...get a full tank of gas? It should be the opposite because the car is lighter. But it really seems to slow down as I approach empty. Could it be something with the fuel injectors?

 

Maybe a clogged fuel filter?

 

Just a guess, but since the fuel pump has to work harder, maybe you are not getting quite enough fuel line pressure on low tank.

 

Now that you mention it, i do remember the same thing happening myself. On a full-tank the Max is more responsive as opposed to having 1/4 or less of a tank of gas left.

 

ask russ... didn't he have a theory?

 

Same Here! I thought it must have been just me, my 01 se also feels faster after i fill her up. and feels slower when its running low on gas. What was the theory behind this?

 

Or maybe the new fuel stirs up the fuel inside when you pump fresh gas. I don't know, just a dumb guess that stirring up the fuel makes it chemically more active.

 

mine does the same. my 00 runs alot better after i fill it

 

I agree. It's never made sense to me, but mine feels the same way.

 

maybe the weight of the gasoline adding pressure to the fuel line?

 

Thats what I've always thought....

 

I've always thought that too. My theory was that since I had a full tank of gas I actually pushed the car harder as opposed to having a low tank and subconsciously conserving gas by not driving as aggressive. I figured it was all in my head.

 

Also, the lower in the tank you get, the more you are likely to be fighting impurities. Most impurities will be heavier than gas and therefore will settle. As you get lower in the tank, the fuel pump will have to work harder to push dirty gas through the filter vs pure gas that passes through the filter with greater ease. The result is a slight decrease in fuel pressure.

Just a guess, but an educated one.

 

my car feels faster too!!!!!

 

mine does too

 

its a conspiracy...

 

my 2k agrees!

a conspiracy...huhh...maybe it's just the gas company trying to get you to fill up more often!!!

 

this makes no sense... full tank or empty, using your words the impurities sink to the bottom so whether it's full or empty it'll have the same amount of impurities in the filter.

 

Again, I am speculating. But, the lower the fuel level, the higher percentage stray materials make up out of the total. The force that is required to push fuel through the filter increases as more material must be filtered out. That would seem like a possible cause for fuel pressure drop. I would be interested in testing this. Unfortunately I lack the equipment.
Anybody out there who can verify or disprove this?

 

with a notch below FULL my car ran the 1/4 mile in 14.995... When I run with the car low on gas (another day), I ran my best time.







..but after I fill my tank, it does feel more powerful

 

I've gotta agree. My car does feel more powerful with a full tank.

 

I suppose this is why some people run higher quality/redundant fuel pumps.

 

Guys its called an "azz illusion". No. Seriously, what happens is the heavier a car is, being that the Hp & Tq stays the same for the same car, the faster it 'seems' to go. Here's what I mean-there is more weight transfering to the rear of the vehicle thus making you think it is moving faster than it actually is. Your suspension has everything to do with this "illusion".
Don't believe me, drive a sports car with a full tank then an empty one. You would be hard pressed to find any desernible difference in the 'feel of how fast it is. Or you could try to compare a max with a severely stiff suspension to one with a stock setup(with the same amount of fuel onboard). The car with the tighter suspension gives less thus lessens the 'felt' effects of torque. Even though the maxima is the same in every other regard except the suspension you would think the one without the beefed up sus. is faster unless you raced each other. I'll bet they'd be neck & neck.
This is not some azz brain theory either. At the drag strip it is common to see it happen. Take for example two cars that run 10's or 11's with one being set up with the traditional 90/10 shocks and the other vehicle set up for a not so traditional set up. Two cars that might fit this face off description would be a stang or maro with the 90/10 as opposed to maybe a supra or WRX a stiffer setup. Now if you were to go down the strip in each I bet you would swear the stang or maro was faster with the front wheels off the ground and all you see is sky for the first 60feet, even if the supra or wrx got to the traps a few ticks quicker.
I know some of you will want to flame because it just wont makes sense at first, but do yourself a favor and go test this out. Then if you still disagree come back and say why.

Here's an easy way to test this--drive your max down to a used car lot that has a caprice with a v-8 and give it a drive. It will sure feel light years faster than your max, but its just not the case.

 

My 2000 acts the same way - I mentioned it to a buddy of mine last fill up and he said it was all in my head.

 

It has the same effect for me. But I wouldn't say it goes faster. I would say it's more responsive to the gas pedal. And it seems to quiet down the engine a bit also.

 

I think its easier to explain if it was a consipracy

 

again, this makes no sense... "higher percentage of the total" and "force required to push fuel through" have nothing to do with each other. The pump regulates how much gas goes in the fuel line, not the amount of gas in the tank.

If you fill a 50 gallon drum with water and poke a hole in it the water will run slower as the level goes down, BUT!!! if you put a sump pump in the barrel it'll pump 5gallons/min no matter how much water is in there. Fuel pump = same thing...

Finally, someone said "it's all in your head." ... been waiting for that. Thanks tampamax.

 

It is an middle east conspiracy to get us to buy more gas. Some OPEC engineer got himself hired by Nissan and put this anomoly in the suspension / fuel delivery system so that people would fill up more!!

