benolin

Hey guys, I have searched and searched and it looks like I'm the only one who may be having this problem. When I bought my Max, before I did ANY mods to it, it would get 26-27 mpg. NOW, with the budget y-pipe, K&N filter, throttle cleaning, and 93 Shell... I got about the same, 27-28. My problem comes in since I have put copper NGKs and changed the fuel filter to a 300zx's. Now my mileage is at 23 mpg! What is going on?! Anyone have any clues? I'm not getting any codes from the ECU at this point. Thanks in advance.

--Ben.

Maxima95Tuner

Ok, 23 MPG is still hella good. Im getting under 16mpg. Mods do decrease mileage. A better flowing exhaust means more fuel can be pushed in and expelled quicker. Also, when you upgrade the fuel filter and plugs, you are still getting more fuel to the engine. 23mpg is still really good though, so I wouldnt sweat it.

benolin

My 96 SE I had running upwards of 30mpg! Something isn't right and I need to figure out what it is. I drive 60miles each day, so fuel economy is important to me. If you're getting 16, don't drive so hard!! Thanks. Any other clues? Does the 300z fuel filter increase the amount of fuel running and maybe it's running really lean?!

plurco

iTrader: (1)

I find that any slightly spirited driving reduces milage greatly. I burned a tank going to atlanta last weekend and I got 400 miles to the tank. Druing the week while I am cummuting, half of my time is spent on the interstate and half on 2 lanes. I am lucky to get 330 miles to the tank.
and BTW I burn on average 3 tanks of gas a week, so I hear you on the gas milage thing. I drive 150 miles a day

benolin

There has to be a way of getting our cars to run really fuel efficiently. I have hit 30mpg before, just have to figure out how to do it on my new one? Suggestions?! Thanks.

gto400no1

iTrader: (5)

I think it might have somthing to do with that filter, it probably lets more gas through it causing the decrease... thats still good milage ur gettin, maximas only get 22-23 mpg, me on the other hand am only getting 15 or so mpg and thats even when i take it easy on the acceleration.

formz

It doesn't matter how much gas (to an extent) that gets let through the filter. The more gas != worse gas mileage. The reason is this:

With a clogged fuel filter (IE: less gas going to the rail) the fuel pressure increases, which means everytime an injector opens, it sprays more fuel in.

With a new (or larger filter in this case) the fuel pressure would drop, and although there is more fuel in the rail, since the pressure is lower, the injector won't spray as much fuel.

Since the maximas use a return type fuel system, X amount of fuel is in the rail, but X-Y amount is returned. In a returnless system like the new celicas/matrix, X amount of fuel enters the rail, and 0 gets returned, so a larger fuel filter could have a negative effect on gas mileage in that situation.

Hopefully that makes sense to everyone.

sean96SE

I would think most mod's would improve gas mileage since the system runs more efficiently (less resistance, better airflow), except maybe the fuel filter. Unless of course you mod and drive the car harder.

formz

Well with more power comes more gas. You don't burn the same amount of fuel at 190hp that you do at 210. Just doesn't happen.

benolin

I would think with making the engine breath better, it would run more fficiently. I don't think more horses has anything to do with how much fuel it's being given. I drive really conservatively... ie... 65 on highways instead of the posted 70 speed limit. I can only figure it would be the filter. That's the only variable I cahnged and saw a big decrease in the mileage. Unless the copper plugs decrease mileage too. 23mpg is not acceptable being all highway

--Ben.

DAVE Sz

iTrader: (2)

The winter HAD something to do with it. You probably didn't check the gap on the plugs before you put them in.

formz

No in your case horespower was not the issue. I was just explaining to the other guy that when you have more horespower, you burn more fuel. (Unless you physically lean it out.)

astil

iTrader: (1)

First thing that I d say you should do is check if there are any codes in your ECU. If you ve got a KS code or an O2 sensor that is bad ,your mileage would suffer.The described upgrades may have nothing to do with the problems. I also installed a 300zx fuel filter ,and my mileage didn t drop...I also have coppers ,and my mileage is decent...Again, it may be a code that you don t know about due to the fact that the CEL is not on. Even for some people 23 miles doesn t sound bad, I d say that 27 would be more acceptabile.
If you live in the cold areas ,the cold temp may hurt your mileage. I see mine decreased a bit ,due to the winter temps.

plurco

iTrader: (1)

well I just filled up and checked my milage and it was right at 20 miles per gallon.

