We have now arrived at the longblock assembly and, for the production of power, our principal goal is to minimize friction. But that is something of an oversimplification of our mission statement. There are many other practical aspects we have to address, beginning with the engine block.
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The block is, in effect, the case that has to contain all the loads brought on by the development of the power we are seeking to build. If you are building an engine for any serious competition, the first move should be to sonic-check a selection of blocks. This allows you to throw out any that may have thin sections in high-stress areas and to sort out the best of the selection at hand.
After you choose a suitable candidate, the main bearing housings need to be checked for alignment and size. Here, the housing diameters need to be sized to ensure the bearing crush is slightly on the tight side, so as to avoid any chance of a spun bearing.
The bores then need to be considered. The boring equipment in many machine shops locates off the old bore, and thus replicates any error there may have been with the bore’s alignment over the mains. The best way to handle things here is to bore on a machine that locates on the main bearings, and reference from there. Also the block faces need to be trued-up square and parallel with the mains.
For certain classes of racing, other jobs may also be necessary. One that comes to mind is accurately locating the tappet bores and resizing them to the largest allowed by the rules.
Bore finish is also very important, and it’s mostly dictated by the type of ring material used. Unless you have positive experience to the contrary, follow the ring manufacturer’s recommendations to the letter.
The Rotating Assembly
Depending on the extent of the quest for power and the budget available, there is much that can be done in the pistons, rods, and crank departments. Before we delve too much deeper into short-block tech, it’s worth taking a look at all the parameters that define the bottom end assembly, and factors that we have some control over.
One of the primary considerations when designing an engine from scratch is the bore/stroke ratio. If maximum power/torque is the primary criteria then, for a given displacement, a big bore and a short stroke gets the job done—it allows bigger valves to be accommodated. However, for a production engine, emissions and packaging (engine length) usually dictate longer strokes and smaller bores. When Formula 3 was a pushrod one-liter class breathing through a single 28-mm restrictor bore/stroke ratios, winning engines were typically in the region of 1.8:1 (bore 1.8 times the stroke). The short stroke allowed for very high RPM. The last one I built (went into my 1-liter Ford Anglia) turned a useful 11,000 rpm. A modern production engine such as, say, the Ford four-valve 5.4-liter Modular Motor, that powers the 500-hp Shelby 500 Mustang, is at a 0.85:1 ratio.
Cranks are essentially available in three grades: cast, forged, and billet. Cast is the least expensive, billet is the most expensive, and forged is in between. Many production vehicles employ cast-iron cranks because of ease of manufacture and wear resistance. These won’t take either the outright power or abuse forged ones will, but they are usually okay for a reasonably strong street performer.
A forged crank is a step up the ladder and, for more money, provides greater strength. Although factory cranks are usually of lesser-grade steels, the steels most commonly used in aftermarket cranks are 4130 and 4340. Depending on the cost of the crank, these can be had with surface hardening, which gives a very good wear life.
Last on the list are billet cranks. These are made from a block of steel because there is no appropriate forging available from which to machine the desired design of crank. And they can be very expensive.
With aftermarket cranks becoming a lot less costly, it becomes very practical to increase the stroke of an engine just to get more displacement. Buying a new stroker crank also means you have a stronger crank, with 100 percent of its fatigue life still available. If a stroker build is in the cards, the next step is to choose your brand of crank wisely. I have dealt with Crower, Scat, Lunati, Callies and Winberg to good effect and can recommend these companies.
The downside of an increased stroke is that the increased piston speed means reduced RPM potential. If we are building just to make the most power from the engine rather than to any race regulations limiting displacement, then the advantage of more cubes from a longer stroke more than offsets the disadvantage of reduced RPM.
more than offsets the disadvantage of reduced RPM. Just how many RPM the rotating assembly can handle depends on the crank stroke and the strength of the connecting rods that will be used. A good cast-steel crank and a decent set of rods should be good for a mean piston speed of 4,500 feet per minute. To figure that in terms of RPM, take the stroke in inches, divide it by 6, and then divide that answer into the limiting mean piston speed. In the example we would, for a 3.75-inch-stroke crank, have 3.75/6 = 0.625. If we now divide 0.625 into 4,500, we get 7,200. That is our limiting RPM. If the stroke had been of 3.5 inches, then the RPM for the same 4,500-ft/min average piston speed would have been a shade over 7,700 rpm.
If top-quality race parts are used, the limiting mean piston speed can be increased to 5,000 ft/min and, for a drag-race-only application, even higher. As of 2010, a front-running 500-inch Pro Stocker can turn more than 10,000 rpm. With the typical stroke of around 3.7 inches, this equates to a mean piston speed of a blistering 6,200 ft/min. This capacity may be fine if you are not expecting to put more than a total of 10 minutes of race time on the parts involved.
