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Air filter cfm ratings

bp71k5

bp71k5

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I'm working in finding an adequately sized air filter for a semi-custom cold air intake (I have an open element filter now) and the cfm ratings listed for stock air filters for an 89 k5 are in the 90-155 cfm range.

If the TBI engine requirements for this same 89 k5 is in the ~490cfm range, how could the stock filter be rated so low?

I have a filter setup from a 98 Cadillac that I'm considering that is rated for 320 cfm which is tons more than the k5 filter. Maybe I'm not looking at these numbers correctly?
 
Where are you finding the specs?

90 sounds way, way too low if others are rated higher.

What are your limitations in size? There are two truck sizes, not sure which one you are checking on, but the "standard" height filter is about half as tall as the larger ones that were on some trucks.
 
It looks like however they are being rated, is consistent, as a Wix also lists the same CFM.

This is the one I was thinking of: http://www.oreillyauto.com/site/c/detail/MGD3/MGA46220/02512.oap?year=1983&make=Chevrolet&model=K5%2BBlazer&vi=1050523&ck=Search_air+filter_1050523_-1&keyword=air+filter It's ~twice as tall as the one you linked to, but "rated" for less than the ones that are smaller, still 85CFM.

Hopefully someone can chime in, no idea how or why they would come up with such low numbers, it has to be how they are measuring them, and not an indication of what engine size they will support.

According to this: http://www.gnttype.org/techarea/turbo/airfilter.html the large filter should have enough area to flow for a 350 at 5000RPM, if you factor in thickness, which doesn't seem quite right. I'm sure that is a very simplified calculator in any case.
 
Pretty sure that is wrong. I went from a stock 14 x3 paper air cleaner to none on my 427 that makes 600hp and has a 1050 cfm carb and gained 8hp. If there was that big of a restriction it would have been a much bigger gain.
 
That filter is about an inch longer in length and width than the air filter used on my 1.6L Hyundai. (it's also rated for double the CFM...you don't get double the CFM from adding two inches)

I'm thinking it's nowhere near large enough for a real motor in any case.

Here is one for the newer trucks, for the 5.3L: http://www.oreillyauto.com/site/c/d...ch_air+filter_1445445_3742&keyword=air+filter I'd check the 6L, if it's the same filter, I'd say it's enough for your 350.
 
dyeager535 said:
That filter is about an inch longer in length and width than the air filter used on my 1.6L Hyundai. (it's also rated for double the CFM...you don't get double the CFM from adding two inches)

I'm thinking it's nowhere near large enough for a real motor in any case.

Here is one for the newer trucks, for the 5.3L: http://www.oreillyauto.com/site/c/d...ch_air+filter_1445445_3742&keyword=air+filter I'd check the 6L, if it's the same filter, I'd say it's enough for your 350.
Click to expand...

Pretty sure my 8.1 has the same filter
 
I think I'd base my selection on what motor the filter was coupled with, not the CFM ratings. I didn't have any luck finding how those are measured.
 
I have had good luck switching to the 80s 305 filter top on a TBI.
 
If going for the circular style, yep, go with the "tall" stock filter/housing.

If going with something custom, the flat one I'd think could be easier to build something for.

There are also late 80's Firebird filters which are cone shaped, which would probably work good for dust as they sit vertical. Pretty small diameter though.
 
I don't know if that calculator works, but if the filter area calculation is correct, the smaller one is pretty close to adequate.

CID = 350
Rpm = 5000
350x(5000/20839) = 84 square inches.

Filter size of 10.6 x 7.9 inches = 83.5 square inches

Am I doing the calcs right?
 
Why try to reinvent the wheel? Keep the filter in the factory location using the tall filter then just plumb the snorkel to a cold air source and be done.
 
The cfm needed for a 350ci engine would be:
(engine displacement) x (max rpm) / 1728
1728 is how many cubic inches are in a cubic foot

So a 350ci engine with a max rpm of 2500 would be:
350 x 2500 / 1728 = 506 cfm
 
Yea, that's an option. It's just tough to find those things in the wrecking yards lately for some reason. I also kinda like the looks of the newer style.
 
bp71k5 said:
Yea, that's an option. It's just tough to find those things in the wrecking yards lately for some reason. I also kinda like the looks of the newer style.
Click to expand...

You could always use a Caprice/Impala SS air box and filter set-up. These cars have an LT1 engine and breathe very well.
 
dyeager535 said:
That filter is about an inch longer in length and width than the air filter used on my 1.6L Hyundai. (it's also rated for double the CFM...you don't get double the CFM from adding two inches)

I'm thinking it's nowhere near large enough for a real motor in any case.

Here is one for the newer trucks, for the 5.3L: http://www.oreillyauto.com/site/c/d...ch_air+filter_1445445_3742&keyword=air+filter I'd check the 6L, if it's the same filter, I'd say it's enough for your 350.
Click to expand...



The 6L is the same filter. Sounds like a smart way to figure out what could work.
 
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