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Tech Talk: Kart engine tuning. Squish – part 2

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The squish band has been a very important technical solution for two-stroke engines. It has increased performance, reliability and combustion efficiency (also reducing fuel consumption and pollution). When tuning an engine we can surely start with a good check of the squish band.

Types of squish
Squish bands can be of different shapes, like oval or circular, can vary in the width of the band and in the inclination of that same band respect to a horizontal line. Finally the minimum distance of the squish band from the piston crown is also an important parameter that can be varied. The first parameters are generally fixed by the engine builder, except machining the cylinder head, which is something quite complicated and that can lead to irreversible errors. Usually it is better to have different cylinder heads proposed by the engine builder. On the other hand the minimum height of the squish band can be varied easily, even though measures must be extremely precise.
Measuring squish height
Take your engine as it is, with the cylinder head well tightened, as indicated by the engine builder, to the cylinder. Take off the spark plug and start measuring the minimum height of the squish band. To do so take a tin wire of about 1,5 mm of diameter and no less then 1,2 mm. Bend the wire and insert it inside the hole in the cylinder head where the plug is usually positioned. Make sure that the end of the string is touching the internal part of the cylinder and has not entered inside any port. Some take measures at the exhaust port side, some laterally, on the lateral transfer ports side. I have always preferred the exhaust port since to me it is always the more critical area. Anyway take measures always on the same side so you will be able to vary the squish band measuring always the same parameter.

When the wire is inserted properly, turn the crank-shaft so the piston goes upwards to the top dead corner, the wire is squeezed between the cylinder head and the piston crown. Take the wire out and you will find a small edge (thicker part) on the extreme end of the wire, that is determined by the wire going in the open zone between the piston ring and the piston crown extreme external edge. After such thicker part you will see that the wire has its narrowest zone, that is what you must measure.

Take a calliper, possibly an electronic one, that will give you measures up to a hundredth of a millimetre. Use the narrow area of the calliper to make your measure. Repeat such procedure two or three times to check that values are all the same. Be careful non to squeeze the wire when measuring with the calliper. Also repeat the whole procedure by cutting the wire on the edge eliminating the already squeezed area and squeezing a new part once again.

Values of the squish and effects
Values of the squish vary from engine to engine and can give important effects to engine performance. Always check with the engine builder the indicated values from which to start. Acceptable values can go from 0.70 mm to 1.0 mm. Values over or under such range can determine reliability problems and limits in performance. For example a value superior to 1.0 mm can reduce the squish band effect and result in bad combustion and insufficient performance. Reducing the squish height to less then 0.7 mm can on the other hand determine over-heating of the engine and hitting of the piston crown on the cylinder head.

In fact when the engine is running, especially at very high revs, the deformation of the con-rod, the crank-shaft and crank-case, and of course the small plays of the bearing, especially the crank-case ones, can reduce the 0.7 mm to less then 0 mm. The hitting of the piston crown on the cylinder head will block the piston ring and engine seizure will be extremely possible. If not so, surely engine performance will be greatly reduced since the piston ring will not be able to work properly and seal the combustion chamber from the crank-shaft. Usually interesting values of the squish are around 0.7 – 0.85 mm for 100 cc kart engines, and around 1.0 mm for 125 gearbox kart engines.

Varying the squish value
To vary the squish value we must act on some copper circular gaskets present between the cylinder and the cylinder head. Such gaskets are generally of 0.1 or 0.2 mm of thickness, but for really precise regulation you should find 0.05 mm gaskets. So after a first measure of the squish band that is for example 0.9 mm, you can open the engine and take away a 0.1 mm gasket to try to have a 0.8 mm squish band. Always measure the squish band again after having taken away or added any copper gasket, since the final value of the squish could eventually be different from what you expect by simple calculations.

Also you could take away another 0.05 mm gasket if you were looking for a final 0.75 mm squish value. To ensure good measures always try to tighten the cylinder head onto the cylinder with the same strength, which means the same couple screwing the head nuts. Badly tightened heads of cylinders can determine great variations of the squish value and bad performance or reliability of the engine. Be careful that a ruined cylinder head, with a spoiled squish band, can give wrong indications for squish measures.

