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Track testing: Transmission ratio adjustment

Written By Carlo Forni

Installing the Predator Carlo Forni
Installing the Predator
Carlo Forni

Last month Carlo reviewed several solutions for changing gear ratios more efficiently, and this month he tried out the Predator system at Viterbo.

First session: 83 teeth
We proceeded with our test using the Predator system as we needed something that could be mounted and dismounted quickly. On our main test track in Viterbo, we started the first session with a base set-up, 140cm rear carriage width and 139cm front carriage width, neutral caster and camber setting. We decided to use our LeCont LH08 tyres, putting together performance with durability. Pressures were set at 10psi. The air temperature was 23°C, a nice sunny day. We set the transmission ratio with the front sprocket having 12 teeth and the rear sprocket at 83, which is an average ratio at the Viterbo track.

Impressions during this session were of a good speed at medium and high revs, but difficulty exiting slow corners where revs went down below 9000 revs per minute. Maximum revs obtained along main straight reached 16,129 compared to a maximum limited revs value of the Super Rok set at 16,700. A few hundred revs were missing, but this was not the main problem. Acceleration in slow corners, in particular a corner with a slightly uphill exit, was definitely a problem and a lot of time was being lost.
Second session: 85 teeth
We decided to add two teeth on the rear sprocket and try to have better push at low revs. Actually the effects of increasing the number of teeth on the rear sprocket are both higher revs at the same speed and greater torque, but for a smaller range of revs. In fact when running along a bend, considering we should be running more or less at the same speed even using different transmission ratios, we will be at higher revs if the rear sprocket has more teeth. This will help avoid too low revs, where torque is insufficient. Also the torque curve narrows (on a graph with speed as the horizontal axis) and increases its peak if the rear sprocket increases in teeth and diameter. This means we will have a stronger push but for a more limited range of revs.

Feelings exiting slow curves were immediately better. No lack of torque and immediate good acceleration. Lap times decreased by 0.3s, which is surely a very valid improvement, indicating that what was gained in acceleration at low speed was not lost at mid or high speed. Maximum revs was 16,365, with an additional 236 revs compared to the previous session.
Mathematically the increase in maximum revs should have been higher (over 350 revs), but most probably the torque over
16,000 revs decreases quite fast so there is anyway a loss in the way revs increase along the straight. Remember it is Torque which affects acceleration and not simply Power.

Third session: 87 teeth
We lastly decided to try to exceed in the transmission ratio, as we wanted to find the limit of the engine. This is where the increase in performance (acceleration) at low speed is offset by reduction of performance at mid and high speed.
Obviously with the 87 teeth rear sprocket, acceleration at low revs was very good, but there was no real need for the additional torque compared to the previous session. On the exit of very slow curves in fact power was transmitted to the ground with difficulty and I had to control the acceleration working softly on the pedal, which meant not being able to use all the torque available. On the other hand at mid and high speed I had the feeling that maximum torque had already gone and the engine was a little bit stuck, with reduced acceleration.

To confirm this, the best lap time worsened by 0.4s compared to the previous session and 0.1s compared to the first session. Max revs reached 16,601, close to the rev limit of the Super Rok, but the increase in maximum revs does not necessarily indicate an improvement in set- up of the engine.

Testing as described confirms that tuning of the engine cannot be done based on the maximum revs obtained along the straight being equal to maximum revs reachable by the engine. What is important is the acceleration at low and medium speed, which are the speeds at which we run on most of the circuit, exiting every corner and in every single acceleration. So maximum revs are always just indicative and can also differ slightly depending on the braking point of the driver.

Since we have been working with increasing the number of teeth on the rear sprocket, which also means greater diameter, a final hint is to remember it is best to reduce the number of teeth on the front sprocket when the diameter of the rear sprocket is too high. This helps produce better transmission of power and torque through the chain, but also avoids the chain and sprocket hitting the kerb when cutting through a corner.

For more information on the Predator email

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Track testing: Transmission ratio adjustment

By Carlo Forni

The Predator sprocket carrier on Carlo’s TonyKart
The Predator sprocket carrier on Carlo’s TonyKart

The new Predator sprocket carrier, and a number of new instruments from Wildkart, one of which a floating rear sprocket carrier, have recently arrived in our garage.

