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.

Bigbox V4 (top) and Sport (below)
External Muffler The solution – also used on the Gori, Ancillotti, Casa Performance and now versions of Ron Moss’ Avanti box pipes – is to add an external muffler to cut down on the sound, allowing more freedom with the internal box design.
The slim tail-can reduces ground clearance slightly, but it’s not as low as the downpipe of the TSR Evo
Rather than mount the muffler at the back of the engine, where it is vulnerable to vibration and hitting the bodywork due to suspension travel, BGM have used a short, larger diameter tailpipe and turned it sharply at the back of the box. This leaves the muffler underneath the scooter, aiming towards the flywheel side of the scooter.
Gas exits through a slash-cut tailpipe behind the flywheel cowling
 
Flange for fitting the tailcan features a slotted hole, allowing you to slightly vary the tailpipe exit angle
What does it sound like?

Personally, I think it sounds better than the V4 but watch the video above. You be the judge.

More fittings than you need Supplied 4-stud exhaust stub is the same as the V4. It still fits 2-stud cylinders. The supplied spring hook is great. Our test pipe was still supplied with a fitting kit for the V4 including the unneeded tailpipe bracket.
Fitting For a quick test we tried both the BigBox V4 and the BigBox Sport on Smurf’s RT195 which has been converted to reed-valve induction using an LTH reed by Scotty of Team DSC. Smurf’s scooter was running a TSR Evo modified with an additional MB rear muffler support.
With the added weight of the muffler, the lower casing bolt fitting becomes even more essential
Fitting the BGM V4 for dynoing was no trouble: running boards off, lift the cowling a little and bolt on the BGM stub. The BigBox stub offers the worthwhile advantage of four fitting points to suit RT cylinders. For the purposes of testing, the BigBox was offered up to the swan-neck bracket and aligned easily. The design is good, allowing movement of the chamber fore-and-aft to accommodate cylinders with packing plates etc. We didn’t bother with the tailpipe support or the bracket underneath the casing for a quick test. The BigBox Sport was a hand-in-glove fit onto the same stub and engine bracket and quickly fitted. The external muffler is the difference between the two versions. It adds just under 400g to the total weight of the exhaust but without a tailpipe connection to the engine casing you now have more weight supported at fewer points on the engine. As such you should regard the bolt-on bracket that fits into a thread under the engine casing as essential. If you leave that bolt out then you are asking for trouble. If the aluminium casing thread is damaged or absent (on some early casings) then get a Timesert or Helicoil steel thread inset fitted.
Choice of finish? The Sport box is available either painted silver or un-coated; which shows off the brazed main engine brackets. Dan sprayed lacquer on the plain test one to protect it but that started to burn off very rapidly. Personally, I’d get a silver-painted one just to save a job.
Tail can support The tail can is supported by a strap bolted to a bracket welded to the downpipe. It features a captive nut for ease of fitting.
On the dyno

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.

How about on piston ported engines?

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.

Pros

• 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

Cons

• 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

Verdict

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.

Sticky

Thanks to Smurf, Alan @ Diablo Moto, Dan @ Total Revamp and Scooter Center for help with this article.

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