GN250 - 300cc upgrade?

[The Everlasting]

I believe they are limited to 15HP for a learner and then it gets progessivly bigger as you move on. Much better than NZ's system IMO where you can get on a 70HP aprilia RS250 for a first bike :S

Oh okay,i thought they had no restrictions...

Yeah their system sounds better and safer.

[imdying]

What do you think the expansion chambers on a two stroke actually do???

I think a better question would be what do you think a two stroke expansion chamber actually does??

[hmmmnz]

give it a go, a cast iron barrel is a cast iron barrel,
i dare say the piston and rings are probably sub standard, but if you keep all the original gear, if it goes tits up,
then well just stick all the original stuff back on

good luck

[quickbuck]

I think a better question would be what do you think a two stroke expansion chamber actually does??

Well, could tell you, but it would be a thread hijack... Besides which, Google is still there.... Just, by the sound of it!

[quickbuck]

Okay,
Bugger it.... Hijack alert.......

You know that changing the exhaust pipes on your two-stroke motorcycle can have a marked effect on the engine's power characteristics, but do you know why?

Simply put, it's because the two-stroke exhaust system, commonly referred to as an 'expansion chamber' uses pressure waves emanating from the combustion chamber to effectively supercharge your cylinder.

In reality, expansion chambers are built to harness sound waves (created in the combustion process) to first suck the cylinder clean of spent gasses--and in the process, drawing fresh air/gas mixture (known as 'charge') into the chamber itself--and then stuff all the charge back into the cylinder, filling it to greater pressures than could be achieved by simply venting the exhaust port into the open atmosphere. This phenomenon was first discovered in the 1950s by Walter Kaaden, who was working at the East German company MZ. Kaaden understood that there was power in the sound waves coming from the exhaust system, and opened up a whole new field in two-stroke theory and tuning.

More can be found HERE

[imdying]

Yeah, that's basically wrong (the metaphor), but I suspected that you'd run across something like that.

Even if it weren't, it has no relevancy to DB's post in any case.

[quickbuck]

Yeah, that's basically wrong (the metaphor), but I suspected that you'd run across something like that.

Even if it weren't, it has no relevancy to DB's post in any case.

So you have a better explination??? And there are actually different explinations as to why strokers "come on Pipe"?

You do realise bolkting a Turbo to an engine is classified as Supercharging as well, don't you?

[SS90]

So you have a better explination??? And there are actually different explinations as to why strokers "come on Pipe"?

You do realise bolkting a Turbo to an engine is classified as Supercharging as well, don't you?

The thing is quickbuck, that a "supercharging effect" does not constitute "supercharging" (anymore than a "forced air" air box does)

There is a lot of rubbish written on the internet about 2 strokes (and like many things, the same rubbish gets repeated time after time), just by different people.

I assure you that an expansion chamber does not "supercharge" a cylinder (super charging "effect" not withstanding)

I am of the opinion that the first person to write of the "supercharging EFFECT" (in English literature) was AG Bell, when he reffered to this "super charging effect", and many people (mistakingly) took this to read that a two stroke with an expansion chamber was "super charged".

The fact is, that it isn't.

I am not alone in thinking this.

The FIM certainly don't think a 2 stroke is supercharged...... they banned supercharging quite some time ago.

An expansion chamber is a busy piece of kit, and indeed we can use the resonance to do many things that assist evacuation of the crankcase ( crankcase scavenging), as well as plugging the exhaust port, after pushing back (some) of the over scavenged charge,(which is part of the cylinder scavenging phase) and in the case of a reed valve inlet, hold the top reed open to operate the inlet boost port. (Using the negative vacum created in the header after the returning pulse wave)

Among other things.

[imdying]

So you have a better explination???

I don't need one, why don't we just stick with the facts, they're out there if you choose to look for them.

And there are actually different explinations as to why strokers "come on Pipe"?

No, there is only one reason. But back to your original post, there is no compressor in an expansion chamger, and thus no 'charger' of any sort. Utilising pressure waves in an expansion chamber is not supercharging, turbo charging, or any other form of charging. The guys metaphor is crap, he would have been better to just stick with the facts.

You do realise bolkting a Turbo to an engine is classified as Supercharging as well, don't you?

Although it's an irrelevant straw man, turbocharging is a sub category of supercharging, but nobody refers to them as such as it strips the clarity that the two seperate terms give.

[Mikkel]

Naturally aspirated: the pressure on the intake side of the engine is, roughly, equal to atmospheric pressure. I put in roughly since a slight overpressure may be present due to clever intake designs and such.

Forced induction (turbos and superchargers): the pressure on the intake side of the engine is significantly higher than atmospheric pressure due to a compressor - driven either mechanically or by the exhaust gasses.

On two-stroke engines it becomes a bit muddy compared to four-strokes, since the air/fuel mixture is effectively being "forced" into the combustion chamber by the piston's downward stroke - i.e. it isn't only being sucked into the cylinder.
However, unless there is a compressor compacting the mixture the overall gas density will be equivalent to atmospheric pressure. This has to be the case since the piston displaces an equal volume inside the cylinder irrespective of direction of travel.


Oh, and a GN250 isn't a sportbike...

[imdying]

Want to muddy it even further... so called 'chemical supercharging' ala NOS... no compressor again, but more oxygen added that can be obtained from atmospheric pressure alone.

[Mikkel]

Want to muddy it even further... so called 'chemical supercharging' ala NOS... no compressor again, but more oxygen added that can be obtained from atmospheric pressure alone.

Yes, but I wouldn't ever call it supercharging. After all, the pressure remains roughly at atmosphere. http://en.wikipedia.org/wiki/Nitrous...bustion_engine

It would be just as valid to consider adding stuff to the fuel supercharging. I.e. not at all, IMO.

[imdying]

Agreed, same end result != same thing.

[quickbuck]

......there is no compressor in an expansion chamger...................

.

Okay, SS90 has given a pretty good rundown...
But, there is NO compressor?? Apart from a sound wave!

[nodrog:Me]

Hi the shipping is £30 which is about 66nz dollars, and the hp gain is 15.5kw=20.77Hp to 19kw=25.4 a difference of 4.6hp if the figures are to be believed.i was interested that someoe here said the gn's are made in china ,didnt know that.
As the bike is 1992 and original peformance had dropped i havent checked the compression but the top speed is well below the 85mph shown here http://www.classicmotorcycles.org.uk...zuki_gn250.htm like maybe 75mph now , i thought maybe it would be worthwhile giving it a go.As to the quality of the bits ime not sure if thers going to be ok or will fall apart , at least if they do i will have the original pistons etc.With regard to the learner laws i have a full licence and could ride any capacity cc after returning to biking after a number of years and chose the gn250 because it was cheap to tax and run etc ,.Anyway one other thing would the carb need to be adjusted??/ Thanks kiwi bikers for all your replies , i will let you know of the outcome whenever i get the parts and a bit of nice weather to do the job (lots of snow at he moment haha.byeeeeeee

dont you have any friends in England to ask? you're not related to Skidmark are you?