Please note: These pages are part of my RS 125 Workshop Page on my web blog and they are not related to the manufacturer Aprilia/Piaggio. These are self-written repair instructions, hints, tips and tricks for the old Aprilia RS 125 with two-stroke engine until year 2012. This blog is operated by a private individual without commercial background and without any benefits. There is absolutely no warranty. Any liability for any kind of damage that may be related to information from this website is totally denied. Use at your own risk.

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There are several issues on the airbox that can be changed or have to be considered:

  • Air filter mat pressure drop
  • Air intake hose pressure drop
  • Air box resonance
  • Intake noise

There are totally different approaches and opinions on how to tune the air intake system, either the stock airbox gets optimized or everything gets changed to achieve maximum air flow. The reduction in pressure drop can be noticed at very high engine rpm (the pressure drop increases approximately as the square of the volume flow). The overall performance increase is still quite small, there is no magic trick to achieve lots of hp here.

Please note: Changing the air intake or air filter might void your vehicles license, log-book or registration, depending on your local laws. In the European union, any unapproved changes made on the air intake system without proper certification will void the homologation of the bike.

Replace filter mat

You can optimize the original air filter by replacing the filter mat with a filter mat with a lower flow resistance. Suitable air filter mats are available from TwinAir, Polini or Pipercross, and other manufacturers also offer universally suitable filter mats. The air filter mat installed as standard also actually does its job very well (subject: maintain standard condition).

How the airbox works

The air filter box is a dynamic system with a resonant frequency in which vibrations in the air filter are intentionally used. As soon as the amount of gas flowing to the carburetor decreases after the engine intake stroke, the moving air mass in the intake hose presses on the air volume in the air filter box due to its inertia, thereby increasing the pressure for the next intake stroke. This is called a resonator. By using those effects, energy which would otherwise be lost as noise is used and the overall noise of the bike is dramatically reduced.

The intake sleeve therefore absorbs the noise by being part of the resonator. The whole air filter box can be described approximately using the formulas of a Helmholz resonator. The resonant frequency f is calculated from the speed of sound c, air filter volume V as well as length L, hydraulic diameter d (of one pipe) and total cross-section A of the intake sleeve as follows:

f = c / 2 / π ⋅ sqrt(A / V / (L + 0.8 ⋅ d))

This shows how the resonant frequency f, at which the effect is effective, changes: Both increasing the cross-section A of the air intake hose and shortening the length L will lead to an increase in the resonant frequency, i.e. to a shift in engine charging by the resonator toward higher rpm.

Removing the intake manifold completely (“desnorkeling”) is not recommended, as the bike becomes noticeably noisy and runs significantly worse; power losses of approx. 1.5 kW (2 hp) was measured with a dyno run [1]. It also sounds kind of ch

Optimizing the stock air intake hose

The stock air intake hose has two openings with a total cross-section of about 690 mm² at the smallest point. The smallest cross-section in the 28 mm carburetor is 615 mm², but in the 34 mm carburetor it is 908 mm². For the 34-mm carburetor, it makes sense to modify the intake sleeve for the high rpm range.

The cross-section can be increased quite easily by cutting off the front part of the air intake hose and inserting the air intake hose into the air box the other way around. The smallest cross-section is then about 1200 mm², the air mass in the intake sleeve is thereby somewhat reduced but still present.

As already described, this action causes the resonant frequency of the air filter to increase sharply, and there may be a noticeable loss of power at low engine speeds.

Third party air intake hose

Some customizers have now produced their own intake hoses by using a 3D printer, which can be used instead of the original intake hose; these intake hoses are available from Abstract Racing or Morley Designs, for example. These intake sleeves have an extremely large cross-section.

Hartrusion air intake hose

I also developed an air intake hose myself, also made for 3D printing. It consists of two pipes like the stock part and uses two intake funnels at the inlet. The cross-section of each pipe is 680 mm², providing a total cross-section of 1360 mm². The shape is curved due to the small installation space at the air filter, in order to be able to draw in the air without any obstacle even when the fuel tank is lowered.

This air intake hose tries to keep the resonator frequency as close as possible to the frequency of the stock air filter. Since the cross-section is much larger, the length of the hose had to be increased accordingly, which was generally difficult in the limited installation space.

The strange-looking shape is due solely to the attempt to make optimum use of the available installation space without necessitating work on other parts.

The intake sleeve must be printed using flexible material such as TPU (softer than 95A) and can be intalled plug & play into the air filter cover, without removing it.

The length of the hose means that the intake noise is quieter, and initial tests also confirm that the resonator function continues to work, with good power delivery in the low and medium rpm range.

You can download the STL file for printing here:

The small flat surface on the inner end has to be placed on the printer surface, those flaps and the end of the funnels have to be supported with some support structure.

It is essential to use a very soft TPU for printing, simple TPU filaments with 95A hardness are still too hard. Usually a printer with direct extruder has to be used. You can find my print settings with a possible filament in the Anycubic 4Max Pro 2.0 article.

Bibliography

[1] EEKNOWS, “Airboxes R vs SP,” TZR Forum, Jan. 21, 2013. http://pure2strokespirit.net/forums/index.php?topic=512.msg5023#msg5023 (accessed May 29, 2021).

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2 Comments

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  1. Resonance calculated for standard inlet is about 3800rpm while it gaves good result at max power. do you know why it happens?

    1. Hi, first of all, I’ve not checked what I’m going to write now so it might be wrong. Resonance means the system will accumulate all induced forces or pressure waves to oscillate on that frequency. There will be a phase shift in higher frequency between pressure frequency coming from the carburetor to the frequency of the resonating air box, this creates the effect which is needed here. But there will also be a lot of practice and try & error