This blog is a follow on from the blog Choosing a Yasuni Exhaust for a Sports 70cc Kit
In that blog we tried out a few different exhausts on our Piaggio Zip scooter. In previous blogs we had already fitted a Polini Sport 70cc kit to our little air-cooled Zip and raised the exhaust port slightly to mimic the Polini Corsa 70c kit (a more highly tuned version of the Polini Sport).
We had also fitted a 21mm dellorto carb, a Stage6 variator and clutch, and a PM Tuning gearup kit.
Since we're going to be tuning higher now and the engine will consequently be likely to be running at higher revs it's time to ditch the crank in favour of a much stronger Stage6 HPC crank which is capable of running reliably at a very high rpm.
This time we're going to continue tuning our Zip by ditching the Polini cylinder kit and fitting something a little angrier. The bike still needs to be reliable enough for daily use and built within a budget so we plumped for the new Stage6 Sports Pro MKII Kit
In theory this cylinder is still in the same mildly tuned 'sports' class of cylinder as the Polini Sport we're ditching and seems like it might not be an improvement especially as we modified our Polini Sport cylinder to get some extra power out of it (click here)...
but Stage6 are making surprising claims that 15-17hp is possible out of the box with their new MKII Sport Pro kit, so it seems a good time to run one up and see where they are getting their figures from :)
What exactly is a 'Sports' Cylinder Kit?
First of all let's clarify what we mean by 'Sports' cylinder... a 70cc sports cylinder is basically a cylinder kit which can be bolted straight onto a moped engine with an absolute minimum of other alterations. As an example a Polini Sport or Malossi Sport cylinder kit will bolt straight onto a standard moped and will work just fine with a significant increase in power whilst still using the standard crank, standard exhaust, standard carb etc. and without compromising reliability of the scooter in any significant way.
You can if you want tune it higher by adding a sports exhaust or perhaps a slightly larger carb, but you don't have to. You can bolt it straight on and it will work.
A mid race cylinder kit on the other hand could be kicking out up to 15hp. This is much higher than you'll ever see from a sports cylinder but this comes at a price. In order to achieve the higher power the engine has to rev very high and you will need to match the cylinder kit to a decent mid-race exhaust, you'll need a larger carb and a much stronger crank to handle the higher revs. You'll also need a decent aftermarket variator to work well at the higher rev range and the scooter transmission and fuelling will normally need to be setup by someone experienced in moped tuning as the powerband will be much narrower and unforgiving of any setting up errors.
A mid-race cylinder kit would not work well at all on an otherwise standard moped, so how are Stage6 claiming both that the new Sport Pro MKII kits are a sports kit *and* that they are capable of 15-17hp ?
Most peoples' reaction is either that it if it really does make 15hp then it is not a sports kit at all but a mid-race kit, or that it *is* a sports kit but the quoted power figures are way exaggerated (it wouldn't be the first time a manufacturer has exaggerated bhp figures!)
So what is it really ?
Well it's both.
Stage6 cylinder kits have a reputation for achieving wide powerbands at a state of high tune. Normally a wide powerband only comes at the price of lower peak power... manufacturers of geared bikes often have to use exhaust power valves to achieve this wide powerband at a high state of tune. A power valve effectively changes the height of the exhaust port of the cylinder at high revs so as you initially get the low end wide powerband of a 'sports' type cylinder and then the high end power of a 'race' cylinder at high rpms (the best of both worlds!).
You may recognise the graph below from a previous blog but it demonstrates this wide powerband perfectly.
The blue line shows the earlier (now discontinued) Stage6 Sport Pro kit. You can see that the Stage6 Sport Pro here is initially making peak power at lower revs than both the Polini and Malossi Sport kits *and* the peak power is higher than both of them. This is impressive in itself but you can also see it continues to make this peak power all the way up to 10,500rpm long after the Malossi and Polini peak power has dropped off. This tells us that this kit could not only be set up to run as a very good low revving sports kit (outperforming both the Malossi and Polini kits) but due to it's obvious ability to rev it's likely to also respond well if we were to fit a more highly tuned exhaust (e.g. a Yasuni C16) whereas with the Polini and Malossi it looks like we've already seen pretty much everything they have to offer.
Ok, let's take a look at how the new Stage6 Sport Pro MKII cylinder kit actually performs...
The first graph shows the Stage6 Sport Pro MKII kit against the (modified) Polini Sport kit from the previous blog.
