Most Common Two Stroke Engine Bike Myths

two stroke engine bike myths

The grain of truth here is that if the piston rings are never seated against the cylinder walls by a proper break-in, they won’t seal and the engine will never develop full power. On the other hand, how fast should break-in be? Do you take out your new zero-mile bike up the interstate? No. A normal break-in, as described in the maker’s manual, and performed with understanding, is all that’s needed.

No matter how fine the surfaces produced in manufacturing on cylinder walls and crankpins, they are like the Alps in comparison with the much finer profiles that proper break-in will create. Break-in is the final machining operation. The oil films that will support moving parts in operation may be as thin as 1.5 microns (.00006 inch), so to avoid piercing these’ films, the Alps of manufacturing must be scrubbed down to an even lower height by the process we call break-in.

A normal break-in calls for a period (usually 500 to 1000 miles) of controlled operation in which the engine is never steadily, or heavily loaded. You would not, for example, climb long hills at full throttle and low rpm. The idea of break-in is to impose short periods of various loads, separated by recovery periods. While the Alps are at work, knocking each other down, wear particles and heat are produced. The recovery periods allow the heat to dissipate and allow the particles to flush out from between surfaces and be swept away to the oil filter. Once the break-in is complete, the engine oil and filter are changed. That’s it.


This one is history. Remember the guys with all the scoops and ducts, the black paint, and the extra, welded-on fins? Back when most engines were air-cooled, there wasn’t enough cooling to allow continuous full-power operation without burning up, so savvy tuners over-jetted,_ using the extra fuel’s heat of evaporation as an internal engine coolant, and to limit combustion flame temperature. Jetting to the chemically correct mixture (at which every molecule of oxygen in the air charge i1-reacted with hydrogen or carbon from the fuel, leaving no extra fuel unburned) gives maximum power and maximum heat release, but on that jetting, poorly cooled engines would run hard for a couple of laps, then cook. That heat would cause the intake of air to expand and lose density, thereby causing power to fall off. Over jetting, by limiting this heat, allowed engines to lose less power. This is why. even today, air-cooled 500 MX engines sound so rough; they are over-jetted to the point of misfiring when cold so they’ll keep more of their power when they’re hot. More cooling would help these engines.

Modern liquid-cooled engines can be overcooled to the point where they lose power simply because too much heat is being absorbed by cool metal surfaces in the combustion chamber, leaving too little heat in the combustion gases to generate pressure to act on the piston. Tuners learn by experimenting with what coolant temperature gives the best performance. This is why you’ll often see race bikes with their radiators partially taped over when going out for practice on a cold morning. Conversely, you’ll also notice that Yoshimura has decided its Suzuki 750 Superbikes need more cooling than they have, and has provided two extra oil radiators.


The fragment of truth here is that the rpm of the torque and horsepower peaks is usually higher than it’s appropriate to use in most street or highway situations; you use less power than you have. Yes, a racer keeps his tach in the region of peak torque and horsepower to get the most from his engine. But some street riders (and even a few racers) assume that if a little is good, too much ought to be just enough. You hear them winding their poor, suffering engines far into the red zone, above the torque and horsepower peaks, above the good performance-just to hear the noise. And they are going slower than the rider who knows where the peaks are.

To make power at higher rpm, the engine must be given the ability to breathe up there with longer cam timings, refined porting, perhaps bigger carbs, and a suitable exhaust system.


It’s true that racing engines often have larger clearances than street engines, but it’s wrong to jump to the conclusion that this is done to cut friction, A racing engine is on the full throttle more than a street engine, so its piston temperature is higher and its pistons expand more. Therefore they need some extra clearance- Street engines must be quiet, which calls for low-expansion cast pistons to run at close clearance. The extra stress put on a race motor often calls for the extra strength of forged pistons, but forging alloys expand more with heat than do casting alloys, and so require more clearance. At operating temperature, racing pistons fit as closely as street pistons: they must in order to present their rings squarely and stably to the cylinder walls. A loose, rattling fit is an invitation to the loss of the ring seal. Also, pistons are cooled by close contact with the cylinder walls; a loose fit means hotter-running pistons.


