OK, this is a weird question, because a full gas tank vs. a near-empty one is obviously slightly heavier. But we're not talking about very much weight here. Our cars have what? 55 liter tanks?

55 liters of water = 55KG
55KG = 121lbs

And gasoline is much lighter than water anyway. So let's say, give or take, a full tank of gas in our cars makes for a 100lb difference. By the old school of thought, removing 100lbs of weight in the car = decrease of 0.1s in the 1/4 mile. And of course that varies from car to car.

So why am I posting this thread? Because from every car I have ever owned, I've noticed them running "peppier" with a full tank of gas than with a near-empty one. I guess what I truely mean by "peppier" is an increase in throttle response, which I guess I also perceived as an increase in acceleration.

It's therefore been a recurring thought in my mind that cars will actually perform better in straight line acceleration with a full tank of gas than with a near-empty one, regardless of the +/- 100lbs weight difference.

Am I on crack, or does this kind of make sense? Does it have anything to do with the fuel pump, and the fact that it can pick up gas at the top of the tank easier than gas at the bottom of the tank? And has anyone done any research on this? I've always wanted an answer to this dilema!

 

i have wondered that before too but i just don't understand how it could be true.


i know at the drag strip the less in the tank the better

 

I have noticed the same thing, a fresh tank of gas made the car feel Pepier. Maybe it has something to do with the emmissions system? The fuel pickup is at the bottom of the tank; however, I think the pump might be picking up air when it's below a half tank, due to fuel sloshing about.

I have a modified fule system and at half tank it sucks air if I punch it hard from a stop. The pickup for the new system is at the same level as the stock pump. So the stock system might be subject to the same problem, just at a slower, less noticable rate since it move less volume.

 

Maybe it's just the weight distribution. since the tank is in the rear, and this is a front heavy car, the weight shift to the rear on acceleration would be minutely greater with a full tank, making it feel peppier?

This is me thinking out of my ass though, so don't take it too seriously.

 

Weird, my car feels "peppier" when I have less than 1/4 tank left. It feels like an M1 tank with a full tank.

 

My car feels peppier when my car is freshly cleaned also

 

I'm with you on this one. I can feel the additional weight when full.

 

I can absolutely notice that my cars feel slower when the tank is full. I lose ~ .2 & 1-2mph at the strip in my mustang when its full vs. my regular 1/8th tank.

 

I gotta agree with you on this one

 

 

?????? HUH?????? Maybe 20 years ago this might be a problem. All car gas tanks come with an anti slosh type fuel sender unit, this retains fuel right till its empty. There is even barriers in most tanks to slow fuel from moving from side to side. My

 

The reason why your car feels peppier when you have a full tank of gas is because in all cars water acculates at the bottom of your tank, no much but enuf. It gets there from condensation on the inside of your tank cause of all of the tempature changes. Also the gas you pump into your car isnt always pure, and has its own impurities. Also any crap in your tank will settle down to the bottom. My best experience is if you run your tank to almost empty then fill it about half way up with fresh gas b4 you go to the track. Having a few gallons in the tank helps the fuel pump get at the gas easier too.

 

That doesn't applie to new cars. With modern fuel injection & fuel there's no harm in running today cars to empty then filling up. Older cars there's some truth to it, & even then the cars need to be like 10 or more years old.

My car feels alot faster with the fuel light on. I fill it up, I feel the difference.

 

I guess this doesn't apply to my car, since I'm not stock. My car accelerates much quicker than the average TL/CL.

 

I guess this doesn't apply to my car, since I'm not stock. My car accelerates much quicker than the average TL/CL.

 

I find that my ride runs "peppier" when I have a hoochie mommy with me...

 

Not true. Here is a post I posted a few months ago:

 

in addition to post #16, water can and does enter your tank by virtue of the fact that all underground fuel storage tanks leak. ground water seeps into these tanks polluting the fuel. also the tanks are inherently at 50-55 deg F. near with fluxuating r.h. they do have times when they are under 1/2 fuel and can be subject to significant condensation on the inside of said tanks. this water is pumped into customers vehicles during fueling . most prevelant in stations which are lightly frequented.
i always fuel at the busiest station i can find as they are refilled more often and experience less leakage.

 

So wait, water leaks into underground fuel tanks? I find that hard to believe since quality control would be hard to do. Since you would have to dig up the fuel tank every so years just to fix the ever-growing-problem since water does errode many materials. I don't think so.

 

Did you read the data? They used a 200 gallon tank just to blow up the data. Now shrink this 10x and you have the normal tank size of SUVs and even lower... cars. That would account for less than 100th of an ounce of water. Negligible if you say.

