Air Filter Requirements

[George Snyder]

I have a 5.0 345 BHP Crate Engine with the Oval Cobra Air Filter. While doing calculations for Air Filter Area I think I have a problem?

Area required is CID X RPM divided by 20,839 right?

302X 5500= 1,661,000 divided by 20839 = 79.71.

Filter is 52.75 in. X 1.125 = 59.32. Thats 25% short of the standard required volume? Am I correct?

This 5.0 does run great because it can't breath!

I do not have enough clearence for a 3" high element.

Any ideas? I will install a K&N, but I don't think it is enough.

Thanks, Bud

[RACER X #99]

George,
I just lowered the engine in my car by 2 " for the same reason as you. I want to install a bigger air filter. I also bought an oil pan that is only 61/2 " deep.

Cranky

[George Snyder]

RACER # 99,

I read that when I did a search on the subject before posting. Sounded involved. It seems that others have not incountered our problem by the lack of responses.

I may have come up with a solution. I think an 11" K&N X Stream assembly will fit under the hood. It should provide an adequate flow rate for my engine.

I e-mailed K&N for approximate flow rates for 21x9.5 Oval filter # E 1960 and the X Stream unit. I will just have to wait for their responses before deciding. I am hoping it will be close enough.

It is a shame to abandon the Cobra Air Cleaner, and if I just had a little more space, I could use an E 1966 (3" Vs. 1.813) which would solve the problem easily.

Thanks for your response and I hope to meet you at Valley forge June 7th.
Bud

[RACER X #99]

George,
Since the motor was out of the car anyway I just went ahead and cut the mounts out and made new ones that are lower. I also notched the frame where the belhousing juts out for the starter mount.
I was also thinking of buying a larger hood scoop to house the 14" air filter but that was before the old engine expired from old age.

I plan on going to Valley Forge but not sure if I will bring my car. I don't like driving on highways.

Cranky

[ST]

I swithced out my S&H air cleaner (8in. x 2in.) K&N filter after they told me it only flowed 301CFM"s.
I've since mounted the 14in. x 3in. with the drop base and increased it to almost 800CFM.
It fit just fine. There's a pic in my gallery.

[NROTOXIN]

George,
The K&N E-1966 is a 6 inch filter, E-1963 is 3 inches. I think you can get them to make a filter to FIT your needs....ask them.


E-1963 K&N

[Mr.Fixit]

removing the 14" x 3" K&N air filter from my chevy 396 is good for 2 tenths of a second in the quarter mile, which is just better than two car lengths. Putting the base only (with custom retainer coathanger wire) back on, sans filter, is good for a tad bit more.

[George Snyder]

ST,
I can't see anyway I can get a 14" assembly on. I viewed your pic gallery, but remember you have a 427, not a smaller 302. It also looks as if a drop base would make it worse, as it would hit the dist.

NROTUXIN,
You are correct on the 1966 being 6'. I stand corrected. K&N will build about any size air cleaner you may want, but the cost would be unsudstantiated.

Mr Fix it,
I can believe what you said. If you spin that 396 at 6k you need 114 minimum and at best you were at 131. Remember this is one case were more is better.

There are many options that could be used, not limited to a custom air induction system. I am selling myself on the X-Stream Unit as the cheapest choice. It should do very well in this application as far as a functional air scoop delivering a direct flow onto the top of the Filter lid. The cost isn't too high either.

[RACER X #99]

Someone posted flow results for air filters not too long ago and the K&N was not the best flowing. I know they look kool but It is just a lot easier to throw away a used one and install a fresh one instead of cleaning the K&N. I think the best flowing filter was made by Wix.

Cranky

[Wayne Maybury]

Using the formula above, I would require a 103 sq. in. (358 X 6000)/20,839 but my filter is only 40 sq. in. (1.5" tall and 8.5" in diameter)

I am not familiar with this formula so I am now simply asking a question, not pointing a finger at anybody. Does this formula refer to the total area of the filter element or the apparent area? For example, my filter is only 40 sq.in. but if I cut it apart and open up the filter element, it will be much more.

I question this since I also measured the size of the bores of my Holley 750 DP. They are 1.6875" which gives me a total area available of about 9 sq.in. not even taking into account the area lost by the shafts and butterflies. If the carb can flow with 9 sq.in. why do I possibly need a 103 sq.in. filter? I realize that my small chrome filter is not ideal but using this formula it is totally inadequate.

Wayne

[Tom Wells]

Hi Wayne,

Maybe this'll help:

K&N filter FAQ

What's probably not as obvious is to compare the pressure drop (a way of measuring flow restriction) across the filter vs the carb.