We can see that similar to the previous graph of the S6 MKI Sport Pro kit, the MKII kit is making slightly more peak power than the Polini Sport kit *and* making it at lower revs. This means it will do at least everything that the Polini Sport can do and firmly fits the criteria definition of a 'sports' cylinder. If we left the standard Piaggio CDI fitted the revs would be limited to 10000-10500rpm and it would then become almost identical to the Polini Sport kit (albeit a more powerful version of it), we could probably even run it with a standard crank... but we can also see that above 10,500rpm Stage6 power curve is much wider meaning that it would be capable of achieving a much higher top speed than the Polini kit as the revs will still be increasing long after the Polini kit has run out of power due it dropping out of its powerband as the rpm increases.
But what of the Stage6 15hp claim? Well let's see what happens when we fit a more highly tuned exhaust.
The following graph shows the same graph again but additionally with the same kits run up with a Stage6 Pro Rep pipe and Yasuni C20 exhaust to represent a wide spectrum.
Once we get past the level of mild sports tuning these Yasuni exhausts really do start to excel, there really is nothing that comes close. The Stage6 Pro Replica and Stage6 Pro perform very well with anything that likes to rev high and are a very good buy and good second choice if you are on a budget, but ultimately the Yasuni exhausts always steal the show. To be fair we probably should have tried an R1200 Stage6 exhaust, that might have have been more interesting!
If you look at the Polini Sport results you can see that no matter which pipe you fit you can't really get the peak power any higher than when it was running at a 9,500rpm with a humble Leo Vince ZX pipe.
Compare this to what happens when we fit the Yasuni C20 exhaust to the Stage6 Sport Pro MKII kit... although it ran perfectly well as a sports cylinder, with a high end Yasuni pipe fitted the beast has awakened! We are now running with a very significant increase in power and the scooter is revving close to 14,000rpms which represents a massive increase in overall top speed. This isn't for everyone and certainly not suitable for everyday use, but the potential is there if you want it.
But it's still not 15hp, so where did they get that from ? Well power quotes usually refer to the power which is being generated at the crankshaft (it is after all the engine power which is being quoted), and a rolling road dyno can only show the power being developed at the rear wheel. The transmission system, wheel, gears etc. sap in the region of 17% of the engine power. For meaningful testing we have to use a specially designed toothed belt drive system to obtain proper power curves (you can't really do any indepth engine power testing with the variator fitted). The system causes an additional small but relevant power loss. In a moment when we remove it and refit the variator we see the peak power jump a little up to 12.5hp at the rear wheel. 12.5hp before ~17% drivetrain losses gives us 15.1hp at the crank. We can see therefore where the quoted figures have come from as we have achieved this straight out the box without any really tweaking or playing about. It should also be noted that this is an air-cooled cylinder kit, if we had used a liquid cooled Piaggio moped we should have seen an improvement of at least 2hp. Normally if you remove the cooling fan from an a/c Piaggio it will immediately run 1-2hp higher. Obviously it wouldn't last long like this but it goes to show how much more efficient it is to turn a water pump impeller rather than a large fan!
The following is a complete graph for the Stage6 Sport Pro MKII with all the pipes we tested...
Just to remind ourselves, here are the test results we saw previously with everything the same except using our Polini Sport (ported to copy the Corsa ports) instead of the Stage6 cylinder.
To put things into context we refitted the variator and set it up roughly without spending more than 15 minutes on it. In the graph below our Piaggio Zip is shown as it performs now compared to how it was at the end of the last blog
The Yasuni C20 pipe is a bit intense for every day use so we used the Yasuni C16 again.
It should be noted that for these sorts of runs we use a load cell dyno which can be programmed to accurately replicate real life results including wind drag. At higher speeds wind drag becomes very significant as speed increases (the increase in drag increases as a square of the increase in speed) and without these compensations we find results are not always replicated reliably in real life.
For the Polini kit the scooter was setup reasonably well which you can see from the horizontal line which shows that the full peak power was available across almost the entire speed range. For the Stage6 cylinder kit it hasn't been setup so well, over 12hp is available between 30mph and 50mph but it drops down to around 10hp at higher and lower speeds. This missing power is due to the transmission not being setup as well as it could be and is referred to as 'free power' as it should be possible to regain much of it simply by setting the scooter up better. We will cover this in a future blog!
Regardless of this our little moped is now significantly more lively. 12.5hp on such a small light air-cooled moped is enough to effortlessly power wheelie and because of the high over run of the cylinder when coupled to the Yasuni pipe we are seeing very close to a genuine 80mph with just a basic gear kit fitted.