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Discussion Starter · #1 ·
Why would a K & N air filter improve fuel economy? To say that reducing the restrictions to intake air flow will improve an engine's efficiency, is similar to saying that opening the throttle further (the biggest restriction to air and fuel flow) would improve fuel economy. A motor sure turns over easier when it's pulling less vacuum (ie throttle open). The only way I can see it improving fuel economy is on a carbureted engine where it causes the overly rich mixture to be leaner due to reduced restriction. If so, you could get the same results by simply rejetting.

Any opinions?

Tim

84 CJ7, 258, HEI, M/C 2100 carb, 5 inch lift, RS9000's, 33x12.5 BFG M/T's, 4.56's and Detroit softlocker, full cage & belts, Xenon flares, Dana 44 rear, GM dual diaphragm brake booster
 

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Discussion Starter · #2 ·
By the way, I'm not trying to bash an excellant product, which incidentally I use, but wondering whether peoples expectations regarding this product are correct.

Tim

84 CJ7, 258, HEI, M/C 2100 carb, 5 inch lift, RS9000's, 33x12.5 BFG M/T's, 4.56's and Detroit softlocker, full cage & belts, Xenon flares, Dana 44 rear, GM dual diaphragm brake booster
 
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You forget that by opening the throttle you not only let in more air, but more fuel. K&N is less restricive on the air flow coming into the throttle.

Jason

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OIIIIIIIO'81 Scrambler & '88 Grand Waggie
 

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Discussion Starter · #4 ·
I agree entirely. So if it improves fuel economy by letting more air in, that means it's leaning out the mixture. You don't want that unless it was already too rich, nor do you need an air filter to perform that function.

84 CJ7, 258, HEI, M/C 2100 carb, 5 inch lift, RS9000's, 33x12.5 BFG M/T's, 4.56's and Detroit softlocker, full cage & belts, Xenon flares, Dana 44 rear, GM dual diaphragm brake booster
 
G

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Increasing fuel economy is like the same as making something more efficient. A K&N is a more efficent way of letting air into the engine which makes less work for something so therefor you get better fuel economy. On fuel injected engines the computer is going to adjust for the change in air flow of a k&n filter, which in turn will not make the mixture much leaner or richer. The easier an engine can allow air to enter and exit the more power it will make and the more efficient it will be. A k&n alone will not give huge gains but every little bit helps.
Duneslider

Duneslider
 

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there was a report in one of the 4wd mag where they dino couple motor, by added a k&n filter, hardly any inprovement was notice. The only way a K&N would improve is that your old filter is in just bad shape to restrict flow. Even a cheap filter would have more than enough flow that the motor can use at one time.

brownbagg
 

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I dunno if your right dune, but that makes sence. I was wondering the same thing and thinking.

"I can lean out my mixture with my mixture screws."

Less resistance means the engine spends less energy getting the air into the engine.

Great explanation. Sell it to K&N.

 

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Discussion Starter · #8 ·
My reasoning is that most driving is done at part throttle. The biggest restriction the engine senses is the partly closed throttle. Opening up the air cleaner, if it does in fact flow more air, is just like opening the throttle further. So if the K & N causes more airflow at part throttle, then you would naturally compensate for this reduced restriction by closing the throttle further, and having the same quantity of air and fuel fill your cylinders. Thus, no savings in fuel economy. The reduced restriction to air flow should only help when the throttle is fully open, as do most performance modifications that improve flow. Unless you are improving the efficiency of burning the existing fuel (better metering,spark, chamber shape, turbulance), cramming more air and fuel in is not going to help fuel economy. It certainly will help generate more horsepower, but more power thru increased air flow means more air and fuel are burnt to generate that power.

You don't throttle your engine thru the exhaust system, unless you are talking about an engine brake on a diesel, so improving flow thru the exhaust system will reduce the workload on the engine (easier exhaust stroke), as well as scavange exhuast from the cylinder more effectively (the second of which will allow more air and fuel in and therefore burn more air and fuel making higher horsepower at the cost of fuel economy as well). Just some thought, and simply my opinion. I'm no engineer.

Tim

84 CJ7, 258, HEI, M/C 2100 carb, 5 inch lift, RS9000's, 33x12.5 BFG M/T's, 4.56's and Detroit softlocker, full cage & belts, Xenon flares, Dana 44 rear, GM dual diaphragm brake booster
 
G

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You are right. It will make it easier to flow. Making you not open the throttle as much, and thereby not using as much gas. MPG is how engine effeciency is most often measured.

But you can think of it this way. You cannot flow any faster than the most restricive path. Therefore if you have the fastest flowing exhaust, polished and ported head, equal length headers, big fast flowing K&N, etc. etc., but a very restrictive intake manifold.....you aren't going to flow any better then the intake manifold.

Jason

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OIIIIIIIO'81 Scrambler & '88 Grand Waggie
 

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Any measurable improvement in mileage is due to a lower restriction of air through the filter, but not because of lower pumping losses - the energy used just to pump air through the engine. It is that carburetors are very sensitive to air pressure differentials. Think of the choke plate; close off the air inlet to the carb and pressure in the carb throat drops. This literally sucks gasoline out of the carb, making the very rich mixture needed for cold starting.

A restrictive air filter will do the same thing to a lesser extent, causing the engine to run a little richer than it was designed to, and reducing mileage accordingly. Put on a new, top quality filter and mileage improves.

Now, a stock engine with a clean filter isn't going to be improved significantly by a premium filter. A modified engine that can suck lots of air can get an improvement at the higher end, but not in the normal operating range. But all of these projections are based on the carb being tuned perfectly for the air cleaner that's on it.

Any new air filter will register an improvement over a dirty, clogged one. Probably a K&N's best feature is that it can catch a lot of dirt before it starts to significantly restrict airflow.

I might have been born at night, but it wasn't LAST night.
 

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Discussion Starter · #11 ·
My point is that the biggest restriction to air flow at anything less than wide open throttle is the throttle plate,not the air filter. So alterations in the air filter will not have significant effect at part throttle.

As well, if you consider the resistance to air flow thru the intake system as frictional drag on the engine (which it does), thus reducing it's efficiency (in what sense,a) that it limits the flow of air and ultimate horsepower potential, b) or that it decreases fuel economy), how else do you throttle an engine and maintain the correct mixture other than intentionally introducing an impediment to airflow (ie the throttle plates)? Less throttle opening=less air and fuel= more mpg (under some conditions, efficient rpm range).

Geeze, I must have better things to do with my time! Thanks for entertaining this silly discussion!

Tim

84 CJ7, 258, HEI, M/C 2100 carb, 5 inch lift, RS9000's, 33x12.5 BFG M/T's, 4.56's and Detroit softlocker, full cage & belts, Xenon flares, Dana 44 rear, GM dual diaphragm brake booster
 
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