This article is the first of two that speak to modifying engines for racing motorcycles. Ignore for the moment that most improvements in lap times come from improving the handling of the motorcycle or, more irksome, riding better.
Photo – Adrenaline Sports Photography. The time for smooth predictable horsepower.
There are many choices involved in modifying an engine. Perhaps at the top of the list is the decision whether or not to cheat and/or spend a lot of money. An informal survey by The Army of Darkness Ministry of Intelligence revealed that many Superstock racers feel they either are not, or would not be, competitive without cheating or spending a lot of money on their engine.
However, we’ve found that bikes with mostly stock engines can be easier to ride while bikes with built-up engines require discretion with the throttle hand, are more difficult to get into a corner because of high compression, or out of a corner because of a lack of torque in favor of top-end. Our experience suggests it is the tractability of the engine, or how easy it is to use the power getting in and out of corners, and not so much the straight-up horsepower number, that favorably affects lap times. Our goal is to maximize the bottom end, or 9000-12,000 rpm on a 14,500-rpm engine. While having a high maximum bhp number is always nice, we will usually trade three or four at 14,500 for three or four at 12,000.
In the bigger picture, most folks report much higher horsepower numbers than we have found for our hardware. Our undefeated 2003 A bike (a GSX-R600), which finished second overall for the season in the WERA National Endurance Series, has only 6.0 more horsepower than a Superstock-class YZF-R6 on the same dyno. It does have a bucket-load more torque. Guest riders have stepped off our bikes impressed at the handling and power delivery and yet by most horsepower standards they are frail, consumptive, matchstick girls. Stepping back even further, we’ve all seen some hero on a 600 give the 750s a run for their money in sprint races. All engine work is expensive so think long and hard before diving into engine modifications. If you are just starting out, skip the rest of this article, change the oil every so often, and concentrate on your riding. Chances are the guy who is beating you by 10 bikelengths down the straight is doing it with a better drive, not with a built engine.
Most sanctioning bodies in the U.S. now allow some engine modifications for Superstock racing classes. For these classes you are looking at a valve job, increasing compression/decreasing squish, and some internal friction reduction (bigger bearing clearances).
(photo courtesy Cooper Performance) – An unsuspecting head is bolted into a fly cutter to have eight thousandths of an inch lovingly removed. Consider that this head has a 12 hour valve job, a painstakingly set compression value and will either be destroyed when one of the 4mm valve stems breaks or relegated to street bike use after a season of racing or so.
Superbike classes are a whole different story. It’s a moral imperative to do everything you can to the engine that will improve your lap times. Read lots of books. Ask lots questions. Do lots of math. Try to determine what a particular modification might actually do for you. A good place to start appreciating the devious world of motorsports is with Carroll Smith’s "To Win" series of four books: Engineer, Design, Tune, and Screw. Alexander Graham Bell’s books on tuning two-strokes and four-strokes are at the very least entertaining. Phil Irving’s Tuning for Speed may seem a little dated, but there’s gold in there. John Robinson’s "Tuning" book offers perhaps the best practical explanation of what’s going on in a modified engine. Kevin Cameron’s "Sportbike Performance Handbook" is excellent. The SAE has a book called "Design of Racing and High Performance Engines" that has some great stuff in it, including pictures of "between the Vee" V8 exhaust manifolds from late 1960s Formula 1 cars. At least one major modification to AOD engines resulted from sifting through the paranoid delusions of the crackpots and self-important hammerheads who infest the World Wide Web. But as the unknown was labeled on the maps of old, "here there be dragons…"
Modifications can be loosely grouped into five categories, the first two of which are addressed in this article: Reducing friction, increasing compression (squeezing the swept volume of the cylinder into a smaller combustion chamber volume results in more work done by the expanding, combusting gases), increasing displacement (bigger bore or longer stroke), increasing the rpm (spin it faster), and increasing volumetric efficiency (the ability to completely fill the cylinder with air and fuel during different rpm ranges). Maximizing an engine’s potential in each category might seem the way to go, but many modifications can work against rideability, safety, and reliability. The following is a mixture of our practice, and an overview of areas where a curious or foolhardy person might devise their own solutions. Much of the success of the Army of Darkness came from pursuing various hare-brained schemes. Some worked, some didn’t, but since nobody told us we couldn’t try something, we often did. The best result came from the vast experience gained in getting off the couch and trying things with the brain/nervous system/soul, damage from spending too much time in the dyno room at Battley Cycles notwithstanding.
