Fuel Starving issue on my 3.6 AX car

[rothaus]

I have a 71 911 with a 3.6 964 engine, which is my autocross car.

Since beginning of the season I'm getting some issues that I loose power, when it's hot or after long sweepers. First I thought I have not enough fuel (1/4 or less), However, also have the same issue with 1/2 tack of gas.

Not sure what else could be, any hints what to check ? I'm hopping to get some tips from the Pro's.

Thanks for any input.
Engelbert

[Dave Kost]

I would pull the gas tank filter. If there is any blockage or silt dirt, rust the tank them must be cleaned. I would then flush the line back to the engine.

[Trackrash]

Carbs or original EFI?

[tctnd]

'71 fuel tank does not have a swirl pot. Unlike carbs, your efi needs an absolutely solid, consistent supply of fuel at the correct pressure all the time. You can install a newer tank with an internal swirl pot or fabricate an external header to insure consistent delivery. Hope this helps.
regards,
Phil

[rothaus]

The engine has the original EFI from the 964.

Thanks for the input.

Do you have more info on the

external header

?

Thanks
Engelbert

[rothaus]

Found it, it's also called Surge Tank.

Thanks, that makes sense.
Engelbert

[tctnd]

I admit "external header" isn't very descriptive. Essentially this would be a small, narrow, additional fuel tank arranged so that the pickup will never be dry. Picture a piece of 2"
tubing about 14" long with the ends closed, mounted vertically in the most convenient place you can find. It will have 3 fuel connections; one at the top, one in the middle, and one at the bottom. An ordinary low pressure fuel pump (running continuously) is used to deliver fuel from your existing tank to the center (inlet) bung and a return line from the upper fitting carries the excess back to the tank. The bottom (outlet) is connected to your existing efi pump which functions in the normal manner.
Short interruptions in fuel delivery from the main tank will be buffered by the contents of the "external header" which, because of it's shape, is unlikely to ever see it's drain uncovered... unless the car is upside down, which is a more serious problem.
You can test the fuel tank hypothesis by simply running your next autocross with the tank topped off; the problem should go away.
regards,
Phil

[rothaus]

Thanks, that makes complete sense.

We have a practice session next Saturday, and I will top of the tank and do the test.

Thanks again
Engelbert

[JohnJL]

Hi Englebert,
I had the same issue but my episode ended badly when the motor detonated when sttarved. My solution was to build up an external surge tank as mentioned above. Works great.

[JohnJL]

In addition to Phils description above you want a 4th conne tion to the surge tank, into which you will connect your fuel return from your regulator. That way the surge tank stays full all the time and only returns back to your stock tank after your engine fuel requirements are taken care of.

You need to be mindful of the pumps you choose. I us3 a pair of Aeromotive a1000s.

The surge tanks are readily available from Summit and Pegasus. More volume is better but heavier. Mine is about 2 quarts.

[rothaus]

Thanks John, good info.

I will report back how things worked out. I will run this weekend with a full tank, since I cannot get all the parts, and have no time to install before the weekend.

For the surge tank, at least 2 liter.
Bottom out to EFI pump.
Top out and return to main tank.
2 side feed from forwarding pump, and the other one return from fuel rail.


Thanks
Engelbert

[rothaus]

Looks like the issue solved. I did run the last event with a full tank, that solved the problem. Now I get the parts to make me a surge tank.

Thanks all for the input.

Cheers
Engelbert

[JohnJL]

This is how I did it: Track car development update - surge tank, splitter