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MG MGB GT V8 Factory Originals Technical - Air Filter - 2' or 3' ??

Hi, Ive just fitted a "Costello" bonnet to my GT and was wondering if there is any benefit to engine breathing/power, by fitting a 3" filter to my low rider instead of the present 2". Engine is Stage2 Fast Road spec. Danny.
D R Smith

I can't remember where I got it from but here is a formula for working it out.

A - effective filtering area
CID - cubic inch displacement
RPM - revolutions per minute at maximum power
A=(CID x RPM) / (20839)

Example a 215 CID Rover engine with a horsepower peak at 5500rpm
A=(215 x 5500) / (20839)
A= 56.74 sq in

If you are sizing a panel filter, multiply the width of the filter area (not the rubber seal) times its length. If you are sizing a round filter, use the following formula to determine the height of the filter.

A - effective filtering area
H - height
D - outside diameter of the filter
3.14 - pi
.75 - the rubber end caps
H=(A) / (D x 3.14) +.75

H = (56.74) / (14 x 3.14) +.75 = 1.290 inches

Hope it helps.

Bruce Mills

You evidently found this on the K&N website, as your post is very similar.
They also say...

"Keep in mind, this is the minimum size requirement. To extend the service interval and to provide an even greater volume of air to the engine, install the largest filter that will fit in the space allotted. If the space above the engine is restrictive, perhaps a remote filter arrangement could be used to gain space."

Wayne Pearson

Thanks for info Bruce and Wayne, i suppose my question should have been does a 14x2 lowrider filter restrict the engine at full power??(6k) Just wondered if anyone had tested different sizes of filter whilst on a dyno. I could get a 4" under the bonnet now. Danny.
D R Smith


The answer is yes.

My mostly stock 215 with 14 X 2 K&N and drop base dynoed @158hp at the rear wheels. Took off the K&N and left just the drop base, made 162hp. If you plan to turn more than 5000 rpm and/or make more power, then you really need more filter area. 14 X 3 will be plently.
Carl Floyd

Thanks for info Carl, i remember reading somewhere that a guy reckoned a 2" wasnt flowing sufficient air at top end of rev range, but couldnt find any evidence anywhere to back that up. My engine will produce power up to 6k hence query. Danny.
D R Smith

Hey D R,

"Ive just fitted a "Costello" bonnet to my GT "

I would love to see some pics of that bonnet!
Carl Floyd

Pics in camera at present (non digital), but will get one off to you asap Carl. Combination of power bulge and ST air dam makes it look the Biz i think :-). Danny.
D R Smith

This thread was discussed between 08/08/2005 and 10/08/2005

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