The V4 BGM Big Box on Loz's RT205
The V4 BGM Big Box on Loz’s RT205


With the release of Version 4 of the Lambretta Big Box exhaust, have BGM finally made the best Clubman-style exhaust on the market?


SLUK is the first scooter publication to check out the changes as we stick three versions of the Big Box on the dyno to compare them.


Text and images: Sticky


V1, V2 and the brazed V4 on right
V1, V2 and the brazed V4 on right


Why a Clubman?


Here at SLUK we do actually ride scooters on the road, so we like to think that we know what we are talking about. I’ve personally ridden thousands of miles with BGM Big Box exhausts and intimately understand the benefits of a well-designed box pipe. Namely:


  • Original looks (no fat mufflers)
  • Original sound
  • Good ground clearance
  • No bodywork modifications required
  • Broader spread of power than most expansion chambers


A specific expansion chamber exhaust will often better a box pipe, but usually in only a narrow rpm-range. What you lose in peak power with a Big Box you gain in rideability. Round-town riding is effortless, but there’s also enough over-rev for the engine to hold high speeds in optimum conditions.


It’s very little surprise that so many Lambretta riders are switching to using this style of exhaust.


In my experience, I really liked the performance of the Big Box, particularly when combined with the BGM RT kit, however my first V1 exhaust fell to bits on the Tre Mari in Italy.


Currently I still have a V3 Big Box on my SX which I used in Ukraine in 2015 and remarkably – if you saw what they called roads – it still survives intact.


Lambretta riding in Ukraine in 2015 using a BGM Big Box exhaust (some swearing!)


Exhaust Fitting Tips


Here are a few SLUKing tips we’ve picked up over the years if you are fitting a Big Box (or other Clubman exhaust) and want it to last.


  • Don’t miss out any fasteners. The tailpipe bracket and the bottom bracket bolt that go into the casing are all vital to the structural integrity of the exhaust.
  • Check frame clearance. If any exhaust touches the frame under suspension travel then it will break. Either adjust the fit or the shock length or send it back.
  • Don’t always blame the exhaust. Shortened or raised cylinders, or some kits that have been manufactured with exhaust ports at funny angles all present a challenge for exhaust fitting. The adjustability of the Big Box design actually makes it one of the best Clubman designs in this respect.


Chassis clearance was fine on Loz's lammy, but frame dimensions vary more than the exhausts do.
Chassis clearance was fine on Loz’s lammy, but frame dimensions vary more than the exhausts do.


What is new?


The biggest change is a new European manufacturer utilising new production methods, but there are also many small detail changes.


The main objective has been to improve build quality and fit while reducing noise and retaining the same performance level which has proved so popular.


Details include:


  • Wider main bracket attachments
  • Brazed rather than welded brackets
  • Revised silencing
  • Redesigned brackets to increase up-down and fore-aft adjustment
  • Easy-fit threaded main bracket retainer instead of nuts (like V3)


Standard Big Box features include:


  • Unique, 4-stud fixing flanges supplied to suit standard or RT barrels
  • Supplied TS1/Monza/Imola flange
  • Greater adjustability to different barrel heights compared to any other Clubman exhaust.


Bracket area been massively expanded on the V4. Don't forget the bottom bracket bolt!
Bracket area been massively expanded on the V4. Don’t forget the bottom bracket bolt!


BGM big Box Spotter’s guide


V1: Shortest tailpipe, welded fixed nuts on main exhaust bracket, all welded construction.


V2: As V1 except longer tailpipe, larger opening into downpipe with ‘top hat’ sleeve on exhaust flange, all welded construction.


V3: As V2 but with simplified downpipe with no ‘top hat’, no welded nuts on main exhaust bracket, instead a double-threaded plate is used, all welded construction.


V4: As V3 but now with new ‘swan neck’ bracket with vertical slots, brazed construction.


V1 downpipe.
V1 downpipe.
V1 & V2 welded nuts. V3 & V4 use threaded plate
V1 & V2 welded nuts. V3 & V4 use threaded plate
V2 downpipe and Top Hat flange
V2 downpipe and Top Hat flange
V1 V2 and V3 use the left Swan Neck
V1 V2 and V3 use the left Swan Neck


How does it sound?


We dyno tested three of the four Big Box versions on Loz’s long-stroke small-block RT195 engine (62mm stroke GT crank giving 205cc).