Nismo3112

iTrader: (12)

I would crap my pants if I got 23mpg....

sean96SE

True, but simply giving better exhaust flow does not increase the size of our 3.0. So it's the same engine, the power is just being translated to the wheels more efficiently. Imagine trying to squeeze the exhaust piping down to 1" diameter, the car would go nowhere fast and burn plenty of fuel in the process. Thats what I've always thought anyway.

formz

What you have to realize is this: When you put mods on a car (even just intake/exhaust) it increases your ability to burn more fuel.

Exhaust expel more gas, faster, which makes more room in the combustion chamber for more fresh air. More fresh air = more fuel needed to burn.

And intake brings in more air then the stock one, so that's pretty self explanitory.

When you make more power, you burn more fuel. Even if all you do is add an exhaust system to it.

waveridr85

iTrader: (4)

ok once again....
when the temperatures get colder, there are two things that cause our milage to drop.
#1. Fuel becomes denser
#2. Air becomes much more denser

With denser air the ecu reads that it must add additional fuel to keep the a/f ratio in porportion. The fuel the engien uses is already dense but it also adds more dense fuel to equal out the ratio. This is what kills milage in the winter

benolin

It just bugs me that it was only present after I installed the fuel filter and changed my plugs. It doesn't seem normal even if it IS cold out. I don't remember there being any change in mileage last winter in my other car. This sucks. I'll let you guys know when I get back to 30mpg

sryth

Check the codes...maybe check the gap on those plugs, too.

I highly doubt it's the fuel filter. It's not as if the fuel filter is the bottleneck in our fuel system. As for pressure, the fuel system is pressure regulated...pressure should be approximately the same, regardless of the fuel filter size. I'm sure our fuel pump can handle pressurizing a few more CCs.

exhip95

formz, please read your post again and think about what you said. You don't sound like you know what you are talking about.

A clogged filter will not decrease the pressure enough to cause any noticeable drop in MPG. It will decrease the volume of fuel flow at a given pressure which would produce a lean condition especially at WOT. This would increase detonation and the ECU would retard the timing to compensate and that will cause a drop in MPG. The fuel pump will make the same amount of pressure with or without a clogged filter. The only advantage the 300zx filter has over the stock one is an extended service life. They have the same amount of flow. I only put one on because its a PIA to replace and doing it every 15,000 miles got to me. I do not consider it a performance upgrade.

The fuel injectors will only do what the ECU will allow them to do unless there is something physically wrong with them in which case you would have a CEL you could not get rid of. The ECU knows what they are doing by looking at info coming from the o2 sensors and others. Their output is not determined by just the fuel pressure getting to them.

I won't even comment on your last paragraph. It would be a waste of my time.

Don't talk so much. Stick around for a few more years and learn some more defore you try to come off as some sort of professor or something.


benolin, I have all the mods I can get for the engine (without getting crazy) and still get around 26MPG almost all city in my '95 GLE with 140,000 miles and a tired tranny. And I have a heavy foot. Do yourself a favor and put the OE NGK pfr5g11 plugs back in. They are the plugs the factory ignition system is designed to use and they work the best. I have tried just about every plug that fits and the NGKs are the best for the VQ. My only b!tch about them is the price. The coppers can make a stronger spark but that is not always an advantage. Plus, they don't last anywhere near as long as platinums.

sean96SE

formz,
For an example, an engine that burns 90 percent of the air and fuel mixture is much more efficient than one that burns 60 percent. So it'll make more power from the same amount of fuel - because what isn't burned in the combustion process becomes waste and gets sent out your tailpipe.

