Hi guys,
Thought I'd just stick up a little post about the CDA and sealed filters in general, we feel the CDA is by far the best sealed filter we have had experience of and it stands head & shoulders above the rest. Here's why:
How do they work?
The theory behind all sealed induction kits is the same in that they take air directly from the front grill (or wherever the intake pipe is mounted) rather than from the engine bay.
The intake air therefore is much cooler than it would be if taken from the engine bay (we all know how hot our engines / engine bays get when we're giving the car a good thrashing).
Colder air has a greater density which means more fuel can be injected (automatically adjusted by your ECU) so therefore there is effectively more fuel to give a bigger explosion in the combustion chamber which means ultimately more power output.
A cone filter by comparrison (even one protected by a heat shield and fed by cold air routed from the bumper) will suck in warm engine bay air which has less density and therefore will result in less fuel injected and less power from the combustion.
Size is everything
By their nature, sealed induction kits can easily be restrictive - this is more likely than with a standard cone filter as the intake air has a lot further to travel to reach the engine. If the filter cannot flow enough air for the engine, the engine will end up having to 'suck harder' to get it's required air which will more than likely result in a power loss rather than a power gain.
So the diameter of a sealed induction kit is very important - image trying to flow water through a hose pipe while somebody else flows it through a drain pipe, it's obvious which will flow the most water.
The BMC CDA we have gone for has a diameter of 15cm and to be honest it only just fits in the engine bay so this is pretty much the biggest size you could go for, so the maximum amount of air possible is flowing through the filter body.
BMC do another CDA which has a diameter of only 13cm and this is suggested for fitment to 1.6 engines or less - so that 2cm does make a BIG difference.
(according to the figures on the picture above d_stroy_r, presumably that filter is 132mm diameter so therefore it could be potentially too small for the FTO)
Less restriction
So with the maximum diameter possible sorted, the next restriction is obviously where the air flows through the filter medium.
If you look at this picture I've nicked from the Halfords site (UK car parts company), it shows one of their sealed induction kit offerings which happens to be see-through & LED lit (:?) which is ideal to show the type of filter element they use - it is effectively just a cone filter with a tube around it - it doesn't take a genius to work out that removing that tube would allow more air to flow through the filter so although the air which is being taken in is cooler with this filter, the engine is having to suck harder to take it in so any benefit of cold air is probably lost with the restriction.
Below is a cross-section of the BMC CDA filter. Note the purple section which is the actual filter element - this runs nearly 3/4 of the length of the filter (which is 20cm) and is a cylindrical element so obviously the surface area of the filter element is MUCH greater than the above Halfords offering.
A quick calculation shows the filter element (which is roughly 13cm diameter and 15cm long) is the equivalent of 40cm x 15cm, that is a big surface area!)
(picture of replacement element)
Filtration Quality
The quality of the filtration is another very important factor. At the end of the day air filters are there to filter dirt and other crap from the intake air and without them your engine wouldn't last very long at all.
Reducing the filtration would allow more air to pass through the filter and I fear this is the way most cheaper air filters manage to give the impression of a performance gain, but do you really want that? Sacrificing your engine for the sake of a couple of extra bhp?
The design of the filtration element in the CDA with it's much larger surface area means that more air can get through without sacrificing filtration qualities so not only do you get a decent performance gain, you also still have decent filtration and protect your engine from harmfull particles.
Do you think manufacturers of cheap air filters perform any tests or even care how well their filters protect your car? Do you think you could ever prove their filter was partly responsible for any damage to your engine? IMO unlikely, but even so probably not as stringent as the bigger filter manufacturers.
Summary
In short, there are plenty of cheaper alternatives out there, and I'm sure there are some worthwhile alternatives out there too.
Just be carefull when you buy a sealed induction kit that it's big enough not to stifle the engine with restricted intake air, and that it's high enough quality to actually carry out it's original job - filtration.
Our kit
The price of the CDA through us includes the full fitting kit which includes Samco piping for the intake & all necessary brakets / clips etc. as well as full fitting instructions - this is something that we developed as there is no FTO fitting kit for the CDA (there is actually one listed for the "2.5" FTO
but it is just the universal filter with no intake piping - we have since passed on the information to BMC that there is no 2.5 FTO and hopefully they'll correct their records...
)
If you guys wanted to organise a group buy on the BMC CDA we are more than happy to sort this out for you at a reduced rate, we were approached about this previously but so far nothing has materialised.
Hope that helps!
PS. Sorry for the long post...
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