Maybe a silly question about getting better mpg's

Bob Kazamakis

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I know we've got some smart people on here so I thought I'd throw this out there.

All the time I see threads on forums where somebody is asking about how to get better mpg's out of there car. 9 times out of 10 the first response is "get a free flowing intake and exhaust".

My question is why would that help at all? If let's say a stock car's intake and exhaust is designed to flow X amount at 100% throttle then wouldn't it only be using say 20% of X at 20% TPS?

Somebody explain it to me!!!
 

XxApollynxX

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Theory would be less throttle and easier for engine to make the same HP. So flowing more air in/out will benefit it from more power, less throttling to move etc there for more MPG. Theres tons of things such as keeping up on matience like oil/diff fluid changes, running full syn, seafoaming or injector cleanings, fuel filters, just depends on how far you want to go i've seen/heard it all.
 

Bob Kazamakis

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Theory would be less throttle and easier for engine to make the same HP. So flowing more air in/out will benefit it from more power, less throttling to move etc there for more MPG. Theres tons of things such as keeping up on matience like oil/diff fluid changes, running full syn, seafoaming or injector cleanings, fuel filters, just depends on how far you want to go i've seen/heard it all.

My question isn't about how to get better mpg's.

It's about the theory of better flowing intake/exhaust getting better mpg's. It doesn't make sense at all. If a stock setup is made to flow to redline at WOT then how would a higher flow setup benefit an engine at say 10-20% throttle?
 

willizm

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makes sense to have more free flowing air in and out of the engine. Think about it like this. If you have a really dirty air filter it could hinder your MPG. In the exact opposite mannor an even free'er flowing intake would make it more efficient than stock. I don't think it's going to make any significant differnce, but maybe on the high end 1mpg.
 

Ron Vogel

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For mileage, yeah headers are the best bang for the buck. Good timing advance, and warm motor will get a complete burn too. Free flowing intakes are good as long as they don't drop intake air temps too much...too cool intake air will give you a slower burn.
 

Oreif

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A free-flowing exhaust and intake allows the air/fuel mix to flow into the cylinders with less work, then burn more efficiently (less scavenged exhaust remaining in cylinders due to decreased back-pressure), and push it out easily. This would cause the engine to actually work less. So you would create the same power required to move the weight of the vehicle but at a lower RPM. (less power is wasted to move the air in and out and transmitted to the wheels) Since the engine is making more power at a lower RPM, your fuel flow rate is less. Thus increasing MPG.

The only problem is that the increased efficiency and power cause the a molecular change in the drivers right foot (left foot in Japan, Austrailia, etc.) and it becomes heavier. Negating any MPG increase. ;)
 

Bob Kazamakis

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First; right hand drive vehicles have the same pedal setup as here; its just on the other side of the car. Right pedal accelerator, center pedal brake and left pedal clutch. :bigthumb:


So you're saying even at low rpm and throttle input it's flowing better or are you talking WOT?
 

CrunkMaro

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Jun 17, 2010
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fluid dynamics

allowing air to escape quicker (better flowing exhaust) to a certain point (determined by fluid dynamics and limited by the PHYSICS) allows for the 'vacuum' to perform better

allowing more air to enter allows more oxygen molecules to enter the chamber which allows for more atomization of the fuel which allows for a cleaner more complete burn which allows for more POWER STROKE and also decreases the amount of unburned fuel. oxygen is the limiting factor (reagent) in combustion engines...fuel is the excess (impossible to burn 100% of fuel in the chamber...well not impossible but not practical for most cars out there...long story)


WOT is a whole nother game...using commanded AFR etc etc

during normal driving, o2s notify pcm of air/fuel changes and adjust accordingly...

just putting an intake/exhaust on a car won't really change THAT much...maybe 1-2. get a tune and its a ll the difference.

its early...lil confusing but basic idea is there.
 

Bob Kazamakis

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Let's stick to the intake theory for now.

So an NA engine only pulls so much air at say 20% throttle right? So wouldn't a stock filter/intake be able to supply more then enough air at 20% throttle? Or are you saying at 20% throttle with an aftermarket intake will somehow allow an engine to draw more air in at the same throttle percentage?
 