The Predator is made of two round aluminium components fit together to create the rear sprocket carrier. One main part is mounted on the rear axle while the second disc is fitted on the first with the sprocket well set between the two. Just by rotating this second disk round its axis on the first disk the sprocket is quickly squeezed and blocked in its position between the two disks.Instead of tightening the three or six bolts we only have to tighten one.

We also wanted to know if it was heavier than traditional systems. The weight of this component acts not only on the overall weight of the kart, but it is also a rotating element which will have rotating inertia (resistance to accelerating) that can slightly reduce acceleration. This aspect is very limited though as the diameter of the sprocket carrier is small it depends on squared value of the radial distribution of the mass of the element.

When it comes to comparing the Predator with a Magnesium rear sprocket carrier things change quite a lot since this material is extremely light…and expensive. An OTK Magnesium sprocket carrier weighs only 253g!

The price of Predator is not low, at £82.50 plus VAT, more than double a traditional system, and it weighs 497g. However it is a great system to save time and effort every occasion you have to change rear sprocket. Also the way the way the Predator holds the sprocket seems to be more efficient than traditional system. The Predator holds sprocket between the two rings, gripping it all along a circumference. This gives greater resistance of the sprocket to bending when hitting a curb.

An OTK Magnesium rear sprocket carrier I found on the web cost around £67 plus VAT, closer to Predator price, and weighed 253g. The decision is over weight on one side and ease of changing the sprocket on the other. A magnesium Predator system would be the top solution, but I guess the cost would be pretty high.

Wildkart floating sprocket carrier
Another interesting solution for rear sprocket carrier is the Wildkart floating system. The component is extremely well refined with five cylindrical inserts that allow a small movement of the sprocket. This solution allows better functioning of the transmission system made of rear and front sprockets and chain. Especially around corners and exiting them, when the rear axle is strongly subject to vertical and torsional forces, and so bends and twists, the rear sprocket position changes compared to the engine and front sprocket. The alignment of the front and the rear sprocket with the chain is momentarily lost and the wear of the three components can increase. Also the smoothness of the transmission decreases and power is lost because of these deformations. Thanks to the floating system the misalignment and power loss can be reduced. This enables the engine to accelerate better and gain revs more quickly.

Tools from Wildkart
Wildkart is an Italian company, based near Treviso and Vicenza, born from Metalfil, an Italian company which worked has in precision engineering in aeronautics since 1996. After working for several years as subcontractors and after accumulating experience in the most sensitive areas of mechanics, the owners decided to expand into kart parts.

The web site at is full of information and photos of the many instruments and kart components, both traditional and innovative.
Two useful tools are the chain aligner, which easily aligns the front and rear sprocket, and the axle disassembler. The first is positioned with a screw on the rear sprocket and reaches the front sprocket to verify alignment. The second is a fantastic tool which must be positioned at one end of the rear axle. Hitting the tool without having to hammer directly on the axle is a great advantage and avoids any damage or deformation to the axle.

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Inside the Two-Stroke Engine


Here’s to a chap called Joseph Day, the man who came up with the basic principles of the two-stroke engine. Never heard of him? I can’t say I am surprised, neither had I until I researched this feature.

It started in the late 1800s when Day wanted to build an engine that didn’t infringe the patents held by Karl Otto who had made the first four-stroke engines. Obviously he succeeded, coming up with a design that was incredibly simple. And, more importantly for generations of racers, he had managed to make an engine that fired twice as often as its four-stroke cousin and hence had the potential to produce twice as much power.

Let’s not run away with the idea that the two-stroke engine was a huge success from the off, Day’s initial design used two flap valves, one below the carburettor and the other in the top of the piston which as you might imagine stopped working as soon as carbon built up. It took another couple of years for one of Day’s employees, one Frederick Cock, to realise that they could get rid of this piston-flap and instead use to skirt of the piston to cover and uncover passages to transfer the gas from the bottom of the engine to the top.

Fuel in – Power out

Even after the advent of the piston-port early two-strokes were weedy underpowered things, they worked… but not well. They were held back by the woeful inability to clear the burned mixture out of the cylinder to make room for the fresh charge; little fresh mixture equalled poor power.

