Post

[Commissioner]

I was feeling sorry for the lonely AC Cobra forum page, no new post in 3 weeks, so I thought I'd drop this to keep it company.

[GreenCobra]

I have a question about air filters. The foam one in my Stellings and Hellings filter housing has given up the ghost. Dirty, crumbly. So I am looking around for a replacement and checked in with K & N.

To fit in the S & H housing and under the hood I need a round filter that is 8" OD and less than 2" tall. The closest match was a filter for a Mazda, 8" X 1.75". Part # E-9181.

Mazda - pretty small engine..would a Mazda filter flow enough air for a 427? So I email K & N tech and ask about the flow charateristics of the Mazda filter.

Very nice response that, in summary, said that the K & N cotton gauze filters flow 6 cfm per square inch and that the Mazda filter is rated at 263.89 CFM.(don't ya love that - out to the hundreth of a CFM!!) The tech consultant said that I could calculate my engine's CFM needs using the following formula:

Cubic Inches X Max RPM / 3456 = CFM of engine.

For a 427 that come out to be about 800 CFM. So looks like the Mazda filter would choke the engine.

So I did a little research on foam filters. They must have much higher flow rates than cotton gauze, right? Otherwise my engine would be suffocating. Well, apparently not. It depends on the foam density but high performance foam filters actually flow a little less efficiently than the K & N.

I went to the Cobra Country classified listing and saw that a lot of the engines are equiped with the S & H filter housing and a filter that looks like a K & N, about the same size as the Mazda filter.

So how does that work? My engine sure doesn't feel starved for air and I am sure those ones in the classifides don't either. Maybe the K & N standard of 6 CFM per square inch of filter surface is too conservative?

Any thoughts/suggestions? Thanks, GreenCobra

[DocDirk]

Well from what little I know, the air filter CFM rating would only affect my engine at and above the RPM that asks for that much, usually my peak HP RPM....where I rarely drive the car. Also it's likely that the air filter will flow more total volume than it's rated for as the rating is likely for flow at optimal resistance from the filter material, thus being suboptimal at high RPM's but not actually choking enough airflow that you'd notice it in any but a racing circumstance wherein every pony counts.

In a week or so I'll have my engine dyno'd with my current 500CFM filter and again with a new extreme top filter that flows 900CFM, and will probably see a few HP difference but not enough to make it worth building a custom drop base for the street.

Ask folks who race, they'll be the ones searching for every missing HP - would they tell us they modify the intake filter from a street stock one for optimal performance??

Regards!

Dirk

[Alex Donghi]

Commish
It does seem that the AC club has petered out a little. maybe we can get a lunch together when it cools off. My problem is I have a lake house and we are there almost every weekend