 

That's funny that you would post something like this, I started to do the same because I thought that it was only my car that was doing that.

I always try to keep my car with at least 1/2 tank of gas if not more, I love the feel of it when it is topped off.

 

Add to that example. Put a fine screen (a filter ever,) over the intake hole for the sump pump. If there is nothing but water in the drum, nothing changes. However, if there are flakes of rust, steel shavings, organic debris, etc, these will collect on the screen. The result is that the pump has to work harder and therefore pressure coming out of the pump is lowered.

Somebody with a fuel pressure gage, please try this. On a low tank, measure fuel line pressue across the RPM range. Then, under similar conditions (air temp, engine temp, etc) measure the same pressure with a full tank. I bet there is a slight difference. This difference will not overcome the added weight during a quarter mile, as many have proven.

 

Caprice feels faster because of the high torque, especially at the low end. It is not as fast becuase of lower redline, lower high end torque, and higher weight. Torque is what you feel. Horsepower vs weight is what makes it fast.

I had not thought of the suspension shift making a car feel faster. I will have to pay attention to that next time I have 3 or 4 people in the car.

 

I have noticed that before on my car. Until you mention it, it doesn't do that to my car anymore. I think it has to with the atmospheric weather change. That's my theory.

 

Hey, I posted the original message. I should have clarified that it feels more responsive, not that it feels faster.

Anyway, I'm a meteorologist in Florida. I agree that atmospheric conditions do affect engine performance. But since our weather really doesn't change much from day to day, I'd have to rule that out as an explanation.

 

I can see were you are coming from, but the theory would not apply in this case. 1st problem with that example is that the screen/filter, in this case the fuel tank, is plenty big and more than sufficient to filter the fuel in the tank.

2nd problem is that there is no way there is enough crap in your gas tank to sufficate the the fuel system and load up the pump(making it work harder to pump the same amount of fuel). If there is then you got a serious problem, but this is highly doubtful.


[QOUTE=Ben]
Somebody with a fuel pressure gage, please try this. On a low tank, measure fuel line pressue across the RPM range. Then, under similar conditions (air temp, engine temp, etc) measure the same pressure with a full tank. I bet there is a slight difference. This difference will not overcome the added weight during a quarter mile, as many have proven.[/QUOTE]


Once again, it has to do with more weight in the car with all else being the same. It is especially critical as to where the weight is added. So once you add however many lbs. of fuel that much more weight is transfering to the rear of your car. With the same amount of Tq being applied to the ground and the added weight it simply mutiplies the effects of the 'illusion' that one is now moving faster.

 

I am having a hard time understanding how the wieght shift could seem to affect the butt dyno the way that you say. I am not trying to flame, I am trying to understand.

A tank of gas weighs about 100 lbs. It is roughly under the rear seats. It would seem that putting 100 lbs in the rear seat should have the same effect as filling the tank if the weight shift is the source of the illusion. To the contrary, the more weight I have in the back seat (for example, my overweight bro-in-law,) the slower my max feels. More weight is shifting to the back without any change in torque.

How does this fit into your description? Again, I am not trying to flame, I want to understand this.

Another problem, I can see that the initial weight transfer to the rear would affect the drivers sense of acceleration. However, does this weight continue to shift after the initial move? It seems like it should reach a balance where it would no longer effect the butt dyno.

 

[QUOTE=Ben]Caprice feels faster because of the high torque, especially at the low end. It is not as fast becuase of lower redline, lower high end torque, and higher weight. Torque is what you feel. Horsepower vs weight is what makes it fast.



I think you misunderstood my post. And i'm not sure what to make of yours, but I believe we are getting at the same point. The higher the weight the more Tq is required to 'get' the car moving faster. Whether from a dig or from a roll you feel the effects of the relationship between Tq and weight more in the caprice than in a Max. However, if you were to drive a Caprice and an SS Impala you would not feel so much of the effects of the Tq to weight relationship manifesting itself in the Imapla as opposed to being in the Caprice. That's where the suspension comes into play.

You had a good idea about passengers in the car. One could have 'a' person sit in the back of the car and that should have the same affect as filling up. Just as long as the passenger isn't like 200+ lbs. In that case the car would be noticably slower. BTW how much does a gallon of gas weigh? Take that answer and mutiply by how many gallons it would take to fill up and you have the ideal weight of your geunie(sp?) passenger.

 

[QUOTE=tampamax]Gas weighs about 6 lbs per gallon. So, about 90 lbs for my normal fill up.
But, if I add 100 lbs to my back seat, I don't notice a change. I do notice a change when I fill my tank.
That's why I think there is something else at work here.

 

"not being mean or sarcastic"
So, you're saying that you have tried this today or yesterday? Or are you speaking from what you recall the car responding like when you had 100lbs in your back seat at some point and time?

 

Previous occassions.

 

the problem with your addition is that if you fill the tank the "stuff" doesn't float around, it's still jammed hard against the fuel filter/screen, so I can't see how adding gas to the tank relieves any "flakes of rust, steel shavings, organic debris" that collects on the screen.

basically, putting gas in doesn't clean the filter/screen, so I'm confused....