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Re: Boats ans Seagulls.

Posted: Thu Jan 11, 2018 2:54 pm
by NaughtyBits

Not without drastically altering port timing - and not likely for the better...


Sounds like you've been talking to Charles at some point. But there's a step or two missing in there, maybe ...?

Re: Boats ans Seagulls.

Posted: Fri Jan 12, 2018 2:33 am
by headdownarseup
I probably have missed out one or two things along the line somewhere,perhaps someone with far more experience at this could expand on what i've missed, but this modifying lark can easily become quite a personal thing too. Some things it has to be said work well, and some need quite extensive testing and changing before you could say you were happy with the end result.
This is why i'm quite reserved at keeping things at a more grass roots level without going too mad with it with baby steps at every level of development until you've reached the limits of what might be called safe and still have a relatively reliable engine that isn't going to self destruct at wide open throttle.

Boats and engines come in many different shapes and sizes. Seagulls are no different in that respect, but they're still just a basic 2 stroke design with some room for improvement if you know where to look. That's half the fun with modifying something for greater performance. Do your own thing with it, but always with an eye on the essentials. Don't want to push the envelope too far into the unknown, but i see a 102 as unexplored territory that maybe a lot of people are afraid to get into partly because the cylinders are a pain to work on, and a few other things besides. Square block seagulls are easier to modify by comparison, most self respecting seagull tweekers know this, but despite the design of a 102 having some drawbacks over their square blocked cousins, with sensible "upgrades" i think there's still a lot of untapped potential in these old roundblock 102 engines.

It's true, i have spoken to Charles about certain things in the past as he's probably the one person we all know within easy reach with enough experience to know what works with seagulls and what doesn't. I've also spoken with an engine builder i used to use when i was racing minibikes. Two quite different sides to the same questions with slightly varying answers from both.That, coupled with my up and down experiences of 2 wheeled racing i'd like to think i knew a good enough understanding of the basics to get me out of trouble without sounding big headed about it. I will say this though, i never stop learning from some of you guys out there. It's what keeps my interest going with this silly hobby of ours. Each day that i can learn something new is a good day for me :P

If you can wait a few days i'll show you something i'm working on just for explanation purposes. Purely a "mock up" you understand, but you should then get a better idea of what we've been talking about here.

Sounds simple in principle except that we're talking about a rectangular shaped exhaust port,(not really square as such) and often with a bridged section down the middle of it. For want of a better description it's more like 2 smaller rectangles phasing into a round pipe of a chosen diameter.
For a bespoke expansion pipe to work well enough in a 102 engine, the exhaust flange is quite a critical area that needs a smooth transition from rectangle to round. There's some complex mathematics involved with working out the area of the ports dimension against which is the best option for a headers ID and length to best suit a particular rev range you're designing this bespoke pipe to work at. The flange is just one area that needs a bit more thinking about on my part.The specific angles inside the expansion chamber itself all play a big part in whereabouts peak power is going to occur, perhaps a motorcycle pipe might not be the best solution to some of these problems though as these pipes were specifically designed to work in a different environment and with a different purpose. So now i have to ask myself what i want this proposed power pipe to do in my seagull engine.Do i want flat out high revs power, or would something in the slightly lower rev range work out better overall. I feel that there will have to be compromises, but i think i can get something to work fairly well in the 102's exhaust department. It would be much easier if the 102 exhaust port was round in shape purely from a fabrication point of view, but as they're mostly rectangular in shape it makes things a tad more interesting, but i understood what you meant with your suggestion of tapered square tubing etc. Rectangle into round will work, i just haven't found the right way to go about it yet. Once i've worked out the basic dimensions of the pipe itself with the diffuser and reverse cone, then measured a cylinders exhaust port duration for a better idea of where i'm going with this pipe and what i want it to do, i'll work out the overall "tuned" length and see if i'll need a standard shaft or long shaft and go from there. I might come back to you about a few things as i will need some idea of what your boat's engine is capable of revving at wide open throttle and maybe compare some of my figures with yours before i settle on a ballpark figure to work from. How's that sound?

Any more ideas on this more than welcomed.


Re: Boats ans Seagulls.

