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MG MGB Technical - K & N Conical Filters

Does the K & N Conical Filter allow sufficient airflow to enable a modified engine to perform optimally.

Engine is uprated 1860, with Piper 270 camshaft and pair of HS4's.

Read somewhere, may have been Steve S, saying that these conicals were not quite up to the job, on an uprated engine.

Having just trashed the emmissions rubbish, do not wish to return to a strangulated engine by another route.

Yes, the conical filters don't have sufficient airflow capacity for such an engine as yours. I'd recommend a set of 3 1/2" X 6" K&N airfilters. They'lll flow enough air without creating too much turbulence.
Steve S.

I already have a pair of the round K & N filters which must measure a good 2" deep by a circumference of at least 6".

These are the usual round filters seen on English cars, unfortunately those twin SU's are too close to the brake servo to fit on mine.

Was thinking of fitting them remotely, if they will provide the airflow required.

If I have to buy new filters, it will be darned expensive, so please say these will do.

Go to this link at the K&N website and scroll down near the bottom. It has a formula that will take the engine displacement and RPM into account and give you the minimum filter area needed. According to this you need about 32 square inches of K&N filter area for our engines. Keep in mind you have two filters so I would suspect that you could halve that number for for each filter. I do not remember the formula for figuring the surface area of a cone however so that may be a little tricky. However that being said I remember hearing that our engines create sonic waves due to the siamese ports or some such thing. IIRC this is the reason to use the deepest filters as the shorter ones bounce the waves back in a manner that's not helpful to performance. I don't fully understand the dynamics of what is truly going on ther so maybe Steve will chime back in. I run the 2.5" deep filters as I used the calculation on the K&N site and found that just one of them would be more than capable of supplying enough airflow.

Jared Snider


Sonic waves are covered on Mini and use of cones


My flow-bench buddies have put the conical K&Ns to the test and have concluded that they do indeed restrict flow compared to larger filters.

However, I have dynoed a lot of MGBs with the conical K&Ns and made back to back comparisons with other filters and have found on the dyno that the K&Ns make the best power by about 1-2HP.

So the flow bench and the dyno contradict each other. I suspect the reason for this is because the amount of flow is one factor involved with power and the speed of flow is another factor. I haven't done the math in awhile, but I recall the K&Ns flow enough until the engine hits 6000+ RPM, and very few people buzz their engines that high. Most of the cars I dyno make their peak power around 5000RPM and won't support revving past 6000, so the K&Ns do fine.

Another factor is that the offset chrome base of the filter set creates a restriction to flow. At higher RPMs, this is a negative factor to performance. At lower RPMs, however, this can be a positive factor in that a restriction will speed up air flow (and low RPMs there is sufficient air being supplied) which improves atomization, cylinder filling, etc. and leads to good power production.

I've probably stirred this up enough for now...let's see what others have to say.

Carl Heideman

I almost forgot, if you go with a aftermarket filters make sure you have some way of smoothing the airflow as it goes into the carb throat. The stock base plate has a radius incorporated into it. With taller filters you can use short ram pipes but with the shorter filters you need to use stub stack suck as these from

Jared Snider


The conical filter won't restrict flow enough to make a difference in a mildly hotted engine that is primarily street driven or even occasionally driven with bad intent!

If I recall correctly, the interference is minor, about 3/8" or so. If true, you can take your phenolic spacers down to a shop and have them cut in half. Once done, you can install the "regular" replacement (early model) filter elements. I had great luck using the baseplate, bolts, grommets and spacers from the coopers unit with a custom top-plate (homemade from scrap aluminum stock with snips and a drill).

That way you already have the air horn in there.

An air-box could always be fabricated with a large filter and a hose. The advantage of this would be two-fold. One filter to buy instead of two. Nice cool air instead of the pre-heated air from the engine compartment. The cool air may even offset any losses from the tubing.

I have the dual "uncovered" coopers in my "B" (chrome bumper, no space issues) but will be designing an air-box due to the hot air problem. Probably to take air from the fresh air system (nice ram air effect) as I don't need the heater.


This thread was discussed between 21/09/2004 and 24/09/2004

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