Jason, the wiring system I'm putting together will almost entirely use
sealed high quality switches from www.longacreracing.com (40 amp / 12 v
capacity). They have a high current capacity double pole single throw
switch (called a magneto switch) that you can use to switch ignition and
other accessories on two separate circuits. The only relay I plan to use
is in the OD circuit, and that's because the safety switches in the tranny
cover are low current capacity switches. I am going to use the modern
blade style 30A fuses, fuel pump by itself on its own fuse, ignition by
itself on its own fuse, other accessories on other fuses. The idea is to
look at what happens if I blow a fuse / short circuit a circuit. If I blow
one of the accessory circuits, the accusump or OD may not work, but the car
will still run and I can still finish the race to deal with the problem
later. There isn't much automotive that will create more vibration than a
race car, and vibration is not the friend of a relay. My concern is to use
the KISS method. Relays will allow a low current switch to control a high
current load. If you are manufacturing cars, and have a bunch of switches
in the steering column, they pretty much HAVE to be low current
switches. In a race car, you can spend the extra money and take up the
extra real estate on the dash with a high quality, high current capacity
switch. This simplifies the circuit and therefore makes it more
reliable. Every extra connection or device increases the chance of
failure. Being a computer professional for me the prime example of the
increased failure chances is in computer hard drives. They have incredible
MTBF (Mean time between failure) stats, but when you connect many of them
to a file server, you exponentially increase the chance of hard drive
failure. You can take drives with a MTBF of over 10 years, connect many of
them to a system and reduce the MTBF of the system to under one
year. Since you have redundant hard drives, this isn't a big concern. On
a race car, you don't have the redundancy so it IS a concern.
This method has served us well in numerous race cars (other than the OD,
this is the first OD race car we're running - again, we look at close ratio
gears as more reliable).
I have seen other race cars use relays successfully and reliably, so they
certainly are not out of the question. It is my opinion that the high
capacity switch is the better solution, though.
- Tony Drews
At 07:32 PM 4/2/2003, Jason Sukey wrote:
>Humm, I'm learning a lot here, thanks for the input. So does this mean
>that running the fuel pump power off a relay is not a good idea?? All
>modern cars that I've worked on seem to have a fuel pump relay, so I was
>under the impression that it was the way to go. Is wiring it straight
>to a high quality switch the way to go??? Thanks for the education!!
>Jason
>-----Original Message-----
>From: Bill Babcock [mailto:BillB@bnj.com]
>Sent: Wednesday, April 02, 2003 7:57 PM
>To: 'Jason Sukey'; 'Brad Kahler'; fot@autox.team.net
>Subject: RE: fuel system questions
>
>Relays, push buttons. There's a reason why these kind of systems were
>not
>commonly added to cars until there was solid state electronics to fail
>all
>as one monolithic molten mess. Reliability. Unless you get very
>specialized, wetted contact versions, relays are painfully unreliable
>when
>you put light current through them (I don't even know where you could
>get
>good relays any more--they are completely obsolete). Relay switch
>pressure
>is light (a solenoid in the energized position, and a light spring
>de-energized. Bang it around in a race car and you're asking for
>trouble.
>
>Not that you'd really need a relay--you need a double-pole, single throw
>momentary push button for the starter. The second pole can bypass the
>pressure switch. Or you could use a single pole one with a second hot
>wire
>bypassing the switch as long as it's on the hot side of the ignition
>system (in other words, not interrupting the ground connection).
>
>Still, I think you're better off with a K.I.S.S. (Keep it simple,
>stupid)
>approach.
>
>As far as priming your float bowls, your race car should have two pumps
>that you turn on with two switches that are separately fused. They
>shouldn't be wired to the ignition switch--they should come from your
>master switch. Sometimes you need to turn off the fuel to the pumps
>while
>the car is running--when you have a float needle stick for instance and
>the carbs load up halfway through a race. Sometimes you want to turn
>your
>pumps on without the ignition--when you're priming, or when you want to
>empty your fuel tank into jugs to see how much gas you've got.
>
>If you want to cut the fuel off when the car goes upside down, then you
>can use a mercury or pendulum switch to do so. But don't expect
>increased
>safety. That comes from simpler things that work almost every time. Or
>very sophisticated systems with known reliability built into the design.
>
>These kind of solutions tend to wind up disconnected or jumpered out the
>first time there's a problem starting the car and you suspect it might
>be
>the new gizmo.
>
>A good fire system, a backup extinguisher, a good rollcage, a simple
>electrical system with shut-offs and safety switches that work every
>time.
>I don't know how many times I've seen cars with alternators that use a
>battery cut off switch that disconnects the battery but won't kill the
>motor if it's revving. There's a really good article called "When user
>interfaces kill" subtitled something like "How John Denver Really Died"
>that examines well-meaning and reasonable-sounding safety and functional
>modifications that actually increase risk. Worth finding and reading.
>You
>could probably find it with a google search.
>
>
>-----Original Message-----
>From: Jason Sukey [mailto:jasonsmf@cs.com]
>Sent: Wednesday, April 02, 2003 2:49 PM
>To: 'Brad Kahler'; fot@autox.team.net
>
>Funny you brought this topic up, I am currently looking into the same
>options for the tr6 autox'er. As far as the oil pressure switch, I
>would
>think you could wire up a relay that bypasses the pressure switch when
>the
>starter is engaged. I'm also looking into a way to maybe have a push
>button primer switch so that if the carbs have been off the car and the
>float bowls are dry, I can get them filled back up without having to
>crank
>the starter all day first.
>Jason Sukey
>
>-----Original Message-----
>From: owner-fot@autox.team.net [mailto:owner-fot@autox.team.net] On
>Behalf
>Of Brad Kahler
>Sent: Wednesday, April 02, 2003 12:32 PM
>To: fot@autox.team.net
>Subject: fuel system questions
>
>All this talk about fuel pumps has me wondering about another aspect of
>the fuel delivery system, inertia switches.
>
>Currently Susan's spitfire does not have one installed, nor does it have
>a
>low oil pressure switch that would cut off the pump either.
>
>Seems like it would be prudent to install one or both of the above
>items.
>For the inertia switch I'mwondering if they pose problems on race cars.
>Pegasus sells a unit for $65 that I would hope would work on a race car.
>Any thoughts on this one?
>
>As for the oil pressure switch, seems this should be used no matter
>what.
>I'm assuming that when first cranking the engine over to start oil
>pressure would be generated fairlly quickly, especially with the use of
>an
>accusump to aid in starting. Am I missing something here?!
>
>Any thoughts would be greatly appreciated.
>
>Brad
>1957 TR3 vintage racer
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