Rover SD1 Snippety Bit - Why is the Mouth of an Efi Air Filter so Small?

Or - Is it really a Good Idea to fit a K&N style Air Filter to a Rover SD1 Efi?

Dave said: I've long wondered why the first orifice into the Rover SD1 Efi air filter is so small?

Ramon replied: Me too Young Sir, but giving it some grey matter, it now seems glaringly obvious.

To put things into context check out the images (not quite to the same scale - but you get the idea) confirming that the diameter of the smallest part of the bell mouth of the Rover SD1 Efi Air Canister is much smaller than the diameter of the rest of the Single Plenum input tract including the AFM hose (shown), the AFM to plenum hose and the plenum intake tunnel. In the case of the Twin Plenum engine intake tunnels, even twice as small again. Hence the question, as to Why?

Whatever the volume of air required by the RV8 engine, from idle to full load, the flow of air must be relatively unhindered because its a "Big Sucker". To check out the suckability of your Rover SD1 Efi engine, place the palm of your hand near the air intake at high RPM?

Stop Press

A Major Review of Rover SD1 V8 Electronic Ignition System

Description & Analysis Here:

Components & Testing Here

Without even attempting to challenge the traditional use or purpose of a bell-mouth entry tube, but simply noting that because the inlet diameter is smaller then the air must move faster through what is effectively an orifice tube, than through the rest of the intlet tract. At the same time the pressure inside the inlet tube increases.

Additionally, due to the tapered shape of the inlet tube, the air also slows down gradually with less pressure until it has the opportunity to expand rapidly when the in-rushing particles meets the much larger interior of the filter canister.

This has two immediate advantages. 1) the highest pressure is at the bell mouthed entrance to the air filter and this helps to push air right through the intake system whenever an inlet valve is open. and 2) As air pressure falls immediately it enters the filter canister, so does the air temperature. (Our Aircon systems do the same when high pressure refrigerant passes through an orifice tube resulting in a dramatic fall in temperature.)

The same function also increases the dew-point of the air inside the filter enclosure so it become damper and more dense just before it enters the Air Flow Meter. Both these conditions - Cooler and Damper air maximises the number of air molecules measured by the AFM which in turn favours improved engine performance. Such phenomenon is commonly observed when engine performance noticicably improves in stormy, wet or cool climatic conditions.

Taking this collection of thoughts one step further, if such such deliberate design decisions were intended to maximise Rover SD1 Efi engine performance, then by dumping such original equipment hardware in favour of (so called) after market improvements such as a K&N style air filter could well be the equivqlent of favouring snake oil ahead of traditional medicine. Hence the alternate title to this piece .

Is it Really a Good Idea to fit a K&N style Air Filter to a Rover SD1 Efi?

Indeed, I do seem to recall a forum conversation somewhere in a parallel universe, someone was bemoaning the restrictive effect, or was it just plain lack of performance, from K&N's. Just goes to show, the Old Time designers knew a thing or two?

Mind you, The above is only valid if one is convinced by the rough science and if one wants to believe that even the design of such a simple thing as a bell-mouthed air inlet tunnel was deliberately arrived at, rather than throwing together a random collection of available components from the left-overs bin of previous marques

And even if one is a believer, it's also entirely possible that a pinch of salt could be appropriate but to be sure in your own mind, why not read through the above for a second time and then tell me it does not make sense. Otherwise also feel free to contact me regarding Errors and Omissions.

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Ramon Alban


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