One thought is that early production runs of an engine can make more horsepower because the molds used to cast the cases are not showing signs of wear. The sections and structures in the castings line up better, crystallize to their proper strength, and can be torqued together without anomalous distortions. There’s a sneaky suspicion that in the waning days of a model’s production a few more pop cans get stirred into the melt instead of the good stuff. If you have an early one, good on you, but mostly you’ll eat what you’re served by the junkyard so it’s not worth losing sleep over.
Another pursuit, perhaps not worth losing sleep over, is "blueprinting" an engine. In theory, if you sift through enough boxes of new parts you can find the ones that are closest to the original sparkle in some designer/engineer’s eye, so that you can put together an engine that is optimal. Assuming there are manufacturing variations large enough to measure one could in theory collect a batch of parts that will sum up to a greater whole than a set of parts assembled randomly at the factory. In actuality manufacturing has progressed to the point that the differences in mass-produced performance-niche motorcycle parts are subtle-to-non-existent.
We gave up measuring parts years ago because they end up being identical within the capabilities of our measuring devices. It is quite satisfying to measure four brand-new pistons and find them identical to 0.0001 inch. A fascinating study of the progress of modern design and manufacturing can be found in the rarefied air of the business section of any respectable bookseller, entitled "Dr. Deming: The American Who Taught The Japanese About Quality" by Rafael Aquayo. The subtle differences that may exist seem small enough these days that it never seemed worthwhile to find parts that were different enough to bother.
One fascinating situation is when manufacturers start talking about their own manufacturing variances as the explanation for why their Supersport bikes are faster than showroom bikes. Besides the fact that this is a gross misrepresentation of what is actually going on, they are advertising that their machining and quality control practices are bad enough that wide variations in parts are being made. Although we have seen our fair share of poor quality, it tends not to be of the "poor dimension" variety. Virtually all of the various types of engine parts which have come through the AOD garage have been dimensionally identical.
Engines have a tremendous amount of internal friction from metal parts sliding past one another, and shearing in lubricating films. Improving the surface finish of parts, reducing pressure of parts bearing or moving against one another, and separating lubricating shear layers by increasing lubricating clearances are the basic ways to approach modifying for reduced friction. Friction reduction has the attractive benefit of increasing power throughout the rev range, compared to compression and volumetric efficiency mods, which tend to add and subtract power in different parts of the rev range.
A worthwhile way of reducing friction is to measure and replace the crankshaft main bearing and connecting rod bearings with sizes that will give the maximum prudent oil clearance. An immobile layer of oil sticks to quickly moving metal parts. Consider two parts and their attached oil films that are moving past each other, say a crankpin and a connecting rod bearing shell, If you can get these films separated by a volume of unattached oil, the layers don’t "shear" or drag past one another, so less friction. Free horsepower. Some advocate going with a clearance larger than specified in the manufacturer’s Book of Words, the manual that describes a particular engine. We have not been so bold, as endurance racers we are a fretful, nervous bunch who lie awake nights considering the enormity of an extra 0.0003-inch. The extra large clearance might be taken up with overtightening the rods anyway, but more on that later.
Tim will often spend hours on the connecting rod torqueing process. Here he is measuring the length of the connecting rod bolt (under tension) and comparing it with the length of the bolt before he installed it to determine the stretch of the bolt. He then compares the actual stretch with predetermined values (which vary from bolt style to bolt style) to achieve his desired results. This compulsion should pay off with having the roundest possible connecting rod bearings with the least amount of internal friction giving free horsepower and lower oil temperatures.
