Prep for Road Atlanta Part 1

One of the biggest problems with the last race at CMP was fuel starvation. Even though the RX7 gets great mileage, using less than 8 gallons during the 51 minute race, below 3/4 tank the car started to hesitate coming out of corners. This problem got progressively worse as the race progressed, partially leading to a drop in lap times of over 5 seconds. To solve this problem, I inverted the fuel filter, and converted it into a surge tank using a second fuel pump left over from a previous project. The basic diagram looks like this;



As you can see, fuel is pulled out of the fuel cell with a low pressure, high volume pump . The unit I'm using is a Carter 72 gph pump that is roughly $65. Next, fuel (and sometimes air) flows into the top of the surge tank. The Carter pump flows almost twice the fuel as the Holley HP pump, so a vast majority of the fuel is flowing out of the top fitting, back into the fuel cell. The idea is that also flowing out the surge tank are any air bubbles that might have gotten in. The tank acts to separate any air before it gets pumped into the fuel rail. Filtered fuel flows from the bottom of the surge tank into the HP pump, which goes into the motor. Interestingly, you don't need a very large surge tank with this setup because the surge tank is only being emptied as fast as the engine can consume fuel, not as fast as the pump can drain it. Excess fuel from the engine re-fills the surge tank, giving several seconds of uninterrupted fuel supply.

Another step I took was to relocate the fuel pickup hose more towards the center of the fuel cell. Not only will it take longer for the pickup to become uncovered, the surge tank should provide several seconds of fuel without any coming from the fuel cell itself. I guess we'll see at Road Atlanta.

Construction of the surge tank is pretty straight forward. Ideally, a good tank should be constructed from aluminum and welded. Good tanks can be purchased for less than $200, but I wanted to build my own because of the AN lines I'm using. All told, I've spent about $35 in fittings on this project. Starting from scratch, expect to spend $150-300.

For my tank, I drilled two holes in the top of the filter, and used a good aluminum epoxy putty to secure the fittings. The rubber lines in the pictures were used to dyno the car. SS lines will be used before Road Atlanta.




Also, the car made it on the dyno for the first time. An initial run showed what the car had at CMP, 196hp and 256ft/lbs. After tweaking the timing setup from the distributor, a second run yielded a more livable 214hp and 276ft/lbs. Not bad for a car that put out 225bhp stock. Expect some porting and a few tweaks in the next few weeks.

Coming soon: Part 2
4.11 to 3.64 gear swap, more power, less weight.