This one derives partly from Number 6 above, and partly from the different riding qualities of different engines. A big single seems to have impressive torque when you ride it, but on the dyno, a twin or four of the same displacement almost always has as much or more. It’s just that a big single feels so torquey. The feeling comes from the flywheel mass, and from the ability of a slow-turning engine to produce thrust without rpm. A single needs big flywheels to idle, and in rough going, those heavy flywheels may carry it through where a twin or four might bog. A single usually has moderate to small valve sizes, so its torque is given at low rpm. That being so, it also usually has very conservative cam timings-timings that give full torque at some wonderfully low, putt-putt speed like 3500 rpm. On a four, snapping open the grip at 3500 produces nothing but a cloud of fuel fog from the carbs and a sickly drone from the exhaust.

Sensibly designed engines take advantage of their natural strong points. A multi-cylinder engine has a lot of potential valve area, and so is usually designed in such a way that its torque and power are given at higher rpm. A single, with far less cylinder-head real estate in which to set valves, is, therefore (usually, but not always) designed to deliver its torque at the low speeds that small valves favor.


It’s true that during acceleration, extra power is consumed in speeding up moving parts. Between two otherwise identical bikes, the one with light flywheels will usually have some edge in acceleration, but not in top speed This process can be taken too far. When Honda raced its RC166 six-cylinder 250 back in the mid-1960s, and again when it tried to race its NR500 oval-piston four-stroke in the early 1980s, crank mass was reduced practically to nothing. The result? These engines were tricky to ride because they could Stall between downshifts, or indeed any time the clutch was pulled.

Recently some enterprising engine fanatics decided to build a V-twin out of car-engine components, but they left out the flywheel. Result? Their engine had such violent variations in crank speed-owing to a lack of rotating inertia to smooth it out that it often stalled on the dyno, or tossed its valves. Although it had the potential for nearly 100 horsepower, it developed only a third of that in tests.

Any piston engine needs a flywheel capable of storing enough energy at idle speed to compress the charge on the next cycle without stalling. If you plan to run at higher rpm, you can get away with less flywheel, because energy storage in a flywheel increases with rpm. But you can go too far-as Honda once did.


Internal combustion engines have a mechanical efficiency of 75 to 85 percent. This means that 15 to 25 percent of the power delivered against the pistons by combustion pressure is wasted as internal friction. Engines last a long time before they are worn out, so it’s clear that most of the time, moving parts don’t touch each other in a metal-to-metal fashion, but are separated by a more or less complete oil film. This being so, most of that 15 to 25 percent friction loss occurs in the oil films supporting the parts-between pistons and cylinders, between shafts and bearings, etc. If oil viscosity is reduced a lot to cut this loss, the oil films fail, and scuffing occurs.

Therefore, what do the makers of mystery coatings and lubes expect us to believe? Oil viscosity-the source of most engine friction-is what makes lubrication work. Without viscosity (and the friction that goes with it) seizure would be instant. This simple fact rules out the most extreme coating and additive claims-the ones that say things like, “Boosts engine power by 15 percent!” Some small part of total friction is caused by actual surface-to-surface contact at areas of peak pressure, such as cam nose-to-tappet, or between piston rings and cylinder wall near TDC where piston velocity is near zero and combustion gas pressure is maximum. In these areas, surface-to-surface contact occurs but, because wear is very slow, it is clearly quite minimal.

All right, then. Shift our attention to a surface-to-surface contact-that a small fraction of total engine friction. Here, surface coatings can work. That is why they are included in the additive packages of nearly all engine oils-in the form of metallic compounds that adhere to metal surfaces. When surface contact occurs, it is this metallic compound that scuffs and shears-not the parent metal under it. The coating heals itself, but the oil additive is eventually used up and must be replaced.