A value and effect
Values of the squish closer to low measures determine better performance especially at low and mid revs, whilst higher values help at high revs. Also low values determine higher engine temperatures also because reducing squish band also acts as an increase in compression ratio. Higher temperatures of course always means more critical functioning and less reliability of the engine.

Also the effect of smaller values of squish height is that combustion works better because fuel is mixed better with air, so more gasoline is burned, better performance is obtained and, as already said, temperatures increase. This also asks for a richer mixture when regulating carburetion compared to a an engine working with higher squish values.

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Tech Talk : Asphalt – part 2

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We have seen how asphalts are made and laid on the ground, and also the different types of asphalts depending on the kind of stones (inert) and bitumen (binders) that are chosen. The different types of asphalts are selected according to the needs and particular uses that have to be made: asphalt for racing tracks, roads, highways, draining asphalts.

Characteristics of the asphalts

Draining asphalts are a great solution and invention for highways where high speeds really do not work well with water, especially when paddles are present. Aquaplaning can also become a problem, but also visibility is improved with draining asphalt.

The limit of such an asphalt is though that it has a lower compactness compared to traditional asphalts, this means that its resistance to wear is also reduced. In fact draining asphalts are called “open mixture” asphalts, because the empty spaces inside of them are equal to around 20% of the entire volume, whilst traditional “closed mixtures” have this percentage equal to less then 8%. This way these kind of asphalts have a better mechanical resistance. Draining asphalt was used once in the 80’s on the Formula 1 track in Belgium, Franco-Champs. The result was a real disaster, the track did not support the vertical and incredible lateral forces of the racing cars. The Grand Prix was cancelled that year.

Also, draining asphalts are permeable to water, but also greatly to air, which means that the positive effect with wet track conditions becomes a negative one considering the ground effect, so grip is greatly reduced for single-seaters.

A good compromise is to produce an asphalt with a rough surface that determines good grip, and a good drain of water, that will pass through the channels present between the peaks of the surface. Asphalts with completely smooth surfaces are no more made, but of course too rough surfaces generate high tyre wear and a weakness of the peaks on the plane of the asphalt. To avoid such limit binders of extremely high mechanical characteristics are used, such as bitumen with polymers.

To measure the roughness of the asphalt a particular procedure is used. A certain amount of sand is distributed on a circular area on the asphalt. The rougher the surface the more the sand will deposit inside the asphalt and the smaller the circular area will be. The area will have to generate in fact a smooth surface. Finally the radius of the area is measured. The greater the radius the smoother the asphalt is.

Racing circuits and roads

We might think that laying and compacting asphalt on a racing track is easier than on a road or highway since we have more time and no traffic passing on the track, except when having a race or testing. This is not actually true. Of course the perfection of a race track must be at maximum levels: no bumps at all! On the other hand though traffic and dirt on the asphalt help compacting. In fact when heavy cars, or even trucks, run along a road they press vertically on the asphalt. Also dirt avoids the new asphalt to stick to the tyres and be lifted and torn away. On the opposite side racing tracks are usually extremely clean, with no dust or dirt, and are also quickly cleaned by the new extremely soft rubbers of the tyres of racing cars. The forces on the asphalt are not so strong downwards along the vertical direction, since racing cars are not very heavy for obvious performance needs, so compacting of the asphalt does not really occur. Transversal-horizontal forces are on the other hand very strong since grip of the tyres is so high. This generates the tear of the asphalt sideways and an extremely strong wear of its surface.

Following this criteria karts are really the most destroying vehicles for what concerns tracks. The weight of a chassis plus engine plus driver is extremely reduced, whist the lateral grip is extremely high and the soft sticky rubber of the tyres pulls off bits and pieces of surface. Asphalt in curves is often, when new, torn away and worn quickly if the choice of the binder is not right and the compacting phase is not completed properly. Also motorbikes wear badly the asphalt surface since they are not heavy, but have a narrow area of contact between the tyres and the track surface. Once the deterioration of the asphalt starts the only solution is re-asphalting. Another partial solution often used in kart tracks is to put in the worn areas particular resins that harden quickly (within a night), but that are very expensive. You can recognize them because they are white coloured. They are also particularly smooth, but still have good grip characteristics.

