Do I need a cold air intake or is stock OK?

[1984Laredo]

Building a 4.7 stroker with the Russ ported 7120 head, polished '99+ intake, BBK 62mm TB, and Banks header with 2.5" exhaust.

My current engine (pre-stroker) has the factory TJ-style air intake tube that crosses over the top of the engine and factory air box and "trumpet".

I was about to pull the trigger on ordering a CAI (either the aFe Momentum GT or the Banks Ram Air) until I started reading info on multiple sites about the downsides of these style filters in letting more dust pass through vs. stock filters. Also, the claims were made that the stock system is plenty sufficient for the needs of these engines. The numbers I've seen is that the STOCK throttle body can flow 450 cfm, which is more than enough for a stock engine or even a stroker, according to the CFM calculations. (I have yet to find CFM specs on the stock air intake tube/air box, but read that it supposedly is close to the flow of the stock TB. )

So, just for giggles, if you do the math on CFM needs of an engine, the formula would be CID x RPM (redline)/3456=CFM at 100% volumetric efficiency (VE), at which none of our engines operate. It seems the general consensus is that stock engines operate at +/- 85% VE. I have read that our strokers are more along the lines of 87-88% VE.

Even so, for worst case scenario, let's do the calculation at 100% VE. for a 4.7 stroker, it would be 287 cid x 5200 rpm= 1,492,400/3456= 432 CFM at 100% VE. (And my engine will NEVER see any RPM above 4500 as long as I own it.)

So is my understanding of this correct? - that no matter what size CAI or throttle body we want to throw on there, the max CFM that this engine could possibly flow would be that 432? At that point, it's just a matter of knowing it the stock tube is restrictive below that point, I guess.

I'm not trying to argue any position one way or the other here. I'm truly seeking any relevant information on whether or not a stock system could provide the CFM needed for my engine or if a CAI is even a worthwhile expense. I admit my very limited understanding of this, and, unfortunately, there are so many contradictory information (well, seems to be mostly people's opinions presented as fact) on the science of this.

[Cheromaniac]

It depends on where you're going to be driving your Jeep. If it'll be frequently off-road in dusty conditions, keep the stock air filtration system. However it WILL restrict performance since a stroker demands a lot more air.

[1984Laredo]

It depends on where you're going to be driving your Jeep. If it'll be frequently off-road in dusty conditions, keep the stock air filtration system. However it WILL restrict performance since a stroker demands a lot more air.

I guess that's the part that I'm having trouble wrapping my brain around. The calculation in my original post is made using the cubic inches of a 4.7L stroker and reflects the maximum amount of air that can physically flow through that volume of an engine (naturally aspirated, of course) at its redline RPM. Stroker or not, that's the max that the engine can physically flow. So, if that maximum flow value is still less than the flow capability of a stock air box, what is the need for a larger intake?

[bleedsblue]

Like you I have tried to find specs on the CFM of the stock airbox with no luck. In the end I am sticking with the stock system as I want my Jeep as quiet as possible.

[jsawduste]

CAI’s are overrated.

The CFM capacity of any engine is always reduced by the VE. Beyond the area of the stock box you have the surface area of the filter itself to contend with.

The factory boxes are plenty big enough and many of the designs feature a cold air inlet that actually draws outside air, not hot underhood.

Sure there are reasons to change the intake. Water intrusion being one of them. Airflow increase is not one of them.

If you really think the factory box is limiting flow. Than open up the inlet or drill a couple holes in it.

I know, manufacture XYZ says their system doubles HP and attracts better looking women. BS

[1984Laredo]

My gut instinct is that the stock air filter is probably the most limiting factor in the entire stock air box setup. I really wish we could find some numbers on it, but they don't seem to be out there.

I just looked at the specs for a Wix air filter for the stock TJ style air box, part # 46081. The cfm rating for the stock filter is only 210 cfm. I was shocked it was actually that low.

[bleedsblue]

I did a little digging. I am in a 2003 WJ and this shows the WIX factory replacement as 320 cfm...I dont know where they got the numbers, but this site may help your research.

https://www.fleetfilter.com/filter/46213.html