Modern Vespa

As a typical two wheel enthusiast we are all interesting in squeezing out more performances from our scooters. I found some Piaggio technical notes on Torque and HP for various Leader engines.

Document 1 in pdf format:
125 cc, Air Cooled, 2 Valves
150 cc, Air Cooled, 2 Valves (e.g., ET4, LX150, Fly 150)

125 cc, Liquid Cooled, 4 Valves
200 cc, Liquid Cooled, 4 Valves (e.g. GT Granturismo )
250 cc, Liquid Cooled, 4 Valves (e.g. GTS, Piaggio 3 wheel MP3)

There are lots of posting about how 150cc bike with Malossi 187 cc (190 cc) cylinder kit will compare with GT (200cc) and GTS (250cc). Therefore I graphed the following:

Document 2- Comparison of Torques + predicted torque for 187cc bike. For 187cc, I took the Torque for 150cc and just scaled it by (187cc/150cc).

Document 3 - Torque comparison doesn't tell the whole story because bikes weight differently. ET4 weigh 236 lbs, GT weights 308 lbs, GTS weighs 326 lbs, and MP3 weights 450 lbs. LT150 weighs approximately same as ET4. This graph is essentially Torque per (weight of bike and weight of a passenger). For a weight of passenger I used a nominal values of 150 lbs. For graphing purpose, at the end, I took the liberty of scaling all graphs by a constant number i.e., (236lb+150lb). This is just an overall constant I used for all graphs, and doesn't affect the comparison. It is like converting from metric to british or mph to kph.

Document 4. Comparison of HP + predicted HP for 187cc bike. Using similar idea as in Document 2.

Document 5. Comparisons of HP per (weight of bike + 150 lb for a passenger). Again I scaled everything with a constant.

Conclusions:
a) Granted MP3 is a three wheel, so it rides totally differently. But because of it exorbitant weight , its effective HP and Torque are diminished. Up to about 6000 RPM, its HP and Torque are comparable to those of 200cc Granturismo. After 6000 rpm, it is worse than 200 cc.

b) 187cc kitted bike should out perform both 200 cc like Granturismo and MP3 in both HP and torque for up to about 6500rpm. I estimate 6500 rpm is about (35-45) miles per hour. After 6500 rpm, the order of performance (best to worse) is 250cc like GTS, 200cc like Granturismo, MP3, 187cc, and stock 150cc

c) It is well known that 187cc kitted bike could be further enhanced by using Malossi Variator (lighter roller weights for better performance in low rpm while compromising performance in higher rpm) and more freely breathing muffler (probable helps in higher rpm), etc. I can only guess what that comparison will be like. But conclusions B and C confirm why 187cc kitted bike with performance mufflers can do wheelies while stock 200cc and 250cc can not. What would be really impressive is if someone posts dyno test for a 150cc bike kitted to the maximum.

It must be raining there too

I think you forgot to take into account the average weight of an MP3/GTS/GT rider is about 250 pounds or more. It's too bad the dyno can't also figure out the speed increase provided by the windscreen.

The problem with your comparison is that you are assuming a linear improvement in torque when kitting up from 150cc to 190cc.

The kit claims to increase your displacement to 187cc (190). But actual displacement is calculated by measuring bore x bore x stroke x .7854. Since the 190cc kit increases ONLY the bore diameter and does nothing to increase the length of the stroke, it is not an efficient increase in displacement. There will be some gains, but it will not perform as well as a true 187cc displacement engine would.

Consequently, you cannot project the torque and potential output linearlly.

The LEADER 150cc engine produces 11.5nm (8.51 ft/lbs) of torque. The LEADER 200cc engine produces 17.5nm (12.9ft/lbs) of torque. Your predicted peak torque for the kitted engine is 12ft/lbs at 6k rpm.

The kit is expected to deliver gains of up to 20% over stock. At most, the kitted engine will produce 10.5 ft/lbs of torque vs the 8.51 ft/lbs of stock.

Therefore, you need to recalculate your power to weight performance data to take into account the actual predicted performance curve.

Skrapiron can be a pebble in your shoe,( ) but there's some truth in there. Speaking as someone with a kitted ET4, I'd have to say that the GT would outpace me most times, but not everytime. Remember, most GT/GTS/MP3 riders are WAY heavier than me.
I'd love to dyno the bike one of these days though.