Small Honda engine

[napoleonb]

Being much more of a lurker than a contributor to this wonderful site i feel now the time is right to start working on a contribution for the near future.

Like most of you i've got more hobby's and interests then spare time with machine acces. I'm a teacher at a Secondary Vocational Eduction in the Netherlands, teaching mechanical engineering. Unfortunately at work the machineshop is used (read abused) quite alot, so i can't leave a setup standing for days and have to check every machine before i can machine something. :wall:

At home i don't have enough room, nor the required funds, to begin my own (hobby) machineshop. This in a nutshell is my dillema.

Now the introduction is over here are my questions:
One of my hobby's involve litle 50cc mopeds from Honda especially the SS50Z, see the attached pic, hhich i almost restored to its original state.

I would like to design a 1/2 scale version of it, almost every part of the real version has been measured and 3d CAD drawn by me. As far as my knowledge goes and i deemed necessary for the future. Of course many parts are still lying around for future reference.
For the simpler parts i can just scale them down by half but the surface area and volumes of course could give problems that way.

Because i want it to be a runner i need some advice.
1 Should i try to remain true to the scale in regards to bore and stroke (39,0 => 20mm and 41,3 => 20mm) wich gives it just over 6cc volume.
Or should i aim at a more traditional volume for example 5cc?

2 Should i then copy the cam lobes and valves with the same profile?

3 With some pain i could reproduce the original carb, but my guess is that most of the measurements are surface areas whereas the required air mixture volume demands even smaller quantities.
Perhaps i can test the engine with a carb know to work on a 5cc engine first and then try building my own carb.

4 Am i missing something here?

With kind regards,
Napoleonb

 

[Tin Falcon]

First of all welcome to the forum.
Second of all getting into this hobby is just something you need to do if you want to. you have to think outside the box or inside a closet some times.
I would have loved to start out with a 20 x 30 shop dedicated to machining and have a Bridgeport mill and a standard modern 12 x 36 lathe. ..... but for most of us that is not reality. I started with a $300 Chinese import 7x10 lathe on a bench in the corner of my basement. some folks have a lathe in a closet.
I have since taken over the shed room in the back of my hose and have machines in several places around the house and yes I am married and have a family. I made pen and pencil sets for a while paid for the lathe a band saw and a nice sander.
IMHO you have the training and experience (as did I USAF Machining ) get what you can afford and have room for and make it work . there are guys that have Sherline. and keep them in a toolbox or tote and carry them as they travel on an airplane. (non comercial)
A freind of mine makes great little engines on a sherline.


As far a the Honda project. IMHO you should be fine scaling it down. I would not worry about whether the displacement is 5cc 6cc or some odd figure what does it matter.
I see no reason to change cam profiles.
the fastener sizes may not scale exactly so just get close. and I would use a carb of known design. scaling the carb may or may not work well.
I am no IC expert but I think you will be fine.

if you have not done so read my getting started in Modlel engineering thread. IIRC in about HMEM sub fora.
Tin

 

[jasonh]

Napoleon:

1) 6cc versus 5cc - not a lot in it. If you use piston rings, and you don't make your own, your choice of bore may be dictated by the sizes available to you.

2) The cam profile should/could scale linearly. One rule of thumb I've heard is that valve lift should be about 20% of valve diameter- past 20% you don't get significantly more gas flow.

3) To start with use a carburetor from a similarly sized model engine. Once you know the engine works you can build your own carburetor if you want to. Removing variables is good when doing new things.

IMO: The amount of work involved is scale invariant - indeed as things get smaller they often get more fiddly and more difficult to work with. So - pick a scale where you find the parts comfortable to work with.

Jason H.