Racing cylinder 80-125 ccm, stroke 51 mm
for Vespa 50-125/PV/ET3/PK/XL/Ape by SIP Scootershop
Welcome to the royal class. This class includes all cylinders with 125-153 cc that are operated with a crankshaft with 51 mm stroke and more. With these cylinders, even a trip to the Scooter Run in Elba is not a world tour in the road tuning sector. In race tuning, it brings tears of joy to the eyes what QUATTRINI, FALC and Co. coax out of their engines. These cylinders dominate every Vespa racing series worldwide. Whether at the Quartermile in Stockach on Lake Constance, the 150 m races in Italy or at the lap races like the ESC in Germany or the Scootentole in France. All scooterists who use them on the road will have more victims than opponents at traffic light starts and the like.
We call these cylinders Vespa 90-125/PV/ET3/PK80-125/XL/XL2. They fit all Vespa 90/R/SS/100/125/PV/ET3 and the big PKs: Vespa PK80-125S/PK100-125XL/PK125/ETS/N/XL2.
In combination with the matching crankshaft, cylinder cover and longer stud bolts, it can also be used to power all small smallframes with 50 cc from Vespa 50 V5A1T 92877 upwards.
The more powerful the cylinders become, the more attention must be paid to the other components. Especially in the areas of carburettors, gearboxes and racing exhausts, a lot has happened in recent years. Tuning experience is therefore recommended for race-tuned cylinders. Also to get to grips with the up to 30 possible hp.
Technique tip: "Setting higher
Example: POLINI 130 with GS piston and MMW MALOSSI head
The original POLINI cylinder has timing of 170° exhaust to 110° overflow, i.e. an advance of 30° (170-110 = 60/2 = 30°) and the original piston has a compression height of 36 mm. The POLINI timing is rather boring in the original. To make the whole thing a bit sportier, the cylinder must be set higher and then, of course, at least the "higher setting" must be turned down again at the top. In addition, the POLINI has an edge at the bottom of the overflows that prevents sensible machining. Therefore, even with the original, the edge at the bottom was turned off and a correspondingly thick seal was added.
The desired example is 190° outlet to 130° overflow. In order to achieve the target timing, the cylinder would have to be raised by 3.3 mm and the exhaust pulled up by an additional 1.1 mm. "This is because a GS piston with a shorter compression height is to be used. The compression height of the GS 130 piston is 30.5 mm, that of the original POLINI piston 36 mm, the difference is therefore 5.5 mm. The starting point of 5.5 mm minus the 3.3 mm by which we want to raise the cylinder anyway and can now save, results in 2.2 mm at the bottom. Since the sealing surface at the base of the cylinder also stands in the way of a sensible expansion of the overflow, this is also reduced by 1.0 mm. This means that we have now lowered the whole thing by an additional 1.0 millimetre and we now have 2.2-1.0 mm = 1.2 mm by which the cylinder only needs to be turned down at the bottom.
In order to avoid manufacturing tolerances, i.e. that the boost port may open up during turning, turn off more at the bottom, as an example 2.2 mm, and add a 1.0 mm foot seal. If you do not want to plan the housing, turn off 3.2 mm at the bottom and include the 1.0 mm gasket. Nothing stands in the way of an outlet spread of max. 70 % of the bore, measured as chord dimension and with a sensible outlet shape.
Attention: Values are only reference values and do not replace measuring/testing on your own cylinder. Keyword: POLINI manufacturing tolerances.
Customers ask - SIP answers
"Why do I have to adjust the ignition when tuning?"
The "correct" ignition timing is a compromise between performance, heat, consumption and exhaust values. Engine tuning in most cases increases engine speeds, which requires a later ignition timing. Too late an ignition timing results in the engine not being able to "rev freely", i.e. part of the power potential would be given away. In addition, and much more dangerous, the mixture in the combustion chamber could ignite (detonate) itself in an uncontrolled manner - the famous "ringing". An unpleasant side effect of any increase in power is the increasing heat development of the engine. Due to a later ignition timing, an engine remains colder during operation.