BGM have released a new version of their popular BigBox Lambretta exhaust called the Sport which has an external underslung silencer. Is it actually Sportier than their V4, or is it just a name?
While testing Kevin Becker’s KillerCase Simonini 260 for my Lambretta Kit Book (on sale now in the SLUK Shop and at Scooter Products) I was offered rides in two formats: firstly with a TSR Evo expansion chamber, and secondly with an evolution of the popular BGM BigBox.
By this stage, the Bigbox was already in its 4th incarnation, with changes arriving to both improve quality and cut down in noise.
The challenge that all box-pipe manufacturers face is a direct balance between noise and performance. If the exhaust has a freer-flowing design and a larger diameter tailpipe then it becomes uncomfortably noisy for those looking for standard-ish looks and sound.
If you redesign the box exhaust internals to cut down on the noise then the usual result is that performance suffers a little. As such the BGM V4 – while probably the most robust version – is not the most powerful.
Personally, I think it sounds better than the V4 but watch the video above. You be the judge.
BigBox V4 results on the dyno were very positive, with the BigBox Sport making more power and torque even earlier in the rpm range. Between 5,600 and 7,200rpm there is a consistent gain of 1 to 2hp which is a 10% improvement in power exactly where you need it. The only deficit of the Sport is that it signs-off a little earlier, reducing over-rev compared to the V4. Conspicuously both versions maintain the odd double-humped curve that earlier versions of the BigBox didn’t seem to have. The dyno curves suggest that this new version will probably produce similar improvement over the V4 on any kit with moderate exhaust port timing, but I suspect that it still won’t work that brilliantly on higher-revving kits with exhaust timings nearing the 190-degree mark.
We also tested the BigBox Sport (red) on a piston-ported engine (RT 195) and unsurprisingly it offered similar advantages over the BigBox V4 (blue). It makes more power lower down the rev range and slightly curtails over-rev.
• Nicer, bassier exhaust sound without being too loud
• Improved power and torque (approx 2hp) across a wide range of engine rpm on the engines we tested
• Excellent bracket design, fit and adjustability
• Maintains the box exhaust advantage of requiring zero bodywork modifications (no cutting of rear runner board)
• Fits all LI-series models
• Still one of the few exhausts designed to fit with the 4-stud exhaust port layout of the BGM and forthcoming Gori aluminium kits
• Likely to work better with larger capacity motors than the V4
• Suits kits with low-ish exhaust port timing aimed at touring
• Fitting kit includes an excellent spring puller
• Heavier than the BGM V4
• Now only supported in three places (cylinder, swan neck and under engine bolt) rather than four (also the tailpipe) with the V4
• Too early to comment on durability
• Not suited to kits with lots of exhaust timing (e.g. Casa SS)
• TS1-type stub no longer included in package
Without giving this exhaust a long road test it’s too early to comment on its durability. In every other respect (performance, style and sound) it looks very promising.
We’ll keep you updated on progress as we put some miles on our sample and we’ll also be testing out the Quattrini version when that arrives.
Thanks to Smurf, Alan @ Diablo Moto, Dan @ Total Revamp and Scooter Center for help with this article.