Guest Column - Why Big Bang Engines Work

The piece we ran by Sean McConnell on an alternative approach to big bang engines kicked up quite a lot of debate about the merits of big bang engines vs screamer firing orders. As luck would have it, this week's episode of Bob Hayes' utterly outstanding MotoGPOD podcast featured an excellent discussion on the history and theory of big bang engines by Scott Jones. The segment was so well-written that we asked Scott if we could run it here, as a counterpoint to Sean's article. Scott, whose outstanding photos of the 2007 Laguna Seca race we were privileged enough to show last year, graciously allowed us permission.

We think this is one of the best explanations of how, and more importantly, why big bang engines work we have seen. We hope you agree.

Why Big Bang Engines Work

By Scott Jones

One way to look at MotoGP is as a continuing struggle to balance ever-increasing horsepower with a contact patch about the size of a compact disk. While common sense says that tire technology and suspension tuning have the most influence over where the limit of traction lies, for the last two and a half decades an element of internal engine architecture has had a profound effect on rear wheel traction and overall rideability.

MotoGP fans hear the big bang engine mentioned frequently, but what is it, and why does it improve a rider's control? The first question is easily explained. The second is not.

The idea behind the big bang engine is to move the pistons closer together around the crankshaft and make them fire in quick succession, rather than every 180 degrees as in an engine with a rational firing order. And instead of firing each cylinder individually, you fire two cylinders at a time, creating two large bangs per firing cycle instead of four smaller ones. The concept has its roots in flat track racing, where Harley big-bang V-twins dominated for decades. The premiere class saw big bang engines in Suzuki test bikes in the early 1970s. Cagiva tried their Bombardone engine in the mid 1980s, but it was Honda that made the big bang design an integral part of road racing with their 1992 NSR500. With this bike, Mick Doohan handily won the first four races of the season and would likely have run away with the championship had he not crashed so badly in practice at Assen and missed the next four races.

When Honda unveiled their big banger in 1992, the sound was dramatically different from the rest of the bikes on the grid. Other manufacturers brought audio analyzing equipment to the track to figure out what Honda was doing differently. The audio trick worked and soon afterward, the other teams were playing catch up, both on and off the track, putting their own big bangers into development to compete with the NSR500.

In the modern era, however, one can no longer tell for sure just by an engine's sound if it's a big banger or a rational firing order screamer. The many technological advances MotoGP has seen since the early 1990s have removed the distinct audible difference of this engine design. Engineers now choose inline or V layouts; single or multiple crankshafts; spring, mechanical or gas-powered valves; and so on, and each manufacturer must decide if a big bang layout works better with their other design choices than a rational firing order. The 2007 season saw bikes on both sides of this fence. One company even switched sides in the middle of the season! This is the protean nature of MotoGP: the bikes are always changing as teams search for whatever combination of technology will produce even a slight advantage.

But as development of the big banger continued in the 1990s, engineers had to determine what degree of separation was optimal for the overall engine architecture. For example, the first responses to the 1992 NSR500 were said to be a Cagiva that fired its pistons 66 degrees apart and a Suzuki that found 15 degrees to be the magic number. The only constant was that big bang engines, combined with the technologies of the day, made for lower lap times even as some very clever people wondered why this design made such a big difference to a bike's rideability.

Given nature's bias against things that are out of balance, the big bang engine is a crazy idea that should create more problems than it could possibly solve. You may remember the right hand rule from basic physics. Imagine an engine's crankshaft spinning in its case and turn your right hand in the same direction the crankshaft is turning. If you extend your right thumb, you have just identified a vector, or gyro effect, that the spinning crankshaft is generating.

The faster a crankshaft is spinning, the stronger that vector is and the harder it is for the rider to turn the bike away from that vector. Dual crankshaft engines produce a better handling motorcycle by reducing the engine's internal gyro effects. But adding a heavy, throbbing big bang to the firing cycle complicates this situation with its lopsided pulse. Such imbalance, especially in machines with as many moving parts as a motorcycle engine, usually leads to unwanted vibrations, which lead to inefficient function, which leads to loss of power, which leads to decreased performance and slower speed. Then add to the equation that the increased torque from the double piston firing and the close proximity of the timing requires a heavier, stronger crankshaft, bearings and gears. You have a heavy, less efficient, imbalanced engine making fewer horsepower than the engine with the rational firing order. But this big banger just happens to have the advantage of increased rideability.