Of the trio, the V1 was both loudest and tinniest (the baffles had started to come loose) and the V4 was the quietest and bassiest of the three. Listen to the video below…


VIDEO | BGM Big Boxes compared


What about performance?


Unsurprisingly BGM Big Boxes tend to work really well with BGM RT kits, standard engines or any piston-ported cylinder with moderate exhaust timing (e.g. below 180-degrees).


For reedvalve engines with wilder exhaust timings you will get much more peak power from a fully-tapered Gori GP, but these sacrifice the low-rpm punch of the BGM to attain power at higher rpm.


We are aware that small internal changes made during production has had some effect on production, so there are slight differences between the versions, but no greater than we’ve seen between different examples of the same cone-formed expansion chamber.


In our test:


  • V1 produced marginally the widest spread of power.
  • V2 produced the best mid-rpm performance, but sacrificed some over-rev.
  • V4 produced the best peak power but suffered a commensurate loss at 5,250


Essentially, there are slight variations between the versions but the behaviour is identifiably all from the same ‘family’.



BGM Big Box test1


Why a Version 4?


That’s a good question, and one we put to Ulf Schroeder from BGM:


“We stopped selling the V3 in 2016. We were still unhappy about exhausts falling apart so we re-started the project from the beginning.


  “Mark Broadhurst originally designed the MBgm exhaust looking into every aspect and the result was the the V1. The loose downpipe fixed with springs should absorb many vibrations (at least in theory) and it is without doubts the best box pipe on the market, making maybe not the highest power but is the most enjoyable one to ride giving power everywhere.


“The prototypes he designed were very quiet and had no problems falling apart. However it is a different story to let an industrial maker produce big volumes and sell big volumes to all kind of customers with different skills and feelings for their bikes .


“We joined the project and looked for a manufacturer who would be willing to produce small batches of 300. Europeans were either not interested or asked silly money for the tooling so we went to Taiwan and found a company who made Vespa exhausts for 20 years that many people in Europe sell without any problems.


 “Once on sale we started to get some returns. Every return we analyzed, discussed with Mark and tried to improve with the next batch. I can’t count how often we went to Taiwan to discuss with the producer. Still we got returns and we saw exhausts that had cracks like a cobweb.


“We worked on the bracket and Alex designed a way people can fix the exhaust without force moving it in every direction. We discussed with people working in the car industry about the force on the bracket and the vibrations and redesigned the front bracket plates as well (Version 3) and the returns went to  a level we were really happy with (about 1%).


“What we were still not happy about was the noise. Some exhausts were okay, some quiet and some too noisy. We tried to find a professional noise designer but all refused because a two stroke exhaust “is a strange animal” just as Tino Sacchi once warned us.


Ulf from Scooter Center (right)
Ulf from Scooter Center (right)


What is different?


“Okay, we went back to school and started again: what should have taken only three months took nearly a year and many visits to Spain and discussions with the actual exhaust manufacturer that is in the business for years. He put all his experience in the project and the result is the V4 that is on sale now. The exhaust has a different muffler design that makes it quieter and the production process is different using brazing in many areas instead of welding. The brackets have been redesigned and a strengthening plate is fitted under the front bracket. The strengthening plate between the exhaust and the rear tube is redesigned and brazed. This brazing process takes much longer and is much more expensive than welding but hopefully worth every penny.


“Every exhaust that is sold by official BGM dealers has got a guarantee of one year from the day printed on the receipt. If anyone has problems with their exhaust within the first year they should get in touch with the dealer they bought it from. They will analyze first if it has been fitted and used correctly. If it is a quality issue then the exhaust will either get a quality repair or will be exchanged.”



Were there metal quality problems with the V1 to V3?



“Concerning the metal and the spider-webs we sent off the steel to the Technical University in Aachen because we thought it might have too much carbon inside. The result was they told us that the steel is totally okay.


“They are using SPCC steel so we suspect it could be a problem that the welding is done with too much heat. If you look at the colour of the steel on the V1-V3 the colour has changed in all areas where it is welded not only in small spots but big areas.


“The Spanish guys braze it and even the two big center sections are now in normal steel colour after welding and not brown as the Taiwanese ones.


“I never saw these spiders-web cracks on the V3 made by the same Taiwanese company. We insisted to take more care on the welding and use less heat so maybe already that was enough to not make the steel brittle.


“I think with the V4 and the new production process the spider-webs should belong to the past.


Buying from SLUK Shop supports this site

  • 1
  • 1