Not for a race car. In production cars you see some relays because they
have dinky little light current switches in the steering columns. there's
also a fair amount of solid state circuitry as interlocks and other
complex functions and it's cheaper and easier to use relays in these
control circuits. They also save weight and wire cost--old style wiring
harnesses are heavy and have a lot of copper. a relay signal line can be
very light and still control a high current. The next stage will be
multiplexed wiring in cars, where all the switch functions occur over just
a few wires. Not something you'll ever worry about in a LBRC.
That's way too much useless information. Bottom line, use a good switch
that greatly exceeds the current rating of the device and you'll be fine.
you can find 30 AMP switches anywhere. Your fuel pump will draw about
five.
-----Original Message-----
From: Jason Sukey
To: Bill Babcock; fot@autox.team.net
Sent: 4/2/2003 5:32 PM
Subject: RE: fuel system questions
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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