And sometimes a given advantage is significant enough to outweigh its disadvantages. Loss of power is one thing, but as we know, MotoGP bikes have more power than they can efficiently use. With power to spare, some vibration in the right place just might be a good thing.

It is, according to a 1992 article by Kevin Cameron, who explained the big bang engine's benefit as an effective compensation for the increasing grip found when radial tires replaced bias-ply tires. Remember those amazing high side crashes we used to see so often with the 500s? The frequency of high sides reached a peak with the introduction of radial tires because the radials would grip like crazy until a certain point, then suddenly break loose, only to grip again just as suddenly a split-second later. This process happened faster than a rider's perception could detect it, which meant that one moment he was riding at the limit and the next he was flying over the handlebars. With bias-ply tires, the transition from full grip to full sliding was gradual, and the rider could feel the limit of traction approaching as the rear end of the bike started to hang out more and more. With the new radial tires, however, by the time he found the limit of traction it was too late.

But how can the firing scheme of the engine give the rider better feedback about rear tire grip? There are at least two theories about this.

One theory suggests that while human perception is much too coarse to tell one millisecond from another, a MotoGP tire endures its very stressful and very short life being profoundly affected by many forces, one distinct millisecond to the next. Not only is gravity forcing the tire against the road surface, but as the bike is leaning from side to side the tire is fighting severe lateral pressure. The bike is also braking frequently and, most importantly for this discussion, accelerating with tremendous power.

Under acceleration, the aforementioned milliseconds 'feel' different to the tire because it senses each rotation of the crankshaft not as one steady force but as an ever-changing pressure. As a cylinder fires, for a very brief moment that force pushing the tire forward is at its most intense. But quickly that force decreases as the piston slows down a bit on its way to the bottom of its stroke. Once there, it reverses direction and heads back up on the exhaust stroke. If no other pistons were involved, it would continue to slow down on the intake and compression strokes until the next ignition forced it to accelerate again. When you fire pistons individually and every 180 degrees of crankshaft rotation, the rear tire feels a cumulative affect of these brief forces of acceleration as a relatively regular and steady pressure - good for times when conditions are ideal, but bad when, say, the power of the engine combines with other factors to cause a loss of rear tire grip. The tire suddenly breaks free of its grip and spins until grip returns, which it always does, and often so suddenly that the rider experiences a high side.

Instead of the rational firing order's more even pressure under acceleration, a force that makes the tire grip grip grip until it suddenly breaks loose, the big banger delivers huge, torque-filled pulses due to two cylinders firing right before the other two cylinders. The engine's entire power cycle is compressed into a brief double pulse that at high revs momentarily overcomes the tire's grip and causes a very brief slip. After the double bang, the tire recovers its grip as the pistons decelerate and progress toward the next combustion strokes. This first theory says that the series of ultra-brief slips creates a unique situation when exiting turns. As revs climb, lateral forces on the tire cause these micro slips to become sideways movement of the bike's rear end. The rider can use this gradual increase in lateral slippage to sense the limit, just like he could with a bias-ply tire's gradually increasing slippage. Thus the big bang engine makes a radial tire mimic the best quality of a bias-ply tire, its gradual slipping as it approaches the final limit of traction.

This was the theory back in 1992 when Cameron explained the big bang engine's benefits in Cycleworld. It was also thought that compressing the timing of all combustion strokes gave the tire rubber time to rest up between big bangs and thus maintain top performance longer into the race. But as recently as November, 2007, Michael Scott wrote: "The tire people have yet to understand why a syncopated drumbeat of firing intervals should be easier on their rubber than the steady, even pulse of an inline four, but it is clearly so." After many years of considering why the big bang engine delivers its undeniable benefits, some very clever people are still wondering exactly why it works.

