cliffy

I recently went K&N on my DD, considering the same for the XJ but what good does a higher cfm filter do with the same air. I went to the junkyard and drilled a bunch of holes in a factory air box aiming between ridges as best I could. I feel like the throttle response is better, but it was just today so has anyone else done it? Pics tomorrow......

s346k

I cut the entire front and side out of my airbox when it was still on there.

DEN1956

yes..i cut a large square opening in the front of my air box,i siliconed in a piece of ali mesh in the opening i had cut to catch large items like leaves etc,i have not noticed any differents but it gets the air into the engine faster.and an easy mod.

mschi772

Hate to break it to ya, but the stock air box with paper element already flows all the air the stock 4.0 could ever need. The only way you pump more air through the whole system is by altering everything.

Smoother intake (all the ridges in that sharply bent rubber elbow are doing more to slow air flow than a paper filter ever did), widened throttle body, port/polish head, quality header, mandrel (or at least compression) bent exhaust, quality muffler.

Even then, very little difference is actually made. Sometimes the way things come from the factory really don't have much room for improvement without tons of effort and lots of money.

Fred/N0AZZ

At least you won't have to buy another air filter again, just clean and re-oil. As for the holes I would leave stock if it were me.

cliffy

Thanks for the feedback guys, @mschi I intend to eventually replace the throttle body with one that has been bored out and deleting the flexible hose with something that is smooth. Not sure how I am going to do it yet though. I don't drive the thing a ton but am just tinkering with things while I piece together my front end lift parts and get the new floor pans in.

mschi772

You can get silicone elbows that are smooth to use or I've heard of guys sliding 2.5" ID exhaust tube bends into the elbow to create a smooth flow while preserving the rubber ends and stretch.

If you do that, bored throttle body, 99+ intake for kicks (I've never seen solid proof of the theory that it's a superior intake, but it certainly can't hurt), port/polish head (I think with a 95 you have one of the technically best heads for this situation iirc), good header, 2.5" header to downpipe, reduce down to 2.25" somewhere just before or after the muffler to preserve velocity as gases cool... Then you'll really be moving air.

s346k

so you're saying a 6 cyl engine with 4 liters of displacement making 190/220 is getting all of the air it could possibly need through an opening literally the size of a half dollar?

negative ghost rider, the pattern is full.

TRCM

Really ??? What is your basis for this claim ??

Unless you are spinning it @ 6000 rpm, yes, it's plenty of air.

Have you ever measured air flow on a flow bench ??

Do you realize the opening all the air goes thru is only maybe 5" in diameter, and that is enough to flow 1500+ CFM easily ????

On some cylinder heads, you can get 300-400 cfm thru a 2.02" diameter valve...and that isn't even a wide open hole...it's got a valve and valve stem/ shroud in the way, and the air has to make a turn in the process to get around the valve.


Now, based on published flow numbers from a stock head, the air flow at .350" lift is ~ 207 cfm per cylinder (or 245 cfm for .650" lift), so you'd think that number x 6 would be the airflow required, but it's not....you are never sucking air on on all 6 cylinders at the same time.

So, lets just say you are sucking air in on some portion of the intake stroke on 2 cylinders at the same time (since on an inline 6, the basic cycles are 120 deg apart)....then you'd only need an available airflow of 414 cfm........for the whole engine at any given time. Note: I said available airflow.....not needed/required. There is a difference. The head can flow 245 cfm @ .650 lift, but you'll never see that lift or flow in anything but maybe a full on race engine.

That's about what a mildly ported big block chevy intake port flows....for 1 cylinder thru a 2.25" valve. Small holes can flow more than you think.


Now, these are assuming optimal airflow, and as we all know, on a stock engine you are nowhere near optimal.

So I'd say yes, that half dollar sized hole you talk about is very likely more than enough air flow for a stock engine being used in a normal fashion & at normal RPM.


Now, if you want to get formulas out and all, then using this one:

Air flow (CFM) = (Horsepower × 0.625) × 1.1104

Your 220 hp engine only needs ~ 153 cfm.....and that half dollar sized hole can definitely supply that


.

s346k

why does removing the air intake setup altogether (from the throttle body) yield more power?

