Skoda 1.3 Engine - Air Filter improvements?

[djaychela]

My car is a 1300cc Skoda Group A rally car, with the Skoda 8v 136-type engine. Engine spec is currently quite vague (it was a running engine when I bought it, and was running on Weber Alpha 40mm throttle bodies and DTA management), due to financial and time constraints I've not been able to strip it down and make any measurements (this will be happening in the near future), but the engine made a claimed 116bhp at 7300rpm on the original setup, and went very well between 3500 and 7500rpm. Apologies for not being able to provide this information - my plan is to get it set up and mapped with the new setup and then do a planned rebuild to replace bearings, etc., as the engine runs very well but has done a fair mileage.

This engine is now being changed to Group A-legal spec with an original-spec Skoda manifold (which I am matching carefully to the ports within the Group A regs), but my main area of concern is the air filter setup. I have previously run my cars using the OEM-spec air filter box and ducting (with a K&N panel filter) and nothing else.

Original spec air filter box and duct to single throttle body.

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Original spec air filter box and duct to single throttle body, side view

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However, the Group A regs allow the replacement of the filter and ducting. I have spent some time looking for replacement filters from various sources, but the shape around the inlet flange on the throttle body makes it difficult to select a filter of any decent size - the throttle mechanism sticks up above the flange. The largest K&N I could find (and bought to check fit) is shown below.

K&N filter, the only fitment which will physically fit on the inlet.

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Front view of K&N filter

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This K&N seems to be very small to me, although the car will only be making around 100bhp. My original plan was to make up a flat "plate" type filter, but the position of the throttle mechanism meant this would be very complex. Unfortunately there isn't much height available between the inlet and the bonnet - the OEM inlet 'neck' is quite 'flat' in profile; the profile of the round flexible tubing from the air box becomes quite squashed as it approaches the right-angle turn to go down into the throttle body (I tried photographing this but none were illustrative).

Note: Pics are NOT from rally car, but from same-model recce car as Rally car is currently in bits. Rally car has straight strut-brace bar between strut towers, running parallel to rear of rectangular box on top of engine (with "Skoda 1.3 MPI" on in first pictures), further limiting air filter space.

My question is this, really - should I forget about the losses here and put it down to experience, OR spend a fair bit of time making up either a new elbow and piping to a larger 'cone' type filter or a flat 'plate' type filter box that will take a suitable larger K&N 'ring' type filter? Are there any calculations which give a rough idea of what size/area of air filter will be suitable for a given engine?

Darren

[Guy Croft]

If I read this right you are be considering a direct fitment of a K&N or similar to the intake. If you do that you must provide a seal route of ambient temp air to the filter or the result will be power loss not gain because you will be feeding air at under-bonnet temp to the engine. The hotter the intake air the lower its density and the lower the charge mass and power (that it is less dense and flows at higher velocity is outweighed by the loss of density).

It is vital to feed the air to the engine via a sealed duct and encased filter and it makes no difference whether the filter is mounted in a sealed airbox on the engine or in remote container.

I should point out that whilst the performance of the excellent K&N filters may well be superior to the OE filter element (and I stress genuine OE) in the laden (dirty) condition it may well be that in the clean state it offers no power gain at all, and from the photos it looks like the OE setup does already offer a sealed airbox with filter inside fed with ducted air.

GC