Their saviour was Dr E Schneurle who in 1925 worked out a way of directing the mixture coming into the cylinder to help blow the burned gas out of the exhaust pipe (the loop scavenge principle). In time engine builders found that with the right size and shape of port they could scour nearly all the burned gas out while filling the cylinder with plenty of new charge. The trouble was that though the loop-scavenge was good at scouring the burned gas from the top of the cylinder it also wasted loads of the fresh mixture by blowing it straight out of the exhaust before it could be used. A good four-stroke engine was better than a good two-stroke every time, producing more power and using less fuel. Motorcycle manufactures used two-strokes but usually only at the cheap and nasty end of their range because they were inexpensive to produce.


Towards the end of the 1950s MZ two-stroke racing motorcycles were hugely quick. Their competition knew that their superior speed had something to do with their exhaust system but didn’t know exactly why. This superiority was down to Walter Kaaden who had been employed by MZ in 1952 to work on exhaust design and alternative porting arrangements for the two stroke engine. Although he did little to improve on the basics of the Schneurle principle except to add another port to further improve cylinder filling, he did however, manage to work out the principles of a brand new two-stroke exhaust.

Kaaden got MZ to build an engine with multiple ports big enough to allow a huge quantity of fresh mixture in to completely scour the cylinder. So much mixture was being blown into the cylinder that a large quantity would spill out and starts to disappear down the exhaust pipe. Ordinarily this would be a disaster giving horrendous fuel consumption for little gain. However, Kaaden had designed a revolutionary exhaust that used a strong positive pressure wave to shove the gas that would ordinarily have been lost, back into the engine by a just before the piston shut the exhaust passage off.


MZ might have retained this fantastic advantage for years but for the defection of their top rider Ernst Degner who spectacularly joined Suzuki after escaping to the West during the Swedish round of the 1961 World Motorcycle championship. With him he took MZ’s chance to win the world crown but much more importantly he knew much of the detail of Kaaden’s work.

Once the secret was out the whole world began using the expansion chamber on their two-strokes. The first ones gave loads more power, but only over a narrow range and even by the end of the 1970’s the concept still needed development. What the designers were striving for was a two-stroke exhaust that gave power over a wide rev-band. Today they are pretty close, look at any modern system and you will see that it is a collection of different tapers, one blending smoothly into another, each working harmoniously with the next to make the exhaust work efficiently over a broad rev-range. Importantly for both road and racing they also learned how to silence the exhaust without losing too much power.

Quantum leaps

Liquid cooling has proved to be a boon for the two-stroke motor, the ability to control the temperature of an engine within strict limits has allowed the boundaries to be pushed. For example they can be given ignition timing figures which would cause an air-cooled motor to be marginal on cooling, plus they can be built with tighter tolerances meaning that they last longer.

The other quantum leap for the two-stroke engine has been advent of the exhaust power-valve. Since the beginnings of engine modification, tuners have realised that the bigger a hole (port) is the more gas could be moved about. They also realised that if you moved the ports up and down you would radically change the characteristics and power of the engine. The trouble was that if you made an engine with the ports placed and sized for good power at the bottom end of the rev range, it would be restricted at the top end and vice versa. It was Yamaha that developed the brilliant idea of having a rotating gate that effectively changed the height (and hence the timing and opening duration) of the exhaust port. They did this with a little servo-motor which took it cues from the ignition system, as the revs rose the servo-motor twisted the gate open. The resulting engine developed great bottom end power. As the revs rose so did the effective port height, changing the exhaust timing to figures more suitable for high-revs so that it delivered at the top end too.

Rotax have simplified the system to great effect and their kart engines use a simple diaphragm controlled sliding ‘gate’ which has proved to be just as effective.

Gone but not forgotten?

Alas the end is probably in sight for the amazing two-stroke engine, increasingly tight emission regulations have all but driven them off the road and environmental campaigners don’t like the idea of them being used just for racing either. The modern two-stroke is a far cry from its noisy, dirty and smelly predecessors but the very fact that they burn their own lubricant means that they will never be as clean as some want. We have a good few years yet, but the end is probably in sight… Enjoy them while you can.