It’s possible to fill an entire chapter on the subject of connecting rods, but I do not have the luxury of space to do that. Here, I need to cover the important basics, starting with the way we discuss rod proportions. Because engines vary so much in size, simply quoting the rod’s centerto-center length tells us little about its geometric proportions. The most important element of a rod is its length in relation to the crank’s stroke. This is called the rod/stroke ratio. What is used can have a significant effect on the engine’s RPM capability and its output. Illustration 12-1 shows how the rod/stroke ratio is defined. To come up with this the stroke, (A) is divided into the rod’s center-to-center length (B). If, in this case, the stroke was 3 inches and the rod 6 inches, the rod/stroke ratio would be 2:1.
So what is the optimum rod/stroke ratio? If you ask around, you hear all kinds of theories as to what it should be and what is optimal. Most of what you are likely to hear is disinformation. The most likely piece of info you might get is that a long rod/stroke ratio is good for high-RPM power, and a short one is good for low-speed torque. The first part may be so, but the second, concerning short rods, is highly suspect. What defeats the situation is a combination of rod angularity and friction.
In simple terms, the longer the rod is, the more mechanically efficient the linkage system becomes. By the time ratios get to the 2:1 mark, there is little to be gained from further rod-length increases. There is one saving grace for the shorter rod ratio: The higher the compression ratio goes, the less negative effect there is with a short rod.
Pistons can be had in three distinct material groups. These are, in order of preference for a high-performance engine: high-silicon aluminum casting, hypereutectic casting, and forging. Pistons for a regular street application need to be inexpensive to manufacture, exhibit long wear life, and run quiet. Casting a piston in the first place allows for the production of a cheap blank. Alloying the aluminum with a substantial amount of silicone makes the casting harder while reducing the expansion coefficient. Regular cast pistons take just so much abuse and then fail, usually catastrophically at that.
The next step up the ladder is the hypereutectic cast piston. The term hypereutectic means that there is more of an alloying element present than will disolve in the parent metal. In this case the principal alloying element is again silicone, but in such proportion (11+ percent) that there are free crystals of silicone present. This makes for a harder piston and, given suitable section thickness, a reasonable strength.
Depending on the bore size (smaller bores dissipate piston-killing heat faster) a hypereutectic piston can be used up to normally aspirated power levels of 100 hp per liter (1.65 hp per cubic inch). Such power levels need to be reserved for engines with bore sizes up to about 70 mm. For the 4-inch-plus bore size of a V-8, power levels need to be contained to 70 or maybe 80 hp per liter to retain the structural integrity of the piston.
Last on the list is a forged piston. Most manufacturers work with two different alloys: one with a lesser expansion rate and which is harder after heat treating, and a second alloy for race purposes, which exhibits slightly greater expansion and less hardness, but is tougher.
As may be expected, the physical form of the piston for a high-output application is important. When selecting a piston, you need to look for a well-supported skirt and pin boss along with drilled oil-return holes in the oil ring groove, rather than slots. The compression rings need to be placed as high as possible, consistent with the application. Go too high, and the rings and top-ring land suffer thermal degradation (burning and melting). Nitrous or forced induction calls for a lower top-ring placement than normally aspirated applications.
There are many piston ring manufacturers. Each has a wide and often bewildering range of rings from which to choose. As you might expect, cheap rings are giving up something compared to what you may get if you pay more. Usually low cost means a shorter life, but that should not imply that any ring set that is not expensive is substandard.
Many good ring sets cost less than you might expect because they are produced in such huge quantities. To ensure a reasonable life expectancy, make sure the rings are a suitable, heat-treated steel alloy or have a wear-inhibiting, moly-plasma spray coating. If the application is nitrous or supercharged, a ductile iron or, better yet, steel ring is needed to combat temperatures and shock loading. If you are going for highend race rings, the titanium nitride steel rings (gold in color) are about as wear resistant as you can get.
With ring styles, the less mass the ring has, the better. For a typical 33 ⁄4-inch (or bigger) bore, ring widths of 1/16, 1/16, and 3/16 have been common for replacement pistons. But the need to meet mandated fueleconomy standards has brought about the development of ring packages that seal better and also have lower friction values. Both Mahle and DSS have very affordable ring packages that feature lighter rings with thinner sections.
The point here is: The thinner the ring, the more effectively it works in a high-performance engine. With today’s titanium nitride coatings, ring wear is no longer a challenge so, other than cost, rings of a very narrow cross section are not a problem. The 1-mm, 1-mm, 3-mm ring pack is becoming the choice of many engine builders, but section thickness is far from the only factor to take into account. There are various styles of top ring, of which the most common is the flatface type. Probably the most complex is Total Seal’s top rings, which function as shown in Illustration 12-2.
have done much back-to-back testing of this type of ring, and it does do what is claimed of it: a virtually total seal of the cylinder. The advantage of this type of ring is that it nearly provides a 100-percent seal with a working-end gap of as much as 0.060. Such a wide gap puts it well out of the range of possible seizures due to thermal expansion causing the gap to entirely close, making the ring nearly a press-fit in the bores. Because it represents a failsafe deal and because results are consistently topnotch, I use Total Seal top rings in all but the cheapest builds that I do.
