Cant Decide

[DIYMark]

Im in the process of deciding what to buy - a 50cc or a 67cc (alleged 80cc) kit.

My situation is this...

I can buy both for nearly the same cost (in fact for me the 50cc kit cost $10 more! but big deal - that isn't a deciding factor.

Once I have the kit I will fit it with a tuned, tapered header, expansion chamber and a dellorto rep. 19mm carb.

Now the problem is, once I have a kit I would LIKE to balance the engine. The problem is that I dont have a spare conrod to weigh the big/little end and thus I would have to split the crank. Spliting the crank maybe hard because I don't have a press - just a vice and big hammer However once apart I can assemble it easily as I have a lathe and the necessary measuring equipment. I could also guess the mass of it (weigh the little end than add the little end times 0.666 to get a rough final mass?)

Now my problem is, does anyone think It's possible to split the crank with a hammer without damaging the crank itself (ie use a piece of rod to drive out the crank pin with the crank mounted on a copper slab to protect it, etc)?

Now this only applies to the 67cc kit as the 50cc kit should come balanced - but me thinks that there is still room to improve on the 50cc engine balance? is this true?

One other thing is (not sure where Im going with this)... I will port my engine for all out torque (even experiment with tuned intakes) because with a single speed setup you need the torque.

To make sense of what Im trying to get at here, lets come up with the X factor - let this be the port width divided by the engines displacement...Then a engine with a bigger X factor for a given displacement (so a wider port) will have more torque...As the wider the exhaust port = more torque.

Now, for a fact the 50cc and the 67cc engines both use the same exhaust pipe, thus same exhaust bolt pattern thus same exhaust port size! Now that means that the 50cc engine will have a bigger "X" factor than the 66cc engine - however the 66cc engine will create more torque as it burns more fuel... this is where Im stuck. I dont know what engine will have more torque - are they the same? or one has more torque than the other?

Hang on i got an idea... if you calculate the 2 X factors (one for the 50cc and one for the 66cc) it turns out that the 50cc X factor divided by the 66cc X Factor equals 1.375.

Now, the only difference between the torque of the 50cc and the 66cc engine is the piston diameter. So If you take the piston diameter of the 66cc and divide it by the piston diameter of the 50cc it turns out to be 1.381.

So from this crude calculation I think that the 66cc engine will make more torque (as 1.381 > 1.375) - even with its "smaller ports to the engine size"?!

What are your thoughts guys?

Can you split a crank with a hammer with no damage if your careful?

Is the 50cc engine balance good or does it still need improvement?

Would the 50cc or the 66cc engine produce more torque given that they both have the same exhaust, carb and both are balanced... even though the 50cc has bigger ports to its displacment than the 66cc?

 

[Speedy Wilson]

I'm a noobie but have built many a small block chevy engine. As the old hot rodders say" there is no replacement for displacement". Seeing as how in an auto engine when the stroke is longer than another engine with the same displacement it usualy means more torque but an increase in cylinder diamater increases the compression ratio which in turn will yeild more torque. I think the 50cc would have to be wound up to make it on the topend where as the 66cc engine would peak at a slightly lower rpm. a thought has occured to me also. Gm has a vortec head in two valve sizes, 1.94 in 1.50 inches ex. and 2.02 x 1.60. Now the smaller valve size head I have found to make more torque but the big valve head a little more torque but more topend hp.
This whole way out there thought comes to this: the small exhaust port on the 66cc actually may help make some torque compared to opening it up bigger. I would just do a very light port clean up,remove the casting flaws and smooth it out without opening it up any. Open the intake just a little but not much. Tuning the intake runner length will help but it's really trial and error.Ive seen snow mobile engines run elaborate exhaust mods so you may be on the right trak there. On cars a longer intake runner usually yeilds more low end torque but get to long and it will fall off bad. As far as crank splitting I haven't been into any China motors yet. I just started messing with bikes and am going the diy/weedeater way for now. Hopefully airflow dynamics from hot rodding can be useful to a small engine builder. Sure hope so. I would take the crank to a machine shop at about 11:30 or 12 noon. Why you ask? Alot of times a guy is off the clock for lunch and if he can make a quick 10 bucks for 2 minutes work he will. Do it mid week,most guys are broke then.

 

[DIYMark]

That's another good way of putting it - the smaller port would allow the gases to NOT flow as good and then create more pressure on the piston (even when the exhaust port if open slightly).

Right now I'm thinking of going for a 66cc, balance the engine, bigger carb, tuned exhaust and open the exhaust port side ways - this would then allow me to have more displacement (than a 50cc) and still have a exhaust port smaller than what would be on a typical dirt bike (even after widening it), which would help out with what I wrote above). The gearing would probably be a 44 tooth sprocket and after balancing I should turn out at least 8K RPM which is around 55km/h (as fast as I would want a mountain bike to go!).

OH Crap, major over sight, with the 66cc I don't think my HT supplier has the high comp. head!? If they don't It looks like ill get the 50cc as they do sell the high comp. heads for the 50cc but then again, if they in fact do have the 66cc head ill get the 66cc engine - so far my choice is dependent on what engine head I can get lol - Ill ask him for a quote or even if he sells them.