Which brings us to a more current theory, that of Yamaha's chief engineer, Masao Furusawa. At the end of the 2007 season, Furusawa gave a presentation in which he explained his theory of the big banger's benefits in terms of the engine's internal harmonics. Instead of reintroducing gradual lateral slipping to help the rider sense the limit of grip, the quick dual pulses of the big bang engine create a much different harmonic state during acceleration.

Just as the tire in the previous theory can sense subtle differences in the forces of the accelerating and decelerating piston, the crankshaft in Furusawa's theory creates a noticeably different harmonic signature when the pistons are fired in close succession.

Again we are dealing with milliseconds and distinct changes in the movement of various engine parts during those very brief periods. Friction causes the piston to decelerate after combustion, and so does the necessity of reversing direction when the piston approaches top dead center and bottom dead center. Again, think back to beginning physics and how something moving in a given direction at a given velocity wants to keep doing just that. After ignition, the piston is moving very quickly down the bore until it reaches the limit imposed by the connecting rod. Rather suddenly it is forced to stop, and then head in the opposite direction. As it is moving down the bore, it has considerable inertia and kinetic energy, both of which have to be accounted for as part of the reverse of direction. Both forces are absorbed by the crankshaft, something Furusawa called 'inertia torque' to differentiate these forces from combustion torque, the force created by the ignition of the fuel mixture.

Just as wind resistance increases exponentially as speed climbs, so inertial torque increases as the revs climb. So while inertia torque is a factor in your road bike, you don't operate its engine at sufficient revs to notice a problem. But MotoGP engines run fast enough that inertia torque becomes a very noticeable problem indeed.

To explain just how this problem manifests itself on the track, Furusawa used the metaphor of tuning a radio. As you turn the dial searching for the desired station, you hear the noise of signal interference decrease as you approach a spot where the signal strength is at its peak. The noise is still there, but the interference is low enough that you hear a clear signal. In Furusawa's metaphor, the rider is using the throttle to tune in a good signal to the rear tire. If there is too much noise between the throttle and the tire, the rider doesn't sense a strong signal and has little notion of where the limit of traction lies. If the interference is low enough, the rider has a good connection to the tire via the throttle and can get closer to the limit of traction.

Inertia torque is noisy, and thus interferes with the rider's ability to tune in a good signal. Enter the big banger, which reduces the periods of inertia torque by compressing the firing pattern. As the crankshaft is absorbing inertia and kinetic energy from the pistons and connecting rods, it's doing so in a smaller window of its revolution. The result is a longer period of each revolution that is noise free, or relatively so, giving the rider a stronger signal between the throttle and the rear tire.

In spite of their troubled 2007 performance, Yamaha had done much testing of different big bang engine schemes, and according to Furusawa, all results backed up his inertia torque theory. Is the big banger question answered once and for all? Maybe, and maybe not. Scientific history is full of explanations that were accepted one day, only to be refuted with more complete data the next. That two such experienced and clever individuals as Kevin Cameron and Masao Furusawa have had such different ideas about why the big bang design works suggests that we may never know, in any definitive way, the big banger's secret. Given, the two theories presented here are decades apart, and the older theory is based more on speculation than on testing data. But Michael Scott's recent remark about the tire people still not knowing why the big banger is easier on their tires makes plain that there are still big bang mysteries to be solved. We may never know everything, because the dominant package in MotoGP last year was a screamer, not a big banger.

In spite of the big banger's successes since 1992, it has not always been the design of choice in the premier class. As a prototype series, MotoGP finds its engineers constantly re-evaluating older technologies as new ones arrive on the scene. The big bang engine is no exception, and it has come and gone according to influences such as continued tire development and rule changes. The switch from two-stroke to four-stroke engines was a major reset for designers, and the early 990cc engines were for the most part screamers as engineers sought maximum horsepower to complete with the holdover 500cc two-strokers in 2002. But when Ducati announced it would enter the premiere class, it went back to the big bang engine right away. In February 2002, Ducati Corse Director Claudio Domenicali explained the decision like this:
"...further analysis led us to decide that the best solution was a 'double twin' and therefore we designed an engine with four round pistons which, thanks to a simultaneous two-by-two firing order, reproduces the working cycle of a twin. This will generate the 'big bang' effect, making the rear tire work in a way that extends its duration and improves rider feeling when exiting curves."
While that first MotoGP Ducati showed promise, it wasn't until Ducati replaced their big bang engine with a screamer in the 800cc GP7 that they won the world championship.