What are we left with? Coatings are unlikely to provide measurable power gains. Honda, in recent research with very carefully applied (no spray cans!) coatings of tried-and-true solid lubricants, found power gains, if any, must be less than one-half of one percent.

Where is the value, if any? If there is a value, it would have to be during break-in, when local pressures and temperatures are very high, and surface-to-surface contact is frequent. If parts break in to smooth, polished surfaces, they will be able to carry heavier loads later on, and certain coatings may help in this. If you feel like making an experiment out of your brand-new engine, then go for it- but only after a careful reading of the terms of your warranty.


Time and again you will hear this-that parts moving back and forth consume power. It’s true that the crankshaft has to accelerate the piston from a dead stop at the top and bottom center, up to peak piston speed at midstroke, but the crankshaft gets that power back in the act of slowing the piston back down again. Energy is neither created nor destroyed-it is simply exchanged between the crank and piston. In this case, the crank slows slightly as it accelerates a piston (or valve, etc.) up to maximum speed, and it speeds up again slightly as it gets the energy back again at the other end of the stroke.

Of course, the weight of the moving parts creates inertial resistance to these accelerations and decelerations. A half-pound piston, accelerating at 3000 Gs, imposes a load of 1500 pounds on the wrist pin, rod, and main bearings, and that in turn slightly increases the frictional drag on the engine. Most of an engine’s friction drag occurs between piston, rings, and cylinders. The rest-some 10 to 15 percent-occurs in the rotating bearings on the crank, rods, and valve drive. Since only about 15 percent of the engine’s power is eaten up by all sources of friction, and only 10 to 15 percent of that is bearing friction, we have perhaps 15 percent of 15 percent equal 2 percent of engine power consumed in the crank bearings. If we cut piston and con-rod weight by 10 percent (not so easy to do) we may gain 10 percent of 2 percent, or one-fifth of one percent-too small to worry about. Then why do designers and tuners work hard to cut reciprocating parts’ weight? Bearings last longer under lighter loads or can be resized smaller for slight economy gains. Con-rod bearings are among the hardest-worked parts in an engine, so lighter pistons and con rods mean longer rod-bearing life. In the valve train, lighter parts will continue to follow the cam profile up to higher rpm than will heavier ones. Light valves, rockers, pushrods, or tappets are created to prevent valve float-not to reduce friction. Finally, fast-moving parts in the engine store energy, just as a flywheel does. When the engine accelerates, it takes some power to increase the average velocity of con rods and pistons. The less those parts weigh, the less power is consumed in speeding them up.


First, there’s the element of wishful thinking. For the people who prefer Uri Geller to the insights of thermodynamics, it’s always entertaining to believe that some backyard inventor has created perpetual motion. And, after all the other mischief that the Bad Guys have fomented, it’s easy to believe They have put the lid on this miracle carburetor.

However, the truth is mundane. A carburetor sprays a fog of droplets of various sizes into the intake. The smaller ones manage to vaporize before compression and ignition, but some percentage of bigger drops are still little liquid worlds when combustion starts, and remain that way, evaporating furiously, all through the cycle, finally being ejected, blackened, and only partly burned, out the exhaust. Also, gasoline is a mixture of chemical species, some of which are not very volatile- especially nowadays. The volatile fraction evaporates easily, but the heavy stuff can separate out as it whizzes through the manifold, remaining liquid. This heavy stuff and the big droplets are therefore at least partly wasted. In an old, inefficient machine, in need of a tune-up, this fuel might be as much as 15 to 20 percent of the total burned.

Anyone who has worked around laboratory research engines has seen the evaporator carburetors normally used on them; they are heated by steam to ensure that no part of the fuel remains unvaporized. This type of carb is used because it eliminates the unwanted variable of fuel vaporization.