The pressure drop across the carb is needed to help vaporize the gasoline, so you can see some pressure drop even at WOT.

Conversely, you don't want ANY pressure drop across the filter if at all possible since this would reduce engine efficiency. I understand that 1 to 1.5 inches of water is the max pressure drop desirable in an air filter.

A case in point - one of the local Cobra drivers dyno'd his car with and without the air cleaner. On this day, this particular car picked up 40 hp with the air filter removed! Clearly a case of a restrictive air filter - too small for his engine.

If you make it all the way to the bottom of the page you'll probably be the resident expert on air filters

Enjoy,

Tom

[wilf leek]

Interesting thread.

I too am limited for space under the hood, and have the Cobra oval air cleaner.

When on the chassis dyno, we did check for differences with and without the original paper filter - little to see. Certainly the AF ratio stayed the same as well.
I now have a K&N filter in there - the #1963 I think - anyway it's around 1 3/4" tall.
In theory, according to the established equations, I need a 14" dia 3" tall filter, but it just won't fit, and crossmember issues mean I can't drop the engine either.
Still it seems to do just fine with the filter I have. I hooked up a vac gauge and at WOT/high revs, there appears to be little manifold vacuum - about 1" from memory. Would this indicate that the filter is not, after all, too restrictive?

[George Snyder]

This is the reply from K&N. I wish they would have given me the flow rates for the E-1963. I will ask again.

Hi George!

The engine needs approximately 488 CFM. You want enough CFM to match the carburetor. The X-Stream 11" Assembly flows approximately 698 CFM. This would be enough to use on this vehicle. The advantage to having a hood scoop when using the X-Stream Assembly is that the air is coming in from the top. This feeds the air directly into the venturis of the carburetor.

My 302 has a B303 Cam, Edelbrock Performer RPM, 650 DP Holley 4777, max RPMS is 5,500.

I also read old posts about the X-Stream and having adverse effects by sucking fuel from the Carb at higher speeds? I am undecided wether it does or not after at the end. Good reading, but way over my head.
Bud

[Tom Wells]

Hi Wilf (sEG) ,

Let me venture a guess here: your air filter element isn't the bottleneck

An engine is in some respects merely an air pump. If you want more HP you need more airflow - of course it helps if the air contains a bit of vaporized gasoline (or is that vapourised petrol ).

Since the removal of the filter didn't change any power readings, my guess would be you have some other limiting factor. Compression, cam, ports in the heads, manifolds etc etc. In other words, if you want more HP maybe a supercharger?

As to space, if you look at my gallery some would see "a scoop big enough to drive a VW into" where others would see HP

Maybe not as elegant as I'd like but certainly functional

Tom

[wilf leek]

Tom - you are correct. Due to height restrictions I had to settle for a Performer rpm intake, would have preferred a Vic Jr. This, I suspect, is my limiting factor.
But it makes darn near 500BHP at 5500rpm anyway, with minimum 400lbs ft of torque from 2000rpm up, so I am not too fussed! Great street engine. Love strokers.

Sooo - restating my original point - if I can make near 500bhp on a 408 stroker with deap breathing heads (Vic Jrs) then that there oval air filter seems to be doing a reasonable job, and is not as restricitive as folks may think???

The air filter sits right up into the scoop, poking through the hole in the bonnet.

[Wayne Maybury]

Tom
OK, I made it to the bottom (the bottom for now ) but I do not feel like an expert. I originally wanted to use the large oval finned aluminium air cleaner. I actually purchased one from Finish Line but it wouldn't fit without major modifications to the hood and the filter itself. Paula was very understanding and took it back and replaced it with the rather small round chrome air cleaner which fit without any modifications.

I removed the foam filter element and replaced it with a paper one that was actually too tall to allow the hood to close. I carefully removed about 1/2" or so from the filter leaving just enough space to safely close the hood. I have considered fabricating a replacement for the lower part which would give me an additional 1/4" to 1/2". The outside diameter is 8.5".

I am really confined by the space available to do much else. Now I find that my filter is less than half the size it should be, based on the formula used above. This is certainly much more interesting that the eternal bb vs: sb debate so keep the information coming. Maybe we can all become experts.

Wayne

[Tom Wells]

Hi Wilf,

Clearly you haven't got any significant restriction if you're measuring 500hp

Hi Wayne,

Maybe Wilf's got a point: if the dyno says the air filter isn't a restriction, that sounds good to me!

Now both you guys got me curious. When my MPFI is completed, maybe next week, I'll see if I can get a dyno reading on my engine with a 2" high element and a 4" high element, both from K&N. I'll let you know if there's any difference.

Don't hold your breath, though, as my scheduling for the Cobra sometimes is, uh, optimistic

Regards,

Tom