Engine Build - The Outer Limits

Understand Stroke Limitations and Piston Speed
There have probably been more stroker motors built in the last 10 years than there were in the previous 90. The proliferation of inexpensive, stronger-than-stock cranks has brought an entry-level stroker kit within almost every hot rodder's financial reach. But there are limitations as to how much stroke can be used. Any time stroke length is increased we either have to accept increased piston/rod accelerations (and therefore loads) or a reduction in redline rpm. For the most part we can calculate about where to set the engine's redline rpm from a given mean piston speed. The formula is simple: Mean Piston Speed (feet per minute) x 6 divided by the stroke in inches. Fig. 1 shows the formula and an example.

The next problem that needs to be addressed is balance. The longer the stroke, the more difficult it becomes to achieve internal balance. Sure, we can go the external balance route, but this is a bandage fix because it exaggerates the bending moment on each end of the crank. This is not such a worry on the rear of the engine, but it can be on the front, where a lot of crank snout is hanging out there for accessory drives. Also, the longer the crank's stroke, the less its torsional rigidity. All of this adds up to a more flexible crank, thus putting greater demands on the crank damper. At the end of the day, tortional vibrations and bending moments can kill a crankshaft faster than the loads produced by power and rpm. Effective dampers are readily available for common stroker configurations; this won't be the case of the more specialized big-inch combinations. The route many top engine builders take here is to employ the custom damper building service offered by ATI. Going this route allows the damper to be zeroed-in on critical torsional and bending moments.Another factor to take care of is the crankcase. Firstly, oil is picked up by a long-stroke crank easier and sooner than a short-stroke one. This means that a good pan design is a must. Along with this, the effects of block distortion and ring seal have to be catered for. A good pan vacuum is a great help here. Not only does it ensure a better ring seal, but also, the lower crankcase pressure encourages the oil to drop out of suspension quicker, thus cutting windage losses.

Maximize Rod and Piston Capability
Like every other part in a maximum-inch motor, the specification and design of the rods and pistons become more critical. Let's look at pistons first. Because we're trying to get as many cubic inches out of the block as possible, there is a need not only to accommodate the longer stroke, but also to try and get a good rod-to-stroke ratio. That means trying to accommodate as long a rod as is possible. This inevitably pushes the wrist pin high up into the ring pack. For most practical purposes, a wrist pin centerline-to-piston deck height (compression height) of 1-inch is about the practical limit. Sure, if you elect to use two-ring pistons the compression height can be cut further, but not by much. Also because of the long stroke that will inevitably be involved in a big-inch motor, the piston weight becomes more critical if reciprocating loads are to be kept to a minimum. This is a little easier said than done because a long stroke (and a rod often too short for the job) usually means the piston skirts and cylinder walls are side-loaded to a far higher degree. This situation calls for a structurally stiff piston. The pin often becomes a part of that structure. Those super-lightweight pins seen regularly in short-stroke, high-rpm engines usually just don't cut it in big, long-stroke mountain motors.As far as rods are concerned, we find (assuming a race-spec rod to start with) these pins do not grow in cross-section as quickly as whatever stroke increase is involved. The reason for this is that as stroke is increased, the rpm involved drops, so piston speed and acceleration peaks remain about the same. The result is a mountain motor rod that won't be as fat or weighty as you might expect it to be.

Optimize Heads and Cylinder Filling for Power
As the engine's cylinder proportions are increased, it becomes harder to extract a proportional increase in power. Just after World War II, it was considered that, in normally aspirated form, 125 hp-per-cylinder was about the limit with the materials, oils, fuels, and technology then available. These days, better materials, modern fuels and oils, flow benches, and valvetrain dynamics have allowed the 125hp figure to be exceeded by a big margin. Currently, a good mountain motor can put out over 200 hp-per-cylinder, but it hasn't, nor will it ever, come easily. The reason for this is largely a question of proportions and geometry. It is better, if you are attempting to build a given size engine, to have a lot of smaller cylinders rather than a few large ones. At first, this does not seem to make sense, so let's go through the logic to demystify things.

Even if all the proportions stay the same, it is harder to fill a large cylinder than it is a proportionately identical, but smaller cylinder. An example here will serve to illustrate the situation. Let's take a 180-inch single-cylinder engine as a starting point. Let us also assume that this engine has a bore/stroke ratio of 1.2 (bore is 1.2 times that of the stroke) and the intake valve is 50 percent of the diameter of the bore. If this 180-inch engine has just one cylinder, the intake valve would be 3.252 inches in diameter, the valve's area 8.306 inches, and it's circumference 10.22 inches. For a cylinder head equipped with this valve to breathe effectively, it would need to be lifted in the order of 970 thousandths, minimum.

Now let us consider an eight-cylinder engine of 180 inches. This engine, with the same proportions, would have an intake valve diameter of 1.626 inches. All eight intake valves would have a combined area of 16.612 inches and a total circumference of 40.88 inches. The lift needed to get near maximum flow would be only 487 thousandths. In a nutshell, this means the eight small cylinders of 180 inches have approximately twice the breathing power of one big cylinder of 180 inches, even though the proportions of valve diameter-to-bore remain unchanged. Also, if we assume 5,000 feet per minute maximum piston speed, the maximum rpm of the one-cylinder engine would be 5,500 where as it is 11,000 for the eight-cylinder engine. There are other factors involved, but from this you can see the reason why it is harder to extract power from big cylinders rather than a greater number of small ones. You can also get an idea of how ultimately these and other limitations call a halt to the amount of power that can be had per cylinder.