In other words, by fitting a free-er flowing exhaust and removing Catalytic converters, the exhaust gases can be allowed to leave then engine more freely, meaning less energy is wasted by the engine forcing this waste out, so that 'spare' energy can be transferred to the crank and increase output power. Only having forced induction would make a sharp decrease in MPG.

guido32

I'm in the same boat benolin. I just cleaned my throttle body, installed new plugs, and dropped in a K&n air filter (oem size). My gas mileage went from 24 to 20 with no change in driving habits. I'm confused to because the weather was cold before I did this work, and it still is now, so weather isn't a factor like most posts have stated. Later

Cool J

i just filled up on gas last night and i calculated i am getting 15.7mpg.....wtf.....that's terrible

-=PK=-Maxima

but i'm getting a code 0304...so hopefully when i change my KS i'll improve on the gas milage. check for codes.....you might have a bad knock sensor as well, or maybe a bad o2 sensors.

Cool J

just checked it today and i am getting an 03 04 too so i guess i will have to replace my knock sensor. car has 81k miles so i am guessing it has never been replaced. Hopefully once i replace it I will get better gas mileage and performance.

formz

Haha are you kidding me? You haven't built too many high powered cars have you? The ECU can't control physics.

Take this as an example:
A common DSM upgrade is a Walbro 255 fuel pump. When you put that in with the stock fuel system, the pressure increases to about 53psi (stock is about 40.) Now, by your logic the ECU could still control how much fuel gets into the motor, but it can't because it's ONLY CALIBRATED TO THE STOCK FUEL PRESSURE. ECU's don't self adjust to fuel pressure changes, you need an aftermarkt FPR for that. So the car runs extremely rich. the ECU only has a certain range of operation, and it WILL try and lean the car out, however it can't because it is excessively rich.

When you start tuning cars for a living, come talk to me. Until then, just shut your mouth.

EDIT: And I didn't say a clogged fuel filter DECREASES pressure. YOU said that.

formz

Actually that's not really true. The combustion chamber is a fixed space. It doesn't increase or decrease in volume aside from the up and down motion of the piston. When you have a bottle-neck in an exhaust system, some spent exhaust gasses still linger in the combustion chamber after the exhaust stroke, leaving LESS room for fresh air and LESS room for fuel. When you take that bottleneck out, you decrease the amount of spent exhaust gases in the combustion chamber, making MORE room for MORE air and MORE fuel.

sean96SE

What you're saying here seems correct. Having MORE room for MORE air and MORE fuel would, in this case, be a result of removing the twists and internal passages in the exhaust system. However the fact remains, the pressure in a highly restrictive exhaust system works against the engine, which expends energy to push the exhaust through. Not saying either of us is right or wrong here since there are so many variables involved, it's just interesting to consider. Either way the change in fuel economy would probably be pretty small.

exhip95

I was not joking.


I didn't say anywhere that a clogged filter will decrease pressure, I said it will decrease the volume of fuel flow. There is a very big difference. The pressure regulator (vacuum controled, not electronic at all), which you seem to have forgotten about, regulates the pressure AFTER the filter. Any surplus pressure/volume would be vented back to the tank through the return line.

After reading several threads you have "contributed" to It seems the only car you really know anything about is a DSM. We are not talking about a DSM here and I will not get into discussion (****ing contest) with you about one.

I have been building/tuning all sorts of cars for for the better part of 30 years. Probably longer than you have been around. In fact, I have a '72 240z ( well, most of the original body anyway) with a 500hp 383 small block chevy, 4 speed, narrowed corvette rear section and a full cage in my garage right now. It is set up as a road car, not a drag car, and I give my friend in his ZO6 fits. That is just my current toy. I have had others that were really nasty but I have other interests besides cars that require money too.

Please stop tring to baffle people with BS that is not on topic.

sryth

It always amazes me how heated a conversation about fuel filters can really get.

formz

sean96SE: by what you are saying then no exhaust would be the best exhaust. And we know that is FAR from true. An engine needs backpressure to run. So think of the extreme cases.

exhip95 : I talk about DSMs because they're what I know best. Engines are engines no matter what car they're in they all work the same. Just because you've been around cars longer than I have doesn't mean you know more then I do. I made a living building and tuning cars, and have put many 4,5,600hp cars on the Dyna Pack we have and turned them loose on the street. BTW... that swap in that Z is nothing special at all, and you know that.

plurco

iTrader: (1)

Ok I am going to give you the benifit of the doubt on this one even though I have seen this myth explained away many times. Argue your case from a technical standpoint.

sryth

Backpressure...good? I've heard that one too, but have heard little to back it up. I think people confuse 'backpressure' with high exhaust velocity...which can be good depending on the application.