One AMA crew chief revealed that as much as 10 bhp in a Superbike came from the oil, at $165 a liter. In another informal survey, most engine builders have a preference for brand of oil, though most have seemed a little nervous about using "qualifying" oils, or watery "aircraft" oils in an endurance bike, fearing premature wear, or outright failure. Our preference is one of the top-shelf brands and originated from the oil not smelling like burned peanut butter after eight or 24 hours. We have not gone further than this, as we go through as much as 30 gallons in a season. As there is so much smoke surrounding the capability of oils and additives we have stuck with what we know for reliability. Oil made specifically for racing motorcycles is probably the best bet, as automotive oils are formulated to preserve the catalytic converter and not the wet clutch or various transmission parts. It is an avenue with a lot of potential, and builders should satisfy their curiosity. Remember that everything you read on the Internet is true, of course.
Streetbikes often reach a point after a lot of miles where they make more power and run cooler than when new, presumably because initial machining defects have worn down. It makes sense to duplicate this process by polishing parts that move against each other, or grinding surfaces with a fine abrasive just enough to wear off high spots. One area that needs some work usually is the cam journals. Suffice to say that the cutter that bores the head assembly to fit the cams isn’t always the sharpest tool in the chest. Figure out what the cam clearance is supposed to be, find an aluminum oxide (blocky particle shape that shaves material, compared to silicon carbide abrasives that are much harder but pointier, and get stuck in the polished surface, particularly aluminum) abrasive powder with a particle size one-third that of the clearance, make a runny paste with some oil and rotate the cams in the assembled head (with the valves removed from the head) until they rotate freely (which might take some time), and clean everything as if you were going to eat with/on it later. Making substantial changes in clearances would be a Bozo No-No, but polishing off the high spots here and there makes sense.
For some strange reason, oil pumps on modern motorcycles feel like a pepper grinder crushing gravel. Polishing the guts of the oil pump so that it feels like it’s only grinding pepper helps for peace of mind at least. Another thought is that a "worn" oil pump makes more horsepower as it reduces pressure in the crank and rod bearings so there’s less oil shear, and less friction. Although we haven’t tried it, some engine builders use a lighter spring in the oil pressure relief valve to reduce the internal high-rpm oil pressure and reduce the amount of horsepower consumed by driving the oil pump. Your mileage may vary.
Our YZF600 (remember the "ThunderCat"?) had a coating on the piston skirts to reduce friction, straight from the factory. Pretty neat but who knows if it did anything. There are various snake oils and coatings available, but we haven’t messed with them. However, one exciting development is the affordable reality of diamond-like carbon coatings. There may be something to this as a friction reducer. Currently, Arrow Precision in England is coating valve buckets and claiming an 80 percent friction reduction in one of the highest-friction areas in the motor. Think of the cam lobes screeching across the buckets, fighting the valve spring pressure. Without pressurized lubrication, only the film strength of the oil provides lubrication.
The most overlooked area to reduce friction, perhaps, is by systematically and properly torqueing the engine together. Over the years, engines have lost a lot of weight. In particular, the cases have thinner and thinner construction. When the 1997 GSX-R came out, the puzzling thing was that making changes to the engine was a little unpredictable. In fact, you could just take an engine apart and put it together and gain or lose 5.0 horsepower, with apparently no rhyme or reason. It turns out that the cases are thin enough that just following the directions in the manual is not enough. Nowadays it is necessary to understand in a profound way just what exactly is happening to the cases or rods when you torque a set of bolts. The number on the torque wrench doesn’t mean much for a number of reasons. The type of lubricant (oil vs. molybdenum assembly lube) on a bolt, or the absence of lubricant, will make for drastically different torque readings. You have to decide on a method (say beam-type torque wrench with oil on the threads, to a certain reading) and ascertain if the assembly distorts, and adjust as necessary. Following the directions for torqueing the cases and rods in the Suzuki manual will surely lose at least 5.0 horsepower. Buy a good beam-type torque wrench (we stay away from the clicky-type, your religion may vary) and a bore gauge, a box of plasti-gage, and figure out, for yourself, the proper method of assembly.
