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I was talking to a well respected Yorkshire scooter tuner and dealer today and mentioned that I was thinking of putting A BGM on my Rapido 225. He said not to as the tail pipe is too small and stifles the top speed on capacities above 200cc. They modified one on a customer's bike with a bigger pipe and that cured a "loss of 10 mph from the top speed".
Anyone heard of this or had similar problems? I thought they were a good product and ,no, he wasn't just saying it to get me to buy one of his pipes.

Heard similar & watching the TS1/BGM big bore clubman thread with bated breath... Lucky I wasn't holding my breath as I would have been dead months ago... So, If I understand correctly what you're saying is that the back end of the box has to be cut open & a larger diameter tail pipe mated on to realise the capabilities of the top end...?

Makes sense to me. Lots of gas to move per second with higher capacities and revs.

Back pressure increases with square of gas velocity so it's more significant than you'd guess.

Ron moss uses 25mm tail pipes. I'm pretty sure the new gori gp50 has a 20mm though and that revs very freely. There are some graphs on a thread about clubmans.

PSC GP200 » Fri Jun 17, 2016 10:52 pm
I was talking to a well respected Yorkshire scooter tuner and dealer today and mentioned that I was thinking of putting A BGM on my Rapido 225. He said not to as the tail pipe is too small and stifles the top speed on capacities above 200cc. They modified one on a customer's bike with a bigger pipe and that cured a "loss of 10 mph from the top speed".
Anyone heard of this or had similar problems? I thought they were a good product and ,no, he wasn't just saying it to get me to buy one of his pipes.

As you happen to ask:

This is good to read as it confirms what has been stated previously on this Forum & probably read by some with cynical disbelief.

Bearing in mind that most Lambretta enthusiasts would agree that racing has been @ the forefront, generally, for achieving increased power from the engine, you can imagine that the tailpipe diameter is a fairly critical consideration. There has to be a trade-off between power/torque & reliability by ensuring that heat build-up is not caused by too small a diameter.

Having said that, the diameter gets to a point where it is not only the cause of heat build up but restricts power/torque as well!

I can recall a ‘top tuner/rider of the 1970’s @ the time advocating precisely the size I would consider unsafe nowadays! They were ‘different times…’

On a reasonably set up large block engine with a kit, bigger carburettor & efficient expansion chamber/Clubman type exhaust, I would imagine Ø22 mm bore to be restrictive. Yet, I hasten to add that is my own opinion based upon experience

An inch bore or Ø25 mm would be a much safer option.

Im no expert and this is my experience only.
I had a BGM clubman, 28mm open mouth delly on my Rapido Race 225. Gp200 box 19-46, 15st on board with the wind resistance of a small galleon.
Top whack 68 GPS , but it was easy to ride, quick up the gears and when others were slowing down uphill, mine didnt drop out of the rev range.
I always felt that were was more in the engine so I changed to a PM25, gained 8 MPH but lost the bottom end grunt and it needs to be kept in the rev range up hill.
so it really depends on how you want it to perform.
I have since fitted an aftermarket reed & TS1 piston to improve fuel efficiency, which cleaned it up and improved the bottom end but at the cost of a little bit off the top speed.
Mine has been changed by iterations without an overall plan, my advice is to decide what you want , find one tuner and work with them to get a set up for you.
But its been fun getting here.

I completely agree that it depends on what spec of motor you're running on but my experience of running club mans on 225s suggests that the internal baffle layout is more of a concern than tailpipe or u-bend. If the gasses can't get through the baffles fast enough then tailpipe size is irrelevant. I've bought a BGM clubman to try at some stage but can confirm that the wrong type of clubman will result in heat building up and causing issue. I run a big cc motor on a Ron Moss clubman, which would quickly suffer heat buildup with a 48mm Af Clubman (older uk made version).

That said I have a pal who runs the cheap and cheerful AF clubman on a low revving/touring iron 230 and that rallies without issue. I would expect the BGM to allow considerably greater volume of gas out than the AF.

As said, a move to an expansion may benefit but that all depends on what result/ride you are looking for.

Adam

Adam_Winstone » Sat Jun 18, 2016 10:07 am
I completely agree that it depends on what spec of motor you're running on but my experience of running club mans on 225s suggests that the internal baffle layout is more of a concern than tailpipe or u-bend. If the gasses can't get through the baffles fast enough then tailpipe size is irrelevant. I've bought a BGM clubman to try at some stage but can confirm that the wrong type of clubman will result in heat building up and causing issue. I run a big cc motor on a Ron Moss clubman, which would quickly suffer heat buildup with a 48mm Af Clubman (older uk made version).

To add to my earlier comments & clarify, whatever is the ‘bleed’ on the system, whether baffle plate or tail pipe, the CSA (Cross Sectional Area) needs to be considered.

I had in mind the ‘usual’ arrangement of a ‘Clubman’ type exhaust where the internal end of the tailpipe is supported by the baffle plate.

In my ignorance, I am unaware of any other method that might well have a baffle that somehow incorporates such a small CSA as to be the restrictive bleed.

^... quite right. The issue that many seem to overlook is that expansions will normally have no internal plate baffle so a like for like tailpipe diameter comparison doesn't take into the account the added restrictions (to speed of getting the same volume out).

Adam_Winstone » Sat Jun 18, 2016 2:28 pm
^... quite right. The issue that many seem to overlook is that expansions will normally have no internal plate baffle so a like for like tailpipe diameter comparison doesn't take into the account the added restrictions (to speed of getting the same volume out).

Around Northamptonshire, it was ‘de rigueur’ to cut open an exhaust & remove the baffle plate. Even on a standard exhaust, the ‘seat of the pants’ dyno indicated a little more top end was available, but they seemed to get there quicker

My first ‘competition exhaust’ was an old Ancilotti ‘cut & shut’ to suit a Ø 50 mm pipe (too big on a 200 back then, I know!) but with a one inch conduit tailpipe. Obviously, very free flowing, it would probably work very well today on a 240 ish with good transfers….