Choosing a Yasuni exhaust for a Sports 70cc kit...


About Yasuni

For a standard 50cc moped engine there is a wide variety of exhausts available, most of them budget.
In a previous blog we established that at this entry level of basic tuning almost any low end sports exhaust will make a worthwhile difference, but we also established that as soon as you start to tune your moped engine any further the exhaust becomes far more important and the whole performance of the scooter depends critically on exhaust choice.
Yasuni is a very well respected exhaust brand which specialises almost solely in development, tuning and production of scooter exhausts aimed at both the road legal up to top end race markets.
They manufacture a wide range of choice of exhausts for each scooter application, each at a different state of tune and with a different purpose in mind. They also have an impeccable race pedigree and reputation throughout the industry and their pipes are to be seen in any race paddock around the world.
So in short, they ought to know what they are doing :) Let's take a look more closely!

Visibly the Yasuni quality is immediately clear to be a significant step above the budget pipes, everything is neatly hand TIG welded. The spring attached header pipes mean the exhaust is almost unbreakable even when poorly fitted under stress and the rest of the exhaust features details like O-ring seal slip mount silencers and other features normally seen only on much more expensive exhausts.

But how good are they really and what is the difference between them all?

yasuni exhaust

For this blog we are going to use the Piaggio Zip air-cooled moped which we used for the following blog...
(click here...)
The setup is basically a Polini Sport 70cc cylinder kit with a gear-kit and Stage6 variator fitted. The exhaust port has been slightly enlarged (as described in the blog just mentioned) to allow it to rev a little higher and the carb was upgraded to a 21mm dellorto carb to make sure that the carburation wasn't holding back the potential of the Yasuni pipes tested. This is a common, sensible sports 70cc setup achieving around 10hp (even on an air-cooled Piaggio). It's reliable enough for everyday use, cheap enough to do, and on such a light moped will perform extremely well. By setting it up properly and selecting the correct exhaust we ought to be able to make it stand out from the crowd :)

How we tested

Mopeds are of course equipped with a variator transmission. Engines develop different power at different rpm's and the purpose of the variator is that it can be adjusted (tuned) to hold the engine rpms constant at the speed at which the engine develops peak power. This means that in theory whenever you mash the throttle wide open the engine spins up to the exact revs at which is makes the most power (peak power rpm) and stays there, meaning that maximum power is immediately and constantly available.
In reality the variator can only hold the revs constant until the moped reaches a certain speed. Once this speed is reached the variator is said to be 'fully changed out' and the road speed can only continue to increase if the engine speed also increases in direct proportion in the same way as the engine speed increases as you accelerate in a car or conventional motorbike. This means that although it's true that the transmission can be set up so as acceleration is identical whether 10hp peak power occurs at 8000rpm or 11000rpm, the ultimate top speed will be significantly higher where the peak power occurs at higher revs.
For a much more detailed explanation of this see this blog.

For testing purposes we want to see what power the engine is making across the whole range of rpm so we need to disable the variator for this. If you're familiar with our other blogs you'll know that to do this we remove the complete variator/ clutch drive system and replace it with a custom fixed drive system which we use just for testing purposes.

Click here to enlarge...
yasuni c16

Ok, so let's take a look at the results!
We added in a Leo Vince ZX/ ZX-R and an Endy pipe as a comparison. The Leo Vince ZX/ ZX-R was included because it's a very popular exhaust which is fitted to many 50cc standard cylinders and widely believed to work well with a 70cc sports kit.
In a previous blog we did similar testing on a 50cc standard cylinder with the results as shown below...

yasuni power curve

You can see that on the standard 50cc cylinder the Leo Vince ZX/ ZX-R performed well, but as soon as we've increased to a 70cc sports kit the ZX is now starting to be completely outclassed even by the Yasuni Z which is widely thought of on the forums as being only suitable for a standard cylinder.
The Endy pipe was put in as a typical budget pipe. It's actually ok on the 50cc, it's not the most powerful but not bad for the money, but again, now that we've started to do even just mild tuning is completely outclassed.
In contrast we can start to see the potential of the Yasuni pipes with our more highly tuned scooter and where all that in-house development work starts to come into play!

So which one to choose?

We've already established that we want a high peak power together with high revving for best top speed. If we don't care about top speed we could go for the Yasuni Z. It makes very nearly the same power at 9,400rpms as the Yasuni C16 does at 11,000rpms. The powerband of the Z is broad and it would be extremely reliable running at just 9,400rpms, but as we've already established the final top speed wouldn't be as high as it would at higher revs, although another advantage of the Z would be because of the low revs it should run happily on a standard crank
But we didn't set out tuning our ped to settle for a reliable compromise :) and the C16 looks the obvious choice :) it makes the most peak power, runs at fairly high revs (not too high so as to loose reliability) so should have good top speed too.
The Yasuni C20 and C30 exhausts are designed for much more highly tuned (very high revving) cylinders/ mopeds and we can see from the graphs where they are being held back by the polini sport kit. They are simply too much for it. At a later date we will upgrade our Polini Sport kit to something much more aggressive and repeat the test, at which point we expect to see these more highly tuned pipes really start to come to life!
The other pipe which stands out alongside the C16 is the Yasuni R. Although it doesn't make quite the peak power of the C16 in this instance, it comes close and has a wider powerband. The C16 is quite 'peaky' and the extra power will only be useful if we can set the transmission up well enough to hold it in this smaller rev range.
Higher peak power is only better if it is fully useable and sometimes peakier pipes can loose a little initial pull-away acceleration unless great care is spent setting it up- something we want to avoid if this is going to be a low maintenance everyday setup. For this reason we decided to try both pipes on the bike after the variator was re-installed to see which worked best.

Final Results

Click here to enlarge...
yasuni exhaust Piaggio

This is what we ended up with. This is with the moped set up as best as possible (spending no more than 30mins simply tweaking jetting and rollers) with both the Yasuni R and C16 pipes fitted. For the C16 we used slightly lighter rollers to raise the revs a little as the previous power graphs demonstrate it making its peak power at a slightly higher rpm.

The C16 choice has paid off here with it subtely outperforming the R throughout the entire speed range. The most significant difference is at the top speed end where of course every last scrap of power is vital in order to continue to accelerate the moped against exponentially increasing wind resistance and frictional losses. Because almost 10hp is available as almost a flat horizontal line across the entire speed range this will actually be a very quick Polini sport moped and could considerably outperform more powerful mopeds which are not always so easy to set up due to lower quality more difficult to setup components
It should be noted that if we were using the standard Polini kit without having altered the porting to tweak to rev more highly, it is almost certain that the C16 would have been a bit too much for the kit and we would have gone for the Yasuni R instead. In this case though the exhaust porting has allowed the cylinder to start to see the potential of the C16 race pedigree!

Bear in mind that if we had used a liquid cooled Piaggio moped instead of air-cooled you could expect to see 10% to 12% increase on all of these power readings.

Next time we'll be continuing to tune our Piaggio Zip by ditching our Polini Sport cylinder kit and replacing it with something a bit more angry :)

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