Here’s an interesting electrical question/poll.

[RodgerH]

Originally Posted by Double Venom

Just following this potential learning thread, but for what it is worth, every Cobra/Street Rod should have an INERTIA Switch (cut-off switch) built into the system.
DV

Thanks for sharing your personal opinion. Anything to support why it is superior to an oil pressure switch?

Here's my story:
My wife and I personally have been in two "rear-ended" accidents in different cars, the first one quite severe (the car was totalled). In neither accident did the engine stop running (both cars had automatic transmissions). Makes me question the effectiveness of an inertia switch.

Rodger

[coinop101]

GET THE HELL out !!!!!!!

[RodgerH]

The profanity is not necessary.

Want to try again to explain what you mean?

Rodger

[RallySnake]

Roger,

You have to imagine the worst case scenerio. The idea is, if you are knocked unconscious in an accident, you don't want anything to create a spark that might ignite leaking gasoline and fry your hide. If the transmission is knocked out of gear, the engine could keep running. Even if it stalls, the fuel pump could keep running. Your dash switch is only good if you can turn it off. The inertia switch is supposed to protect you in this situation. To do that, it must be wired to cut off the fuel pump and the ignition.

I know that the inertia switches in my Lotus Elise and my Jaguar S-Type are both wired this way. For racing requirements, an external safety switch should do the same thing.

RallySnake

[Double Venom]

Rodger, an Oil pressure switch may turn off your motor if somehow you motor just quits, but it will not turn off the fuel pump.

As fAR as your switch not turning off the engine in a rear end-ala serious accident, that is highly unusual. The dealer would most likely just tell you as bad as it was, it wasn't a hard enough hit to "kick it".

Think of a TILT mechanism in a pin-ball machine, you can beat the heck out of the machine getting that last little "English" on the ball and it wont "tilt". Next time you hit it just right and it shuts off!

Most inertia switches are built on the same principle, a small internal cup of some type with a ball bearing centered in the cup. It wont "tilt" until the ball is knocked out of the cup and grounds out the system it is hooked to.

DV

[RodgerH]

DV,

My oil pressure switch is wired in series with the fuel pump relay. If the engine stops (or otherwise loses oil pressure), so does the fuel pump.

I'm not arguing an inertia switch wired to shutoff the fuel pump (or the ignition?) is a bad thing; just trying to decide if it is "better" than the oil pressure switch I presently have.

Pegasus offers both the inertia switch AND the oil pressure switch as fuel pump shutoff safety devices. Maybe it would be safest to have both.

inertia switch: http://www.pegasusautoracing.com/pro...s.asp?RecID=87
low oil pressure switch: http://www.pegasusautoracing.com/pro...s.asp?RecID=88

Rodger

[Double Venom]

I see...
I ASSUMED it was just a low pressure switch, didn't realize you had fuel in line/series too.
My 2-cents is; Both would be an excellent investment.
DV

[RallySnake]

I don't like the idea of the oil pressure switch cutting off the fuel pump. In some hard turns on an autocross course or racetrack, you might get a big surpise. Usually though, by the time the fuel drops low enough in the carburetor to stop the engine, a lot of damage could be done. I like the idea of a very bright red light hitting you in the eyes when this happens. It's much more immediate.

RallySnake

[RodgerH]

Originally Posted by RallySnake

.....by the time the fuel drops low enough in the carburetor to stop the engine, a lot of damage could be done.

The purpose of the low oil pressure in series with the fuel pump relay is to shutoff the fuel pump when the engine stops, NOT to stop the engine (by letting it run out of fuel) if you have low oil pressure.

Rodger