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TKART magazine How To | Importance of squish
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18 November 2017
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Squish is a key parameter for the performance and reliability of the engine. In general, the lower the squish, the greater the turbulence created in the combustion chamber, with the consequent optimization of carburation. One must be careful though, as a too low of a squish risks to lead to the impact of the piston crown with the cylinder head, especially in high rotation ranges, causing overheating and engine failure. Viceversa, an insufficient squish does not allow the full combustion of the mixture, thus losing performance.
TKART has already analyzed these aspects ina prior technical article yet, since even a tenth of a mm of modification of the squish can entail substantial variations of performance and potential problems of the engine’s reliability, it will be useful to analyze here in detail the procedure for a careful and precise measurement. Without neglecting the techniques that avoid the measurement being influenced by the movement and the deformations of the engine’s components.
Squish and all the parameters which determine it
The additional turbulence produced by the squish in the air-fuelmixture improves and accelerates combustion, thus optimizing the engine’s performance.
The sensitivity of a parameter such as the squish entails that its value may change for various reasons. In particular, some small variations may occur in the event of substitution of the piston or of the mainbearings, the big end and little endof the connection rod rather than the piston head. It is true that, today, the engine’s components are increasingly precise in tolerance levels and, therefore, the substitution of the above-mentioned elements, theoretically, should keep the values of the squish unchanged; but a measurement of such parameters is always appropriate after the engine has been overhauled.
Given the influence of the squish on performance, the tuners vary its value according to the needs, the circuits, the type of gear, tires and the contact patch with the road and also of the pilot and gear ratio. Very important in the definition of the squish is also the type of fuel used.
The measurement of the squish is done by utilizing a solderwire with a diameter greater than 1,5 mm (ideally about 1,8 mm). This wire is placed inside the combustion chamber, then, by rotating the crankshaft the wire is pressed down to measure the exact minimum distance between the cylinder head and the piston crown.
Going into more details, the optimal procedure for the measurement of the squish provides for the dismantling of the head of the cylinder and inserting a piece of solderwire which length equals the diameter of the cylinder (54 mm, i.e. the cylinder bore). The wireshould be positioned paralleled to the piston rod: in this way, the imperceptible (yet present) play movement deriving from the inclination of the piston which rotates around the piston rod will be avoided.
At this point, the head of the cylinder is reassembled, tightening the screws correctly (one at the time, in a cross pattern with the correct torques) as if the engine had to be reassembled correctly for use.
After that, grabbing the crankshaft by the two sides (spark and sprocket) make it rotate back and forth a couple of times. Then, with a pair of long pliers, extract the solderwire and measure with the caliper the lower part, which is after the “indentation” which formed at the two extremities of the solderwire due to the remaining space between the piston crown and the elastic band.
The solderwire must be stretched along the diameter of the piston, parallel to the piston rod so as to nullify possible play or movement.
The head must be reassembled and tightened as if the engine had to be used. On the Modena engines, the tightening torque equals 1,8 Nm
After turning the crankshaft for a couple of times, with long pliers, remove the solderwire.
After turning the crankshaft for a couple of times, with long pliers, remove the solderwire.
For the Modena Engines engine, used in the service, the average value of the squish indicated by the company is 1 mm. The range goes from 1,2 mm to 0,8 mm, limit values used in some cases during international races
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