porting for lower rpm?

[trillian]

I was just reading a few threads and a thought / question occurred to me. It seems the 240, 270s etc are being ported to run properly at higher revs, like 13.5K etc.

Would it be possible with a bit bigger displacement, like the 300, to go for lower rpms, like 8,000- 9,000 and still have enough power? The thinking behind it being to get a more pleasing and quieter sound. Also, not everyone needs to get 2000 rpm headspeed.

[doitalldal]

I think you maybe asking a bit much at 8-9000 what you want is an engine tunned for a heli not a car. It is possible to lower the powerband and also make the engine tunned for more tq not rpm which we need for the heli. I had a std zenoah 29cc in my girl friends trex 600 Helibug conversion and tried to use the 20t pinion gear to give a nice low engine rpm at a reasonable head speed. It is for training about 1650-1700 but this meant with the 20t on the 115t mod1 main gear the engine was only doing about 9450 ish rpm. It would not climb out when require safely and was very slow to respond. So I raised the head speed to 2000-2100 which made hell of a difference it climbed out ok but was a bit loud and in your face for a beginner to learn on with 2100 head speed but the engine was only doing 12 000 but it was on the powerband for a std engine. This was all in corfu last yr at the RC hotel so could not rework it out there as all my gears were hear in uk. I am now going to rebuild it with 16t pinion gear and that should give her what she wants.
I had a engine built last yr for one of my trex700 gassers and that has a tq tune and balanced and lightened and should be producing near 6 hp so will run great with the lower 12 000 rpm and still have enough power for all I will throw at it as the 6hp will be at 18 000 rpm so I will have a lot more than a std engine at 12 000. I hope this helps the engine was built for me by a bloke in uk and was a reasonable price. Drop me a PM if you want any more info and I will help if I can.

[Doug Darby]

Wow... 6 hp at 18,000 rpm...
you should look closely at the dyno sheet power and torque curve to make sure its still making power at 12,000 rpm.... Most engines power and torque peaks are not much more than 1000 rpm wide at the peak and fall off significantly below and above.
sounds interesting. ..

[Toxic Al]

The answer to your question is yes, but it is not as effecient as a lower ratio higher rpm motor/ gear ratio. My engines can be built anyway the customer needs. Just to be clear there is not one of these engine producing 6hp without a tuned pipe period, i don't care what these guys are saying. peace, Al

[trillian]

Well, I just threw out a figure of 9000 rpm, the point being just the idea of going for more displacement, like the 29cc engine but optimising it for good power at a relatively low rpm.

My experience of small two-stroke engines with motorcycles back in the 1980s, my brother had a Suzuki 80cc motocross bike that had lots of power at high rpms but just next to nothing below that, so you had to really work the shifter to keep it in the power band or it just dropped off to zilch.

[fastflyer20]

I often run my TRM VX300 at 1600 HS in my Goblins with a 7.03 ratio yielding 11,250 engine RPM's. This engine has the widest power band I have seen in a Zenoah, it will then rev into the low 2000's and make monster power. You may want to try the VX300, Al can tweak the porting for even a little more lower RPM power.

Watch what style clutch you try. A metal clutch will not grip as well as a Zenoah clutch, exaggerated even more at lower RPM's