TRCM

I am talking strictly about airflow into an engine...my info & observations/numbers are with NO intake system.....but the half dollar sized hole you are talking about in the intake system will still flow more than enough air for the engines needs, even after subtracting the values for head loss and flow turbulence (friction & directional changes).


On a stock engine, removing the airbox/intake will generally yield slightly more power, but that DOES NOT mean they were a limiting factor in power production/capability, which is what was being stated above. From my experience, the 'perceived' power increase from removing an intake system on an otherwise stock engine is not really verifiable on a dyno. Sounds meaner, but true numbers...meh.......

And while the intake may have limited what the engine was making, it is still capable of supplying the airflow needed by the engine. Think of it this way....you buy a slurpee, and if you suck on the straw a little, you get some nice cold icy liquid in your mouth. But if you suck on the same exact straw very hard, you get a lot of cold icy fluid, and brain freeze. Now, the size of the straw didn't change, but the flow thru it did because the demand on the other end of the straw changed, and even though the flow changed a bunch, the straw was still able to supply without getting bigger. So, while the intake may limit the power being made when it is installed at any given rpm, when you raise the rpm (ie increase demand), it is still able to supply the airflow needed to the engine. At any given differential pressure, the intake system may be a slight restriction due to flow anomalies, but even at max D/P, it is still capable of supplying all the air the engine NEEDS.


On almost any engine, even full bore race engines, the airflow the engine CAN obtain/produce will always be more than the airflow the engine NEEDS to make power based on fuel and valve/spark timing.


Just like a jeep...it only needs ~ 100 hp to do 75 mph, but you have 200 or so available.....to make sure you CAN get to 75 mph under almost any condition. Likewise, a race engine can develop airflow above what the engine actually needs, and this is to ensure it always has enough air under any condition and thus can maximize power production.


.

Waynerd

My 98 had no airbox with a cone filter when I got it. I put a stock box on it I had and threw a k&n stock style filter in it. No noticeable difference other than sound.

s346k

so.....trcm, what exactly are you arguing? I haven't disagreed with anything you've said. however, there is a huge, gaping hole in your ramblings. how much the engine *needs is 100% subjective.

and I go back to my original statement. a stock 4.0 will make more power of it can get more air than the factory airbox provides. it's not rocket science. every engine ever made would do the same given identical test criteria.

I'm not sure what you're trying to argue.

TRCM

what I am 'arguing ' is your statement that the stock airbox will not supply enough air for the engine. It is plenty for a stock engine being driven in a normal fashion. Even a stock engine being driven hard I'd say.

There is no subjective to it.....a stock engine at max rpm will require a finite amount of air to mix with the fuel and ignite.

On an average engine, the head is capable of flowing 25-30% MORE flow than the engine needs for combustion. I have no numbers for how much the intake manifold & throttle body choke flow....if the air can't get to the head due to the intake manifold, then the engine will never see it (and the intake system has nothing to do with this restriction).

A stock cam has .424" lift, and at that lift, a stock head flows ~232 cfm. So let's say based on averages, the engine NEEDS ~ 174 cfm for adequate combustion per cylinder, and since you are only sucking on 2 cylinders at a time, that total engine air flow calculates to less ~350 cfm at any rpm the engine can spin to.

That half dollar sized hole that is really more like 3", will flow way more air than the 350 or so cfm the engine NEEDS.

Look at it this way....that hole is the same diameter as the throttle body, but it doesn't have a shaft & blade in the middle....so if the throttle body can flow the air, that hole for sure can flow it.


What I am trying to get across, is even using max lift numbers, which are way more air than the engine can even burn, the intake system can still flow it.



.

cliffy

I am getting a lot of good info that is for sure. I do intend to get rid of the flexible hose and install a bored out throttle body as budget allows, so there will definitely eventually be greater air flow going in. The mods so far have been drill the holes and remove the baffle that goes from the air cleaner box top into the flexible hose. I still need to grab the K&N filter. My XJ is a spare car for me so although my alterations aren't complete it isn't super critical right now either. Thanks again for all of the feedback.