With the introduction of GM’s amazing new LT6 5.5-liter engine and its flat-plane crankshaft, the industry is abuzz about flat-plane crankshafts. While these crankshafts offer some interesting and substantial beneficial breathing characteristics for V8 engines that do help power, there are also significant detrimental results that are a direct result of using a flat-plane crankshaft and these less-than-desirable results need to also be discussed. That’s what this story will address.
Ed. Note – This writer would like to officially thank Jack Kane for his generous input and guidance through this description of the fundamentals of flat-plane crankshaft engines. (If you would like to read more from an engineering perspective on piston engine technology, you can find this at Kane’s website epi-eng.com.) Additionally, thanks go out to Bobby Kimbrough, General Motors, and Ford Motor Company for graphs and photos.
Starting At The Beginning
Let’s start with a couple of easy definitions. A flat-plane crankshaft is one in which all the crank pins – four in the case of a V8 engine, are aligned in a single (or flat) plane, 180 degrees apart, with two pins up and two pins down. With a cross-plane crankshaft, the pins are positioned every 90 degrees. So, with the number one pin straight up, the second pin would be 90 degrees, number three would be at 270 degrees, and number four would be at 180 degrees relative to number one. Imagine that there are now two planes perpendicular to each other that intersect each of the opposite pin locations, forming a cross.
In order to save space, we won’t get into all the tuning benefits of a flat-plane crankshaft. Suffice to say, that there are major beneficial aspects to a cylinder firing order that evenly alternates between the left and right banks. A flat-plane crank engine will always fire alternate banks as in L-R-L-R-L-R-L-R. This places firing pulses on each bank exiting the exhaust 180 degrees apart, allowing the engine tuner to take great advantage of this through induction and header pipe tuning.
A cross-plane crankshaft does not enjoy this advantage. We’ve included drawings of both a flat-plane crank and cross-plane crankshaft cylinder firing to reveal this much more succinctly than we could with a lengthy written description. But in essence, with a cross-plane crankshaft, the sequencing of the pistons reaching TDC produces a firing order in which two cylinders in each bank fire sequentially.
To illustrate that, we’ve chosen a small-block Chevy as the example with the number one cylinder in the left front position. With a 1-8-4-3-6-5-7-2 firing order, you can see that 8 and 4 cylinders fire consecutively on the right bank and 5 and 7 do the same thing on the left bank. This produces a much different progression of cylinder firings compared to a flat-plane crank engine with a L-R-R-L-R-L-L-R sequence. This makes 180-degree exhaust header tuning on each bank impossible unless the header pipes were connected across the engine in the classic bundle of snakes approach as popularized by the ‘60s GT-40 engines, for example.
Based on this description, it would appear that all 90-degree, V8 engines should be designed using a flat-plane crankshaft to take advantage of this obvious tuning benefit. From a tuning and a distinctive 180-degree exhaust note advantage, that would make sense. But as in most things in life, there are very few free lunches, and the flat-plane crank idea extracts its own price in terms of an inherent lateral vibration situation.
Rocking The Boat
This leads us to some interesting and not-so-intuitive reasons why a flat-plane crank presents challenges to the engine designer. There are several distinct vibratory forces that can occur in any engine, including primary and secondary forces, primary and secondary moments, and pitching and rocking moments. This discussion will address the primary and secondary forces and some of their effects.
A primary vibration occurs once per revolution. This occurs when the piston and rod assembly moves vertically (defined here as up and down the cylinder bore). To offset this force, the crankshaft designer creates counterweights to counterbalance the forces generated by the accelerations in the reciprocating motion of the piston, pin, rings, and top half of the connecting rod assembly.
A flat-plane crankshaft still needs counterweights to counterbalance the primary forces exerted on the respective main bearings, and to reduce the bending loads applied to the crankshaft. With all eight piston and rod assemblies weighing as close as possible to the same, the aforementioned primary force is minimized. However, there is a second-order vibration that causes problems for a flat-plane crankshaft. Let’s look into why this occurs
In any rotating assembly, the engine uses a connecting rod between the rod journal and the piston. When the rod journal begins its downward travel away from top dead center (TDC), the connecting rod begins a movement laterally as well as downward. This creates an angle formed by the connecting rod relative to the piston. This angle makes the connecting rod effectively shorter in length as it moves downward.
When the crank pin arrives at 90-degrees After TDC (ATDC), the piston has actually traveled farther than halfway due to this effective shorter connecting rod. Then from 90-degrees of the crank pin to Bottom Dead Center (BDC) of the crank pin, the piston travels a shorter distance. This discrepancy in travel distance (and resultant differing acceleration forces) creates what is called a secondary vibration.
This secondary vibration occurs twice because, on movement from BDC to the crank pin halfway point (270-degree position), it again travels a shorter distance than from the 270-degree position back to TDC. So this secondary vibration occurs twice for every 360-degree rotation of the crankshaft.
The three lines in Graph 1 show the primary in blue, the secondary vibratory forces in green, and their combined effect in pink. The primary forces have their maximum amplitudes at the 0-degree (“upward”) and 180-degree (“downward”) positions. However, the secondary forces have their maximum “upward” amplitudes at the 0-degree and 180-degree positions, and their maximum “downward” amplitudes at the 90-degree and 270-degree positions.