The secret, mileage-increasing carburetors operate in exactly the same way- by heating the intake charge to obtain full vaporization. However, most auto engines already heat their intake manifolds to some degree with radiator water and exhaust-gas heat to obtain good drivability during warm-up. Some new bikes and snowmobiles now have carbs heated by engine cooling water, to ensure fuel vaporization. Heating the intake air more than this may indeed result in a small gain in mileage-but it is in direct proportion to how many of those oversized fuel droplets your carburetors are putting out. Heating the intake air a lot to the point of complete fuel vaporization also expands the intake air enough to considerably reduce power. When Mystery Carb Corp. publishes mileage gains, it is always on big, old, unsophisticated V8 engines- never on modern, fuel-injected equipment. Those older engines, with blob-spray carburetors whose jets and metering rods are worn to a huge oversize, may indeed give much better mileage with a Mystery Carb.

Will Mystery Carb double your mileage? (Will my mileage quadruple if I use two Mystery Carbs?) No, sadly, it can’t.


At one time, plain bearings had not yet been developed to their present excellence, and building a race engine with them was an invitation to bearing troubles. Mercedes-Benz designed all its racing engines in the thirties and fifties with complex, extremely expensive built-up Hirth cranks-all so they could use reliable, proven rolling-element bearings on mains and rods. Plain bearings were something cheap for economy designs- but, not suitable for high performance. People who grew up reading about all this came to believe that ball and roller bearings are the only things for high speed. Rollers are “cool.” I believed it for years.

Meanwhile, automotive shell-type plain bearings were rapidly being developed to high reliability and, indeed, had long existed in aircraft engines in a form able to carry extreme loads. Once such bearings could be mass-produced, and the technology for designing with them was understood, rolling bearings and the complexities they involved were quietly put in the attic. At high speeds and loads, there is little advantage to rolling bearings over plain ones in terms of friction loss, and plain bearings are actually capable of carrying heavier loads and greater misalignments. The one-piece cranks used with plain bearings are far, far stronger, and more reliable than the multi-piece type used with rollers.

You may now ask, “Then why are automakers turning back to roller cams, and why are they again investigating rolling bearings for cranks and rods?” Their reasons lie in fuel economy, and in the way, cars are used. Cars spend most of their time at very small throttle openings, often running only partly warmed-up in commuter service. Under these conditions, rolling-element bearings have an advantage in lower friction.


The compression ratio is a major variable in making power. The higher the ratio, the further you are expanding the burned gases, and the greater the energy you are extracting from them. Unfortunately, there are limits to this process:

(a) As you go higher, the gains diminish. It’s a bigger step from a 3:1 ratio to a 4:1, than from 12:1 up to 13:1.

(b) Raising compression makes your fuel more likely to knock, rather than burn normally. Heavy knock destroys engines and must be avoided.

(c) When you raise compression, you make the combustion space smaller.

To be efficient, combustion must be rapid, so designers try to create rapid motion in the air/fuel charge by cleverly directing the intake streams, using squish bands, and so on. Very high compression leaves little room for air motion, so it can actually slow combustion down. As compression rises above 12.5:1 in four-valve engines, there is usually not enough time for a complete burn at high rpm. This means that high compression-really high, like 14:1-only, produces a gain at lower and middle rpm. This is precisely why drag racers use these high ratios-because they need to make their engines “turn the tire” off the start line.

When these extreme ratios are used in road racing, the engine has good punch down low and fades out higher up. This is why they use lower ratios than do dragsters.

The rule of thumb about compression? There is no easy rule. Different engines need different ratios in different applications. More is not always better. Experimentation is the only final answer. In general, the smaller the bore, the more tolerant the engine will be of compression because the flame-travel distance is smaller and combustion is faster. Long-stroke engines, because they have more room above their pistons at a given compression ratio, tend to have more efficient combustion. In general, the higher the rpm, the less likely detonation becomes- because there is less time in which the conditions necessary for detonation can develop-and the higher the C.R. that can safely be used.