Understanding these limitations allows us to make moves that help optimize what can be done within the constraints of having only eight cylinders. Firstly, because filling the cylinders is going to be a problem with any type of conventional head design, we find that bumping the CR as high as possible becomes a priority. To get to high compression without compromising the combustion chamber shape means that the bore of the engine cannot get too big in relation to the stroke. Indeed, it is often found that engines with a stroke a little longer than the bore pay off when a very high compression is sought. But this leads to conflicts within itself. With a long-stroke motor having proportionally less bore, the size of the valves is further compromised.Although few in number, one point going for us here is that the higher the compression goes, the less the exhaust needs to flow to get the job done. Consequently, we can increase the intake valve size at the expense of some exhaust valve size. Another factor is the type of cylinder head used. If we are talking two-valve-per-cylinder heads, a Hemi-style head has a better breathing capability than a vertical-valve head. So the longer the stroke of the engine, the more the use of a Hemi head pays off.

Pay Close Attention to the Valvetrain--or Fail
We have already found that the bigger the cylinder is, the harder it is to fill. As if that is not problem enough, we find there are also some critical issues with the valvetrain that need to be addressed for success with any bigger-than-normal engine. It has already been ascertained that, for almost any bored/stroked motor, a two-valve head is inadequate. This being the case, it is very important that the intake valve is opened as fast as possible to give an air-hungry cylinder the best access possible to the port's airflow. The need to open valves very quickly might lead to the conclusion that very high ratio rockers are a benefit. If that was a conclusion you came to, it would certainly be right. Lift numbers in the region of 1-inch-plus are common for the type of engine we're dealing with here. Where this may sound hard on the valvetrain, let's not forget that the rpm involved with, say, a 730 incher, is significantly less than would be the case for a regular short-stroke 500-inch Pro Stock motor. In this situation we find the reduction in rpm makes finding lift that much easier. This isn't to say that it's still not a problem, but it is achievable, mostly by the use of rockers in the region of 1.9 to 2.0/1.

It is unlikely that most people reading this article will be building 700-800-plus-inch engines. What I am sure of is that all who are building big engines should take this point about rocker ratios and valve lift seriously. If you're sticking with the original heads, whatever rocker ratio worked on the original size engine needs to go up (at least) in proportion to the increased capacity.

Also, those stretching an existing engine need to understand that if the cam's valve opening and closing events were correct for the original engine, they won't be so when the displacement is increased. Assuming the lobe centerline angle was correct originally, you will need to decrease (tighten) the lobe centerline angle by about 1-inch for every 16 inches the engine grows by. In some cases this can make for a pretty nasty looking piston crown because the valve notches could be deeper. Fortunately, there are two redeeming factors here: First, the bigger the cylinders are, the easier it is to achieve a given CR. This means that the piston dome will be smaller for the bigger engine. The second factor that plays into our hand here is that the higher the compression used the wider the optimum lobe centerline becomes. This means the high compression needed for those underfed cylinders actually works in our favor in terms of valve-to-piston clearance. A ballpark number here is that for every two ratios of compression increase the lobe centerline angle needs to be spread by 1-degree. By following the aforementioned advice on valve lift and lobe centerlines, you can really make the most of an engine that has grown significantly in displacement.

Use An Adequate Induction System and Keep It Cool Because a bored/stroked engine has a greater difficulty filling its cylinders, we must (if both torque and top-end output are of any consequence) make sure there is no unnecessary restriction in the induction system other than the intake valve. This means no puny carbs. The induction system must have minimal restriction, thereby leaving as much pressure on the manifold side of the valve to push the charge into the partial vacuum in the cylinder. That workhorse of a 750 that produced such good results on a 350-inch engine is not going to cut it by a long shot on hardware stretched to 440 inches. Why? Because that 440-inch small-block is now looking like a big-block. Any time we have this many cubic inches we need to be thinking in terms of carbs of at least 850 cfm, and probably nearer a thousand. If the engine is to be a street driver and the intake you're planning to use is of the two-plane variety, be aware that it needs plenty of carb cfm because the manifold cuts the carb capacity in two.

It is often stated that two four-barrel carbs on a tunnel-ram intake have a place only on the racetrack. I am not sure where this piece of disinformation came from, but the fact of the matter is that a 2x4 setup can actually make for a more streetable deal than an open-plenum single four-barrel intake.One last important point on induction systems for big inches: You need to be aware that the more air-starved the engine is, the more sensitive it becomes to charge temperature. Building an induction system that locates the thermostat somewhere else other than on the manifold pays dividends beyond what you might expect. On 540-inch big-block Chevys (and that's small by today's count) I've seen 20 hp more from a drop in intake charge of 20 degrees. If you want to go the next step--and this is just as worthwhile--then thermal management of the intake charge from filter to intake valve will pay off. A good start here is to apply a thermal barrier coating on the inside of the head and manifold runners as well as on the lifter valley side of the manifolds base. Also, since a considerable amount of combustion heat comes through the face of the intake valve it's also a benefit to output when given a thermal barrier coating.