In our last article Stage 6 Pro Replica Exhaust Triumphs Exhaust Shootout ! we ran up some performance scooter exhausts on an otherwise standard Yamaha Aerox moped.
Today we're going to run up the best selling of the 70cc big bore sports cylinder kits.
70cc "Sports" cylinder kits tend to be fairly mildly tuned but still offer a large performance increase over standard 50cc without really losing much in the way of reliability. Sports kits are by far the biggest selling big bore kits for mopeds. "Sports" kit manufacturers usually claim their kits make up to around 10hp, but take it all with a pinch of salt.
It isn't really possible to get much over 10-11hp on a 70cc moped without starting to significantly increase the revs, and if you do this you need to move up from a "Sports" cylinder kit to a "Low race" kit, something like a Malossi MHR replica kit or Stage 6 Racing. These kits start to get much more expensive and parts start to wear much more quickly and the scooter becomes much less reliable for day to day use. Low race is about the limit you can use on the road as a functional bike unless you like working on your bike on a very regular basis !
For this article we've used just the main Italian manufactured kits (Polini Sport 70cc, Malossi Sport 70cc and Stage 6 Sport Pro 70cc) because the quality on all of them is unrivalled and spare parts, pistons etc. are readily available for them.
You'll notice that we've also included the Stage 6 Streetrace kit which is a budget 70cc cylinder with milder tuning than its Italian made Stage 6 Sport Pro big brother. It's proven to be a very reliable kit and at just £102 it's also a very popular kit, but is the performance any good at such a cheap price?
There are a lot of Chinese made budget 70cc kits which are sold under various brand names at around this price of very dubious performance and quality, so for those just looking for a budget 70cc kit we thought we'd give the option of a decent quality budget kit for comparison.
We've conducted the tests with the same Aerox moped we used in the previous article for consistency.
In general the results will translate directly to Piaggio mopeds also, the only really difference is that Piaggio engined mopeds normally make +1hp higher than the Aerox with otherwise identical cylinder kits or exhaust pipes fitted.
For the tests we've locked the variator drive so we can see what power the engine develops across the whole rev range.
Our previous article concluded that the Stage6 Pro Replica Exhaust and the Leo Vince ZX exhaust were the best performing moped exhausts we tried when fitted to the otherwise standard 50cc Aerox.
(click to enlarge graph)
We also observed that because both pipes ran well at higher revs they would also be likely to run well with a more highly tuned 70cc cylinder kit fitted so for this reason we started out by testing all the 70cc sports kits with a Leo Vince ZX pipe fitted to see how it fared when faced with more highly tuned cylinder kits.
Mopeds use variators instead of conventional gears. The variator is designed to hold the moped engine revs dead steady by automatically changing the gearing whilst the speed of the scooter increases. If the variator is working well you can set it to hold the revs at exactly the peak power of the moped engine so as whenever you mash the throttle the engine goes straight to the revs at which it makes peak power and stays there. You can adjust the actual rpm that the variator holds the revs at by changing the weight of the rollers.
If we use the above graph we see the standard cylinder makes peak power of 7.4hp with the Leo Vince pipe at 9,300 rpms, so if we can set the variator rollers weight to hold the engine at 9,300rpm then we always have this 7.4hp available whatever speed the bike is going whenever we full throttle.
This would mean that we're only interested in the peak power each cylinder makes and the rest of the graph is irrelevant.
This is pretty accurate but there comes a point when the variator can no longer keep on changing the gearing. This point is reached at somewhere around 30mph for a standard 50cc moped (without gear up kit) and from this point on the speed of the moped can only increase if the engine revs also increase.
So in a drag race up to 30 mph the winner simply will be the one with the highest peak power so long as the variator is set up and working correctly, so Stage 6 Sport Pro would come first, then Malossi Sport, then Stage 6 Street and last in the Polini Sport.
Once we reach this 30mph mark the rules change... with the Malossi Sport, Polini Sport and standard cylinders you'll notice that the peak power drops off quickly after the curves reach their peak, so as soon as you reach the 30mph the revs increase and the power available also starts to drop off. Because you need much more power to accelerate swiftly at higher speeds what happens to the power curve after it has peaked is critical in moped tuning both for final top speed and acceleration between around 30 to 50mph.
So if we look at peak power again there isn't a huge difference... from the standard cylinder making 7.3hp to the Stage 6 Sport Pro making 8.8hp - the upgrade hardly seems worth bothering with at first glance, this is why peak power claims by manufacturers are a bit misleading. Let's look more closely at between 10,000 and 11,000rpm- let's say this represents a road speed between 44mph to 50mph. We can see now that in this speed range the standard cylinder drops from 5.5 to 3hp, the Malossi Sport and Polini Sport now look worth their money dropping from just 7 to 6hp. Despite having a peak power of less than 1hp more than the standard cylinder (up to 30mph) these kits now have 3hp higher at higher road speeds and will annihilate the standard cylinder bike at higher road speeds because of this.
However even with the Polini and Malossi kits the power still starts to drop off the moment the variator has fully moved out and cannot hold the revs constant anymore. The only way to get round this is if the power curve carries on horizontally after it makes peak power... and this is exactly what the Stage 6 kits do.
If you look at the Stage 6 Sport Pro kit it not only makes the highest peak power at the lowest revs (8.8hp at 8,800 rpm) but it is also still making 8.8hp at almost 11,000rpms, generating a full 33% more power than the more expensive Malossi and Polini kits at this speed. What's really surprising is that the Stage 6 street, one of the cheapest 70cc kits on the market is able to match it at this speed.
At lower speeds the Stage 6 Streetrace kit (£102), Polini Sport (£127) and Malossi Sport (£131) are almost identically matched but the Malossi Sport slightly has the edge. The Stage 6 Sport Pro (£124) is significantly quicker than all of them.
At higher speeds (approx 30mph up) both of the Stage 6 kits will considerably outperform the Malossi and Polini kits and they will also have significantly higher top speeds. The Stage 6 Sport Pro and the Stage 6 Streetrace will have about the same top speeds but the Sport Pro kit will get there quicker (is about 10% more powerful).
There is one potential problem with these results which you may be wondering about... In our previous article we found that both the Leo Vince ZX pipe and Stage 6 Pro rep were both extremely good pipes when fitted to a standard 50cc cylinder (hence we used the Leo Vince pipe for these tests). The ZX clearly works well with these 70cc moped sports kits but it's also quite likely that some of the pipes we tested which didn't perform as well on the standard 50cc cylinder as the ZX did were designed for use on a more highly tuned cylinder and consequently may work much better on these 70cc cylinders than they did on the standard 50cc.
For this reason we ran up all of the different exhausts from the previous article on all of these 70cc cylinders. Whichever exhaust we tried, the 70cc sports cylinders all finished in the same order, so whichever exhaust we used the Stage 6 Sport Pro came out top, followed by the Streetrace and so on.
Interestingly one of the pipes which was outperformed by the Leo Vince ZX in the 50cc write up was significantly better than the ZX pipe when fitted to every one of these 70cc Sports cylinder kits.
We'll do a proper write up next week comparing different exhausts when fitted to a 70cc Sports kit.