Polossi 221 build

[nicsar]

Hi there, started to build a polossi for next season. Theres a couple of things that I´d need opinions on.
-I see that opening up the lower part of the cvf in the piston will improve gasflow, should I open the upper, small hole aswell?
-Timings are now 171/124, that gives 23,5 blowdown, is this gonna work with the sip road?
-Xport is 58% wide, should I widen it or will this be good for a allround tourer?

Rest of motor looks like this

bgm 60mm crank
si24 ovalised
sip road
rotary, 120/65 totalinlet 185deg
old clutch and std gearbox

looking for a reliable tourer that will sit at 60 all day with reasonable fuel consumtion

heres a couple of crappy cell pics

[Polossi220]

Hi Nicsar is that a new polini alloy kit you are using I was wondering if the malossi piston would fit looks like I will have a new alloy polossi next year :anbrgin;

[nicsar]

Yes sir, the A piston went perfectly together with the A alu-polini. Clearance was measured at LTH before they sent the bits to me.

[ducksta]

are you using a packing plate, mine would run better on normal petrol and i needed a fast flow tap and a larger carb body it ran really well, good luck

[nicsar]

The head im gonna use has a 2,7 "squish", so with the protruding 1.5mm I should end up with ca 1,2mm squish. I have the .4mm basegasket that came with the kit but I´m not going to use it, as the transfer timing will go up.

I know there is a packer that enlarges the float chamber volyme, but for now I´m gonna go as it is, start with a 130 main and see what happens.

[ducksta]

i ended up on a 140 main jet ran better on normal petrol pinked on the higher octane stuff

[nicsar]

Pinking cant be generalised to a certain setup. Check edens post about detonation, good info there. Lots of different things can cause detonation. Higher octane should reduce pinking, so your situation sounds strange. Heres a quote from the article mentioned above.

The octane rating of the fuel is really nothing magic. Octane is the ability to resist detonation. It is determined empirically in a special running test engine where you run the fuel, determine the compression ratio that it detonates at and compare that to a standard fuel, That's the octane rating of the fuel

Any thoughts on the x port width, opening the piston holes and blowdown?

cheers,

n

[ducksta]

mmmmmmmm something for me to look into then cheers

[Diablo]

23.5 is a fairly short duration for blowdown. I certainly think with 125 degree transfers you could raise the exhaust to 175 without problems. I'd fit a 36t 4th though.
Certainly 185 inlet timing is a tad high. I usually open the port early and leave the front of the port flowed rather than lengthened.
I've never really done any dyno testing with modified malossi pistons even in malossi cylinders so not sure that will even be benificial in your set up.
The exhaust port can be safely widened to at least 65% but check this is doable regards cylinder studs before you go mad with a grinder!
Regards jetting 130mj is thereabouts for sure but don't forget you can get further adjustment by using different air correctors. The don't increase volume but increase the speed at which the volume flows. This still has the effect of richening the mixture.

[nicsar]

Interesting, thanks for some reasonable answers.

I´ve been under the impression that 205 degrees inlet is a "max" value. My smallframe rotary polini is pretty maxed out, making 22hp, has 200 degrees inlet. Wanting safe and reliable allround motor from this and reasoned on 185 degrees. Hopefully I didnt go over the top.
I will raise and widen the x port, aiming for ca 65% and 178 deg timing.

I´ve been thinking in lines "the less obstructions creates better flow", but the whole concept has been thrown upside down in the discussions here in the past days. So really dont know what to think. Would the reduced mass be an advantage? Surely the piston gets a bit weaker, but as its a quite common mod it shouldnt be an problem.

I thought that the advantage of polini over malossi is that it pulls std 4th better, especially as stroked with a low pulling pipe. But a 80kg guy +camping gear tucked behind the leggies of an SS needs a short 4th then thats the way to go.

Im planning on fitting the old style clutch with uprated springs, do you all think thats up to the job?