Although it is termed a compression ring, the function of the second ring is not to seal combustion pressures so much as to act as the first line of oil control. The ring of choice here should, in most cases, be of a Napier Scraper design. Here the tapered face of the ring tends to ride over the oil on the way up the bore and scrape it off on the way down.
The most common type of oilcontrol ring is the multirail-andexpander design. Introduced in the early 1960s, its effectiveness has been proven in many millions of applications. Part of the ring’s oil-control function is due to the outward radial load it exerts on the bore. This, of course, is a frictional loss, so it must be minimized. At this stage, crankcase vacuum comes into play. By pulling a partial vacuum on the crankcase, the need for so much outward radial force is reduced. This can be quite significant. Testing by just changing to a lower-radial-preload ring found an easy 5 hp for a four-cylinder 1,600-cc Formula Ford engine. For a V-8, making such a move can be worth double to triple this amount, depending on the RPM involved.
If you are starting on the development of a particular piston and ring combination, it is a good idea to deal with a piston ring specialist such as Total Seal because they can point you toward effecting good oil control while limiting frictional loss.
Although we have concluded the discussion of oil-control rings, we have not entirely finished the subject of oil control. There is plenty of opportunity to squander power in the engine’s lubrication system. All too often, the assumption is made that the oil pressure for a race engine needs to be really high. There’s an often-quoted general rule that an engine needs 10 psi for every 1,000 rpm at which it turns. Although it’s not quite as simple as that, it is at least a starting point and, for the most part, it’s at least on the safe side. But applying this basic rule to a 1,000-cc, 11,000-rpm engine means way more pressure is needed, as well as a substantial power reduction just to drive the oil pump at such pressures.
Many manufacturers’ stock performance engines survive 200,000+ miles with no more than about 55 psi. With few exceptions, a typical fourcylinder, 2-liter performance engine turning up to 8,000 rpm needs only about 55 psi and maybe only 70 or so at 10,000 rpm. But oil pressure alone is not the only issue here.
Be aware that a high-volume oil pump sounds better on paper than it is in practice. Ask yourself why you would want to install a higher-volume oil pump if the stock one was already into the bypass valve at 2,500 rpm. The answer here is: Maybe you shouldn’t. Increased bearing clearances, more RPM, and hotter oil may justify an increase in oil pressure by adjustments to the bypass valvespring. But that, for the most part, is the only change needed.
As for bearing clearances, we find that in most instances 0.002 for the rods and 0.0025 for the main bearings work well. However, with really good parts and close manufacturing tolerances, these clearances can, as we see with Cup Car engines, be closed up by a half to one thousandth or so (0.0005 to 0.001 inch).
Almost certainly of greater importance than the search for oil pressure is what happens to the oil after it has done its job lubing parts and cooling pistons. No matter how you look at, it the bottom end of an engine is pretty ugly in terms of aerodynamics. The crank and rods can absorb quite a big chunk of power just thrashing around in the crankcase. The number-one job here is to minimize windage and viscous losses by separating the oil from the rotating assembly. The first move is to make sure the pan is sufficiently deep so that the crank is far from being able to dip into the oil.
Probably of near-equal importance is making sure that the oil pump pick-up is always well immersed in the oil. This requires using a horizontal baffle to reduce surge to a minimum. An effective wet-sump system for a serious drag-race engine has an assortment of trap doors, baffles, scrapers, and de-aerating mesh screens. The purpose here is to allow, as far as possible, the crank to run in an oil-free environment. For the most part, drag-race pans can be deep because ground clearance is not so much of an issue but, for a road race car, the situation changes somewhat.
When ground clearance or a bigger budget permits, opting for a dry-sump system is well worth considering. Although far more complex in nature, a dry-sump system allows a lower installation of the engine in the chassis, thus lowering the car’s center of gravity. Dry sumping also allows us to use multiple scavenge pumps with a capacity to not only pull out all the oil but to also drop the pan/sump internal pressure well below atmospheric. Exactly what is accepted universally as the optimum absolute crankcase pressure is still far from decided. But I have seen more than 15 inches of mercury used to seemingly good effect. My own experience indicates that anything over about 2 inches of mercury (40 inches H2O or 1 psi) starts to pay a worthwhile dividend.
Written by David Vizard and Posted with Permission of CarTechBooks