BGM PRO 177 cc cylinder for Vespa PX125-150, Sprint150 Veloce - part 2

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bgm177 vespa cylinder development

bgm 177 - the new 177 - cylinder for Vespa

In this post we have already introduced you to the new cylinder. Today we give you a little insight into the development of the cylinder. From the beginning of April 2016 the cylinder finally available!

bgm 177 - the development of the Vespa cylinder

Developing a two-stroke cylinder is very exciting.
Countless parameters that have to be taken into account, thousands of test bench runs, repeatedly measured, calculated, tested, thousands of kilometers of road tests ...

Everything to get a cylinder as a finished end product that high-torque and willing to perform is, but also at the same time durable is and uses little fuel.
At the same time, it should offer the broadest possible range of applications for touring riders who simply combine it with series components, as well as for the sporty, ambitious tuner who is more concerned with performance.

Here is a little insight into the test series that were on the daily program for the BGM177.



We have tested a wide variety of solutions for the external shape and design of the overcurrent channels.
We created various duct geometries, control angles and cylinder combustion chambers and compared them both on the road and on the test bench.

BGM177 cylinder test


For some of the tests, in addition to an original Piaggio Sprint, we also used an LML engine housing with original inlet membrane control as a starting point.
The Sprint's motor housing is representative of the very small, original rotary valve inlet of the Largeframe Engines. The rotary valve inlet of a PX125 is only slightly larger than the inlet of the Sprint engines and the control angles of the crankshafts are almost identical. In terms of the expected output, this is the “most unattractive” starting position. Nevertheless, we wanted to know what performance can be expected with these basic requirements or, better yet, which performance can be achieved.

We presented the result on a Sprint engine with the 24mm SI carburetor of the PX200 in comparison to a 12HP 200 engine.

So that you can interpret the graph below a little better, just imagine a normal PX200. Up to approx. 90km / h something like acceleration can still be felt on a standard 200. This speed corresponds to about 5500min? ¹ of the motor. From this speed, the torque of a 200 engine drops so far that it takes some time to reach 100km / h (6000min? ¹). If you now compare these speed points of the blue curve with those of the red curve on the graph, you can see that the BGM177 is still increasing torque in this area. In other words, when the 200 slowly runs out of air, the BGM177 accelerates even further.

BGM177 vs PX200



Since the 125cc engine of the Vespa Sprint with the small rotary valve is too restrictive on the power delivery from a certain power and speed level onwards, we used an LML engine housing for various tests and comparisons. The LML motor housing offers the best prerequisites due to the diaphragm inlet that is already present at the factory and the large overflow channels in the housing. As a result, for example, an inlet that is too small or differently designed overflow channels can have no or only an insignificant influence on tests within a test series.

BGM177_9 023

So on the LML housing you can easily switch between an SI carburetor and a large, adult intake manifold.

We performed the tests in this area with a pre-series cylinder with small duct cutouts in the cylinder base
and started with a piston specially made for this test.

BGM177_9 015

The aim of the game was to find out how much space the piston must allow for the overflow duct due to its design in order to ensure good filling. The piston was manufactured in such a way that the clearance around the piston pin in the area of ​​the ÜS channels could be continuously modified by milling.
We tested each of these intermediate steps on our test bench to find out which shape or which combination works best.

Piston BGM177 V2


The test series below was run on an LML membrane motor with a 24 SI carburetor, BigBox Touring and pistons and ducts that have just been processed in different ways.
At this point in time, the cylinder head was still relatively highly compressed. With the cylinder head in series production, we have reduced the compression somewhat in favor of a broader performance curve.

In its original state, the test piston had the contour of an original PX piston. The piston skirt was closed on the red curve (similar to Piaggio, Polini, Grand Sport, ...)
In the blue curve, the piston has been exposed by a few millimeters in the area around the piston pin. The picture above shows such a piston.
The green curve shows the power curve with partially open overflow channels at the cylinder base and the machined piston. The released area of ​​the channels in the cylinder base corresponds to approx. 80% of the possible channel area that the cylinder can offer.

Comparison of the BGM177 piston and channel

  • RED CURVE: Closed piston skirt
  • BLUE CURVE: Half-open piston skirt
  • GREEN CURVE: Open piston skirt

The BGM177 can therefore be equipped with a gearbox from the 200 series engines without any problems and offers significantly better performance than the large 200 series engine.
The PX125 and PX150 motors were equipped with a gearbox from around 1982 that is almost identical to that of the PX200. With these engines, the primary drive can be extended directly in order to convert the gained power into speed.
The use of a primary gear with 64 teeth would be recommended here. Depending on the desired area of ​​application, the overall reduction can then be adjusted accordingly with the coupling pinion. The primary gear with 64 teeth offers the advantage that the bgm clutch pinions with 22, 23 and 24 teeth can be used.
This offers the possibility of changing the engine from “sporty short for the city (22/64)” to “Autobahn - everything that works (24/64)”.

In the next post we equip our LML test engine with different carburetors and intake manifolds ...



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