Posted: Fri Jan 12, 2018 6:15 am
by NaughtyBits
As in - a good starting point might be to take a look at the problems BS encountered (and the changes they began to make to try to solve them), when they began adopting more powerful, efficient power head designs in the later model ranges. You're likely going to be asking far more of a 102 crankshaft/con rod than they were designed to handle.

Much of the best advice I've been given re: tuning the older models referred back to the issues highlighted with the QB's.

Re: Boats ans Seagulls.

Posted: Fri Jan 12, 2018 8:35 am
by Collector Inspector
I admire your enthusiasm Jon. Every now and again over the years the modification to a 102 comes along.

I am watching all this with interest to see if you/we/community can come up with a reliable result.

You mention reed valve.

You will have easier fitment of such into the crankcase......................



Re: Boats ans Seagulls.

Posted: Fri Jan 12, 2018 9:52 am
by Nudge
Sounds simple in principle except that we're talking about a rectangular shaped exhaust port,(not really square as such) and often with a bridged section down the middle of it.
I was just trying to keep my answer simple....
If you wanted to know how I would go about the problem myself.... I would make a pattern and cast a part! Simple fix! :P
Now in saying this you to can cast the part at home.... it's not that hard.
The hardest thing will be sorting out some casting sand... and thats not that hard!

'The simplest of all possible sand molds is a hole or depression in sandy soil. If the soil is sandy enough to resist the hot metal, dry enough to avoid steam explosions, and wet enough with enough binders to maintain its shape, then the metal will accurately take the shape of the depression.

Pouring metal into sandy soil is undoubtedly how the process of sand molding was developed by our primitive ancestors, but it is not a controllable enough process to make much use of in modern casting. Instead of using naturally occurring sand, we need to make our own with desirable properties that can be controlled and maintained. The process is simple for water-bonded sands, which are the most accessible variety for most hobbyists. (Oil bonded and resin bonded sands achieve the same thing in principle, using more complex binders for improved properties.)

To make any molding sand, the first step is to get good sand. Clean white silica sand of approximately 70-90 mesh is a good starting point. Most hardware stores sell a sieved and cleaned “play sand” that will work adequately, though a foundry sand will generally have a finer and more useful gradation of mesh sizes. Olivine sand will also work, and it is usually green or black. The grains should be sharp, not rounded, and the sand should not contain rocks or debris.

This sand now needs a binder. For water-bonded greensand, the binder will be bentonite clay, approximately 10% by weight. Bentonite is available in high purity from ceramics suppliers and in bulk amounts from well drillers and some farm supply stores. Some types of cat litter are bentonite chips, but these will have to be ground into powder before use. A coffee grinder works well, if slowly.

Adding more clay will increase the sand's strength but decrease its porosity, leading to gas defects in extreme cases. Adding less clay will lower the strength but make the sand more porous. 10% is what I prefer, but it is possible to get away with as little as 6%. As long as the sand molds well, less clay is generally better.

The clay and sand will not become greensand without the addition of water. Water should be added slowly to the blended clay and sand, and thoroughly mulled in until the sand feels uniformly damp (but not sticky) and forms a cohesive ball instead of crumbling when squeezed. It takes time for the clay to absorb water, and mulling action is necessary to distribute the clay evenly (but excessive mulling will “wear out” the sand by rounding off the grains, reducing porosity and strength). Sand generally improves dramatically if it is left to sit overnight after mulling in a sealed container, and the sand will continue to improve even more for several weeks after it was first made, as long as the moisture level is maintained.

Too much water in the sand increases the amount of porosity necessary to prevent gas defects and will cause the sand to stick to things. (Dramatically too much water can even cause metal to shoot back out the sprue of the mold from the steam generated! A mold that does this will never produce a usable casting, so it is best to retreat for safety's sake until the metal has cooled.) Too little water will cause the sand to abrade away and lose strength, eventually crumbling. A sand mold can be dried after molding without a loss of strength, but the drying will cause shrinkage, which can lead to gap run-outs and problems with dimensional tolerance. It is generally better instead to use properly conditioned sand and to pour molds as soon as they are made.'

Some of these words have come Alloy Avenue metal casting forum.

I think this lot may need to be moved to its own thread.

Re: Boats ans Seagulls.

Posted: Fri Jan 12, 2018 6:26 pm
by headdownarseup
I agree. Maybe we should move this to another section.
All good though :P

Shall we move over to the "racing" section where we can resume this in the proper place.