 

the car is just happy with a full belly of 93

 

reply post #18,
yes they leak. (i appreciate your skeptisim ) there is a multi billion dollar govt program aimed at reducing ground water pollution in the us. part of this program is identifying UST's and abating the contaminated soils associated with the history of leakage. and when the tanks are not full, hydraulic pressure from surrounding ground water forces the water into these tanks. many of these tanks (at your local station ) are leaking and will continue until the millions of stations nationwide have replaced with updated pressure tested fiberglass units
btw, WHAT QUALITY CONTROL are you speaking of?

 

I guess

 

I agree, the CL just seems a lot "happier" when it's clean and full...

But, It's amazing what your own mind can trick you into thinking...

B

 

On her knees or on the hood...

 

This post reminds me of the debates between octane booster, spark plugs etc. .

 

I do Phase I ESA's all the time. All UST's do not leak. Give me a break

If anything they leak the fuel out, not water in. And if any water via seepage or condensation were to occur within the tanks the amount of water would be so miniscule compared to the amount of fuel within the tank it won't make one damn bit of difference.

 

with my car i can feel a faster throttle response with a full tank but also i can feel the weight when it is full when i take conrners

 

I was told that there is an issue with running cars with too low of fuel because it can shorten your fuel pump life. From what I understand, fuel in the tank actually cools the fuel pump. When it's too low, there's no cooling effect and your pump runs hotter thereby shortening it's life.

 

I believe the pump gets cooled as fuel passes through it. Thus, the amount of fuel in the tank wouldn't matter as the same amount of fuel passes through the pump when it has 16 gallons in it as it does with 1 gallon.

 

same here

same goes for my moms tahoe and dads tundra

 

i know this is an old thread... first of all i didn't read the full thread so i don't know if this might have came up...... it may sound dumb but here's a thought.... could it be when ur tank reaches a certain point (we'll call it 1/4 full, cuz thats when my gas light goes on) that the ECU gets the signal that u r low on fuel... so the ECU restricts the motors performance as well as to try and conserve gas...

 

the removal of minute particles from the body improve the aerodynamics of the car.

 

I should stop using Zaino...it might take 0.00230532 seconds from my 1/4mi times.

 

The only logic I can think of is that the new fuel is COOLER than the fuel in your gas tank, at least in the summer anyway. Engines like cooler denser fuel mixture.

 

I can feel the extra weight when the tank is full. When I have about 2 gallons left the car wants to fly.

 

Ding Ding! We have a winner. This is why your car feels peppier after you fill it up or have a full tank of gas. When your car is running, especially on a hotter day with higher underhood temps, your fuel is pumped up to the fuel pressure regulator and slowly bled off and returned to the tank. This fuel that is returned to the tank is heated by the underhood temps heating up the fuel rail and related components. This causes the fuel in the tank to be less dense (therefore contains less actual fuel molecules for a given volume). When you pump new gas in, it is cool from being underground regardless of outside temps because the tanks are below the frostline so they maintain a relatively constant cool temperature (think your basement stays cool year around) .

Another factor in the fuel tank level making a difference is, the fuel that is recirculated back into the tank compared to the over all volume of fuel in the tank. In other words, if you have a 1/4 tank of fuel and drive for a given amount of time, your fuel will simply heat up quicker/hotter as compared to a full tank of gas which will absorb/dissipate more heat since there is more fuel to absorb and dissipate the heat.


The cooler fuel also when pumped into a hot cylinder is more completely atomized than fuel that is warm. The car feels much peppier but the extra weight of the fuel may take some of that away. But you are feeling the extra power whereas your actual 1/4 mile times may be a tad lower.

Either way, you will make more power with cooler fuel. In fact, if you don't believe me and have time/money on your hands. Make a small metal/plastic box that will fit underhood. Inside the box have a copper line coiled so that it looks like a spring sort of. Run your fuel line into the copper line and out the other side inline. Fill the box up with ice and enjoy more power. This also cools the intake temp when the cool fuel hits the air and we all (hopefully) know what cooler intake temps do.


Cliffs: Eh, why did I type all that when people only read the first two lines of a post anyway lol.
I had a friend who raced (small local stuff) and the cars were regulated pretty close. He had the "icebox" (see next paragraph) built in his car and he swears that's why he won so much. He was always faster on the track than the other cars. His only advantage-cooler fuel temps.

Sorry for the long post but if someone wants to experiment with this box, another variation is running the copper line inside your windshield washer reservoir. I'm not sure what ours look like but if there is room to run the lines through it will help. The methanol in the winshield washer fluid (buy the 40 below stuff for higher methanol content) will absorb heat like crazy. Ever get winshield washer fluid on your hand, it makes it nice and cold. Same for your fuel. I would do it to a track or race car but not on my daily driver but if you like to extract all the hp you can, this will help.

If anyone is interested in this idea, I can try to draw a picture of it.

 

This will sound stupid (and probably is), but I trash talked my way into a race with a recently retired champion grayhound back when my 97 Prelude was my daily driver. The race was between 3 telephone poles (start at one pass the second and finish at the third).

I made a very concious effort to drive the tank down as far as I felt possible to race and make it to the pump.