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This is the squish band area the wider this is the more low end power it makes and less top end it makes the squish band helps mix the fuel and air to burn faster this is why you need high octane fuel ( race fuel ) to make the engine safer and avoid preignition detination on most stock engines the squish clearance maybe 1.5 mm or more this slows the squish velocity so it does not mix the fuel and air a much so you can run lower octane fuel ( pump fuel ) squish clearance is when the piston reachs top dead center the distance from the piston to the squish band area ( the head ) most race engine run between .75 mm 1.5 mm the closer the piston is to the head will increase velocity whitch helps low end and hurts top end so compression is not the only factor when selecting fuel when you go to a big bore cylinder the squish velocity changes this is why you need higher octane fuel and less advance on the timing ( most of the time run stock timing no advance ) pump fuel you have to jet a little richer to make the engine safer and avoid big loads on the engine ( like 5th 6th gear with tall gearing and paddle tires do not use a advancer ) most of the time you can hear the engine ping (preignition detination ) if you hear this stop and make ajustments to jetting , timing , fuel , etc. race fuel (high octane ) you can jet it cleaner and have a safer engine race fuel is recommded when you have a big load on the engine like paddles in the sand
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