Uncompressed Plasti-gage. We used to be able to buy this stuff at the corner auto parts store but it is getting to be increasingly more difficult to locate. Cut a little piece, follow your exact torqueing procedure on the bolts, disassemble.
The idea here is to squeeze the fuel and air mixture pulled in during the intake stroke into a smaller combustion chamber volume. The hope is that if the initial pressure of the material in the combustion chamber is higher than stock, the ultimate pressure of the burning gases pushing down on the piston will be that much higher, and more work will be done. If some is good, then more must be better, no? Increasing the compression will have benefits to a point but then the engine has to do too much work on the compression stroke. The old saw goes that engines with high compression will accelerate well, but tend to fade on top speed. High- compression engines also tend to run hot and, without high-quality fuel, can suffer from detonation and melted pistons.
Example of squished solder on the top of a piston. The thick coating of carbon on the edges of this pistons suggests there is a fair amount of room for improvement.
The basic approach is to skim the tops of the cylinders (or cases on most modern bikes) which brings the piston closer to the head. This results in a minor but effective increase in CR while decreasing the squish clearance. The squish band squeezes gas and air from the circumference of the piston into the center of the combustion chamber. The mixture loitering around the edges of the piston tends to not combust so getting the piston closer to the head will get more of it to burn. Skim too much, of course, and the piston will start hitting the head. The connecting rod elastically stretches on the exhaust stroke as the piston tries to keep going up, while the crank is trying to stop it and pull it back down. If the piston touches lightly, once in a while, the engine will survive. If the piston hits too often the top of the crown pinches the top piston ring which then allows lots of blowby (compression gases leaking past the piston rings into the crankcase). This over-heats the piston and causes seizure. Alternately too much piston-to-head contact will cause the connecting rod bearing to fail. Either way it’s pretty bad. People talk about trying to set the clearance such that the edge of the piston is a little shiny from hitting the head but we’ve only gotten that close by accident.
The combustion chamber. The welding rod is pointing at the squish band area of the head. Keep in mind that skimming the head increases compression and moves the valves closer to the piston, skimming the cylinder increases compression and moves the piston closer to the head and the valves. None of this should be undertaken lightly.
A light piston on a stout aftermarket forged rod is probably good to 0.024-inch, though we tend to go with 0.028-0.030-inch. Bear in mind the piston and valves get very close at a couple points and care must be taken to allow proper clearance between the piston and valves which will be discussed further in next month’s article. A good conservative clearance is 0.030-inch piston-to-head clearance. Older Yamahas needed more clearance due to stretchy rods and long strokes (higher piston speeds) while Suzukis could run less. We always measure this by sticking some pieces of 0.040-inch solder on the top of the piston with a dab of grease, bolting on the head and rolling the crank over a few times. Don’t use a blob of clay on the top of the piston as it is quite difficult to measure the compressed thickness accurately. Take the head off and measure the solder and decide how much to remove from the top of the cylinders to get your desired clearance. In some cases the OEM will have a "racing" head gasket (many stock head gaskets are a three-layer affair, the thinner one is usually a two-layer affair, and some engine builders just use stock head gaskets with the center layer removed) which can serve the same function as milling the cases but without all the hassle of machining.
The width of the compressed Plasti-gage reveals the internal bearing clearance. This can then be adjusted using the various thickness of bearing shells available from the OEM. Slightly bigger is usually slightly better but too big or too small is disaster.
There are a few other ways to boost compression ratios. Increasing the displacement of the engine (either with a bigger bore or longer stroke) will typically increase the compression ratio. Changing the pistons for ones that have more of a dome or material up top that fits the combustion chamber more closely will increase the compression ratio but the more topographical the combustion chamber the harder it is to get good, clean flame propagation. Since stock motorcycle pistons are of a very high quality these days, swapping them out for high-compression pistons is an expensive way to go.
Photo – Adrenaline Sports Photography. Ample mid-range horsepower creates passing opportunities in short chutes.