Ever since 1992, engineers have faced the dilemma: more horsepower at the expense of rideability with a screamer engine, or less power with an advantage when exiting corners with a big banger? Again, each new technology must be taken into account in the search for the current season's best solution. When pneumatic valve systems entered the scene and offered higher revs and more power, once again teams had to re-evaluate the screamer-big banger question. As tires improve, teams must decide if this year's rubber is good enough to allow the abandonment of big bang engines. The big bang pendulum swings back and forth. Sometimes teams reveal their choices, and sometimes they don't. Ducati announced that the GP7 would be a screamer when the bike was revealed at the beginning of the season. Kawasaki announced that their 2005 engine would be a big banger as part of that bike's introduction. Yamaha switched to a big banger in 2004 but didn't publicly confirm this choice until the 2007 season ended. In a recent interview on GPone.com, Randy Mamola said that Honda switched from a big banger to a screamer during the 2007 season. This was likely due to the lack of power relative to the Ducati at the beginning of the year. Honda continued to push the last spring-valved engine in MotoGP to its probable limit of performance, and by the end of the year had the engine Ducati feared most.

So what will we see in 2008? Honda's testing of their new pneumatic-valved engine has not been the success they were hoping for, and it looks like Hayden and Pedrosa will begin the season on last year's spring-valved screamer until HRC can get the required power from the newer technology.

From Furusawa's presentation, it seems clear that Yamaha will stay with a big banger for its harmonic superiority, though in the aforementioned GPone.com interview, Mamola speculated that Yamaha will certainly offer Rossi a screamer at some point if their current package continues to lag behind more powerful engines.

Ducati will almost certainly stay with an updated version of its GP7 screamer. Suzuki seems content with their big banger, but 12 weeks after the Valencia test, teams went to Sepang, where Kawasaki, a big bang proponent since 2005, made news by testing a screamer engine. A team spokesman would not admit that the team is moving away from the big bang engine for competition, saying that this was a test of 'other technologies.'

Other technologies are always a complication. Honda proved that the big banger was the right way to go in 1992. The dramatic change in tires when radials replaced bias-ply tires made the big bang engine a success, regardless of exactly why the different firing scheme worked. But other technologies happen, and something as significant as a fundamental change in tire construction has arrived that may very well make the big banger a thing of the past: electronic control of power delivery. Ducati's electronics package is said to combine GPS data (to tell the computer exactly where the bike is on the track) with live lean angle information so the computer knows just what the bike is doing and can control the engine's output accordingly. Computer-controlled engine mapping is now done not only on a gear-by-gear basis, but also on a corner by corner basis. As electronics get more and more sophisticated, the rider gets more and more help controlling the power, which is just what the big bang engine is supposed to do at the expense of added weight and lower horsepower.

So Kawasaki was using a screamer to test other technologies, rather than considering a switch from big banger to screamer for competition? Does a team need to admit that it would like more horsepower if their electronics allowed their riders to get that extra power to the ground? It seems that a screamer test is more likely a test of the team's electronics, to see if the current computer package is capable of taming the added power the rational firing order produces. Or, in Furusawa's terms, is the new software sophisticated enough to overcome the signal noise generated by inertia torque and give the rider a good throttle-to-rear tire connection?

So the big banger's future is far from certain. Advancing electronic control of power delivery could spell its demise. Or, something like the recent spec tire and spec ECU threats could become reality, forcing teams to abandon the allure of the screamer's greater power and return to big bang engines. The only thing that is certain is that MotoGP will continue to change as engineers search for even a slight advantage of technology. For the foreseeable future, at least, the big bang engine will certainly be one of the many factors they will consider.

Scott Jones is a photographer and writer. You can see more of his photographs over at http://www.turn2photography.com/, or read his blog over at http://blog.scottjones.net/. You can contact him via his website.

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