Can you make a sport cylinder into a race cylinder?

[broyager]

Hi, your measurements for the port map, did you take measurements on the cylinder itself?

Your cylinder definitely has different distances, and intake transfers to mine (malossi sport, minarelli, air cooled), but the internal port shapes are similarish..

49ccscoot.proboards.com/post/447532

I'm surprised by how far the transfers and exhaust port roof(s) are from the top of the cylinder on yours, but I don't know how the engine designs differ.

I've smoothed some corners on my crankcase halves when I upgraded my crank, and cleaned out rough castings in the transfers to the best of my ability with the tools I have, but I'm not sure I made any concrete gains there. I haven't changed the port shapes yet, to any great degree, and haven't had any material taken off the top of the cylinder. I have tried to deck a couple of heads which resulted in a squish of 1.1mm. This seemed to make more torque, but made the engine run more roughly, and it didn't want to rev as easily, so am back to the malossi head with 1.3 or so. My base gasket is 0.4mm, and at BDC, the piston is slightly above the bottom of the transfer and exhaust ports.

[245luigi]

For those that are curious, here are the races from this past weekend:

www.youtube.com/playlist?list=PLD4AfyDDotGkTiqhFgvwS2QtYsrCw0FzX

I didn't have time to implement any of the tips given on this thread just yet so the bike was set up exactly as last September. My stella crank finally failed so I had to rebuild that bike over the winter. Tonight I did raise the exhaust port on my new cylinder 2mm though!

If anyone is interested, we are racing again in August.

[245luigi]

So some updates. Per forum advice, I raised my exhaust port 2mm. So from 26.5 mm to 24.5 mm. I also polished the exhaust outlet.











I removed my old Malossi Sport kit and here's what I saw. Seems like everything was fine. Curious what you guys think if anything.



























I did a quick test ride on the new cyl. Honestly it didn't really feel any different. I imagine I need to tune the CVT though to access the higher rpm. That is next. I did notice my CHT's were lower than before. Before they used to sit at around 370 F. Now they are around 320F if I am lucky. Is that due to the loss in compression from raising the exhaust port?

[Zino]

The underside of your piston has a very dark spot
That would mean the piston crown got very hot and cooked oil to the underside of the piston

[245luigi]

Right? I noticed that. I guess it means my jetting is/was lean. Just slapping on the new one, my CHT's are around 320 now, so 50 degrees cooler than that piston. I would think that would be around perfect now. Then when I deck the cyl for squish, I should upjet? Either way I'm thinking I should go up on my main. But I rode and raced with 370-390F CHT's and not a single seizure!

[bigmatthew86]

370 is a little high but the 320 is a little low. Aim for 350ish. I don’t know how long your straights are but you want it to stay around 350 at wot. So if you’re jetting for the track that might be leaner than jetting for 350 on the road where you’re wot for miles. What cylinder do you have now I don’t know if I missed that?

[245luigi]

Hey Big Matthew! So there is one straight and its not super long. If I remember right I could hit 370-390F on the straight.

This bike is now for track only. Here is an image of the track: lafayettemp.com/the-track It is run counterclockwise. You can pin it from the chicane to the hairpin. Since its .5 miles long, I'd say the longest WOT bit is 1/8th mile.

I removed an unmodified Malossi Sport (pictured used) that I had put on even before racing. I just installed a modified Malossi Sport (pictured new), exhaust port raised 2mm and the exhaust outlet polished.

I just got a Polini Sport in the mail. I'm gonna post pics of the port map and differences between the cyls later today.

[bigmatthew86]

390 is definitely too hot & you’re lucky it hasn’t seized. Try for around 350 (a little higher is ok). I don’t know what all cylinders are offered for you bike but I’ve had good luck with the S6 street race & the Corsa runs good if you’re staying with a sport kit.

[aeroxbud]

I love how the track has named each corner. They are all some of the most famous corners from racing circuits.

[aeroxbud]

I know the Polini cylinder tend to be more powerful than the Malossi ones. At least on the air cooled ones. The water cooled versions tend to be the opposite.
So it will be interesting what you find.

[245luigi]

It will be interesting. I'm not totally sure if I will run it as is (hoping to get feedback here when I post the info).

Unfortunately, it appears I am the only one to at least publicly talk about this cyl on my bike (that I can find on the internet) so I guess I'm the guinea pig!

[245luigi]

Ah and Big Matthew, this is the heart of my problem. For my bike, 2012 Kymco Like 50 2t, there are only 4 70cc offerings, Polini Sport, Malossi Sport, Airsal Sport, and Naraku. Nothing in any kind of race spec, so I'm trying to figure it out with the good people here's help!

[245luigi]

Malossi Port Map



Polini Port Map




The comparison between the kits is interesting. We can quickly see Malossi sets their working area deeper into the case compared to Polini. Polini resolves this by having a different shaped piston (pictured below), 2mm taller than the Malossi at the center and 1.8 mm taller at the circumference. So the Polini Cyl is taller from the piston pin to its top and is more domed. In the pictures below, left is unmodified Malossi Sport, right is unmodified Polini Sport.


Total height difference




Deck Height differences






Duct comparison




Intake is bridged in the Malossi, Polini's intake cutout in the skirt is slightly larger




The transfer cutouts in the Polini skirt are larger





The Polini piston is taller




Transfer area of the pistons




Underside of pistons, Polini didnt include an intake cutout




Underside of heads, Malossi has a lip, Polini's squish goes to the end, Polini's compustion chamber is more conical, while Malossi's is more "curvy"




Polini went with a long reach plug

[245luigi]

With the Polini measurements, I was able to figure out some theoretical durations.

Polini Exhaust Duration 173.45 deg; Transfer Duration 115.7 deg; Blowdown 28.87 deg

My MODIFIED Malossi currently has the following:

Mod Malossi Exhaust Duration 178.52 deg; Transfer Duration 117.23 deg; Blowdown 30.65 deg

Based on the numbers, I would think my modified cylinder is theoretically better. I also calculated the Polini's theoretical squish. It should be at 1.6 mm. I know my modified Malossi is at 1.4 mm. Right now, I'm thinking the modified Malossi could benefit from Polini's cylinder head.

[245luigi]

Its looking like I am going to be pushing for higher RPM, possibly 10k. Further if this all works. But I only have my stock crank or what looks like OEM+ (?) as options www.athena.eu/en-us/crankshaft-pin-12-mm-P155530.htm

What tends to fail when going higher RPM? I've picked up a JASIL racing small end bearing hoping to prevent anything from happening.