IIRC, hight exhaust velocity will help out low end...maybe it was top-end...can't quite remember and my mind is failing me right now...the point is, it depends on the application.

plurco

iTrader: (1)

here I will go first.


Destroying a myth.



Some say that "an engine needs backpressure to work correctly." Is this true?

No. It would be more correct to say, "a perfectly stock engine that cannot adjust its fuel delivery needs backpressure to work correctly." This idea is a myth. As with all myths, however, there is a hint of fact with this one. Particularly, some people equate backpressure with torque, and others fear that too little backpressure will lead to valve burning.

The first reason why people say "backpressure is good" is because they believe that increased backpressure by itself will increase torque, particularly with a stock exhaust manifold. Granted, some stock manifolds act somewhat like performance headers at low RPM, but these manifolds will exhibit poor performance at higher RPM. This, however does not automatically lead to the conclusion that backpressure produces more torque. The increase in torque is not due to backpressure, but to the effects of changes in fuel/air mixture, which will be described in more detail below.

The other reason why people say "backpressure is good" is because they hear that cars (or motorcycles) that have had performance exhaust work done to them would then go on to burn exhaust valves. Now, it is true that such valve burning has occurred as a result of the exhaust mods, but it isn't due merely to a lack of backpressure.

The internal combustion engine is a complex, dynamic collection of different systems working together to convert the stored power in gasoline into mechanical energy to push a car down the road. Anytime one of these systems are modified, that mod will also indirectly affect the other systems, as well.

Now, valve burning occurs as a result of a very lean-burning engine. In order to achieve a theoretical optimal combustion, an engine needs 14.7 parts of oxygen by mass to 1 part of gasoline (again, by mass). This is referred to as a stochiometric (chemically correct) mixture, and is commonly referred to as a 14.7:1 mix. If an engine burns with less oxygen present (13:1, 12:1, etc...), it is said to run rich. Conversely, if the engine runs with more oxygen present (16:1, 17:1, etc...), it is said to run lean. Today's engines are designed to run at 14.7:1 for normally cruising, with rich mixtures on acceleration or warm-up, and lean mixtures while decelerating.

Getting back to the discussion, the reason that exhaust valves burn is because the engine is burning lean. Normal engines will tolerate lean burning for a little bit, but not for sustained periods of time. The reason why the engine is burning lean to begin with is that the reduction in backpressure is causing more air to be drawn into the combustion chamber than before. Earlier cars (and motorcycles) with carburetion often could not adjust because of the way that backpressure caused air to flow backwards through the carburetor after the air already got loaded down with fuel, and caused the air to receive a second load of fuel. While a bad design, it was nonetheless used in a lot of vehicles. Once these vehicles received performance mods that reduced backpressure, they no longer had that double-loading effect, and then tended to burn valves because of the resulting over-lean condition. This, incidentally, also provides a basis for the "torque increase" seen if backpressure is maintained. As the fuel/air mixture becomes leaner, the resultant combustion will produce progressively less and less of the force needed to produce torque.

formz

I didn't say an engine with no backpressure won't work.. never ever did I say that. It just won't make the most power.

EDIT: Actually looking at what I typed I could see how you could get that I was trying to say it wouldn't work at all. Sorry about that I meant it just wouldn't make the most power. It certainly will run. D'oh.

plurco

iTrader: (1)

Seems to me if you can compensate for that lessened restriction on the exhaust side with adequate fuel mapping then that is how you are going to abtain "max power"
I am no Physics wiz, but this does not seem to be hard to figure out.
BTW you did say an engine would not run without backpressure, hence the quote above. Please don't take this out of context, I am not trying to be a wise acre, I am just trying to debate this dead horse of an issue.

formz

No you're right I did say the wrong thing. I meant one thing and said another. You're totally right to correct me.

sean96SE

You've kind of made an assumption here about what I said. While a better flowing exhaust will make an engine perform better and be more efficient, I should have also mentioned that having NO exhaust piping whatsoever would probably have a negative effect.

So yeah looking at the extreme cases, somewhere between NO exhaust pipe and a highly restrictive one is the optimal setup, for performance and efficiency. That is what I meant.

formz

I have no idea what to say. So I'll go with... "ok "