How it works

The two-stroke principle is remarkably simple once you have your mind round it. A four-stroke engine needs two revolutions of the crankshaft to initiate the four separate stages: induction, compression, ignition and exhaust. The two-stroke manages to combine these into a single revolution of the crankshaft.

Let’s start with the piston at the bottom of its stroke.

As the piston rises it produces a vacuum underneath it (within the crankcase), drawing in the mixture consisting of petrol, oil and air. At the same time it is squashing the mixture that is above the piston ready for it to be ignited by the spark-plug.

Just as the piston reaches the top, a spark ignites the compressed mixture. This burns fiercely, forcing the piston down. As the piston descends the underside of the piston is compressing the mixture that had previously been drawn into the crank cases putting it into light compression. As the piston reaches a certain point this passages are uncovered and the pressure (primary compression) shoots the mixture up the transfer ports to replace the gas that just been burned.

The only additional component needed apart from the piston, connecting-rod and flywheels is some sort of mechanism that prevents the mixture from being shoved back out of the carburettor at the time when it should be going to the top of the piston. These devices come in two basic forms, either as a flap (like Day designed), which is pushed open by the incoming air (reed-valves) or as a spinning disc with a cut-out in it (a rotary-valve). The cut-out in the disc lines up with the carburettor when the mixture needs to be drawn in, the plain portion blocks the entrance to the carburettor at other times.


The other major difference between a two-stroke and a four stroke engine is in the way that they are lubricated.

Put simply a four-stroke is effectively built as two separate compartments, fuel and the lubricant are kept separate. The top half deals with the power production and the bottom half with the lubrication. Somewhere on all four-stroke engines is a tank of oil which is positively pumped through the engine to lubricate the moving part, hence they  can use a cheaper plain bearing which rely on a pressurised supply to keep them turning.

A two-stroke motor needs to use both the bottom and top half of the engine in order to run, hence oil needs to be circulated through the motor with the fuel as there is no separate lubrication system. Unlike the bearings in a four-stroke which get a constant supply of ‘pure’ oil, the bearings in a two-stroke are being bathed in a mixture of fuel and oil. The upshot of this is that a two-stroke needs to use more expensive roller bearings which need relatively little lubrication.

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Track Testing: The best kart Axles

Since one of the most important parameters of a kart chassis is the rear axle hardness, and since in the Rok category it can be changed, we decided to carry out a test to verify that theoretical rules referring to chassis set-up were confirmed by track tests.

Start of test: soft rear axle

We started our test with new LeCont CIK H LH tyres, the hard compound homologated by the CIK. These tyres have a superficial tread hardness of 54 DIDC-IRHD compared to the 42 of CIK M LH08 medium tyres we tested some time ago. Since the tyres were hard compound we decided to start with a soft axle, which we know is not the best solution with these tyres as, in theory, a soft axle does not help grip, but on the contrary usually gives a sliding effect to the rear of the chassis. This can help on high grip tracks and with very soft tyres to “free” the kart when exiting corners.

We started our test at the Pista D’Oro track near Rome with not much rubber on the track and consequently quite low grip. We set the tyre pressure at 8psi, quite high for a start, but the cool temperature led us to start with something that could help with finding some grip fast. The set-up was 1400mm rear width and minimum width front. Neutral caster was set. The kart started running with very low grip on the rear tyres. The sliding was very smooth, so quite controllable, but excessive. Only after eight laps did the grip on the rear tyres become acceptable, but still not sufficient. The lap times started at 51.50s and decreased constantly to 50.11s but this was still too slow. Once stopped, the tyres showed a very smooth surface and a total absence of the bulky areas on the tread that usually show evidence of good grip.

The only possibility was to move on to the next step of the test, changing the rear axle from soft, indicated by the letter P, to medium, indicated by the letter U.

Changing the rear axle

Some quick tips on rear axle changing:
• To avoid having the rear axle stuck in the bearing passages, always clean it very well before trying to slide it out.

• Also remember to unscrew all bolts from the axle and loosen disc and rear sprocket carriers.

• Then use a rubber or plastic hammer to hit the rear axle sideways, this will permit to avoid ruining the edges of the axle. In fact this would spoil the axle and also make it impossible for it to slide inside the internal diameter of the bearings.

