1.8 litre, multi point injection K series.

Typical installions: MGF, Lotus Elise, Rover 218is

Figure 1. All cone filter data for the MPi shown.

Apologies for the untidy illustration, but what this diagram does demonstrate is the rather wide vairation in the developed outputs from each of the cone filters available for the K series engine.

I would like to draw your attention to a feature on the torque curve (2) that is evident on all of the filter types investigated with this engine. All of the "performance" filters cause a dip in the torque curve between 3- 3,500 rpm. Whilst none of the filters tested actually robbed power or torque, it is dissappointing that the filters should result in a dipped power curve that corresponds to urban engine use. The most linear torque curve was generated by the fitment of the Pipercross Vector filter- a filter that is clamped directly to the throttle body, rather than fitted to an intermediate spacer tube.

On the positive side, all the cone filters resulted in a useful power boost- both the K&N 57i and the Ramair installations resulted in an 8 bhp gain.

 

Figure 2. Pipercross Vector Cone installation, versus the standard filter element

Adequate power gain, and an impressively flat torque curve can be seen on this illustration. This filter represents good value for money at less than 30 UKP per unit, resulting in a power per pound quotant of approximately 0.13 bhp per pound sterling.

Figure 3. Pipercross Vector cone, verses the K&N 57i kit

Both the Ramair performance filter kit and the K&N 57i kits provide very similar levels of performance. The illustration here compares a representative trace (from the K&N 57i) with that from the Pipercross Vector cone. The Piperccross Vector filter appears to have the advantage below 3,500 rpm, but after this, it's all advantage K&N! The characteristics from the power graphs can be accurately reproduced on the open road, the K&N resulting in a more freely revving engine (especially higher up the rev range), whilst the Pipercross gives a more gutsy low down power response.

Ultimately the K&N and the Ramair filters are the most efficient in the measures recorded here. The Ramair is is the more expensive of the two- 140 versus 100 UKP. Therefore the K&N represents the best value for money of the two @ 0.08 bhp per pound sterling.

Conclusions from the MPi Cone filter tests:

The K&N 57i and the Ramair filters gave the most impressive power gains, but it is the cost differential of 40% to K&N's advantage that sways the investigator's decision to the K&N's favour. The cheaper Pipercross filters represent excellent value for money, liberating 0.13 bhp per pound compared to 0.08 bhp per pound for the K&N. The Pipercross filters also result in a flatter torque curve in the low end of the rev range, that eleminates a small 'flat spot' in the standard car's power curve. This makes the final decision very marginal between the Pipercross Vector and the K&N 57i. Power junkies who drive on the open road should choose the K&N product. Those of us who are stuck in the city may find the case for the Pipercross filter compelling.

HOWEVER

There is one further important factor to note- induction temperature. For these tests, induction temperature was the same for each of the filters tested. In the real world, this is not the case. The K&N and Ramair products benefit from filter cooling from ram air ducts. The Pipercross filters do not possess this refinement. From investigations with temperature probes within the engine compartment, the Pipercross will have an intake temperature perhaps 10 celcius higher than the Ramair or K&N filters under the same, normal driving conditions. Intake temperatures this high would rob the engine of any power gain derived from fitting the filter. Bearing this in mind, the investigators would award first place to the K&N57i. For city drivers, I would advise simply excluding the spacer tube to attain similar performance to the Pipercross PX. I have done this myself, and am very happy with the results.

Read about the impressions of the Pipercross filter and the K&N 57i here.