CMB RS91H ENGINE

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Anders,

26mm stroke x 1.8 =46.8mm rod length.

50/26 = 1.92 rod length to stroke ratio. :huh:

How would you compare the engine to K90's and RS91's in terms of performance?

Ian.
 
Yeh Harry Red Head MAC 91's in a CrapShooter....................... :lol: :lol: :lol: :lol:

OH!!!!! MY BAD...............CMB 91'S in a Crapshooter................ :p :p :p :p :p
 
nitrocrazed said:
Anders_Martinelle said:
Bore 27mm Ian............

They are equal in performance :ph34r:

Anders
Click to expand...
Why would you compare the bore to the rod length? :huh: I can see why rod length to stroke ratio is important:

http://e30m3performance.com/tech_articles/...ratio/index.htm

After 5 pages of discussions it is only equal in performance??!! :ph34r: I guess it will suit people who dont like putting circlips in pistons thru the exhaust ports.... :D

Ian.
Click to expand...
Good design facts that can be applied to our toy motors in these two articles Ian.
 
Jim Allen said:
nitrocrazed said:
Anders_Martinelle said:
Bore 27mm Ian............

They are equal in performance :ph34r:

Anders
Click to expand...
Why would you compare the bore to the rod length? :huh: I can see why rod length to stroke ratio is important:

http://e30m3performance.com/tech_articles/...ratio/index.htm

After 5 pages of discussions it is only equal in performance??!! :ph34r: I guess it will suit people who dont like putting circlips in pistons thru the exhaust ports.... :D

Ian.
Click to expand...
Good design facts that can be applied to our toy motors in these two articles Ian.
Click to expand...
Jim,

Just the results of a quick google search on the topic! :D The kinematics would be a little more complicated by the variation in rotational speed of the crank in each rev on a single cylinder two stroke. :ph34r:

Ian.
 
nitrocrazed said:
Jim Allen said:
nitrocrazed said:
Anders_Martinelle said:
Bore 27mm Ian............

They are equal in performance :ph34r:

Anders
Click to expand...
Why would you compare the bore to the rod length? :huh: I can see why rod length to stroke ratio is important:

http://e30m3performance.com/tech_articles/...ratio/index.htm

After 5 pages of discussions it is only equal in performance??!! :ph34r: I guess it will suit people who dont like putting circlips in pistons thru the exhaust ports.... :D

Ian.
Click to expand...
Good design facts that can be applied to our toy motors in these two articles Ian.
Click to expand...
Jim,

Just the results of a quick google search on the topic! :D The kinematics would be a little more complicated by the variation in rotational speed of the crank in each rev on a single cylinder two stroke. :ph34r:

Ian.
Click to expand...
Yes, that's true, but the principles still apply to our toy motors. In my motor the stroke is .902 & the connecting rod holes center distance is 1.741. The motors displacement is .8966 cu in & the rod ratio is 1.930.

Jim :) :) :)
 
Last edited by a moderator:
Well Jim, I guess it is straying a bit from the 91H topic, but compared to any commercial 90 engines I have measured you have a shorter stroke and longer rod/stroke ratio. Considering there seems to be a trend towards longer strokes in engines moving towards roughly square bore/stroke ratios why did you chose a comparatively short stroke?

Ian.
 
While it is clear from many dyno tests that the rod length/stroke ratio has no direct influence on the power produced by any engine, it does have an indirect influence. I have found the advantages of a long rod/short stroke to be some of the following. In engines where very high revolutions are required, for the same RPM, the piston speed is reduced. While the weight of the rod increases slightly as well as the total reciprocating weight, there is a significant decrease in the forces acting on the crankshaft. This is due to the reduced angularity of the rod at the 90 & 180 deg points. A shorter stroke allows a smaller crankcase volume (better pumping) & a much stiffer crankshaft. Loop scavenging is also improved because of the more frequent passage of fresh charges into the cylinder. Piston rocking & side loading forces are also reduced when moving the wrist pin as close to the piston crown as possible with a longer rod length. The ideal number considered by engine builders is a rod length that is 2X the stroke length. The number in my engine is 1.930 with a rod center distance of 1.741 & a stroke of .902.

Emissions From Two Stroke Engines by Marco Nuti, pages 146 to 149 explains in detail the effects that BORE DIAMETER/STROKE ratio have on scavenging & trapping efficiency in loop scavenged engines. There is also information in the Blair & Bossaglia books. Typical numbers range from .8 to 1.4, with a slight improvement with lower numbers. In my engines with bore of 1.125 & a .902 stroke, the ratio is 1.247.

Jim :) :) :)
 
Last edited by a moderator:
Jim Allen said:
While the weight of the rod increases slightly as well as the total reciprocating weight, there is a significant decrease in the forces acting on the crankshaft. This is due to the reduced angularity of the rod at the 90 & 180 deg points.
Click to expand...
Jim , while there may be a decrease in forces acting on the crankshaft I would consider the increase in recipricating mass as a definite disadvantage in our small engines. Any increase in reciprocating mass is going to reduce available operating rpm and add more stress to the crankshaft, bearings etc. A lower reciprocating mass is all gain no pain.

Jim Allen said:
Emissions From Two Stroke Engines by Marco Nuti, pages 146 to 149 explains in detail the effects that BORE DIAMETER/STROKE ratio have on scavenging & trapping efficiency in loop scavenged engines. There is also information in the Blair & Bossaglia books. Typical numbers range from .8 to 1.4, with a slight improvement with lower numbers. In my engines with bore of 1.125 & a .902 stroke, the ratio is 1.247.

Jim :) :) :)
Click to expand...
Jim, You seem to be running a very oversquare engine which goes against the practice of most GP two strokes and high performance kart engines which use bore/stroke ratios of very close to 1:1. The port area available in a two stroke engine is proportional to the the cylinder wall area not the head area. In a two stroke this favours long stroke/small bore up to a point.

Dave
 
DaveMarles said:
Jim Allen said:
While the weight of the rod increases slightly as well as the total reciprocating weight, there is a significant decrease in the forces acting on the crankshaft. This is due to the reduced angularity of the rod at the 90 & 180 deg points.
Click to expand...
Jim , while there may be a decrease in forces acting on the crankshaft I would consider the increase in recipricating mass as a definite disadvantage in our small engines. Any increase in reciprocating mass is going to reduce available operating rpm and add more stress to the crankshaft, bearings etc. A lower reciprocating mass is all gain no pain.

Jim Allen said:
Emissions From Two Stroke Engines by Marco Nuti, pages 146 to 149 explains in detail the effects that BORE DIAMETER/STROKE ratio have on scavenging & trapping efficiency in loop scavenged engines. There is also information in the Blair & Bossaglia books. Typical numbers range from .8 to 1.4, with a slight improvement with lower numbers. In my engines with bore of 1.125 & a .902 stroke, the ratio is 1.247.

Jim :) :) :)
Click to expand...
Jim, You seem to be running a very oversquare engine which goes against the practice of most GP two strokes and high performance kart engines which use bore/stroke ratios of very close to 1:1. The port area available in a two stroke engine is proportional to the the cylinder wall area not the head area. In a two stroke this favours long stroke/small bore up to a point.

Dave
Click to expand...
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 :) :) :)
 
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
 

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