Rotax 280 Squish

[brownie001]

Hi, does anybody know what squish gap I should have on my Rotax 280 cc motor ?

 

I have 1,8 mm at present but I'd like to know just exactly what the upper and lower limits are.

 

Many thanks in advance,

[bezaboy]

May be worth contacting Martin Matthews at www.MotoSWM.com 

[reggie]

Check out the 2 stroke tuners handbook by Gordon Jennings. I think 1.35mm to 1.5 mm would be a good target.

[turbohead]

It depend on the ignition timing and RON fuel you are using plus fine tuning of the power delivery. On standard timing, ordinary pump gasoline at around RON 95 and a modern 2-stroke oil mix of 0,5-1% you can use anything between 0,5 - 1,5 mm. When these Rotax engines were used in mx and enduro bikes, squish around 0,8 mm was standard. The Rotax trials engines I have measured recently has had from around 1,2 mm to 1,75 mm. When rebuilding my Rotax 280 right now, I am aiming at around 1 mm.

[brownie001]

Gentlemen,  many thanks to you all for your helpful answers.

For me to achieve a 1mm squish gap, I would need to remove both of the shims currently in place and either find/make a thinner cylinder base gasket or machine the top of the cylinder down.

The two shims are 0,47 mm together so around 1.4 mm sould be somewhere within the tolerances mentioned.

I will be using the standard timing of 2.6 to 2.8 mm BTDC, pump petrol mixed to 50:1, as used in my 125.

 

Phil

 

[smelling123]

1mm will be fine, any larger and you won't be taking advantage of the squish effect..

[swm]

Hi,

find a 'squish area' in the head of rotax trial engines 237, 276 and 347 is quite difficult....

 

first of all the shape of the combustion chamber is not symmetric;

the optimistic squish area is more or less a quater of the total piston circle;

therefore the efficiency of the squish area is very reduced....

 

I remember that the efficency of the squish area is related to the ratio of total top piston area and the 'crown' area of squish, the distance between piston and engine head, the angle between the top of piston and the head and the trumpet shape from squish area and the main combustion chamber (please read the Gordon Blair book 'theory of two stroke engine');

in fact the modern trials engines (but also the old SWM250 cross/enduro) use a very, let me say, fast combustion chamber....in fact the purpose of the squish is to increase a lot the turbolence in the combustion chamber (to increase the combustion speed,,,,) without subtracting too much work at the energy balance of the engine system....

I developed a long time ago (immediately after the SWM bankrupt....) some engines 276, 347 and 370 for trial use adopting a high turbolence head and when today I use these engines I say 'beatiful engine this engine....' obviously also the carburettor and ignition setting must be changed....

 

to provide some info:

-squish area ratio 35-45 %

-squish initial thickness 0.8 mm

-squish angle 3 degree

-ignition advance 10-12 degree BTDC (should be 0.8-1 mm before TDC)

-carburation setting with standard carb 50-52 minimum nozzle 95-98 maximum....but the best is use the 26 carb ovally enlarged to 28 and 30 effective area....

 

I hope to be clear and not confused....regards.

 

[brownie001]

Hi swm, thanks for the very detailed explanation. If I ever get round to some hardcore tuning, I'll be in touch. At the moment though, I will be more than happy with the engine if it runs cleanly after the rebuild.

Still not got the bike completely back together.

 

Ciao,

 

Phil