I am going to Autocross my RX7 this weekend. I was told that I should run with the tank full to put more weight over the rear tires and balance the car. I would be adding about 85lbs by running full vs 1/4 tank. If I ran 1/4 would the sloshing cause the rear to get upset?

Full or 1/4?

 

Full, but not for the reason mentioned.

If you have 1/4 tank of gas and there are any heavy G turns, your pickups may suck in air instead of fuel, causing your motor to cut out during and exiting the turn.

 

If it's an FD you better have that thing 1/2 full, MINIMUM. 3/4 full if on slicks.

 

Yeah, i used to race my truck offroad with barely any gas and on big hills itd come close to dying

 

I run my FC almost empty. I never have any perceivable pick-up problems. The weight difference from running it ful of gas is truly significant. Don't believe me? Go run your car hard on a light tank and then fill it up and go back out.

Cory

 

I say completely full, and have additional reasons. For one thing, it's repeatable. When the tank is FULL you KNOW how much is in it, as opposed to when it's at "1/4 tank" which could mean anything within a range. INconsistent.

Another thing is, the fuel sloshes around pretty much undamped. Might as well have some bowling ***** knocking around loose in the back. Remember how carefully you corner weighted the car? Gone. Plus since it's undamped it's inconsistent. Full tanks do not slosh.

 

I have tried doing auto-x with little gas and that let the taill lose abit too much now I always go with full tank. I am using slicks (g-Force R1s)

 

Interesting, Henry Watts recommends 1/4 tank in his book, and to bring a 5 gallon plastic tank of extra gas just in case. I've never noticed any of the symptoms listed above, but then again, I'm by no means an expert at this. So Henry Watts is wrong then?

Steve

 

No, Watts just hasn't considered cars like the RX-7 that can kill engines by running out of gas. I don't think that there is a right answer ... it all depends on the driver's level of skill. Most auto-x'ers try and lighten the car up as much as the rulebook allows. Personal experience tells me that my FD is a gas hog on course .. so I keep the tank at least half full. Everytime that I've looped it has always been a function of my steering/lack-of-braking inputs; never once because the backend was too light.

 

I prefer the balance of the car with the tank as empty as possible, but I can easily starve the fuel pick up with anything under half a tank when on slicks.

 

Depending on the state of tune your car is in and how much power you are making you need to keep that tank at least 1/2 to 3/4 full. The 3rd gen tank is 20.1 us gal and contains no real baffling. Upon hard acceleration you can starve the fuel pump pick up as already stated. If you are lucky the car will jolt and buck, if you are unlucky you will need a new engine after your little attempt to save some weight. I road race regularly, and have been "lucky" once, and don't plan to be "unlucky". My friend, who also has an FD was "unlucky". Don't risk it, if you need more power to make up for the weight difference just up the boost a little, or shave weight somewhere else.
Art

 

Thanks, Art. Like I said, I PREFER the balance with a low tank but am not able to run that way because of the problems you describe above.

 

Call me a wuss but running my car on 1/4 to E , messed up my Fuel pump and that wasn't cheap.

 

I'm just curious but how did that mess up your pump?

 

pumps should move something that is much closer to liquid or at least has a high vapor pressure. Ever heard of a waterpump going bad becasue of cavitation? Same thing with the fuel pump. It can do actual physical damage to the "moving part" whatever it may be in these fuel pumps.

Ever see a boat propeller with pits and the paint missing? That is cavitation. The pump creates a vacuum that when the air "pops" creates a force that can mangle... even metal.

It can also burn up the motor due to excessive rpms. THe same reason 2 gens shouldn't use a pop off valve. THe extra velocity caused by lack of load can ruin the bearings and cause shaft damage (to the turbo) or a dead short (fuel pump)

These are merely popssibilities that I was thinking of. It may be something else entirely though lol!
Rob