Extra ports for transfer duct.

[RogerKnogeTopic starterBerichten: 4]

Hi!

I’m currently working on a friends engine.
He want a fast 50cc on a polini speed engine case and I’m almost done with the cylinder (standard 38.4) but I’m not sure if I should mill out extra holes for the transfers (see picture) 

I have already filed the skirt shorter and thinner at the transfers but do I gain anything from making extra ports in the cylinder?

It’s currently ported with around 170° ex and 120° trans + boost.
So kind of high revving.

Thank you!

[zevenenergie]

Hi welcome on the forum RodgerKnoger.

Those holes should match holes in the piston. Your piston probably doesn’t have those holes and isn’t designed to have them. You’d weaken the piston.

The theory behind those holes is that the pressure in the piston is higher than in the crankcase. That,s because when the piston moves down, the mixture is compressed by mass inertia.

• Deze reactie is gewijzigd 3 maanden geleden door zevenenergie.

[RogerKnogeTopic starterBerichten: 4]

• zevenenergie wrote:

  Hi welcome on the forum RodgerKnoger. Those holes should match holes in the piston. Your piston probably doesn’t have those holes and isn’t designed to have them. You’d weaken the piston. The theory behind those holes is that the pressure in the piston is higher than in the crankcase. That,s because when the piston moves down, the mixture is compressed by mass inertia.

  Thank you!
  I’m aware of the technical things to a point but at the moment the cylinder does not have those ports and I was wondering if I do gain any performance from milling out ports there (and also the piston). Does the cylinder need more air/fuel mixture than the ports already can provide.

  Or do I just lose crankcase compression and it will be a dead horse untill 9k rpm

[WimBijdragerBerichten: 3364]

RogerKnoge wrote:

• Thank you! I’m aware of the technical things to a point but at the moment the cylinder does not have those ports and I was wondering if I do gain any performance from milling out ports there (and also the piston). Does the cylinder need more air/fuel mixture than the ports already can provide. Or do I just lose crankcase compression and it will be a dead horse untill 9k rpm

  ---

  Yes, you can make one or two extra port(s).
  When you make it(or them)exact at the same timing/opening
  as the ports they are already there, than it’s oké.

  You don’t loose compression (under and above) compared
  with the compression before the new port.

  Ofcourse you need finetuning with and testing with serveral yet’s.
  But I think you understand that. 😉

  Gr. Wim.

Een oude brommer is alleen leuk als sleutelen uw hobby is.

[zevenenergie]

It,s a lot of work and you will gain little to non.
It,s better to make banana ports.
A ciao cilinder has plenty room to make them and
they won,t mess with your powerband.

• Deze reactie is gewijzigd 3 maanden geleden door zevenenergie.

[WimBijdragerBerichten: 3364]

zevenenergie wrote:

It,s a lot of work and you will gain little to non. It,s better to make banana ports. A ciao cilinder has plenty room to make them and they won,t mess with your powerband.

---

Hé. Een kreidler cilinder   😉

Gr. Wim.

Een oude brommer is alleen leuk als sleutelen uw hobby is.

[RogerKnogeTopic starterBerichten: 4]

Thank you for the replies!

I decided not to grind extra ports for the transfers.
This is how it turned out in the end. Hopefully it will rip but I have my doubts 😕

[WimBijdragerBerichten: 3364]

Aha, an oldtimercilinder.
38,4 instead of 38,2
So you have a small-end 12mm?

Nice work, all that tuning.
Use the biggest yet that’s normal drivable.
More gasoline is more cooling and lubricate 😉
And look for the best gap of piston ring-end clearance.

Gr. Wim.

Een oude brommer is alleen leuk als sleutelen uw hobby is.

[RogerKnogeTopic starterBerichten: 4]

It’s an old timer cylinder with a new timer 10mm crank.
Dr50 piston, speed engine case portmatched to this cylinder.
Will use a 13mm carb and a gianelli fire (or power, havent decided yet)

I’ll start with a 64 jet and go from there.

Do you know if there is any “real” 38.4 heads with centered sparkplug? All i can find is theese stupid polini heads wich states that they fit 38.4 cylinders even though they are 43mm wide..

[Gebruiker]

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