Size of Air Filter

Virtually all air filters become restrictive at some point.

Some are to the point that you can feel a torque loss right off of idle if you remove all parts of the air cleaner.

One of the most restrictive filter elements I can think of are the oval paper elements supplied for the COBRA oval air cleaners. It has been called an "rpm limiter" by many.

As I recall, the early magazine testing of the 68 428cj Mustangs showed about 1/10 of a second improvement in the 1/4 mile times just by removing the air cleaners for the test.

I think someone back then factored 100hp gain at a .2 second improvement in the 1/4? I'd have to look that up though.

The most efficient filter would be one that sits with the opening straight up on top of a velocity stack.

Exactly what that translates to in HP I don't know.

I can tell you that on a 48 IDA Weber set up, the hp difference is in the 80-100hp area. That was clearly shown on a dynamometer.

Engines are usually so full of compromise to begin with, the only control we often have is where.

quote:

Originally posted by SIG:
Bonjour,
I'm wondering whether the size of my air filter is adequate large for the 351c. The filter element is a K&N E-3505

Dimensions from K&N's site show that filter calculates to 64.6 square inches of filter area. A typical 14" dia x 3" high filter has 132 sq inches, so your filter has less than half the area of a "normal" filter. I would say it is not adequate.

I am a big fan of the K&N X-Stream filter setups. The lids is also a filter that had almost the same area as the ring.

You need an airflow number on the filter to know if it is compatible. Ask K&N for it.

What is the size of your assembly? With the dimensions listed with the E-3505 maybe try the E-1290. It may fit the same assembly(if you like it and want to save some money) and offers more height. I think (can't remember off the top of my head)that this is the one I use with a 13" Ford Racing assembly. If you can make it fit an E-1570 may be better yet. I've seen guys run this with their 429's and 460's. They seem to like it.

Thanks for all the feedback which shows me that I need to change the air filter assembly. I will evaluate your proposals.
Brgds
Hartwig

To really evaluate a filter's effect on HP, you'd need to dyno the engine with it on and off. One thing is the area, another the flow it creates inside it.

When I go boating I have a kind of a dyno, you can immediately read the effect of any change you do on the top speed. In my twin engine boat I had normal flame arrestors on. I replaced them with K&N marine filters that had a filtered area much bigger than my venturi area, but lost almost 1 mph. Took them off alltogether, gained more than 1 mph. So a waste of money

Here is a little tidbit I found on K&N's website. A formula on a recommended filter size:

A = effective filtering area
CID = cubic inch displacement
RPM = revolutions per minuteat maximum power

Using this calculation, a 351 at 6000 RPM requires 101 sq inches of filter area. They also give a method for calculating filter areas. See:
http://www.knfilters.com/filter_facts.htm

Correction mine is an E-1535 with a Ford Racing assembly.

Interesting calculations Dave. I played around on the K&N site some more and find it interesting that with their calculations and using a 6000RPM calculation many configurations seem to suggest a filter that is 3+ inches tall, however many of the filters I looked for were shorter in height. One example would be when I plugged in a 70 Mustang using a 428 the filter was still under 3". I know hood clearance plays a huge role, but many of these calculations suggest that factory air assembly configurations are less than ideal for power or efficiency.