O-tuning Olivier's px200

[sean brady scooters]

guess you have also seen the new Pinasco racing cranks 60mm stroke part number 80827 gg ? ...lovely cranks

[fastfrog]

Yes and they seem better designed than previously. Sip says rubbish about inlet timing (6°ATDC/138°.. obviously that's not what we see...) but anyway they look a bit anticipated and not only retarded...
Olivier only wants an o-tuning thing: no race part inside: we'll keep a standard 57mm crank this time....

[fastfrog]

Really interested in seeing this go.Anything that would pass the Spanish MOT and hopefully cost effective.
Always like to see standard stuff reused.

It pulls like a train and power range has stretched to a much higher limit. I'll have it dyno soon but a previous incarnation (piston you see now on olivier's project, sip road..) gave this:



my V2 pulls more and higher....

[Jack221]

Hey Fastfrog very impressive job! I did a similar "standard" PX200 before but with a 60mm crank a few years ago.

After some trial and error I ended up with Exhaust 175 degrees, Transfers 124 degrees, Mercure 60mm crank inlet duration and standard rotary inlet. Rebored to the 68mm GS thin ring piston. My exhaust port was as wide as I dared grind without cutting too close to the barrel studs (44mm chord width) and full flat across the port top width for max blowdown. The exhaust and the engine all original standard PX200 apart from 16 springs in the clutch to stop it slipping and a banded basket. It went way better than I expected and would do 7500 rpm in third and 7200 rpm in 4th on standard gearing.

What have you done with the head on yours to close the squish without the 60mm crank?

[fastfrog]

My V2 o-tuning has a polini 68 mm piston and a dedicated MMW Head. Barrel is skimmed to get a 1,2mm squish without gasket.
crank is a 59mm conversion with a modified (as shown) inlet timing and home balancing...
I always try to get an 5 to 10° overlap between inlet and transfer timing. Best way to get much power from 3000 to Pmax. that means in my case
T:125° (E:180° BD:27,5°)
I: 125°btdc-55°atdc

[Jack221]

I too use 10 degree as my max overlap on any I tune. Seems to give the right balance and any more would lose high rpm power (and there isn't much more metal in the valve pad!).
180 degrees on the exhaust duration with your oval shaped port is probably very similar to BD area as my 175 degree exhaust with the full width square shaped exhaust port.
When I did my one, my idea was to get at least double the power without buying any tuning parts. I pretty much did it apart from the 60mm crank and that was just to make it easier. Skimmed the original head flat. Used the standard carb and modifed that.
Let us all know how it runs on the dyno!

[fastfrog]

square exhaust is not the best idea. See gordon Jenning's 2 stroke Bible.... Mine is not oval but ovoïd with an almost flat upper limit...
I rather cut the inlet side web above pin and get the 10° overlap requested than doing a too diverging inlet port which kills gaz speed....
zum beispiel:

[Jack221]

As far as the 1970's rule book goes you are quite correct but When you can't get an oval port wide enough, you can break the rules and go to a square exhaust port! The whole problem with the Vespa 200 engine is no matter what barrel or kit you have the exhaust port width is restricted by the gap between the barrel studs. Clearly not an issue on some of the exotic smallie and lammy kits with their auxiliary exhaust ports!

Not relevant for you here but when doing a high rpm tune the square port is preferred, as it gives a sharper shock wave into the expansion giving more pipe effect.

I do the same grinding on the crank when going for the full 10 degree overlap. Although this few mm helps with the numbers......the big end is still in the way and blocking the inlet flow!

[Bifter]

God i wish i knew what you lot were on about

[fastfrog]

Right for the square shape exhaust port at high rpm but lack of progressivity at bottom end/ mid range and hard to ride on a daily scooter. Don't forget i'm using an S-box and not a full expansion pipe: it's still a commuter/ city scooter not an endurance/race one.

God i wish i knew what you lot were on about

We haven't spoken of women yet!