Header, Intake, & Exhaust - Why you don't get any fuel mileage improvement from performance mods

It’s been a while since I promised to write this up, and I’ve been working on this really hard. I wrote several drafts and discarded them cause they were too detailed and complicated, so one would get lost in details and miss the big picture. The one here I believe has the right level of detail, and if anybody needs more, I will gladly provide. :)

A common assumption among car enthusiasts is that if one adds some device to the engine that gives him extra say 10% power, his car will consume 10% less fuel if he does not use the extra power. Seems logical, yes? Would have been, if performance mods were increasing the amount of work produced per unit of fuel burnt. But they don’t. Not to any noticeable degree.

What they essentially do, they allow engine to burn 10% more air/fuel mixture. All of them, be it I/H/E or any of the chargers. Intake allows more air to be sucked in on the full throttle, headers and exhaust reduce the amount of exhaust gases left in the cylinder on the full throttle and thus allow more air in, and chargers just plainly push air…They do not increase the amount of work produced by any single unit of fuel.

Another argument can be made that stock intake and exhaust are just too restrictive and thus consume significant amount of engine power to suck air in and expel gases out. This is definitely true, but performance mods noticeably ease those restrictions only for high rpm full throttle and close to this. Under normal driving conditions pumping losses on partially closed throttle are way higher than all other pumping losses combined. And this is how we actually can regulate how much power engine is producing. Remember, on idle ALL engine power goes to offset pumping and mechanical losses.

Now, why I was telling turbo may give you better fuel mileage? Because to compress the air turbo uses the part of fuel energy otherwise wasted, hence it produces more amount of work per the fuel unit burnt. But, the way you guys do turbos, you will never see any fuel mileage gain. Riching it out and retarding ignition timing hurt efficiency badly.
That’s pretty much it, from automotive engineer point of view.[8D]

[sm=smiley12.gif] [sm=ummmmokay.gif] [sm=why.gif]

werd, homey.

ORIGINAL: Misha

What they essentially do, they allow engine to burn 10% more air/fuel mixture. All of them, be it I/H/E or any of the chargers. Intake allows more air to be sucked in on the full throttle, headers and exhaust reduce the amount of exhaust gases left in the cylinder on the full throttle and thus allow more air in, and chargers just plainly push air…They do not increase the amount of work produced by any single unit of fuel.

.......[8D]

But if 10% more air/fuel mixturecomes in....science states that some finite amount of energy will be created by combusting it...there by increasing the amount of power produced. I'm guessing most people believe that having that extra power (call it horse power) will allow them to reach cruising speed faster or with less throttle...there by saving gas.
Iof course am always testing the coeffiecent of friction of clutch to flywheel to tranny to rubber on the pavement. I got my I/H/E so I could pass cars going up inclines without having to downshift. Fuel savings??? Mileage??? thats just the icing on the cake.

ORIGINAL: Dogmeat

But if 10% more air/fuel mixturecomes in....science states that some finite amount of energy will be created by combusting it...there by increasing the amount of power produced. I'm guessing most people believe that having that extra power (call it horse power) will allow them to reach cruising speed faster or with less throttle...there by saving gas.
Itcomes inatfull throttle and high rpms. It does notcome atpartial throttle. The only way I can imagine some minuscule (still way below measuring error) fuel economy is if one always accelerates at full throttle.

Sorry but your wrong, mods can save fuel milage just because all of them dont and/or all brands dont does not mean it is not there. You can de-cat a exhaust and run from 1.75 to 2.0 inch diameter, replace the stock air filter with a K & N drop in, lower your car to reduce drag, tune it for conservative objectives, run a hotter burning ignition setup, etc, etc. A big milage saver for civic owners but they rarely take advantage of it is the transmission they run. for D series people generally a 96-00 dx/lx/cx Y7 transmission will yeild the best mpg, for B series people generally a 95-01 Integra RS/LS transmission will yeild the best results, for H series use a 96-00 accord LX/DX F22 transmission, and for K series people use a 2WD crv/accord transmission(K24).

It can be done, has been done, it's just when you get right down to it people will never take advantage of it because everyone mods their cars for performance other than milage. Moral of the story, dont bitch about gas milage if you are modding your car period.

