how to stop fuel pump on race car

piggdekk · 03-16-2022, 11:08 PM

Hi,
I'm building a race car with carburators and an electric fuel pump. The set up is very similar to an early 911 with carbs and in fact I might have to do the same on my 911 T if I decide to rebuild it as gr. 2.
The FIA rule book implies that the fuel pump has to stop automatically when the engine stops: "all fuel pumps must only operate when the engine is running, except during the starting process".

This is to prevent that in case of an accident the pump keeps running and fuel gets all over even if ignition is still on.
If the car had an ECU or a crank sensor it'll be easy to link a switch to the pump and make sure it stops when the engine stops, but with a carburated car with a simple dizzy how do people wire the fuel pump to make sure it stops when the engine stops? Should I had some kind of crank sensor or a vacuum switch to power the fuel pump?
thanks
luca

dannobee · 03-17-2022, 02:13 AM

Oil pressure switch

piggdekk · 03-17-2022, 05:28 AM

thought about but pressure does not drop immediately, does it? Is it how people usually do it?

3rd_gear_Ted · 3rd_gear_Ted's Garage

Holding circuit with the relay coil being in series with the on/off key switch.
The contacts on the relay control the fuel pump.

thetorch · 03-17-2022, 05:56 AM

No idea is this meets the FIA rules, but this fuel pump cutoff relay worked great for me when I had carbs. Primes for 3 seconds on key on, then if the RPM isn't above about 500 or 700 RPM (not sure), cut off the fuel pump. Can be wired to the hot or ground on the fuel pump / pump relay. Never failed me. If the motor is still running, however, it will keep the pump running, so it's not the same as an inertial fuel cutoff switch.

https://www.amazon.com/Revolution-Electronics-Electric-Controller-Primes/dp/B00BNZ05JM/

dannobee · 03-17-2022, 09:30 AM

https://www.motortrend.com/how-to/safe-and-efficient-electric-fuel-pump-wiring/

Or ask your local official what he wants to see to cut the voltage to the fuel pump. Oil pressure switches have been used for decades.

piggdekk · 03-17-2022, 11:02 AM

Quote:

Originally Posted by dannobee

https://www.motortrend.com/how-to/safe-and-efficient-electric-fuel-pump-wiring/

Or ask your local official what he wants to see to cut the voltage to the fuel pump. Oil pressure switches have been used for decades.

Thanks, I as wondering what people used in the past and this looks like a sound solution. I need to find an oil switch that fits my case. I thought oil switches were normally closed and would go open if the oil pressure is high, but looks like this GM switch works the opposite way.

piggdekk · 03-17-2022, 11:04 AM

Quote:

Originally Posted by 3rd_gear_Ted

Holding circuit with the relay coil being in series with the on/off key switch.
The contacts on the relay control the fuel pump.

this wouldn't work because the pump has to stop every time the engine stops, not just if the key is turned off (think in case of an accident).

piggdekk · 03-17-2022, 11:05 AM

Quote:

Originally Posted by thetorch

No idea is this meets the FIA rules, but this fuel pump cutoff relay worked great for me when I had carbs. Primes for 3 seconds on key on, then if the RPM isn't above about 500 or 700 RPM (not sure), cut off the fuel pump. Can be wired to the hot or ground on the fuel pump / pump relay. Never failed me. If the motor is still running, however, it will keep the pump running, so it's not the same as an inertial fuel cutoff switch.

https://www.amazon.com/Revolution-Electronics-Electric-Controller-Primes/dp/B00BNZ05JM/

this would be great but the car has got a mechanical tachometer.

mikesarge · 03-17-2022, 12:11 PM

Quote:

Originally Posted by piggdekk

Thanks, I as wondering what people used in the past and this looks like a sound solution. I need to find an oil switch that fits my case. I thought oil switches were normally closed and would go open if the oil pressure is high, but looks like this GM switch works the opposite way.

Or you could use a DPDT relay, and have it cut voltage to FP when it senses voltage at the trigger (oil light)

3rd_gear_Ted · 3rd_gear_Ted's Garage

Quote:

Originally Posted by piggdekk

this wouldn't work because the pump has to stop every time the engine stops, not just if the key is turned off (think in case of an accident).

Your right, need to have the oil pressure switch NC contacts paralleled with the control relay switch NO contacts in series with the momentary key switch contact as a holding circuit to allow start up too or drop out NC contacts when engine quits

piggdekk · 11:22 PM

Quote:

Originally Posted by mikesarge

Or you could use a DPDT relay, and have it cut voltage to FP when it senses voltage at the trigger (oil light)

I think a 5 pole relay (SPDT) would work too.

I could connect power to the fuel pump on 87A so that as soon as the oil swtich closes (engine stop) the coil gets power and switches the pump off. My only concern with this is that it's not a fail safe solution. If the switch for whatever reason stays open (wiring, switch failure, even relay failure) the pump does not stop.
Of course I would need a push botton anyway to prime pump and carbs at start up.

3literpwr · 03-18-2022, 07:16 AM

I'm going to need to sort this for my car also. I bought a wire harness from Tuthill racing in the UK and assumed the fuel pump was wired as such, but after testing, I recognize that both fuel pumps are switched on. Seeing that many of their race cars are FIA spec (2liter, rally etc) they should have a solution. Will ask.

piggdekk · 03-18-2022, 07:55 AM

Quote:

Originally Posted by 3literpwr

I'm going to need to sort this for my car also. I bought a wire harness from Tuthill racing in the UK and assumed the fuel pump was wired as such, but after testing, I recognize that both fuel pumps are switched on. Seeing that many of their race cars are FIA spec (2liter, rally etc) they should have a solution. Will ask.

I'm guessing their wiring wouldn't include it because it depends too much on car set up.
Appreciate if you'll let us know what they recommend.

Mahler9th · 03-18-2022, 08:15 AM

RPM switch and relay might work.

thetorch · 03-18-2022, 08:58 AM

I have played with this micro switch thinking I would add a delay off on my interior lights so I don't drain the battery if I fail to shut a door. It works as a relay that can have triggers, delay on, delay off, etc.

I believe you can program it to power the pump (or relay) with the key on power, and shutoff power to the pump on any signal, such as oil pressure warning, etc. 5 amp rated, and I believe there is a higher rated version.

Minor pain to program (use a 9V and small LED to test and program) but very flexible. Cannot vouch if it's robust enough for your use case.

https://www.amazon.com/gp/product/B015Z2EQCU/