Is this too big (exhaust port)

Karlo Maric /

I was porting the cylinder and got a little caried away so im wondering did I messed the cylinder up. Port is 35 mm wide on a 39.5 mm cylinder and i think the exhaust port is 196 degrees


Re: Is this too big (exhaust port)

yup, 100% too big. 70% of the bore at most for ring life

Re: Is this too big (exhaust port)

Karlo Maric /

Well damn, I guess its time for a new one, thanks for the answer

Re: Is this too big (exhaust port)

You will snag for sure.

If you’re handy you could add in a bridge.

I’ve done that

Re: Is this too big (exhaust port)

Next time mark your ring locations on your cyl so you dont go too far if you are trying to push it as far as possible.

Re: Is this too big (exhaust port)

Any of you guys every run glow fuel model engines?

Familiar with the ABC engines?

If those little engines make crazy power/displacement and run rpms to the moon with ringless pistons, I wonder why we can’t do a cylinder/piston like that in a racing moped build.

No help here. Just thinking out loud.

Re: Is this too big (exhaust port)


add a vertical bridge.

nothing to lose now.

i think exhaust port width should only be 60% of cylinder bore diameter.

Re: Is this too big (exhaust port)

Now that I gave it a little thought, is there any reason a person couldn’t get a slight over bore size piston and lap it into a perfect fit?

Rings are a gas check of convenience. Easy to fit, easy to replace. Allows “sloppy” piston fit and mismatched thermal expansion to keep manufacturing cost low.

But you dont need them.

A perfect piston and bore would be one that the piston swells perfectly into the cylinder at op temp. Full contact would allow much better heat dissipation.

Frow in a couple em oil retaining dimples.

Surely this isn’t so labor intensive that nobody has ever tried it. Exhaust port could be almost 360* and the ringless piston wouldn’t care.

I know it is much easier on a 25cc engine. But if the model airplane engines have them and run 20K+rpm, it could be worth a try.

Re: Is this too big (exhaust port)

Overpriced Parts /

You can’t have full contact piston to cylinder you need a gas oil cushion for piston to ride on on a two cycle gasoline engine but other than some of what you say could could work

But only a small a small bore engine that’s basically a diesel two-stroke that runs very narrow powerband @ 20k rpm and don’t have a ton of torque and the fuel would need to be very thick with oil sort of acting like a ring/rings, The service life wouldn't be very long either

Re: Is this too big (exhaust port)

Tweaker trash /

I wanna start brazing too

Re: Is this too big (exhaust port)

I mean you can always try to run it and see what happens.

Re: Is this too big (exhaust port)

Yessss! Throw it on there and see?

Re: Is this too big (exhaust port)

Tweaker trash /

> Graham Motzing Wrote:

> -------------------------------------------------------

> I mean you can always try to run it and see what happens.

I bet it last about a week and he’ll snag going down a hill

Re: Is this too big (exhaust port)

Quick search,

2.74 cc motor produces 1hp? damn. These rc nitro motors 2 stroke motors have us way beat.

Re: Is this too big (exhaust port)

rc tech why cant transfer to mopeds? pumper carb, knife edge conrod, turbo crank induction(rizatto), sleaved cylinders that drop in(they look like brass with a nicasil lining, high tower super cooling heads, tuned exhaust, high flow intakes and exhausts manifolds...

Re: Is this too big (exhaust port)

The problem is that they don't scale, same reason a 1.5l F1 v6 can run 15,000 rpm and a 5 liter american V8 can't. In order to support the forces the components would have to get bigger and heavier and eventually it's a point of diminishing returns.

Also the funny thing about horsepower, if you are making 1 hp at 20,000 rpm, you are making .2 lb-ft of torque, by the time you convert that to anything useable in human scale, the gears or whatever method you use to convert it adds so much drag and inertia it goes to nothing

Re: Is this too big (exhaust port)

Dont think it has to do with size, a v8 can be made to go xxx rpm just as well. Were talking competition seeking peak performance within a set limit type of motor, type of race, type of vehicle which is a reflection of audiences preferences. Both are design for peak performance, accelleration vs wot endurance fuel consumption. Theres tons of restrictions placed on f1/v8 or any type racing, rules placed to increase competitiveness, safety and speed.

So, regarding torque and hp i give up, way too complicated but those 3cc rc trucks do 60 quick and a 50cc 1hp moped does 19 slow. Sounds just about right considering size n weight.

Re: Is this too big (exhaust port)

there is something called mean piston speed that is a limit for RPM and it really all comes down to stroke length.

They say fast stuff is like 20-25m/s (they being this wikipedia article that i read.)

Only REALLY fast motors (top fuel and F1 stuff) really move faster then ~25m/s.

from some real brief calculations, a puch spinning 12K is moving at 17.2 m/s. I'd say that's pretty badass.

anyways, heres the wiki link!

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