Turbocharged400sbc

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mass produced cars are not often driven at over 75% throttle its less than 10% of its lifeime that it see's over that, so yeah the factory stuff is setup more for the area it's used most often at. but yes, 20% throttle through a restrictive intake with their nice little silencer/resonance chambers only flows so much since its being forced in by atmospheric pressure (cause ya know most people hate a machine that sounds like a machine...they want it to sound quiet...that whole pesky NVH deal that manufacturer's waste so much time and money on)
with an aftermarket/high flow intake the air pressure is able to push more air into the cylinders at that same 20% opening...more air, more fuel = more power....so instead of needed 20% throttle youd be able to accomplish the same amount of work at say 19% throttle...the main area of savings (for fuel econ) is that at that lesser throttle your not adding extra fuel from acceleration enrichment or entering power enrichment... which means your not running at a richer than stoic mixture and saving fuel for a little bit.
yes it does increase fuel econ a tiny bit..but the same hing can be accomplished with even better results if you drive as if there's a raw egg between your gas pedal and your foot, slowly and gently...unfortunatly alot of people are digital drivers...stomp, brake, stomp, brake. not much can help em
 

hellraiser319

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I'd say its vehicle dependant as well. Supposedly on the forums specific, the Aspire picks up a few mpg's from a properly done CAI, and exhaust can help but may not be worth the hassle/$. Other factors, tire pressure, front/rear wheel alignment, brakes, whether or not they are properly operating, not sticking/dragging, maintenance of the vehicle, aerodynamics, and hell certain gas can give you different results.
 
My question isn't about how to get better mpg's.

It's about the theory of better flowing intake/exhaust getting better mpg's. It doesn't make sense at all. If a stock setup is made to flow to redline at WOT then how would a higher flow setup benefit an engine at say 10-20% throttle?

For O.E.M.s its a NVH issue. A cold air intake, free flowing Intake, headers, high flow cat(s), and high flow exhaust all aid in more power and lower fuel consumption (if tuned properl), but make the car to loud and undriveable for the average government test and new car buyer.
 

Oreif

Crazy Little Child
Oct 17, 2008
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Let's stick to the intake theory for now.

So an NA engine only pulls so much air at say 20% throttle right? So wouldn't a stock filter/intake be able to supply more then enough air at 20% throttle? Or are you saying at 20% throttle with an aftermarket intake will somehow allow an engine to draw more air in at the same throttle percentage?

It's not only the amount but the speed at which it travels thru the intake.
The valves open and close on an engine over a known amount of time.
If the air/fuel speed is slowed by a restrictive filter and/or restrictive intake manifold you will have less air/fuel in the cylinder.

If the intake is less restrictive, then the air/fuel speed is faster. Moving faster, more gets into the cylinder creating more power even though the throttle is at the same position.


The term is "Volumetric Efficeincy". Typically most stock N/A vehicles run around 75 to 85% If you get that percentage up to 90-95%, you will increase power and efficeincy.

The formula for this is: CFM = CID x RPM x VE ÷ 3456

So say you had a 350 V8
At 20% throttle you are at 2000 rpm
Your factory intake has 80% VE

Your engine is flowing 162 cubic feet per minute (cfm) of air.

Now you go down to your favorite speed shop and they install a different intake manifold that increases the VE to 90%.

The same engine at the same throttle/RPM is now flowing 182 cfm of air.

That is a 20 cfm increase.

More flow = more power same RPM/throttle position.
Now you have to reduce the throttle/RPM to provide the same power to maintain the same speed. Less RPM = Less fuel flow rate = higher MPG.

(The forced induction (turbo/SC) engines have a VE over 100% because of the pressure differential. You can still increase the VE of a turbo engine by getting a better flowing intake or by increasing pressure.)

Cold air intakes (CAI) work slightly different. As air gets colder the molecules become more dense (smaller in size).
(which is why your tire pressure decreases in the winter)

So you can fit more molecules in the same volume.
If the engine is flowing 162cfm, there are more molicules per cubic foot if they are at 50* than at 80*.
More molecules per cubic foot = increased power.
A CAI does not increase flow just because the air is cooler. The flow rate stays the same, Just more molecules per cubic foot.
It only increases it if it has a less restrictive flow rate than the stock filter and the intake has the ability to flow the slight increase.
 
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