Seasons and settling

One of the main problems in renewing a track surface is that work lasts quite long and the track has to close for such a period of time with great loss of money. The weather conditions such as rain, sun, temperature, humidity must be well considered to permit a good result of the asphalt laying and compacting phases. Also such conditions can increase or reduce the duration of work.

For what concerns the period of the year usually it is best to avoid the coldest seasons when the low temperatures make it difficult to work the asphalt with the right temperature (asphalt cools down quickly when transported or laid down). Also humidity acts negatively on the bitumen reducing the binding effect. In the rest of the year there are no real problems in renewing the asphalt. Times of rest after laying and compacting the asphalts are around 10 days. To increase the compacting effect what can be done is to spread all over the asphalt surface limestone sand to eliminate sticking characteristics and then to drive on the track with heavy vehicles.

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Tech Talk: Kart engine tuning – Part 1

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One of the most interesting and mysterious topics in the kart World is engine tuning. Two stroke-engines are so simple yet have so many parameters to tune and optimize. Performance can change from engine to engine, and from a simple new engine to a tuned one. What is true though is that engine tuning, done of course by keeping engine parameters within values given by technical rules, can give some additional horse power, increase torque, but this will result in 2 to 3 tenth of a second lower lap times. Halves of a second or even greater lap time reduction cannot be determined by the simple tuning of an engine, but is the sum of some other additional parameters like driving capacities and chassis setup (including tire performance).

What kind of tuning?
What is engine tuning, what is there to modify? Well one important aspect is that kart engines, especially the latest homologated ones, have already great performances and are tuned by the engine manufacturer. Finer tuning can be done by the engine tuner, but must be extremely precise in order not do spoil engine performance. Working on different parameters and elements of the engine without the right knowledge of engine functioning and possible tuning tips can only bring to a reduction of performance of the engine. Such reduction will easily be greater then the increase in performance we can generate with the right tuning.
The possible actions on the engine can be focused on some main aspects:

1. Acting on engine geometrical parameters such as: tolerances between piston and cylinder, piston ring gap measurement, phasing, squish height, compression ratio. These parameters can be regulated as indicated by the engine builder to have precise settings and good performance or they can be varied to increase performance (if one knows how to vary them correctly). Most of these parameters will also have to be checked each time a piston is changed or the cylinder is honed, so you will have to learn to deal well with all the phases of basic tuning.

2. Varying some engine components to permit best performance (spark plugs, reed valves, lubricating oil). These elements permit better performance and sometimes also better reliability of the engine.

3. Acting on shapes of the engine as ducts and ports. This action determines irreversible changes to the engine and must be done with extreme care, even more than with all the other tuning actions. Any mistake will damage the engine definitely.

4. Tuning external parameters like exhaust pipe length, transmission ratio, carburetor. The regulation of such parameters is not usually considered tuning, but their importance is so high that really an engine can express all its potential only if all the parameters, these ones included, are perfectly tuned. Also these parameters must be adjusted for example every time we go on a different track. In particular carburetion regulation must be controlled every time we change track, or weather changes, or the weight of the driver changes. Carburetion is probably the main parameter to regulate that sensibly effects both engine performance and reliability.

First step
Since competition kart engines are built to produce extremely high performance, they already are very close to maximum power output as they are when sold by the constructor. Mass production can though generate some small misalignments of geometrical parameters of the engine. The first step is then to measure all the main quotes of the engine. Squish height, piston and cylinder tolerance, phasing (opening and closing of exhaust, inlet and transfer ports), compression ratio and combustion chamber volume.

All engine builders homologate their engine (following the rules of FIA) indicating the basic parameters. These homologation data is present on “Homologation fiches”. The indications on the Homologation fiches are a base from which to start verifying the above indicated parameters. A more precise indication of the values of the engine parameters can be obtained directly by the engine builder or by the dealer that has sold you the engine.

Instruments for parameters check
To examine and measure all the parameters of the engine listed in the first step of engine tuning we have to be equipped with a caliper, a Palmer caliper, a graduated wheel (for measuring of crank-shaft rotation), a comparator for cylinder diameter measurement, thickness measurement plates, a graduated pipe for combustion chamber volume measurement. So find the parameters that are the right ones for your engine and next issue we will start with the first phases of tuning taking each single parameter at the time.