Venturi comparison
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Many people who are interested in the performance of their Vespa engine ask themselves: What else could I do to give my Vespa engine a bit more oomph?
One simple adjuster screw is the shape of the air intake into the engine. The standard SI carburettor comes fitted with a small, fine-mesh air filter, which not only keeps out dirt but also significantly restricts the flow of air into the engine.
There is a solution: following POLINI’s lead, many other manufacturers have realised that the intake geometry can be improved using simple methods. Whilst air ‘with an Air Filter’ has to struggle through the mesh, ‘without an Air Filter’ turbulence is created at the edge of the Carburettor, which narrows the effective cross-section of the Carburettor. The Venturi Air Intake System represents the ideal solution, as it reduces turbulence and thereby allows the full cross-sectional area of the carburettor to be utilised.
We asked ourselves: just how effective are the Venturi intakes we have in stock for you? They range in price from around 30 to 199 euros. We wanted to know whether such a wide range also applies to performance.
This video shows a simple comparison: we’ve received samples of six different Air Intake Systems and are using a test bench to measure whether there are measurable differences in performance between the individual models, or whether these differences are negligible.
The following samples are up for consideration:
SIL-TECH Venturi for SI 24, retail price at SIP: 26.90 euros; made from 3D-printed plastic with a fine-grained, matt finish; two Screws included in the pack, which cannot fall out.
VESP-TEC Venturi for SI 24, retail price: 29 euros; 3D-printed, surface with visible layer lines; one separately encapsulated Screw included.
VESP-TEC DRALL-turi for SI 24, 3D-printed. The spiral-shaped guide vanes are designed to impart a swirl to the mixture. The kinetic energy of the swirl theoretically alters the kinetic energy of the gas column – it will be interesting to see if we can measure this.
MRP Venturi for SI 24–26, RRP: 51 euros; two Screws included. Due to its very similar design, it also serves as a substitute for the Venturi tubes from PINASCO and POLINI.
LLVR Venturi Suitable for all SI carburettors from 20 to 28 mm, RRP €39; made of aluminium, black anodised with laser-etched logo and 1 Screw included, side-encapsulated.
Brand new: CTP / Vesptec Joint development ‘Spacetec’; versions available in 3D-printed plastic and milled aluminium; prices range from €45 to €199 (not shown in the photo above)
At first glance, it is clear that the Venturi – that is, the funnel-shaped section – is designed very similarly across all prototype parts. Only the DRALL-turi stands out slightly.
Measurement procedure
1. Original air filter – the restrictor:
First, on the test bench, we unscrewed the standard Air Filter from the SI Carburettor to check whether it really does make it so difficult for the intake air to reach the Engine. The result is clear: simply by removing the Air Filter, power immediately increases from 19.9 to 22.2 PS
2. Do Venturi tubes really boost performance?
The second step was to prove that the simple Venturi funnels do indeed deliver more power and that the Venturi effect has a measurable impact.
The result: even with the weakest Venturi, the engine gains over half a PS – that’s 2 to 3 per cent of the total power output, which is therefore clearly measurable.
3. A comparison of all Venturi systems
Based on these measurements, we can already draw some initial conclusions and identify the strongest and weakest Venturi tubes. In summary, however, it can be said that all the Venturi tubes are virtually on a par. The difference between the highest and lowest readings is just 0.2 PS. This corresponds to only about one per cent and is therefore right at the limit of measurement accuracy.
Incidentally, the winner was the Venturi from SIL-TECH. This is the only one we know of whose flow geometry was actually evaluated using a computer simulation …
The DRALL-turi, which VESP-TEC kindly provided us with, showed no measurable difference in performance compared with a standard Venturi with a smooth surface.
Otherwise, there are hardly any significant differences between the performance curves. Instead, the Venturis differ primarily in terms of material, mounting and price.
There are both 3D-printed Venturis and aluminium versions available. It is particularly important to secure the rear fixing screw. With earlier Venturis, the screws could sometimes come loose due to vibrations and fall into the Intake Tract. This is by no means a rare occurrence and can result in irreparable damage to the pistons, Crankshaft and Crankcase. Many Air Intake Systems now feature locking mechanisms designed to prevent this risk.
4. A venturi without a Cover looks a bit silly
To round off our comparison test and be able to make a valid recommendation for real-world riding, we also tested two caps as examples. This is because, with an open Carburettor Casing, dirt and moisture can get inside the engine.
PIAGGIO originally supplied metal covers in various heights. Nowadays, numerous manufacturers offer large-volume ‘Airboxes’, which are often produced using 3D printing. Many tuners confirm that the larger the volume of an Airbox, the more engine power it promises.
With the large carburettor casing cap – the ‘Airbox’ – the engine is admittedly not quite as powerful as it would be with a completely open intake, i.e. without any cover and with an open venturi. However, compared with the original Air Filter and metal cover, which produced 19.1 PS, the Airbox still delivers 21.8 PS on the power curve. This represents a substantial power increase of 2.7 PS, or 14 per cent, compared with the original components.
5. The effect of the frame intake rubber – the snorkel to the chassis
One detail that is very often questioned is the influence of the ‘frame rubber’, i.e. the intake rubber or rubber bellows between the Carburettor Casing and the Vespa chassis. So, naturally, we have measured this influence for you as well. The surprising result: there is no measurable difference. Both power curves peak at 21.8 PS, and the torque figures are also almost identical.
For comparison, we used the large-surface-area, high-quality SIP ‘Sock’ Air Filter, item no. 50383000. This can be flanged not only to drum and flat-slide carburettors, but also fits perfectly at the front of the Carburettor Casing on SI carburettors.
6. VMC Airbox
The configuration comprising a Venturi, a large Carburettor Casing Cap and a filter mounted at the front of the Carburettor Casing is very similar to the innovative VMC Airbox. This consists of two generously sized cast aluminium components and also features a small Filter inside, as well as a venturi-shaped taper.
The result can be summarised quickly: the VMC Airbox performs practically just as well as a combination assembled from the components mentioned above. In terms of measured power, it is just 0.1 PS higher.
Large ‘Airbox’ Carburettor Casing Cap and classic Air Bellow to the chassis
Conclusion
The original SI carburettor fitted to the Vespa large-frame models undoubtedly enables remarkable engine performance. An aerodynamically optimised intake system is a key step towards achieving high power output. In our test, the difference between the worst and best set-ups was a good 14 per cent.
If you compare the power gain of 2.7 PS with the cost of around 80 euros for a basic Airbox and venturi kit, this works out at around 30 euros per PS. Compared to many other tuning parts, that’s a real bargain. And best of all: it works well even on standard and lightly tuned engines.
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