We have all heard the adage “there’s no replacement for displacement.” The more air an engine can displace, the more fuel it can burn. Anytime you can add more fuel, more power is sure to follow. This is the reason that turbos and nitrous oxide are so popular. Both force more oxygen into the combustion chamber, allowing additional fuel to be burned. The same concept holds true for displacement.
There are three ways to increase the displacement of an engine: increase the number of cylinders, increase the bore size of the cylinders, or increase the stroke of the crankshaft. The first choice requires a completely different engine block to achieve, so for this article, that narrows the choice down to bore and stroke. Increasing bore size is easy, and is the most common practice. However, an increase in bore size is typically restricted to less than one-hundred-thousandths of an inch (0.100) due to cylinder wall thickness on a stock block. Aftermarket blocks or the installation of cylinder sleeves does allow for more of a size increase, but again that requires a different block or major machine work.
An engine’s stroke on the other hand, can typically be increased by five-hundred-thousandths (1/2-inch) or more in some stock blocks. The result is a large increase in cubic-inch displacement. Using an aftermarket block could allow for even more of an increase, and of course, there are always limitations and other things to consider. An engine builder must consider all factors when designing a stroker engine for a particular application. Chapters could be written covering all of those factors, but for this article, the focus will be kept on the physical (dimensional) and dynamic (operational) properties associated with selecting connecting rod length for a stroker engine. We spoke with Tom Lieb of Scat Enterprises, Trip Manley of Manley Performance, and Kirk Peters of Lunati so we could get their take on the effect of engine performance in regards to connecting rod length. It should be noted that these concepts are based on differing rod lengths using the same stroke (comparing a 5.7-inch rod to a 6.0-inch rod in a 383 ci Chevy stroker) not necessarily rod length in general (comparing rod length in a 383 ci small-block to rod length in a 632 ci big-block) unless otherwise noted.
Rotating Assembly Height
When you increase the stroke of a crankshaft, each journal will rotate on a larger diameter. Think of crankshaft stroke as a circle. The centers of the crankshaft’s main journals represent the center of the circle. The centers of the connecting rod journals represent the outside of the circle. As the crankshaft rotates (circular motion) the rod journal travels in a circle, which has a diameter equal to the stroke.
When a given cylinder is at top dead center (TDC), the rod journal is directly above zero degrees of rotation on the center of the circle. At bottom dead center (BDC), it is 180 degrees directly below the center. Although the big end of the connecting rod (connected to the crankshaft) travels in a circular motion, the small end (connected to the piston) travels in a reciprocating motion (up and down). The connecting rod converts the rotation of the crankshaft into a reciprocating motion of the piston. The total movement of the piston from TDC to BDC is equal to stroke.
Rotating assembly height is equal to half of the stroke, plus the connecting rod length, plus the compression height of the piston. The goal is to achieve a rotating assembly height that will provide the desired deck volume or clearance for a particular application. Deck volume or clearance is determined by finding the difference between rotating assembly height and the deck height. Deck height is measured from the center of the main journal bores to the top of the block’s deck. An engine where rotating assembly height and deck height are equal is considered a zero-deck engine.
There may be multiple combinations of connecting rod length and piston compression height available for a particular stroker engine. A long rod will require a short compression height piston (distance from the center of the wrist pin to the top of the piston crown), and a short rod will require a tall compression height to achieve the same assembly height. Before selecting which combination of rod and piston you will use, there are a few factors to consider.
Once the desired assembly height has been determined, rod length and piston compression height are selected. A short rod will require a taller compression height piston than a long rod would require and vise-versa. The weight of the components should be considered. A piston with more compression height will also weigh more than a piston with less compression height for the same application. A heavier piston requires the crankshaft to have heavier counter weights to offset the additional reciprocating weight of the piston. This may even require additional weight to be added externally to the harmonic balancer and flywheel. When this is the case, the engine is considered to be externally balanced.
Any additional weight incurred by using a longer connecting rod has less of an effect on counter balance weight because the connecting rod is both reciprocating and rotating. Reciprocating weight requires more weight to offset than rotating weight. The difference in connecting rod weight is split between rotational and reciprocating while differences in piston weight is only applied to reciprocating weight. Using a lighter piston will allow for lighter crankshaft counterweights and may not require any additional weight to be added externally. When this is the case, the rotating assembly is considered to be internally balanced.
Lieb, says that many times, the connecting rod length is determined by whether or not the engine builder is looking for an internally or externally-balanced engine.
Piston Design and Stability
While on the topic of piston compression height, it is worthy to note that more compression height will allow for more room between the top of the piston crown and ring pack. Manley states, “Performance engines today are all about power adders. For the tuner crowd it’s boost, and for the drag racer, it’s big-blocks on nitrous. With a short rod, the piston pin is moved lower on the piston, creating a better ring pack for boost.” In addition, more compression height can increase the thickness of material on the deck of the piston, which provides increased strength for higher cylinder pressures created by power adders.
Stability of the piston should also be considered. A longer connecting rod will keep the piston further up in the cylinder bore when at BDC for a given stroke. The small end of the rod, which is connected to the piston pin, is further up the cylinder bore with a long rod as compared to a short rod. Therefore, the piston also moves up in relation to the bottom of the cylinder, adding distance from the center of the pin to the bottom of the cylinder wall.
