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- Exhaust test Happy Black - Vespa PX125 with BGM177
In‑depth comparison – with dyno chart interpretation, hands‑on tips, and a clear, daily‑use recommendation for a plug‑and‑play setup.
If you want to move your Vespa PX125 from stock character towards “torquey and everyday‑friendly”, the sensible first step is a larger‑displacement cylinder. The engine gains noticeable punch and feels more lively – discreet in appearance and reliable in daily use. The bgm PRO 177 was developed precisely for this: wide range of use, pronounced day‑to‑day usability and already a convincing plug‑and‑play base.
At some point you start wanting “a bit more”. One of the classic questions is: box exhaust or expansion chamber? To put this into context for everyday riding, we invited Dana. She started – almost still a fresh license holder – with an otherwise stock PX125 engine that already had a road‑legal (now discontinued) Scorpion exhaust. To evaluate the influence of such an exhaust on power delivery and torque, we invited her and her custom scooter “Happy Black” to our exhaust test.
The video shows the dyno runs, starting with the baseline: a near‑stock PX125 engine, converted to bgm PRO 177, SI24 carb from the PX200, and the mentioned Scorpion. All measures that you can implement in your home garage with little effort.
Cylindrical/conical body for high peak power. Usually requires raised port timings, adapted inlet and often a different crankshaft. Character: late power mountain; lower down there can be a torque dip. Rather for experienced tuners than beginners.
Near‑stock appearance with internally optimized header geometry. Goal: early torque build‑up like stock, but with a wider band and – depending on the box – remarkable peak power. Ideal for beginners and anyone who wants to be discreetly quicker.
The dyno primarily measures torque at the rear wheel; power is calculated from the acceleration time of the approx. 250 kg dyno drum. The dotted lines show torque – exactly what you feel when riding. Power without torque is of little use; ideally you want an early and sustained torque rise that translates into high power at higher rpm.
Measurement note: Runs were done in 3rd gear; the km/h figures in the single graphs are not top speed.
The BigBox range has been continuously developed since 2013. The current BigBox Touring 2 (BBT2) combines very early punch with a noticeable reserve zone at the top. Result: low‑shift, touring‑friendly riding without dropping off too early at the top.
RED: bgm PRO BigBox Touring 2
BLACK: Scorpion
Compared directly to the Scorpion expansion chamber, the BBT2 offers much stronger pull without a hole, reaches almost the same peak power and maintains a wide band that clearly shines in everyday use. The Scorpion’s theoretical over‑rev is hardly usable on this setup; above approx. 7,500 rpm power is too low to accelerate noticeably in 4th gear.
Close behind the BBT2: sporty character with good bottom‑end and strong midrange. The later Polini Box adopted the BBT2’s strengths and made them accessible to a wider audience. Its slightly muted sound on small‑block engines (125–190 cc) goes along with a somewhat narrower rev range.
The near‑identical rise of power and torque makes the Polini very rideable. In the middle it feels minimally stronger, but drops off earlier on top. The pleasant sound is offset by somewhat lower long‑term durability. If you don’t ride five‑figure annual mileage, you’ll usually be fine with that.
RED: bgm PRO BigBox Touring 2
BLACK: Polini Box
Rev‑happy box focused on the upper range – interesting if the setup will be sharpened in the future (shorter gearing, raised timings). The LTH Box is clearly designed for higher flow and feels particularly at home on “worked” engines.
RED: bgm PRO BigBox Touring 2
BLACK: LTH Box Racing
The original exhaust copes surprisingly well with this setup. It delivers a strong rise from low rpm and then – consistent with its restrictive character – tapers off earlier but moderately. A valid everyday choice, but it does not exploit the 177’s potential.
RED: Scorpion
BLACK: Piaggio PX80 stock
On the PX125 + 177 base concept, the box systems deliver the early torque that “pulls at the bars” in everyday riding. Differences are mainly in bandwidth and top‑end delivery.
RED: bgm PRO BigBox Touring 2
BLACK: Polini
GREEN: Scorpion
BLUE: LTH Box Racing
For a robust everyday engine with noticeably more riding fun we recommend this coherent trio:
This set delivers – depending on overall condition – an almost doubled baseline power compared to a near‑stock PX125, remains daily‑use friendly and durable and forms a solid base that can be gradually expanded towards ~20 hp.
For Vespa PX125 tuning in the sense of plug‑and‑play, the bgm PRO BigBox Touring V2 is the strongest solution: early torque, wide band, sufficient top end – exactly what matters in everyday use. In combination with the bgm PRO 177 it becomes clear that both components were developed for each other. The BBT2 also performs very well on other 177 concepts (Quattrini M1X, VMC Stelvio, Super G, Pinasco, Polini, DR). The Polini follows closely with a sporty note and good everyday qualities; LTH is recommended for rev‑oriented concepts. An expansion chamber pays off once cylinder, timings, inlet and, if necessary, the crankshaft are designed as a system for high throughput.
Note: Dyno values were recorded on a precisely calibrated Amerschläger P4. The figures are authentic and serve orientation – not devaluation or overvaluation of individual products. Deviations on other setups are naturally possible.
