Bore stroke porting and displacement

Brad William /

Feb 19, 2019 12:29AM

For the more expert out there, help me with this. My airsal 65cc kit sits on the kitchen table next to the oem 50cc e50 it will replace.

The bore is visibly larger, of course, since the stroke is unchanged. Interestingly, the exhaust port window/timing on the 65cc is, rough guess, 5 to 6 mm higher than the 50cc. Thats huge really. Im not doing the math, but another rough guess makes the actual volume of compressed mixture at ignition almost equal between these two cylinders. I dont know enough about two stroke theory to make anything of this. Its just interesting.

The airsal has a bigger bore..same stroke.. but negligibly greater volume of compressed mixture at ignition. Is it better performing merely because it breathes easier through its other portings.

Jus curious.

Re: Bore stroke porting and displacement

Graham Motzing /

Go to branch

grahammotzing

Feb 19, 2019 11:31AM

in a nutshell raising the port raises the power band, stock puch cylinder powerband is from like 2000-4000 rpm, that kit is probably ported out of the box to 6000-8000 rpm

i'd suggest reading Jennings '2 stroke tuning' as a primer into the world of tuning two strokes. Bell is also a good read and more current, its higher level stuff in some instances but a novice can still follow it pretty well.

If you're going to buy one, I prefer the Bell book.

Re: Bore stroke porting and displacement

Overpriced Parts /

roffman

Feb 19, 2019 11:54AM

> Graham Motzing Wrote:

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

> in a nutshell raising the port raises the power band, stock puch

> cylinder powerband is from like 2000-4000 rpm, that kit is probably

> ported out of the box to 6000-8000 rpm

>

> i'd suggest reading Jennings '2 stroke tuning' as a primer into the

> world of tuning two strokes. Bell is also a good read and more current,

> its higher level stuff in some instances but a novice can still follow

> it pretty well.

>

> If you're going to buy one, I prefer the Bell book.

Stock puch powerband with stock pipe goes to 6,500 +rpm, With aftermarket pipe could go way higher like 9-10,000 rpm

Unported cylinder with stock exhaust with one extra hole in it . Oh yeah junk airbox and snorkel thrown away as usual and mesh filter used, that’s what stops a puch from reving,

Sounds like a lot more then 4,000 rpm to me:

Re: Bore stroke porting and displacement

Douglas Binder /

coclimber

Feb 19, 2019 12:07PM

Really important to consider the effects of the exhaust, specifically the expansion chamber. As the piston is moving up, the return wave from the expansion chamber will hit the port and hold the fresh charge in the cylinder, effectively increasing the volume of the cylinder. These two strokes are fundamentally simple but when you start looking at the wave action inside them, they get complex quickly.

Re: Bore stroke porting and displacement

Brad William /

wsemajb

Feb 19, 2019 10:48PM

I'm using a tecno hunter with this cylinder. Any experience with this? I weld an additional silencer to the end of the hunter. No restriction, just lower decibel. I fucking hate loud. Im not into top speed. 35 reliable mph is just fine. Most interested in better overall steady torque curve 0 - 35 mph. While im here asking.... Carb recommenations? Vm18? 15mm bing? The bing appeals for simplicity sake. But i do have bad memories of wishing i had more than just a needle setting when tuning the 12mm bings that came oem on my two puchs.(edited)

Re: Bore stroke porting and displacement

Brad William /

wsemajb

Feb 19, 2019 11:08PM

How can the bings still be used with kits and aftermarket pipes? The only adjustments to such a wide range of demands imposed, beyond the very specific ones the oem bings were designed for, are needle slots and intake restrictors. Isnt any attempt to match them with different cylinders and pipes just badly limited from the start?(edited)

Re: Bore stroke porting and displacement

Overpriced Parts /

roffman

Feb 20, 2019 03:10AM

> Brad William Wrote:

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

> How can the bings still be used with kits and aftermarket pipes? The

> only adjustments to such a wide range of demands imposed, beyond the

> very specific ones the oem bings were designed for, are needle slots

> and intake restrictors. Isnt any attempt to match them with different

> cylinders and pipes just badly limited from the start?

I get 56-57 mph with 64cc polini reed kit, on a e50, horrible case match, 15mm bing, 90 jet, 217 atomizer, one clip heavy on needle, 14.5° BTDC timing and cheep estroil pipe

I’ve hit 62 mph with better mlm pipe

Bings rule but-so does mikuni, dellorto and other carbs once tuned !

Be off a fraction in jetting and your bike will be a dog but be tuned perfect and you got a small motorcycle

Re: Bore stroke porting and displacement

Bas Autowas /

dutchpuch

Feb 20, 2019 05:37AM

> Graham Motzing Wrote:

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

>

> If you're going to buy one, I prefer the Bell book.

You can download it here for free

Re: Bore stroke porting and displacement

P D /

pdcomm

Feb 20, 2019 11:15AM

> Bas Autowas Wrote:

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

> > Graham Motzing Wrote:

>

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

>

> >

>

> > If you're going to buy one, I prefer the Bell book.

>

> You can download it here for free

Thanks for that^ Bas . :)

Re: Bore stroke porting and displacement

Matt Mahloch -- MBM Customs /

azzkikinvette

Feb 23, 2019 12:09PM

OP cool thing to notice is the amount of time the exhaust alone is open, this is known as blowdown. More blowdown requires a bigger hit from the pipe but when matched properly is the reason 2 strokes are so damn sweet compared to a 4 stroke.

Re: Bore stroke porting and displacement

Bob Selby /

slimhoody1163

Feb 23, 2019 06:04PM

The explanation was not exact numbers but did get the point across...........

Re: Bore stroke porting and displacement

Chöschi21 C.K. /

pistonbully

Feb 23, 2019 06:08PM

> Brad William Wrote:

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

> Most interested in better overall steady

> torque curve 0 - 35 mph. While im here asking.... Carb recommenations?

> Vm18? 15mm bing? The bing appeals for simplicity sake. But i do have

> bad memories of wishing i had more than just a needle setting when

> tuning the 12mm bings that came oem on my two puchs.

12mm Bing is fine, 14 if you have it, you can tune with changing the Needle and Needle Holder, if you don't change them around you can't use an Airfilter that flows more than Stock.

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