• Once the axle slides you can help it with an aluminium tube or bar if the edge of the axle gets stuck just inside the internal ring of one of the bearings. Just put the aluminium tube on the edge of the axle inside the bearing and hit the tube on one side with a hammer.

Test with medium rear axle

We pretty quickly changed the rear axle which was perfectly straight so it slid out of the bearings’ internal cylinders easily. Sometimes when the TECHNICAL axle is slightly bent the process is much harder, and we have to be careful to understand which side the axle is bent and make it slide on the other side to help it come out. We proceeded with the same parameters and nothing was changed in the chassis set-up except the rear axle. The kart immediately, from the first laps, showed much better performance with rear grip increased strongly. Also the greater stiffness of the rear axle helped exiting curves, pushing the kart out of the bends thanks to the axle bending and returning to its straight position with greater strength due to the greater stiffness.

The lap times decreased quickly, and after the first two laps times decreased to under the best laps of the previous session with the soft axle. The lap time was 50.02s on lap 3, and decreased down to 48.98s on lap 8. After that the lap times were more or less constant and we stopped to check the tyre treads. They definitely showed a more bulky surface confirming my sensation of greater grip.


It generally helps to have a stiffer rear axle when on hard compound tyres and a low grip track. But with more grip and softer tyres a softer rear axle helps, giving more predictable behavior from the chassis and smoother sliding of the rear tyres. Sometimes in fact too much grip can lead to the chassis remaining “stuck” to the track with loss of speed and slower lap times.

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Karting Telemetry Basics


Take a look at any kart at a track and nine times out of ten the wheel will be adorned with that familiar lap timing device. Even at its most basiclevel, it’s pretty advanced with split times and engine temperature monitoring facilities. Most drivers couldn’t imagine racing without one.

One of the things that attracts me to motorsport is the fusion of mechanical propulsion with technology. Formula 1 might be the pinnacle in this area but karting isn’t a million miles behind. I’m referring here to the data gathering process which allowing drivers to examine their driving in detail. Welcome to the world of telemetry. One thing I’ve wanted to do this year is compare two drivers and analyse their driving through data acquisition. I contacted Aim Technologies and before long I was being loaned a wealth of equipment.

To complement the Mychron 4 already on the kart, the equipment I received a GPS 05 unit. This is a powerful bit of kit measuring position, speed and lateral and longitudinal forces on the kart at any point of the lap. There was an additional speed sensor too positioned on the rear axle. An expansion box was fitted to allow various sensors to talk to the Mychron 4 unit through one interface. This comparison took place rather ad-hoc during a practice session at Clay Pigeon in Dorset. It combines a fast, sweeping section with a much more technical sector towards the latter part of the track.

The kart was kitted out with the gear and Colin Wright, boss of Team Wright, was my reference. Colin’s held previous lap records at Forest Edge and is generally regarded as an extremely fast driver. Our comparative performances would be analysed by comparing our best laps in Race Studio Analysis. This software allows the user to examine data in both a graphical manner and in an overhead plan mode using the GPS sensor. You can run a movie of the kart around the track, with its position depicted as a cross, and analyse the various telemetry readings from any point.

It’s important to point out that the times in this little test are on the slow side because my kart engine was experiencing one or two teething issues. The good news was that it was consistent in its delivery of power, albeit it consistently slightly lower than expected.

Now for the raw data:

CHRIS                                            COLIN


Time                              39.670s                                          36.750s

Top speed                   61.8mph                                        60.8mph

Slowest speed           30.6mph                                         29.3mph

Highest “G”                 1.76                                                 2.21

Yes, you might have spotted something rather odd. Colin recorded a slightly lower top speed and a lower slowest speed compared to me yet posted a higher overall time. But he also pulled more lateral G forces in the corners, particularly after the long straight heading into Billies Corner.

In fact, looking at the graph traces, Colin maintained higher average speeds across the lap which translated in much quicker lap times. Ultimate top end speed is of no use if it is negated by slower speeds around the rest of the track. The hardware and software combination also allowed me to examine the braking points between myself and Colin. He was braking some 12 feet later into the corner, with sharper deceleration shown by a greater rate of drop of lateral acceleration well into negative territory. In general, the equipment worked well. It was important to ensure that the GPS sensor had a free line of sight to the sky, so it was positioned on the bodywork just in front of the steering wheel.

