Are our air cleaners adequate for our engines??

[DocDirk]

I would welcome any helpful input regarding the impact of air filter surface area and engine CFM demands please!

I took my car to a chassis dyno shop last week and the owner said that my engine would not perform with as restrictive an air filter as I had on it. My engine builder recommended an 850 CFM carb, that's what I'm running.
I've been researching just how to provide the optimal CFM flow for my 533 BB Ford engine and if the filtration numbers I've found online are correct, my filter is restrictive for my engine except at low RPM's. What I've found in online searches sums up to the following:

"K&N" type cloth filters are said to flow about 6.5cfm/in sq.
Standard paper filters are said to flow about 4.5cfm/in sq.
Foam filters are said to flow even less than 4.5cfm/in sq.

I started with this fornula I found online:

Required engine CFM = (CID x max RPM)/3456

Example: 533 CID x 6000RPM / 3456 = 925 max CFM requirement

(Volumetric efficiency of between 80 -90% modification
would make this between 740 and 832 max CFM req.)

So I calculated the actual flow capacity of my air filter, which is 8 1/2" wide, 15 1/2" long and breathes through a 1.75" open side (the filter heighth measures 2" but that includes the top and bottom rubber parts.)

If we generously figure the side area using a rectangle of 15 1/2" by 8 1/2" and height of 1.75" the total filtering square inches equals the product of
2 x (15 1/2 + 8 1/2) x 1.75 = 84 in sq. area.

Since mine is a paper filter it would flow at 4.5cfm/in sq and give the filter a breathing capacity of 378 cfm. (I think that the CFM figure doesn't give the maximal flow rate for my filter, but instead it's the maximal flow rate without restriction.) I have been given to understand that restriction in the flow through the filter creates a problem with the air/gas mixing in the carburetor (reverse vacuum?) - I'll leave it to the carburetor gurus to explain that.

This suggests to me that if I need 850 CFM for 6000RPM on my motor then with a paper filter in my current air filter my engine only breathes without restriction only up until 2668 RPM!??!

Using the optimistic figure of 6.5cfm/in sq listed for cloth filters the total CFM capacity of my filter would be

84 in sq. x 6.5cfm/in sq = 546CFM

This would allow my engine to breathe well without restriction up to 3854 RPM..... decent enough for most of my street cruising.

But if I want to use my engine to the max and have it breath optimally nearer to redline I would have to obtain 850CFM which means I'd need

850CFM / 6.5 cfm/in sq = 129 in sq. of cloth filter to avoid restriction
on engine breathing.

I haven't found any filters than have that kind of in sq. filtering area except the "extreme" filter top ones that use both the top and sides to filter. K&N makes them in a circular but not in an oval shape Summit carries an oval one and it figures out approximately to 150+ in sq. filter surface area, is made of the multilayer cotton and might even breath as well as K&N ..... I don't know. If it even breathes at 6 cfm/in sq. than it would provide engine air demands up to over 900 cfm without restriction. Heck even at paper filtration levels it would provide 675CFM.

This seems a bit crazy since most all the cobra engines I've seen photos of have side filtering air cleaners that would appear to restrict flow by these numbers - where am I going wrong? Are the flow numbers actually much higher?

I have not found out how much the "restrictive" qualities of my air filter affect performance / HP / torque. Has anyone got these numbers? Anyone got comparison Dyno information with and without a restrictive filter? Anyone know what filters are routinely used on engine dyno testing if any?

Thanks for any answers and for putting up with the eternal post!

Regards!

Dirk

[FUNFER2]

Did the dyno guy show you the 02 readings through out the run ?
If so, can you attach a copy ?

I use the oval and when compared to the 14" round filter on the Mustang Chassis Dyno, there was no difference in cfm, low or high rpm.

Are you positive that your fuel curve was appropriate, you weren't lean ?

We did run a test with the fiber material VS paper. The results were the paper out performed the material once the oil was applied. Kind of surprising. I will ask my buddy that owns the dyno for more details.

[sparks]

Put in a k&n ,keep it the size you have. If you can go a little taller do it. If not dont worry about it. You will never feel the differance. Dont waste your money. Cover 70% of your air cleaner up and rev the engine. See if you notice a diff....Sounds like you have at least 600hp now. What pipes do you have ?