The intent of this idea is correct. If you want more power, you’ll have to flow more air/fuel mixture, and that may mean that ports and valves will have to be increased in flow capacity. But is that the same thing as “hogging ’em out”? It is not!

First of all, it is not airflow alone that fills engine cylinders; there must be velocity as well. This is because a fast-moving intake stream can continue to “coast” into the cylinder long after the bottom center-giving a slight, free supercharge. This is the ram effect. Pushing velocity too high with tiny ports runs into losses from air friction. Making velocity too low by making the ports too big just kills the ram effect and loses power.

Second, many engine designs today already have port sizes that are too big for the best power. Making them bigger yet- even with the careful use of an airflow bench-may just kill your midrange without boosting the top end much.

Finally, those who jab a die-grinder into a port intending to make it “real big” will likely offend the Gods of Airflow, doing more harm than good. Many are those who, blowing up their “good” ported head, was forced to fall back on a stock spare, and then went faster.


As in so many other myths, this one dates to the air-cooled days-in particular to when iron Harley race motors ran with 900-degree cylinder head temperature.

Oil’s primary job in the engine is to keep the moving parts from touching each other. It does this by virtue of its viscosity-its fluid friction. Oil is made viscous enough that it will not be squeezed from between the parts even under the highest loads. Any more viscosity than this simply adds drag to the engine.

Oil viscosity falls with increasing temperature. Therefore, engines with ineffective cooling systems thin their oil out badly. To prevent this viscosity loss from reaching the point where moving parts break through the thinned oil film and seize, the engine is given more viscous oil to begin with-maybe even that SOW “gear grease” I just mentioned above. Those hot-running iron Harleys had to start with extra-heavy oil so that, at operating temperature, they would get the same bearing and piston protection that other, cooler-running engines can get from straight 30 oil. Therefore, unless your engine runs as stove-hot as those old-timers did, putting extra-heavy oil into it buys you nothing but extra internal friction-from shearing all that resisting viscosity.


I grew up near an airbase where I was always hearing about the guy who bribed the sergeant to pump him some aviation gas. On that sweet-smelling stuff, his car or bike ran great-no knock and pulled up hills without overheating. The sergeant tells him he can get him jet fuel, too. Assuming that since jet engines are more modern and powerful than piston engines, their fuel must also be something extra-special, our man does the deal. He wonders a little, as he pours the stuff in on top of the av-gas already in his tank, why it smells so much like kerosene. Without knowing it, he has changed the antiknock rating of his fuel from 100-plus, down to somewhere south of 50. Off he goes, his engine now knocking like an old taxi. After a few miles of sustained, heavy detonation, his engine is reduced to smoking junk. Man, that jet fuel is hot stuff- ran so good, burnt my motor right out.

In fact, aviation gasoline of the blue, green, or purple persuasions is excellent stuff in terms of anti-knock properties and is used as the basis for many racing gasoline. Aviation gas contains essentially the same energy per pound as does street gas, and won’t make more power than street gas unless you increase the compression ratio to take advantage of its superior knock resistance. Its big drawback in non-aviation engines is its low volatility, which may keep it from vaporizing to form a good mixture in an unheated intake system. Turbine (jet) fuel is in fact severely pro-knock and is similar to kerosene. Turbines don’t need anti-knock fuel; piston engines most certainly do.

People who say this don’t understand that there is a chemically correct mixture, and that power drops if you go either richer or leaner than this mixture. Yet there is a reason why so many have believed it; back in the air-cooled days, engines needed extra richness for internal cooling, because air-over-fins was so ineffective in getting rid of heat. Therefore, with those engines, you enriched the mixture almost to the misfire point and ran that. The extra fuel prevented the engine from heating up as much as it otherwise would have, and it made more power in a long event. Now, with the coming of liquid cooling, this extra richness is no longer necessary, and tuners jet as close as they can to the chemically correct mixture. That’s where the power is, if you can get rid of the heat.