Absolutely agree on lowering and transmission, and I did not mean those - I was talking about engine mods, specifically I/H/E and turbo- and super- chargers.

Ignition - not on modern cars. Itdefinitelycan helpon your Impala though.

Tuning for conservative objectives - of course, no question about that.

Cat - well, one might want to remove it and then tune engine to run lean on partial throttle - will work, but at the expense of pollution and legality.

Air filter and exhaust diameter- sorry, I just wrote why those will not work...

I’m going to throw in the mechanical engineers POV on this topic. Any I/H/E will in some effect increase horsepower. Lets say you take a 100 hp car and with mods take it to 110 hp. Purely hypothetical. So a common thought would be if my car used to do 60 mph at 4k rpm, then with more hp (and torque) I will be doing 60 mph at 3k rpm. Again all those numbers are purely made up for the example. Unless gearing was changed, this of course is WRONG. For the most part a car, in the same road conditions will do the same speed at a set rpm and gear whether the engine is putting out 100 or 300 hp. Honda spends a lot of time making an engine that performs efficiently at a given air to fuel ratio (AFR). This number is for the most part “constant” when the engine is at operating temperature. When it is starting and the engine is cold, the mixture is rich. This is why you use more fuel when the engine is cold. So what I’m trying to say is this. All of those modifications essentially attempt to do the same thing; move air easier and more effectively. The advantage to this is the more air and fuel you have inside your engine, the more power you are producing. This is what a turbo does. But the more air the car has inside the engine, the more fuel the car will be pumping into the cylinder to preserve that AFR. When you put an I/H/E you are making your car more efficient. But in this case efficient does not mean better gas mileage, it means more power produced per constant volume size of an engine cylinder. A combustion engine is essentially an air pump. All an I/H/E does is make the engine breath better, both intake and exhaust end, which essentially increases air flow, which creates more hp. But to maintain the constant AFR, more fuel needs to be spent in the process. If you want to increase gas mileage, lowering the cars weight and minimizing drag is the best bet. Both will make the engine work less which will decrease fuel consumption. An I/H/E will make the engine breath better, but won’t increase gas mileage.

ORIGINAL: Misha
Air filter and exhaust diameter- sorry, I just wrote why those will not work...


I'd have to argue the intake one, or at least the air filter part from personal experience

my car is entirely stock, with the exception of a K&N filter, and I had the car for well over a year before I got the K&N

I remember the best mileage I got making a long distance haul (CT to VA) was around 32mpg, was in the heat of summer, highway cruising at around 77-80mph the whole way (yay jersey turnpike) with the stock filter (2 years ago)

same trip in march doing the same speed, i got around 28mpg (it was freaking cold) (2 years ago)

same route, and trip in heat of summer again (last year), with the k&n, i managed 36mpg (the record for my car), again, same driving habits, same route, even almost the exact same amount of time (it varied by less than 5 minutes because there was a line to take a piss at the rest stop)

when i made the trip in march again, i got around 29mpg instead of 28 (last year).

whenever I make the trip, I fill up at the same gas station (on both ends), and stop at the same rest stop (pitcher on the way down, stockton on the way up), i always fill up with low test (from shell or exxon), and I always get my oil changed the day before coming down.

there were no changes in weight (afterall, i've weighed about 170-175lbs for years), and generally no changes in traffic (there's always the backup on the jersey turnpike @ noon, and DC beltway @ 3pm)

this year when i made it, with a car full of stuff (i moved down permanently), I managed 32mpg (figure with an extra 300-400 lbs) coming down

my driving habits have remained the same for the duration of driving my car (heavy on the throttle to merge, maintain pedal to keep speed steady for cruising, no cruise sucks)

I always drive with my windows down completely with the weather over 70, and about halfway in the 55-70 range, and closed lower than 55
I never run my AC while driving long distance, and I'm pretty good about my consistency in patterns

oil changes every 3000, tires rotated every 6000, and tire pressures checked weekly

I drive the same roads everyday and each time I calculate mileage it comes out different. There are a lot of deciding factors that come into play with that calculation. But you probably did. Only reason I can think of is with more air available at any given interval of time, your car can compensate quicker with less lag to either lean/rich your mixture.

