Quattrini M3-150-S exhaust…

Forgive them lord for they know not what they do

So, when it came time to fit the engine to our SS90 chassis we encountered a problem with the M3-150-S exhaust.

This exhaust design is specific to the M1L 60 S cylinder because it fits to the specific cast aluminium stub manifold. That manifold has been carefully shaped internally to maximize gas flow. With this design it’s only possible to use an exhaust which wraps around the back of the shock as all the Quattrini smallframe exhausts seem to do.

This routing works fine on race Vespas which use removable fibreglass rear bodies because these ‘fake’ bodywork sections are slightly roomier inside. Additionally, if you can remove the bodywork then it’s much easier to access the exhaust springs on the slip-joint in order to change a wheel.

The problem with using this exhaust layout on a full-bodywork original SS90 is that there’s so little room between the wheel and the frame. Under suspension travel, it is impossible for an exhaust this diameter not to touch. An unmodified exhaust would hit the frame over every bump until it eventually snapped.

The Quattrini exhaust is nicely made and features an O-ring seal on the slip-joint. 

Testing suspension travel revealed that the exhaust touched inside the SS90 frame.

There’s no room to reroute the exhaust as it is already close to the tyre.

Errr, no, that’s not happening. Even my battered SS90 is too precious for that.

So if the frame wouldn’t give, then the exhaust had to. It honestly felt like sacrilege to heat up the Quattrini exhaust with a blow torch and start putting big dents in it with a hammer, but there was no way we’d be able to test it otherwise. Every tap with the hammer felt like adding a moustache and bunny ears to the Mona Lisa. Eventually, we put in a big enough dent that the pipe cleared the bodywork and the scooter would be rideable.

Performance?

Before you all freak out about this, there’s plenty of evidence from the Motocross world that putting big dents in 2-stroke expansion chambers makes much less difference to engine performance than you’d imagine. A dent in the wider parts of the chamber has barely any effect on gas flow. And, depending on the position and size of the dent, it has minimal effect on the reflected pressure waves inside the chamber. Essentially, the expansion chamber still works if the internal geometry is not totally mangled.

Yes we had to dent a brand new exhaust!

First Blood

The great thing about ‘Underdog’ is that it remains insured and road legal, so it’s possible to test and set-up on the road. 

The Quattrini exhaust isn’t overly loud but I still did my best to get away from the residential areas before giving it the berries.

As expected, the 150 ‘S’ engine didn’t disappoint. There was a little area of rich carburation around 6000 rpm but once it cleared its throat it shot off like a greyhound. The power spread felt massively wide and the front wheel was suspiciously light, even in third gear. This little 150cc engine clearly eclipsed the performance of both tuned TS1s in our household by a good margin. Combined with a chassis that weighs around 30% less, acceleration was on the highly entertaining side of ‘brisk’.

On the dyno our last re-tuned 133cc Quattrini M1L-R engine made around 21.5hp with the Franz exhaust. The new 150-S engine made almost 28hp. That’s a substantial increase in performance for only a few extra CC.

Rideability

The downside of this change was a reduction in easy rideability. On fast circuits like the BSSO use, then it wouldn’t matter to lose a little low-rpm pull because you generally need as much peak power as possible. On twisty kart circuits the BSEC endurance series, it is more advantageous to have an engine that pulls cleanly out of tight hairpins in a choice of gears, depending on the grip levels available. I’m sure with a bit more work on jetting and ignition timing we could have cured the low-rpm splutter of the current set-up. But a potentially quicker route was to try the Franz exhaust first, so that’s what we did.

Dyno comparison

If you remember back to our objective from the first edition of this feature: to simply get more of the same, then the ‘Franz’ exhaust configuration (red) of the M1L 60 S perfectly hit our target.

Now we had 24.6hp where we previously had 21.4hp. That’s substantial but not the massive jump in performance the Quattrini exhaust gave. However, there’s much more to this story than the peak numbers.

Firstly, look at the low rpm part of the curve. There’s no spluttery dip in the power between 5,000 and 6,000 rpm. The Franz configuration pulls with increasingly predictable enthusiasm from very low revs, making it absolutely easy to ride whether the track is wet or dry.

Power isn’t everything

Now look at the torque curves of the 150-S with the two different exhausts. The Franz may have lost 4hp of peak power compared to the Quattrini exhaust (green), but the Franz actually makes more peak torque because power is delivered so much earlier in the rev range. On the track it will feel just as fast in terms of acceleration, but the difference in ease of use is like chalk and cheese.

This set-up ticked every one of our boxes for endurance, but it’s also clear that we can fit the Quattrini pipe if we want more power for a bigger track. With a 34mm carb instead of the 30mm we currently run I reckon it’s quite feasible to get close to 30hp with this little engine. For a bolt-together configuration with zero tuning, that really is remarkable.

Low revs = low consumption = low wear

The only negative of making the engine work so much lower down the rev range is that it requires much taller gearing to reach a similar top speed.

Crimaz supplied a suggestion for the gearing we’d need based on the spec of the motor as he knew it. With peak power at around 10,500 revs, we used a 72-tooth primary gear behind the clutch. His suggestion worked perfectly for the Quattrini exhaust, but with the Franz making peak power at only 8,250 rpm we needed to change not only the front primary gear cog, but also swap the one under the clutch for a 69-tooth.

Ordinarily, swapping both primary drive cogs would mean a complete engine strip, but thanks to the Crimaz system we were able to do it with the engine in the scooter without difficulty. The decision to use this primary drive system had already paid off in the hours it has saved us.

Theory into Practice

The results speak for themselves. Our new engine performed faultlessly throughout its first race, coming home to win comfortably several laps ahead of the closest competition. It still wasn’t the fastest engine on the track but some of the faster ones didn’t make it to the end of the race.

What it did do was absolutely hit our target of adding enough power to stay with the faster scooters on the straight while still remaining reliable and easy to ride. It’s difficult to compare races, as the last one was mostly under the rain, but I estimate that the new motor probably knocked a couple of seconds of all our riders’ lap times. Considering we’ve only gone up around 11cc in engine capacity, that’s quite a transformation.

Words & images: Sticky