Spark lead (how far in degrees before top center the ignition spark occurs) is another of the variables that can lead to detonation. The more lead you crank in, the longer you are holding hot burning mixture at high pressure, and the likelier it is to knock. On the other hand, you’d like to have combustion reach peak pressure at the optimum time so that maximum power is given to the piston. The trouble comes when the best tuning for performance is too advanced for the fuel. As you dial in more lead, and the power is coming up nicely, you run into knock.

In drag racing, where the engine runs for only a few seconds, the combustion chamber is relatively cool off the line. A drag racer can run a lot of lead and get away with it for this reason. He may need it, too, for at the compression ratios drag racers use, combustion chambers are tight and slow-burning. Extra compression and spark lead work for them because they don’t run long enough for the excess to cause trouble.

Cranking in more lead remains popular on the street because people figure that if it works at the strip, it has to be the hot ticket down the avenue. Yet time and again these timing advances are dyno-tested on stock engines and fail to show any gain. Why are they made? For drag engines with high compression and combustion chambers so tight that flame speed is slowed down in making its way around all the valve cutouts and bumps and lumps, it’s useful. In a street bike, with its much more open chamber, extra advance is just not necessary or desirable.

According to legend. Old Man Yoshimura used to crank in about 45 degrees on his air-cooled, 1025cc Superbikes and literally go for broke. After a few laps of torrid,.glorious action up front, the engine would fry. Later, like everyone else, he learned to make faster-burning combustion chambers with lower compression ratios that would go fast all day on 10 or 15 degrees less lead. In fact, among experienced tuners, being able to run a very minimum of spark lead is regarded as a sign of an efficient combustion chamber. A two-valve engine running 30 to 32 degrees would be regarded as efficient, as would a four-valve burning in fewer than 30 degrees.


It’s true that it’s hard for a good small engine to beat a good big engine, other things being equal. On the other hand, the limit of power is set by how fast you can turn the engine and still fill and empty its cylinders. Cylinder filling is limited by valve and port sizes, so a 10 percent displacement increase usually doesn’t bring a 10 percent power increase; valves that were the right size for the smaller engine are too small for the big one. The result is that the oversized engine now gives peak torque at lower speed than did the original. The torque is increased because combustion pressure is now pressing down on bigger pistons. Horsepower may have increased, but not very much; with torque being up, and rpm being down, these tv factors cancel somewhat. Often, oversized motors are pleasant to ride because of their generous torque, and because they need not be buzzed to get thrust.


The lure of the exotic is felt by us all, bi some few have to try silly stuff like cold heat-range racing plugs in street engine-or big jets without the other changes that make them necessary in modified engines. Street bikes use warm heat-rang plugs to prevent fouling at the moderate engine heat loads they produce. Race engines push much more heat through their components, and spark plugs must be of a cooler-running design to avoid having; overheated electrodes act as glowing, sources of premature ignition. Putting; cold racing plugs in a street engine is an invitation to fouling. How many times have I heard, at shops, a customer asking the mechanic to “Throw in a set of racin’ jets.” Modified engines, with longer cam timings and < free-flow exhaust systems, often need jetting up to compensate for a sluggish in-take process at low rpm. Some people knowing only that large jets are somehow associated with powerful motors want t( screw big holes into their stockers. The result is blubbery throttle response, black plugs, and reduced power at all speeds. All this is not to say that no street bike; ever needs rejetting. In these times o EPA-mandated carburetion, street bike; often come with fairly severe lean spots and to correct this, jet-and-needle kits an offered by the aftermarket. Sometimes’ these kits work well, but their good performance is the result of repeated dyne and road tests. You can’t get there b) simply making everything richer. It would be very nice if swagger and dash could be substituted for all this fuss) testing and fiddling. (Throw in a set o them racin’ jets! Crank in more lead!) Unfortunately, our swaggering makes little impression on reality. For the swagger-and-dash people, I can only recommend harmless modifications-like generic racing team T-shirts, jackets, hats, and stickers. They go well with myth.

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