[DocDirk]

My car is in the dyno shop since I dropped it off Friday night. I have 1 7/8" restrictive exhaust baffles in it with the old air filter and the Vic Jr. intake. They'll be doing dynos this week, first as it is currently set up, then after changing out the sidepipe baffles for 2 1/2" ID baffles with packing, then again after swapping my intake out for a Performer RPM. This one I'll have them run with the old filter and the new one from Summit to compare outputs. Lastly a Chassis Dyno Tune I hope to have the sheets by next weekend and I'll post them so we can see what changes occurred with the larger side pipe baffles, the dual plane intake and the larger air filter (if any). I rather think the baffles will make the most difference, the intake might increase low end torque and change the curve a bit and the filter ...... well...we'll just have to see.

Why bother with different sized filters if an engine can run on any one of them??

Perhaps all the calculations are just a waste of time...we'll see!

Kevin, I'll try to be sure and have the O2 readout, A/F changes and anything else I can get them to print out for me lol

Regards,

Dirk

[MaSnaka]

DocDirk,
I may have the same air filter you have (oval w/1 3/4" tall K&N element). My problem is the top of my air filter sits right on top of the choke horn on the carb. Not much clearance around the choke horn for any air to get through the primaries with the air filter assembly installed. The top breather design may solve the problem, but it's really the allowable hood clearance that restricts what we can use. My day at the chasis dyno was all for not once the air filter went back on for the final pull. But I was educated by it. Now I know. Still working on the solution.

John

[MaSnaka]

Do you calculate the filter element by the surface area of the element itself? or by the exposed surface of the element? Most elements zig zag in and out creating extra surface area not found in the elements circumference. Just a thought worth clarifying.

John

[sparks]

I think those calculators are VERY misleading...Put your new pipes on(make sure bung is welded in) and have it tuned on the dyno.If its tuned good you will never no the diff. in what AC you use...

[razerwire]

I T G air filters

[RICK LAKE]

DocDirk YOur K&N airfilter uses oil to caught the debry in the air. It also slows down the air intake speed to the carb. You are trying to get a ramming effect withthe aircleaner. Does your air cleaner have the top that is filtered for a straight shot into the carb? If not this will cost you some HP and torque. mostly in the upper rpm range. Before you get crazy, you need to open that exhaust system up to a 3-3 1/2 muffler inlet and outlet in the side pipe. 500+cid breaths alot of air. Do you have a hood scoop on the car? Before spending alot of money, goto the auto store like Autozone or Pepboys and get a 14" airfilter that is just paper. Try for something taller that will fit under your hood when down and give you 1" of clearance to not hit the hood. Have a low profile aircleaner that sits around the carb BUT make sure it clears the throttle linkage to your carb. These help bend the air into the carb. I have a small 10" round that helps with the flow on my motor. There is a differents between a carb and throttlebody. Do 3 dyno runs with the air filter, you will find that a good paper filter works just as good as a $100.00 K&N. Some of this also has to deal with how much oil is on the screen materials. The best race filter I used to run was a 14" low profile that looked like a plate. It had foam (black) and a wire support. Worked great with a hood scoop but was not the greatest for cleaning the air. I used window screen to fix this. Worked great but one back fire through the carb and it was all over for this air filter Don't get crazy, fix the exhaust first, then spend your hard earned money on the other things. REMEMBER it's just a big air pump. Big inlet, big outlet. It needs to breathe.

[DocDirk]

Thanks for all the input folks! So if I understand correctly, it really hardly matters at all what air filter I have on my engine so long as it clears my throttle linkage and my hood?

I wonder why there are calculators for required air cleaner area, and many different filter sizes all for a 5 1/8" carb opening?

You know, I forgot that the cleaner surface is pleated, which should increase the surface area greatly! I thought that was to give more surface area for particulate accumulation. I'm not sure folding a piece of paper into an accordion shape will let more air through it than leaving it flat.

Well I must say I agree the sidepipes will make the most difference. I guess I'm still in a bit of confusion over the air filter but then what's OCD for anyhow?

Regards,

Dirk