I just purchased a evolution .46 NT engine.Comparing this engine to others of its class,I noticed that while it has a smaller bore and rpm rating then other engines,it has greater displacement and a longer stroke.Although I have limited knowledge of engines,it would seem to me that this engine would have greater pulling power than other engines of its size,though sacrificeing some speed.Am I right?

Your premise is correct. “All else being equal”,the engine with the small bore and longer stroke will tend to produce more torque while the larger bore and shorter stroke will develop its maximum power at a higher RPM. However there are other factors that also influence this. Some of these are: Weight of the reciprocating parts. A lighter piston and rod assembly will allow a higher RPM even with a longer stroke. Internal friction (rings vs lapped piston, bearings vs sleeve, etc.) will effect maximum RPM. Higher compression ratio will result in higher power and RPM. The interplay of the different factors make it hard to guess at performance differences. The only realistic way is to run the engines in question "under the same conditions" and compare.

As far as the Evolution 46NT is concerned, it has a displacement of .455 cu. In. (bore=.867, stroke=.770) and a maximum RPM of 16,000. The O.S. 46 FX , TT 46 pro, and Magnum XLS 46 all have a displacement of .455 cu. In.. The O.S (bore=.866, stroke=.772) lists 16,000 RPM as max. The TT (bore =.853,stroke=.7874) lists 17,000 RPM as max. And the Magnum (bore=.860, stroke=.770) lists 17,500 RPM as max. The Thunder tiger has the longest stroke of the bunch but the second highest rated RPM.

All of the .46 size engines that I checked are very close in bore/stroke dimensions. With such small differences, the other factors probably have more influence on maximum RPM.

Thanks for replying Lou.

Horizen Hobby gave me different Specs for the Evolution .46 NT engine.
Bore=864''
stroke=.797''
displacement=7.65cc
It seemed to me using these figures that there was a significant difference in the stroke length and the displacement comparing it to other engines like the .46FX which had a displacement of 7.45cc and a stroke length of .772"using the specs supplied on this website. I realize that other factors also come into play such as prop size in determining the power vs speed ratio.

Thanks again

Using the figures you have, the difference in stroke is only 3%, not enough to make a significant difference. Other factors can easily overshadow such a small figure. Only tests run under controlled conditions can confirm such small variation in performance. There is noticeable difference between the sleeve bearing engines (like the OS la and the TT fp, etc.), and the ball bearing engines (O.S.FX, TT pro, etc.), but within the same displacement, and class there is really not much difference in power between engines. Features such as ease of tuning, durability, idle/transition, (and even appearance) are more likely to determine the choice of engine. Modern engines have such a high output to weight that most aircraft have an abundance of power. Though if power is a requirement, there is no substitute for displacement. If a good .46 isn’t quite enough, get an MDS at .48 cubic inches.

Thanks again,Lou

As far as our engines go there's really no such thing as a long stroke or a short stroke. In the examples that Lou mentioned there's only a 2% difference between the longest and shortest strokes given (as he pointed out). The idea of long stroke=torque comes from the old days of 4 stroke auto engines. By increasing the bore size and shortening the stroke bigger valves could be fitted. This gave 2 benefits. One was that the breathing ability was better and the other that piston speeds were lower so higher revs could be used to take advantage of the better breathing. The valve timing was then modified to suit the higher revs. The end result being more HP at higher revs but a loss of torque at lower revs (the larger valves actually contributed to loss of torque at lower revs because gas flow was too low for decent turbulence).

A similar thing happens with a 2 stroke in that a smaller bore gives less room for the ports so it cuts down their area but other variables (as Lou also said) will competely swamp this unless the difference becomes extreme. An incorrect idea is that long stroke gives more torque because there's more leverage on the crankshaft. While this is true, there's also less piston area for combustion pressure to act on which reduces the force that can be applied to the crankshaft. The two balance each other.

As far as max revs go, the figures listed will be max USEABLE revs. In other words, there's no point in running it any faster because the available HP will be dropping dramatically (and the prop needed would be far too small to be of any use). They'd all quite comfortably run at 20K+ but wouldn't pull the skin off a rice pudding let alone a model [img]i/expressions/face-icon-small-happy.gif[/img]