modified by a tuner, he should have told you the chamber volumes when you purchased it. Incidently, the combustion chambers should have been equalised in volume so that the volume of the largest is no more than 0.2cc larger than that of the smallest. Figure 2.19 Measuring combustion chamber volume.

Burette

Perspex or glass.

41

Four-Stroke Performance Tuning If you are running dished or high-top pistons you can find what increase or decrease in combustion chamber volume they are causing by using the method shown in Figure 2.20. In this instance let us assume that the cylinder bore is 102mm in diameter and the piston crown is 13mm from the block deck, therefore using the formula

this volume should be

which is 106.2cc. However, measuring the volume with a burette we find it to be only 74.7cc. Therefore the lump on top of the piston occupies 106.2 - 74.7 = 31.5cc. Having found these volumes we must go back to the original formula to calculate the compression ratio. Today's engines make extremely taxing demands on the valves. Let us just look at a typical exhaust valve during a single cycle. At the beginning of the intake stroke, during the overlap period, it is subjected to a chilling blast of fuel mixture. Then it is slammed against the valve seat and blasted with a hot flame that will raise its temperature to perhaps l,800째F, before it is suddenly hit by another chilling intake charge. In an average racing engine this can be happening 3,000^,000 times a minute, during which time it is expected to form a gas seal that successfully contains a pressure of around 1,500 pounds per square inch (psi). To exist under such conditions, a valve has to be made of pretty good material, but here we have a problem. Racing valves are made of austenitic stainless steel, which, while being able to withstand high combustion temperatures, has very poor scuff resistance. Therefore they should be used only with valve guides made of silicon aluminium bronze, which is more compatible with austenitic steel as far as antiFigure 2.20 Measuring high-top piston displacement.

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Four-Stroke Performance Tuning

It is important to ensure that there is sufficient clearance for valve stem oil seals between the valve cap and valve guide at maximum valve lift.

seizure. Fitted to the exhausts they do not seem to handle the heat too well, so they quickly harden and fail to provide good oil control. Perfect Circle-type teflon seals are preferred for competition engines. Like neoprene seals they slip over the top of the valve guide, but in race engines that always have minimum valve-stem-to-guide clearance, and hence no valve stem wobble, they do a more consistent job of controlling valve stem oil flow. However, when the clearance increases they work quite poorly, so if you allow your engines to run down like this you may be better off with another type of seal. When fitting these seals over exhaust valve guides it is a good idea to lightly ream them, using a reamer the same size as the valve stem diameter to allow a little more oil flow to the guide. A less hi-tech solution is to fit an 'O' ring seal up under the valve collets (valve locks) and an oil splash shield under the spring retainer. The 'O' ring prevents much of the oil flowing from the camshaft and/or rocker arms from travelling down the valve stem in the gaps between the collets. Instead, most of the oil has to flow out over the valve cap and the oil splash shield fitted under it, then down over the valve spring. This doesn't work on the inlet valve as well as a neoprene or Perfect Circle teflon seal, but it does increase oil flow over the valve springs. Without a good flow of oil to carry away heat, valve springs begin to fatigue and lose pressure. Another solution that works in the same manner as an 'O' ring, but more effectively, is to seal the gaps between the valve collets with Silastic RTV when the engine is being assembled. Note that for this to work properly the top of the valve stem, the collets and retainer must be carefully degreased with solvent and allowed to dry before applying the Silastic silicone sealant. Using this method I have seen oil consumption halved and carbon build-up on the valves almost eliminated in endurance-type engines. In engines with large-diameter inner springs, the old 'umbrella'-type seal can 44 possibly be fitted. However, as is also true when the neoprene and Perfect Circle types

The Cylinder Head are used, there must be sufficient space for it between the bottom of the valve spring retainer and the top of the valve guide at full valve lift. A point not to be overlooked when it comes to valve stem oil control is that some engines, either by design or negligence, allow a lot of oil to 'puddle' around the valve springs with the result that the top of the valve guide may become submerged in oil. Obviously if all of that oil is needed to cool the valve springs, as may be the situation in some overhead-cam engines with inverted bucket tappets, only neoprene or Perfect Circle-type seals will be of any use. However, it is also possible for valve guides to become submerged in oil due to oil drain back holes or dry sump scavenge pick-ups being in the wrong place because consideration has not been given to G forces being imposed by long high-speed turns. Obviously the drain back problem should be rectified, then any appropriate valve stem oil control system can be chosen. Armed with all this information on cylinder heads you are in a good position to make a sensible choice when you go shopping for a modified head. There are, however, just a few more points worth mentioning at this time. Do not allow yourself to be tempted by all the dazzle and shine of the ports and chambers. You are wasting money; the finish should be good and fine, but it certainly does not need to be polished. Some cars have such poor carburettors that fuel will actually collect on a polished port wall. This point must be kept in mind particularly if you are running a competition motor where alcohol fuel is permitted. Alcohol, being less volatile, separates from the air easily, and as the inlet tract is pretty well refrigerated, especially so when running on alcohol, it remains clinging to the port, causing a flat spot and power loss. Before you take delivery of the ported head it should be matched to the inlet manifold. At the same time dowels should be fitted so that each time the manifold is removed, it will, on being replaced, align perfectly with the ports in the head. These dowels need only be one-eighth of an inch diameter silver steel. While the head porting work is being done, other specialised work should also be carried out such as re-cutting the valve spring pads. Therefore the head modifier will have to be given specific instructions by you regarding such things as what valve Oil splash shields can befitted under the valve caps to direct oil away from the valve stems. 'O' rings are also installed under the valve locks to stop oil flowing through the collet gaps.

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Four-Stroke Performance Tuning