My set up is of course a far cry from the extraordinary level of data able to be pulled, in real time, from F1 cars. In that motor racing series, it’s pumped back from the track to engineers both on the pit wall and back at the team’s base. More than 120 channels of information are gathered at a rate of thousands of cycles per second over 100MB broadband pipes starting with fibre optic technology at trackside.

In karting, a fully prepared machine can be loaded up with more than dozen channels of information at a rate of ten cycles per second. That may sound comparatively little compared to F1, but nevertheless it is a highly cost effective performance solution for a karting team to eek out at least a few extra tenths.  And that, of course over race distance, is often the difference between victory and first of the losers.


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Tech Talk: Kart chassis tuning – part 1

by Carlo Forni

Tech Talk. Chassis tuning – part 1

The main and most important item of kart competition is surely the chassis and its tuning and setup. Engine tuning can give some 3 or 4 tenths of a second of lap time reduction, but chassis setup and tuning can make more than a second of lap time difference.

First step, the choice
Not all chassis are the same. Different constructors produce different chassis with different shapes and materials. It is also true that some constructors have their chassis made by others. Anyway when starting to tune your chassis make a step back, first choose well your chassis. The kind of tires used, the weight limits in the category you intend to race in and the height of the driver are important parameters that need to be considered when choosing your chassis. Usually every chassis producer has different models of chassis for different uses. Some models have a higher basic grip on all four wheels or just at the front or at the rear.
It is true that if we have to use soft tires and are a heavy and-or tall driver we need a chassis that has not too much basic grip. Such a chassis we call generally a “free” chassis. On the other hand hard compound tires need weight and height (to a limit) of the driver and a chassis that has already a very high basic grip. I have verified personally that chassis that have a longer distance between front and rear wheels are much more “free” than shorter chassis.

An even better hint for you is to check directly on the track. Go and see some races, verify which chassis are the winning ones in the different categories and choose amongst the first ones that are the most performing in your category. Verify if particularly tall or heavy drivers manage to be competitive with particular chassis. In particular verify if drivers similar to you for what regards physical characteristics manage to be competitive with particular models of chassis. Use all these indications for the choice of your chassis.

Parameters for chassis tuning
After having chosen the chassis we will have to deal with all its parameters. Some are regulations that can be done once and are difficult to be varied quickly during a race. Some other parameters can be varied changing some components of the chassis and finally, the last ones are parameters that can easily be varied on the chassis.

For example seat positioning has an enormous importance in chassis setup, even though it is usually underestimated, but can be varied mounting and dismounting the seat, sometimes even drilling new holes in the seat. Changing seat position changes weight distribution on the four wheels. The material of such component is also important and can change both in its type and thickness. Usually glass fiber seats are used, but also carbon fiber (stiffer and more expensive) materials have been utilized in the last years.

Rear axle type, thickness and material, is another important parameter. Changing a rear axle represents a change in a component of the chassis, a substitution that is not extremely rapid, but can be done quite easily (especially if the axle is clean with no rust and perfectly straight). Other substitutions of chassis components are the braking systems or only the brake pads (materials can be changed and sometimes performance varies sensibly). Also wheel types can be varied in shape and material (aluminum or magnesium) and front and rear hubs (length and material).

Finally camber and caster angles regulation, convergence, front and rear carriage modifications, front and rear heights, tire pressure, front, rear and lateral removable bars (if present) are amongst the parameters that can be varied rapidly when tuning a chassis.

Regulations for every track

You will notice in all your testing and races that every track has its particular characteristics and your chassis will adapt differently on each of them. So some chassis have better performance on some tracks compared to others. Fast or slow tracks, high and low grip asphalts together with different weather conditions such as hotter or colder, vary the performance of each chassis. Regulations can be done to adapt your kart to the different conditions. So you will never find the perfect setup for all conditions, but surely a base from which to start must be found. Especially weight distribution is so important and can be a good start for your chassis tuning. Start from such a parameter and then understand how every other regulation can act on your chassis performance. Some parameters have greater effect then others on performance. Start from the first ones to obtain good performance and then act on the second ones to reach excellence.

