Fuel level for autocross

[cmiovino]

Basic, but controversial topic. We all can agree that less weight is better. However the entire fuel tank is 13.2 gallons and with higher octanes coming in at 6.1lbs, that's a maximum of 80.5lbs in fuel.

The fuel light comes on right around 3 gallons being left, so you need at least 18.3lbs in the car more or less to be running with the light just coming on. I haven't seen any fuel starvation issues doing this personally and other 1st and 2nd gen owners tend to agree if you're on street tires.

Give or take, it's a little bit over 60lbs you can either run with or not, which for the record is located at the lowest point in the car and towards the rear.

The first gen's curb weight comes in at 2,835lbs. I've had mine weighed with me in it and it was around 2950 with me (150lbs) and half a tank of fuel, on the gauge at least. This was also with the spare removed.

Bottom line, it's about ~2920 if it's with the fuel light on or 2980, or about a 2% overall deduction in weight at a maximum. More like 1% or 30lbs if you're running 1/2 vs with the light on.

We even have people running around in grid before runs putting a splash of fuel in, trying to shave off <5lbs. It seems nuts.

Most of the time I'll fill only ~5 gallons before the event, drive an hour there, run, an hour back, and I generally use about 4-5 gallons doing so. I sort of hate doing this because I know 1/2 gallon of that is likely 87 octane from the previous person who filled. If you're putting a gallon or two in, you're getting ~50% 87 octane.

That leads me to the theoretical question that if everything is 100% equal on your run, how much actual time might you be leaving on the table with 30 to even 60lbs extra in the car? Are we talking 0.1 or more towards 0.5? Or is it even not quantifiable?

1% off a 30 second time is 0.3... but times and weights don't work like this. I'd have to assume it's less than 0.3 seconds in that example.

You could even argue that leaving the extra ~30-60lbs in the rear low to the ground slightly helps overall balance, would could make the car turn in a hair better, that could impact times also.

[foshjowler]

I'm not really sure what sort of time you're leaving on the table, where's Andy Hollis to do a test? But if I was to guess, it'd be in the .1 second range when comparing low to full fuel. Low enough to be in the noise, but every advantage is an advantage.


I'll typically run it with the light on, and never had any issues with fuel starving. The biggest difference I find in full vs almost empty is the balance of the car. It shifts that 60lbs or so to the back of the car, and settles the rear. Could be a good or bad thing depending on setup, but I would try and keep it consistent across events.


What you don't want to do is have half a tank as you now have a mass of fluid sloshing around constantly changing the balance of the car. Run it almost empty or full.


I don't always get it right, especially for local events I'll sometimes show up with whatever is in the tank and do no planning ahead of time, but for national stuff, I do whatever I can to show up with low fuel.

[autoracer86]

Book a test and tune event show up with low fuel do a few runs then toss in some gas and do more runs.

See how it goes

[14stu]

From taking passengers, seems like about 0.05-0.1s per 50lbs (course dependent, the more hard acceleration, "dig" corners the worse the weight).

[Locust]

When I had a co-driver for several years we tracked it at about 0.3s difference for a 200lb passenger. So every 1/4 tank of fuel you're around 20lbs or 0.030s. In street class these cars can run a full 4-run event once the gas light comes on except at high grip sites like Crow's or Lincoln. In STX trim or higher I would keep it to between 1/4 to 1/8 a tank.

[Sapphireho]

Originally Posted by Locust

When I had a co-driver for several years we tracked it at about 0.3s difference for a 200lb passenger. So every 1/4 tank of fuel you're around 20lbs or 0.030s. In street class these cars can run a full 4-run event once the gas light comes on except at high grip sites like Crow's or Lincoln. In STX trim or higher I would keep it to between 1/4 to 1/8 a tank.

Crows in Cal?

[cmiovino]

Interesting results this weekend. Car was on empty with ~19 miles flashing on the dash after some distance driving and an event prior to this. Decided just to fill up fully because I'd be driving a hour, running an event with 9 runs, driving back, then doing the same the next day with 6 runs Didn't want to be partial filling ~5 gallons both mornings.

Ended up getting my top finish ever at a local event. This is out of at least 100 events at this particular location location. The first day was my 3rd best out of ~100 too. Tank was virtually full on day one and at just over half on day two during runs.

Definitely not scientific testing and no real "control". I can't say the extra weight was slowing me down that much. Given @Locust's data point, I'd say maybe a tenth or two may have been out there given everything else equal with that full tank.