My understanding is if you allow the motor to run more efficently like ihe and it is buring more completely you will get better mileage an have more even wear...cars are built with restrictions that damage gas milage so if you open up the engine to a more optimal flow it should increase overall performance right? I mean if you have alot of mods then you wont get better gas milage but i would stand by the fact that i/h/e together or seperate can get you better gas milage...i feel as tho that has been my experience

Powermidget,

I/H/E do NOT increase fuel efficiency and do NOT let fuel to be burnt “more completely”. Modern engines already burn about 98-99% of supplied fuel. Those mods merely increase the amount of air that comes into cylinder at the full throttle, and this allows engine to burn more fuel, thus increasing power output. This has nothing to do with fuel efficiency.

Cderalow,

The method you used to measure your mileage is terribly inaccurate. Just for a sake of it, try to calculate and to write down your mileage for several tanks in a row. I guarantee the difference will be more than 10%, most probably closer to 20%.

If an I/H/E did increase fuel economy you can be guaranteed Honda wouldalready be puttingsimilar itemsin their cars already. These days fuel economy is a huge selling point. But they don't so they don't bother.

ORIGINAL: Misha

Powermidget,

I/H/E do NOT increase fuel efficiency and do NOT let fuel to be burnt “more completely”. Modern engines already burn about 98-99% of supplied fuel. Those mods merely increase the amount of air that comes into cylinder at the full throttle, and this allows engine to burn more fuel, thus increasing power output. This has nothing to do with fuel efficiency.

Cderalow,

The method you used to measure your mileage is terribly inaccurate. Just for a sake of it, try to calculate and to write down your mileage for several tanks in a row. I guarantee the difference will be more than 10%, most probably closer to 20%.




I track my mileage with every tank... my current running average is just under 31mpg since June 2003 (includes time with/without k&n)

since the k&n, it's just over 32mpg

ORIGINAL: pimpciv98

If an I/H/E did increase fuel economy you can be guaranteed Honda wouldalready be puttingsimilar itemsin their cars already. These days fuel economy is a huge selling point. But they don't so they don't bother.


They do.

The intake system is in fact great from the factory and that is why I said to install a drop in filter into the stock air box and have you not took the time to look at stock honda exhaust manifolds. From around 1992 on most stock honda manifols use a 4 to 1 or a 4-2-1 header style with the correct size collector. Why do you think stock ITR headers go for so much money? Honda's also use straight through exhaust designs. Think about it, these are all reasons you dont get hardly any extra power from bolt ons, just throttle response.

I am not going to argue this because for one thing I am abovetaking place in pissing matchesover the net but I will say this. You would about have to be dumb to think that a good aftermarket drop in filter is not going to do better than a old dirty pile of **** stock paper composit filter that chokes the hell out of a engine. I am not a "engineer" but I know engines and I know cars. Your talking to a guy that has been into working on cars longer than most on this forum have been alive and anytime you can make a engine work less to attain the same amount of air(because fuel and fuel pressure are adjustable on alot larger scale than air) the better off you are. A perfect thing to look at that is not car related is a vacuume cleaner, turn one on with it set to use the hose attatchment. Listen to it, then cover half of the opening on the hose. The vacuume still sucks in as much air true but listen to how hard it work to do so.

Marty,

Absolutely and I'm not really arguing with you. There is only one small - hmmm - clarification - to what you are saying, including vacuum cleaner example - all those things give some minor economy effect on the full throttleand close to it.

Because some extra air in the cylinder gives you slightly higher effective CR, thus even more slightly improving fuel efficiency plus better flow slightly reduce pumping losses. We are talking something like probably 1-2% efficiency of the full throttle for 10% of power increase. This is already way below our ability to measure it, and if you factor in the percentage of total driving time you use the full throttle, it comes just to nothing. All those good things quickly disappear as soon as you stop pushing it to the floor…


Cderalow,

Can you post the variance between measurements, please? Did you get your car new? Did you make any tune ups when you installed k&n and since then? Or just dropped it there? Was your stock filter clean when you replaced it, or it was there for 100K miles before? Do you mind to install stock filter back and to measure your mileage for some time?