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Engine basics => Engine set up tips => Topic started by: redout on April 16, 2022, 12:24:46 AM
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I thought I would post this here as I have done some Googling and can't find the answer.
In four-stroke engines ( cars and motorcycles), all else being equal, long stroke engines have a lower red-line due to higher momentum stresses in the moving parts and also restricted breathing ability due to the limited cylinder head valve sizes possible in their smaller bores.
I always assumed the same would be true in two-stroke engines i.e. long stroke = lower redline. I don't know enough about two stroke porting to decide how this affects my question but I do recall reading that a longer stroke in a two-stroke engine allows greater height for the ports, albeit with lower widths due to the smaller bore.
Does the opportunity to use greater height ports in a long-stroke ( 2-stroke ) engine mean that these engines may in fact be higher revving than their short-stroke counterparts ?
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Yes, long stroke>more port area>higher rpm. Also, a long stroke with ports optmized for lower power output don’t often scavenge so well. L
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Why would the ports be larger for a long stroke?
Port area = width * height
Width will be proportional to bore, and height proportional to stroke.
Since bore*stroke = constant, port area will be as well.
If anything, the short stroke might have more area, since the spaces between the windows could be a lower % of the total.
Or am I missing something?
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Why would the ports be larger for a long stroke?
Port area = width * height
Width will be proportional to bore, and height proportional to stroke.
Since bore*stroke = constant, port area will be as well.
If anything, the short stroke might have more area, since the spaces between the windows could be a lower % of the total.
Or am I missing something?
From a purely geometric point of view you start from a wrong assumption.
It is not true that bore * stroke = constant.
On the other hand, it is true that piston surface * stroke = constant
And since the surface increases or decreases as a function of the square of the half of the bore, the diameter and therefore the circumference of the piston will not follow a linear trend of the variation.
Massimo
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You are right - too early to be thinking clearly :-[
Lets try again :)
stroke * bore^2 = constant, so stroke ∝ 1/bore^2 or bore ∝ stroke^1/2
port area∝ bore*stroke ,
so port area ∝ stroke^1.5
More stroke = more port area
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..and, you shall not look at degrees but a persentage of stroke. Only that way it’s possible to compare different layouts. But the reality is more cmplex, you cannot just focus on one detail and decide if it works or not. L
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Coming from a "4-stroke background", this is quite a revelation to me !
So engines like the old Webra 61 LS and Rossi 60/61 LS's, which seem to be lower revving engines than their short-stroke versions, were designed with something other than maximum rpm in mind ?
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Yes, sure!
Both engines were designed that time (eighties-nineties) typically for F3A (FAI R/C aerobatics).
And they found that 11 x 7 3/4 props, wich were typical, are too small for vertical manoeuvres.
They expected some more torque and bigger diameter props, by the LS variants.
Try to get some photos on Hanno Prettner and/or his Curare...
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That makes sense. I now feel inspired to learn more about 2-stroke specifics.
And though I had heard of Hanno Prettner before, I didn't realise the extent of his achievements ( nor his reported humility). Some of his models appear to still be available as kits.
Thank you very much to all respondents.