One very subjective thing I'd like to note was slaloms and transitions felt better with a full tank. Felt. I can't put any numbers or times behind it.

I can see how at a national event you'd want the lowest fuel level possible to limit sloshing AND reduce weight. Best of both worlds.

[jcaserta]

The internet seems to think there's about 1/3 of a gallon left of whatever prior person had at the pump. And it's worth noting that the majority of newer cars seem to require premium. And the premium is more like 94 or 95 octane, not actually 93. So if you put in at least 2 gallons, definitely 3, of 93 you're very likely a total of 93 or better even if prior person did 87. I find even keeping my fuel level targeted at about 1/4 tank at start of autox I generally put in 4 or 5 filling up before I drive to the event, so I feel confident I'm getting at least 93.

If I'm doing a lot of trailering, then can use gas cans so that also solves the problem.

When I have a passenger it is very significant. Haven't had many passengers recently but probably at least 0.5 second on a 50-60 second course. And they're usually 200 lb give or take 40.

My guess is another 30 lb/5 gallons is around 0.1 seconds.

[SCFD]

Trying to optimize the car based on fuel levels is probably highly dependent on the driver's skill and competition. You'd need to be a fast and consistent enough of driver for it to matter. Subjectively, I prefer how the car feels with a full tank compared to one near empty. Although every little bit of advantage adds up, I personally never felt my results would've changed due to the amount of fuel in my gas tank.

[strat61caster]

Originally Posted by cmiovino

Interesting results this weekend. Car was on empty with ~19 miles flashing on the dash after some distance driving and an event prior to this. Decided just to fill up fully because I'd be driving a hour, running an event with 9 runs, driving back, then doing the same the next day with 6 runs Didn't want to be partial filling ~5 gallons both mornings.

Ended up getting my top finish ever at a local event. This is out of at least 100 events at this particular location location. The first day was my 3rd best out of ~100 too. Tank was virtually full on day one and at just over half on day two during runs.

Definitely not scientific testing and no real "control". I can't say the extra weight was slowing me down that much. Given @Locust's data point, I'd say maybe a tenth or two may have been out there given everything else equal with that full tank.

One very subjective thing I'd like to note was slaloms and transitions felt better with a full tank. Felt. I can't put any numbers or times behind it.

I can see how at a national event you'd want the lowest fuel level possible to limit sloshing AND reduce weight. Best of both worlds.

Yeah I try to show up with >1/4 tank and I don’t waste any more energy thinking about it until I’m on my way home and deciding where I need to fill up so I’m not overly late to work on Monday.

[gnarjunkie]

Originally Posted by SCFD

Trying to optimize the car based on fuel levels is probably highly dependent on the driver's skill and competition. You'd need to be a fast and consistent enough of driver for it to matter. Subjectively, I prefer how the car feels with a full tank compared to one near empty. Although every little bit of advantage adds up, I personally never felt my results would've changed due to the amount of fuel in my gas tank.

^ this.

I know there's an advantage to less weight for sure and I used to always show up to events on the fuel light. But honestly, I just don't know if I'm consistent and/or fast enough to find the small amounts from something like that. There's always something else I could've done better or worse that makes so much more of a difference in time.

Same with a passenger, I rarely see any improvement with having one or not. Maybe it's that I usually have one, so I'm used to the balance difference. But I've very rarely taken a solo lap and seen a noticeably faster time.

And while I'm probably just not good enough to be running right on the edge of what the car is capable of all the time, I'm not that slow. I'm almost always on the podium in class and often in top 10 PAX with a completely stock GR86 on 225 re71rs.

[Locust]

Originally Posted by Sapphireho

Crows in Cal?

Yes Crow's Landing in California. You're going to pull higher Gs on concrete and increase your likelihood to fuel starve.

[e1_griego]

@Locust please share your washer fluid strategy.

[Autoxer62]

Originally Posted by Locust

When I had a co-driver for several years we tracked it at about 0.3s difference for a 200lb passenger. So every 1/4 tank of fuel you're around 20lbs or 0.030s. In street class these cars can run a full 4-run event once the gas light comes on except at high grip sites like Crow's or Lincoln. In STX trim or higher I would keep it to between 1/4 to 1/8 a tank.

Not all weight is created equal, your problem there is the wrong passenger. I had a friend that had a passenger seat in his EMod car to take "babes" (his term) for ride alongs, he said he went about a half second faster.