This is important if the piston skirt comes out of the bottom of the bore at BDC. The further the piston skirt moves out of the bore, the more piston rock becomes an issue. Piston rock ultimately causes a loss of ring seal. The piston skirt contacting the cylinder wall is what limits the rocking motion of the piston. Adding distance from the piston pin to the bottom of the cylinder improves piston stability on an engine where the piston skirt protrudes from the cylinder at BDC.
As the crankshaft rotates the big end of the connecting rod, the small end is moving up and down. This creates an angle between the cylinder wall and the connecting rod. The severity of the angle is determined by the ratio of rod length to stroke (rod ratio). Rod ratio is determined by dividing the rod length by the stroke.
Common Formulas For Building Stroker Engines
A few formulas you need to know when building a stroker engine:
- Displacement in cubic inches = Bore x Bore x Stroke x Number of Cylinders x .7854
- Assembly Height = (Stroke / 2) Rod Length Piston compression height
- Rod Ratio = Rod Length / Stroke
- Mean Piston Speed (feet per second) = (2 x Stroke x RPM / 60) / 12
All three rod manufacturers that we consulted had a slightly different view when it came to rod ratio.
According to Lieb, “Any angle that does not exceed 20, 21, 22 degrees is a non-event. When you look at a 410 ci Chevy sprint car engine with a 6-inch rod, that angle is pretty severe, and those engines run pretty good.”
A 410 ci small-block’s rod ratio when using a 6-inch rod, will be in the 1.5 to 1.6 ratio range, depending on the bore and stroke combination used to achieve 410 cubic-inches. The maximum rod angle for a 1.5 rod ratio is just under 19.5 degrees, and that falls into Lieb’s non-event category. He adds, “When you get into big-block stuff where you have a 4.750-inch stroke, then you get into some issues.”
Manley pointed out the large range of ratios from 1.87 in the Nissan GTR engine to less than 1.5 in some big-block strokers. “Rod ratio is not as important as other factors,” stated Manley, referring to moving ring location down with a short rod for boosted engines.
Peters suggests using, “As high a ratio as possible,” citing less rod angularity, reduced reciprocating weight due to a shorter compression height piston (remember, although a long rod will weigh more, the difference is not as significant because it is split between rotating and reciprocating mass), and reduced piston rock as benefits.
Rod length and ratio further affect one of the most important aspects of a stroker engine’s performance — piston speed.
It is common to see formulas and calculators that will determine mean piston speed. This is simply the average speed of the piston for the given stroke at a set RPM. Mean piston speed will always be the same for the given stroke, regardless of connecting rod length. Peak piston speed, on the other hand, is dependent on rod length.
[A change in performance] has nothing to do with rod length, per se, it has to do with the relationship of the piston when the valves open or close. – Tom Lieb, Scat Enterprises, Inc.
A common error that is made regarding peak piston speed is assuming that it occurs at 90 degrees of rotation — which is not true. Peak speed actually occurs somewhere around 70 to 75 degrees BTDC and ATDC (depending on rod ratio) due to the angle of the rod affecting piston speed and location. Peak piston speed is higher with a short rod compared to a long rod (stroke being the same), because the shorter rod creates a greater angle.
As mentioned previously, the rod ratio determines at what degree in rotation peak speed occurs. As rod ratio decreases (shorter rod), the number of degrees before and after TDC at which peak speed occurs also decreases (in other words, peak speed occurs closer to TDC). This also means the piston starts to decelerate sooner in rotation with a shorter rod. Therefore, piston speed is less with a short rod on the lower half of the stroke (across BDC) than (refer to the graph provided by Prestige Motorsports).
The significance of a rod length’s effect on piston speed is ultimately dependent on piston speed in relationship to valve events. “The rod length and stroke of the crankshaft determines piston speed,” Lieb says. “[A change in performance] has nothing to do with rod length, per se, it has to do with the relationship of the piston when the valves open or close.”
In today’s engine building, one would use a shorter rod when the engine builder wants to improve the scavenging effect at lower RPM. – Kirk Peters, Lunati
“In today’s engine building, one would use a shorter rod when the engine builder wants to improve the scavenging effect at lower RPM,” Peters stated.
This is true because piston speed has a greater effect than piston position during overlap. Piston speed is near its peak when overlap begins before TDC. A short rod will carry more speed from the peak back to TDC, and again back toward the peak (in other words, there are less degrees of rotation between peaks). Therefore, rod length can significantly affect the scavenge effect due its affect on piston speed. A short rod will increase piston speed during overlap allowing the benefit of scavenging to occur at a lower RPM than a long rod.
The camshaft’s intake lobe opening ramp also follows right along with piston acceleration. A short rod will provide more piston speed on the opening side, but lower speeds on the closing side. The exhaust lobe, on the other hand, is opening and closing on the BDC side of rotation where a short rod provides slower piston speeds. Therefore, a long rod will increase piston speed during the exhaust events.
Stroker engines provide a significant increase in displacement. While an increase in displacement alone will provide for additional power, there are many factors to consider to get the most out of the increased stroke. Connecting rod length is one aspect to consider when designing a stroker engine.
Rod length changes both the physical and dynamic properties of the engine. Factors such as assembly height, engine balance, piston ring location, and cylinder length are physical features that must be considered, while rod angle and piston speed are dynamic characteristics affected by rod length. The dynamic characteristics will change engine performance based on their relationship to camshaft events.
As an engine builder, it is important to take all aspects into consideration, and understand how one component will affect the overall combination. Rod length alone cannot be generalized as providing a certain change to every engine. Rather, any change in engine performance is due to the rod length’s role in changing the dynamic properties of the entire combination.