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Tech Talk: Engine tuning – part 3


A very delicate and important aspect of engine tuning is setting the piston-cylinder tolerance. This determines variations both in performance and in reliability of the engine.

How tolerance acts

The extremely wide range of functioning of two-stroke competition engines, from 7000 to 20000 revs per minute, determine the necessity to work on extremely precise tolerances between all engine components, in particular between cylinder and piston.

Such tolerance represents the space present between the internal side wall of the cylinder and the external lateral surface of the piston. Such space is extremely small, more or less around some 10 hundredth of a millimeter, and variations of a hundredth of a millimeter make some difference in engine performance and reliability.

The function of the piston, as we know, is to transform the force given by the high pressure of the burned gases obtained by combustion into movement. The piston does this with good efficiency if gases are not allowed from the combustion chamber through the space between piston and cylinder into the crank case, before the exhaust port opens.

At low revs the piston of course moves at a lower speed up and down inside the cylinder. Slow speed gives more time to the burned gases to make their way in such passage. This means that a greater value of the tolerance between piston and cylinder determines more pressure loss, and lower torque value. So a lower value of tolerance is needed, since low revs also determine a less critic movement of the piston inside the cylinder.
At high revs the piston has a higher speed and burned gases really have not much time to pass through the space between piston and cylinder. Tolerance can be greater and such greater value helps the movement of the piston inside the cylinder since friction is reduced with greater tolerance values.

Friction also determines the production of heat together with the erasure of the walls both of the cylinder and piston. Friction produces heat and a force acting on the piston opposite to the piston movement. This means that a low tolerance generates friction reducing maximum revs and increasing engine temperatures, with great danger for what regards engine reliability. If, in fact, engine temperature increases over a certain limit detonation (small explosions inside the combustion chamber) can occur and this means also probable engine seizure.


Of course when the engine heats up also the cylinder and piston heat up. This means that both components will expand and diameters of will increase. Generally though it is the cylinder that will expand more so tolerances will increase. On the other hand both components will expand in a not uniform way. For example both components will heat up more in the upper area since it is the area where combustion occurs. This is why both cylinder and piston are slightly conical. Also the cylinder with its various ports will deform itself and the small areas of cylinder between two ports, like the areas between the exhaust port and the boost ports, will somehow expand towards the internal part of the cylinder reducing the diameter in that particular area. This will generate a strong reduction in tolerance between piston and cylinder and a risk of engine damage and seizure. In particular you will notice that there will be extremely polished areas on piston and cylinder surface in the zones where the exhaust port and the boosts are close to each other.

Temperature also means that with different weather tolerance could need to be adapted. What happens usually is that high temperatures tend to increase the risk of engine seizure and low tolerances tend to increase heat production by the engine. This means that with hot weather tolerances should be greater. On the other hand hot temperature means more expansion of the cylinder and increase in tolerance, which leads to loss of torque at low revs. To this we must add that hot air is less dense, which means less mixture enters the engine and power output is reduced. So hot conditions are quite a complicated situation.

Optimizing tolerance

First of all if you need to tune your engine you should know what the basic tolerance between piston and cylinder should be. Your engine builder should give you a good indication. Of course such value will generally be higher then the tolerance “of excellence”, because more tolerance means better reliability of the engine. Anyway start from the value given by the constructor and then try to reduce such value of around 0,01 millimeters. Generally a medium value to start from is 0,10 millimeters.

Consider, as seen in the previous paragraph, that if you reduce tolerance you will obtain better torque at low revs, but also a reduction of maximum revs, which means lower speed along fast sections of the track. Consider what kind of track you will be running the engine in, then make your choice. Be careful in reducing too much tolerance because you will risk engine seizure, and with low tolerance always consider a longer running in of the engine, since the piston will need more time to adapt to the cylinder internal wall.

Looking at a new piston you will see that a layer of graphite (grey) will be present on the piston lateral surface. This graphite will help running in by adapting to the cylinder shape and wearing out where tolerance is slightly less compared to other areas.

Practical measurement of tolerance

We will need all of next issue’s column on engine tuning to go deep into practically regulating tolerance between piston and cylinder. Precise instruments such as Palmer caliper and centesimal comparator will be needed. But also important hints on temperatures of piston and cylinder when measured will have to be considered.