In an inline-four-cylinder engine, for example, the primary and secondary forces are all acting in the “vertical” (up-down) orientation. However, when two inline-four engines are configured into a 90-degree V8 with a flat-plane crank, the secondary forces unite in an interesting way, to produce a severe horizontal shaking vibration
According to Jack Kane, this situation is affected by several factors, including the weight of the piston and rod assembly, but is also most affected by the rod-length-to stroke-ratio, commonly abbreviated as R/L.
This is defined as the length of the connecting rod divided by the length of the stroke. As an example, if we have a rod length of 6.00 inches and a stroke of 3.00 inches, then the rod-length-to-stroke ratio would be 2:1. An R/L of 2:1 (or higher) turns out to be advantageous for flat-plane crankshaft engine combinations as this ratio reduces, but does not eliminate, the additional distance the piston travels in the first 90- and last 90-degrees of travel.
This secondary vibration is present in any crank-rod-piston mechanism. The problem in a flat-plane V8 arises from the fact that the vertical components of the secondary vector on each pin cancel each other out while the horizontal components add together to produce the horizontal shake. We will expand on this later in this story.
There have been many V8 engines built with a flat-plane crankshaft and, according to Kane, one critical elemental key is, the R/L. Among the more recent attempts at taking advantage of the tuning effects from this design is Ford’s 2016 Voodoo engine used in the GT350 Mustang.
This engine is based on Ford’s Modular engine but suffers from some design constrictions. The most glaring is that the Modular engine uses a somewhat restrictive short bore-spacing of only 3.937 inches for the cylinder block. This limits bore size, so in order to increase displacement to 5.2L on the Voodoo engine, Ford engineers sized the bore diameter at 3.70 inches but increased the stroke to an almost-square 3.66 inches.
Unfortunately, because of the Mod engine’s rather short deck height (dictated in part by the large DOHC head width), this limited the rod length selection for the Voodoo engine to 5.933 inches. This created an R/L ratio of a short 1.62:1. In comparison, that is almost identical to a traditional small-block Chevy 350 with a 5.70-inch rod and a 3.48-inch stroke creating a 1.63:1 R/L ratio. Unfortunately, this somewhat short rod length to stroke ratio aggravates the secondary imbalance situation.
The most recent engine that has stoked the flat-crank fires is Chevrolet’s new LT6 5.5L high-RPM engine. Chevy’s approach to a flat-plane crank engine was to build an honest blank-sheet-of-paper engine designed around the use of the flat-plane crankshaft. Chevrolet has not announced the deck height or rod length numbers yet, so we will have to make a few assumptions surrounding those exact measurements. But because this engine was not constricted by existing production LS engine boundaries (except for the bore spacing spec of 4.400-inches), the Chevrolet engineers could take advantage of a couple of design freedoms.
For one, with a given deck height and crank stroke, there are limitations to rod length. With a 3.15-inch stroke and the same deck height as the previous Gen-III, -IV, and -V engines, we’re assuming that this engine enjoys a 6.333-inch rod length that we will explain in a moment. With a 6.333-inch rod length combined with the 3.15-inch stroke, the LT6 would enjoy an R/L ratio of 2.01:1.
At this number or larger, the effects of secondary vibration could be minimized to the point where they could be managed through other means within the Corvette body structure. This does not mean there is not still a substantial secondary vibration present in this engine, though.
Corvette’s chief engineer, Tadge Juechter, mentioned to members of the press during the car’s introduction that during engine development, a spin-on oil filter vibrated and fell off the engine in the dyno cell! This recalled an original, bolt-on oil filter cartridge solution from the Gen I small-block Chevy filter from the early ‘60s.
Another variable that affects secondary vibration in flat-plane crank engines is the weight of the piston and rod assembly. In the case of the new GM LT6, they employed a short skirt, forged-alloy pistons combined with titanium connecting rods, both to reduce reciprocating weight, which allows the engine to rev quicker, but also to help reduce the effect of secondary imbalance.
Of course, hot-rodders being inveterate tinkerers, some have already expressed ideas around adding a stroker package to the current LT6 as a quick way to enhance power. But now that we’ve detailed the engineering around why Chevrolet chose the bore and stroke package that they did, this reveals why increasing stroke would be a terrible idea.
The Next Factors
This next discussion involves a specification called compression height. This is defined as the height from the centerline of the wrist pin to the flat portion of the piston top. This distance is often tightened in aftermarket performance pistons where the wrist pin intrudes into the oil ring groove, requiring an oil ring support rail. This technique is avoided with production engines, both from a cost and complexity standpoint, so compression height is a critical component of the stroke, rod length, and piston assembly scenario.
As an example, let’s assume the LT6 has a deck height of 9.240 inches, which is the same as the current LS family of engines. If we also assume a rod length of 6.333 and a stroke of 3.15, we divide stroke by 2 and add that number to the rod length. This gives us a compression height that is almost exactly the same as an LS3 at 1.332 inches.
If we increase the stroke on the LT6 from 3.15 to 3.50 inches and also maintain a compression height of 1.332, this demands we use a much shorter connecting rod. Doing the math, it comes out to a rod length of 6.158 inches – which at first seems reasonable since this has only shortened the rod length from 6.333 to 6.158 or 0.175-inch.
However, the rod length-to-stroke ratio is greatly affected by the increase in stroke which becomes clear when the math delivers a 1.759:1 R/L that is dramatically different from what we are assuming is somewhat greater than 2:1 and yet still greater than the Ford Voodoo engine’s 1.62:1 ratio.
Hopefully, this discussion has opened up a new level of appreciation for engine design and engineering and also why some ideas that seem fruitful might in fact be detrimental to the end result. The numbers don’t lie.