So awhile back someone suggested that I run a full tank of gas while autocrossing. The reasoning as you can probably tell is better weight distribution (low, in the rear). I'm wondering if anyone else around here intentionally autocrosses with a full tank.

At my last event I was pretty full, and the car did feel a hint more neutral, with less disparity between front + back tire pressures / shock settings. But I'm not a believer. I usually run on fumes, the less weight the better.

The person I was talking to drives an HS Cooper. I wonder if stock classes are better suited for this, since weight distribution would seem more important as there are fewer variables to play with (ie. swaybar + spring rates), AND stock cars' hp/weight ratio is not as important. I run in SM, and every smidge of advantage (lighter weight) I can get against 400+hp Evos and STis, the better!

What do you think?

 

As a wise man once said "To add speed, add lightness"

I subscribe to the theory that you will never go faster by adding weight. Adding weight to the rear may make the car feel a bit more neutral, but you have not done anything to increase the cornering ability of the front, which is the limiting factor in how much cornering speed you can carry (nose heavy MINI wants to understeer). This aditional weight has also slowed you down when accelerating in a straight line and reduced your braking ability.

Jason

 

I've tried it with a full tank, and I didn't like it. But it's more of a personal preference, I like my rear to step-out just a bit. I usually start the day with 1/4 of a tank.

 

I usually run with a pretty low tank. I think a lighter back end lets it come around a little better. I haven't had any fuel starve issues either, and it has gotten very low at some events when I take advantage of fun runs.

I run an MCS in stock class, for now.

 

For most of this summer I had been using 1/4 tank or less. But for the Nationals last weekend I decided to go with a full tank based on observations by others of severe inside/rear wheel-lift during past events. I have JCW struts/springs (one strut blew out with 11000kms on it!) so it's not soft but it ain't race-hard either. The added weight was not a factor on day 1 as we had wet conditions and so speeds were limited. It felt as though the extra wheel on the ground with some weight on it helped the ABS. Day 2 was drier but by then I was down to 3/4 tank. Again I have to say the balance was better overall. I think you want to avoid the 1/4 to 3/4 range... too much space for slopping the gas around and throwing off the balance following a quick steering input.

So fill'er up or leave it almost empty (but not enough to starve your fuel-pickup).

 

Lighter is definitely better. At a track day, I was running with a friend who had an 06 Miata. I was able to pull away from him slightly lap after lap with low fuel. I went out and put in a full tank and he actually started pulling on me slightly lap after lap on average when we went back out. The biggest difference was braking, I wasn't able to brake as late as I did with no fuel(not a massive difference, but, a noticable difference to myself). Sure, its only 100lbs or so, but, every pound makes a difference at the big events(sure, it comes down to driver 99% of the time, but, when you lose by 0.001, you know that extra 100lbs comes into play. Now I just need to go on a serious diet before nats.

 

Power to weight ratio... higher is better.

1 gallon of gasoline equals 5.8 to 6.5 lbs. Weight depends on content of ethanol which weighs about 6.6 lbs per gallon.

Figure low tank vs full tank is difference of about 10-11 gallons.
Or about 60-65 lbs.

For track events you will be driving 10 to 20 minutes per session so have enough gas to last. Running 1/4 tank is good and fill up during rest periods.

For autocross I usually run 1/8 tank and sometimes less so that when I finish my 4 runs I get the light to just turn on red.

 

If my light hasn't been on for a while before my first run, I feel bloated. lol Actually, my goal is to have ---- on the range on my last run or two.

 

I am actually amazed at how well the car picks up fuel in the tank. I personally run the car as low on fuel as much as possible.

Not to be paranoid, but I think that the suggestion to run a full fuel tank might have come from someone who does not do it themselves, and runs your class.

 

You're paranoid.

edit: the suggestion came from HS winner at Nationals stop in Devens this year. If it was just some random dude, I wouldn't have paid much thought.

That said, I'm in agreeance with you guys and still run it empty at events.