Jim Allen
Well-Known Member
- Joined
- Jun 7, 2005
- Messages
- 1,622
Dave, the last time I looked our model boats had neither gear boxes or slipper clutches, so I'm not sure what the comparison is here. I'm not sure what is meant by your second statement concerning the head area, but I think a larger bore would provide more area for port widths, wouldn't it?
Jim
Jim, gears, clutches or no gears, it makes no difference. Mechanical benefits aside, the power output of 4 strokes benefit greatly by bigger bore/shorter stroke for many reasons, bigger valves in the bigger bore being a major factor but on two strokes, a larger bore doesnt do that. Do the calculations and you will see that the longer stroke will give a bigger port area.
If you begin with equal bore/stroke, you have to reduce stroke by 10% if you increase bore by only 5%. With the same duration all your ports will be 5% wider but also 10% lower which means you get approx 5% less area. I'm not going to do the maths but I think you will find that this points towards more time area with long stroke engines for the same duration. This is the reason why, for example, almost all the high performance racing motorcycle two stroke motors have bore stroke ratios of 1:1 or more exactly 1:1.1 . An example of this change in area would be the CMB 90EVO vs the CMB 91 RS. Although both are oversquare engines (which is not good), the RS has the bore reduced by 0.5mm and has a stroke increase of 1mm. Hence the exhaust port width (which is approx is 88% of the bore) has decreased by .34mm but the port height has increased by .65mm. The transfers/boost areas increase in the same proportion.
Dave
Dave, while I agree with you assestment of the numbers, I must ask if you have built & tested motors that are under square, square as well as over square. In the case of oversquare motors the mechanical benefits cannot be overlooked when attempting to achieve very high sustained RPMs (32,000+). I agree that a longer stroke will increase the time area, but that does not necessarily mean greater HP will be generated at higher RPMs. My testing on the dyno shows that smaller depth transfers, wider intake ports, & smaller exhaust widths (67% of the bore) produce more HP at high RPMs, especially in piped engines.
Also with a longer stroke & wider exhaust, a lower taper liner must be used. The mechanical problems of the piston catching or rubbing on the top of the exhaust port cannot be eliminated in this type of setup. This is true even when using oval shapped exhaust tops & cocking the cylinder bore in relation to the crankshaft. Short stroke, large bore motors can use very high taper amounts without problems. I wonder if the continuing problem of chrome comming off aluminum liners in some popular motors is caused by the above??
I have tested these bore sizes with a .902 stroke in piped engines; .956, .975, .990, 1.060, & 1.125. Large time area numbers do little to increase HP if the window geometry is bad.
Jim
Jim
Jim, gears, clutches or no gears, it makes no difference. Mechanical benefits aside, the power output of 4 strokes benefit greatly by bigger bore/shorter stroke for many reasons, bigger valves in the bigger bore being a major factor but on two strokes, a larger bore doesnt do that. Do the calculations and you will see that the longer stroke will give a bigger port area.
If you begin with equal bore/stroke, you have to reduce stroke by 10% if you increase bore by only 5%. With the same duration all your ports will be 5% wider but also 10% lower which means you get approx 5% less area. I'm not going to do the maths but I think you will find that this points towards more time area with long stroke engines for the same duration. This is the reason why, for example, almost all the high performance racing motorcycle two stroke motors have bore stroke ratios of 1:1 or more exactly 1:1.1 . An example of this change in area would be the CMB 90EVO vs the CMB 91 RS. Although both are oversquare engines (which is not good), the RS has the bore reduced by 0.5mm and has a stroke increase of 1mm. Hence the exhaust port width (which is approx is 88% of the bore) has decreased by .34mm but the port height has increased by .65mm. The transfers/boost areas increase in the same proportion.
Dave
Dave, while I agree with you assestment of the numbers, I must ask if you have built & tested motors that are under square, square as well as over square. In the case of oversquare motors the mechanical benefits cannot be overlooked when attempting to achieve very high sustained RPMs (32,000+). I agree that a longer stroke will increase the time area, but that does not necessarily mean greater HP will be generated at higher RPMs. My testing on the dyno shows that smaller depth transfers, wider intake ports, & smaller exhaust widths (67% of the bore) produce more HP at high RPMs, especially in piped engines.
Also with a longer stroke & wider exhaust, a lower taper liner must be used. The mechanical problems of the piston catching or rubbing on the top of the exhaust port cannot be eliminated in this type of setup. This is true even when using oval shapped exhaust tops & cocking the cylinder bore in relation to the crankshaft. Short stroke, large bore motors can use very high taper amounts without problems. I wonder if the continuing problem of chrome comming off aluminum liners in some popular motors is caused by the above??
I have tested these bore sizes with a .902 stroke in piped engines; .956, .975, .990, 1.060, & 1.125. Large time area numbers do little to increase HP if the window geometry is bad.
Jim
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