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Tuning a Polini 70cc Sport Cylinder (Basic Porting)


In this short blog we're going to show some very basic porting.
It should be noted that this is for interest only and should not be used as a guide... if you decide to replicate this you do so entirely at your own risk !!!

Porting at a true professional level is a bit of a black art relying on extensive experience and a profound understanding not least of the mathematics of 2-stroke exhausts. Such experience usually breeds only after years being fully encompassed within a 2-stroke racing and workshop environment and even then only after a multitude of experimental trial and error successes and failures.

Without this knowledge keen enthusiasts still frequently modify their cylinders by copying other peoples' or other manufacturers' cylinder maps. By carefully selecting your cylinder it can for example be possible to duplicate the porting of a much more expensive cylinder.

Today we're going to take an air cooled 70cc Piaggio Polini Sports Cylinder Kit fitted to a Piaggio Zip and modify it to squeeze a bit more power from it.

The reasons for selecting this kit are...

  1. This kit is cast iron- because of this it is very hard and needs no cylinder plating. Aluminium cylinders are very soft and the cylinder has to be plated with a very thin nikasil coating to prevent it wearing very quickly. More care has to be taken when porting aluminium cylinders in case of damaging the plating. Cast iron is for the most part beginner-proof :)
  2. A modified version of this cylinder already exists as the Polini Corsa kit which we can basically replicate.
  3. The kit in it's standard form is at a mild state of tune and easily improved
  4. The whole modification can be done by a relative novice in less than an hour including removal and refitting of the cylinder and with no special tools and will still retain a wide powerband meaning it will work without additional bike set-up work.

Ok, all we're going to do is slightly raise the height of the exhaust port without making any other changes. This effectively decreases the tuned length of the exhaust which in turn should result in a shift of the powerband slightly upwards in the rev-range and in turn a higher peak power. This is basically copying the main difference between the Polini Sport and Polini Corsa kits so is essentially well tried and tested. As you increase the power of any cylinder this is always likely to come at the expensive of a slightly higher wear rate and we're getting to the point where a standard crank will struggle to be strong enough to be reliable, so keep this in mind ! The corsa kits come with an uprated chrome plated top piston ring instead of the standard cast ones for long-life durability but there is no spectacular difference and many corsa kits end up with standard cast rings fitted as soon as it is time for the chrome one to be replaced.

Referring to the picture below mark out an area 2.5mm above the top of the exhaust port. This new height should bring the top of the exhaust port to approx 21.5mm below the top of the cylinder. Measure this height regularly as you enlarge the port because it's easy to loose track of the pen marking.

(click to enlarge)
polini porting

At a push you can remove the extra metal with just a half round metal file. Ideally hold the cylinder in a vice using wood or similar to prevent damaging the fins. It will probably take 15 mins or so to file out but access is easy and it will work just fine. If you have a dremel or dremel copy you can remove the metal much more easily with a small grinding bit in just a few minutes but still use a half round metal file regularly to keep everything smooth and neat and in straight line. You don't need to enlarge the exhaust hole itself on the outside of the cylinder where the exhaust bolts on but make sure it tapers smoothly into the enlarged port on the inside of the cylinder barrel (use a flat file or straight edge to check)
Once you're happy with it be sure to wash out any metal filings before re-assembly

The below dyno printout shows the before and after for the kit we used for this blog...

(click to enlarge)
polini tuning

For a 'free' modification this is quite a substantial benefit. There is a significant benefit everywhere but most significantly at higher top speeds where the power is needed most. At 67mph we're up from 5hp to 9hp !
Keep in mind we haven't optimised the setup in any way for the porting mod. We were anticipating higher power at higher rpms so it would make sense that if we now fitted lighter rollers we should see a more significant improvement. It should also be noted that after modding, the engine was running slightly lean resulting in a required increase in jetting of approx 3 sizes due to the porting.

Next time something a bit more advanced !