hobbs switchs

[grumpyvette]

a hobbs switch is usually a pressure, or vacuum activated switch that completes an electrical connection under pre-set or in some designs operator adjustable changes in pressure, or vacuum levels , air compressors use them to limit tank pressure, fuel supply safety switches on nitrous systems use them to help reduce overly lean operation that can melt pistons and cause detonation if fuel flow drops, below a intended pressure.
they can be normally open and close the contacts at a preset or adjustable pressure. this type is commonly used to kill an ignition or fuel pump if oil pressure falls below 4-5 psi, or fails to reach that during start-up
you can also find NORMALLY closed switch designs that can be used to do things like prevent a vertex magneto from operating , by keeping its kill switch lead grounded until your engine builds oil pressure or darn near any other function you might want to use, to control an electrical circuit based on a pressure level, there are also hobbs switches that operate on vacuum levels, or similar thermal switches that complete or open circuits based on heat levels.

http://www.atpturbo.com/mm5/merchant.mv ... y_Code=BCS

http://www.jmschip.com/jms-adjustable-hobbs-switch/

http://www.translectricinc.com/catalog/ ... no=76579-4

http://www.kipanderson.net/rs/hobbs.htm

I thought I'd share how I fixed a problem on my Shelby Cobra replica.

I'm running a pretty radical cam and the engine is not making enough vacuum to run the power brakes. I tried a vacuum cannister and that didn't really help much. Soooo...I'm left with either going hydroboost or spending around $300-$350 for a vacuum pump. I figured I ought to be able to build something for cheaper - and I did.

Parts List
95 VW Passat vacuum pump from e-bay: $20
15 inhg Hobbs NO (Normally Open) vacuum switch from e-bay: $15
2 vacuum check valves for 3/8" ID hose: $14
SPDT Relay from Napa: $15
Brass t-fitting from Home Depot: $3.50

I opened up the VW vacuum pump and removed the circuit board, replaced it with a toggle switch, and riveted some aluminum to it for mounting:

The Hobbs switch is normally open and closes when the vacuum gets greater than 15 inhg. Hooking the vacuum pump to terminal 87a on the relay makes the pump run until the Hobbs switch closes.

I used the check valves to plumb the pump into my vacuum canister and still have the vacuum line from the intake manifold:

The setup works pretty well except the vacuum switch actually closes at about 12 inhg instead of 15. I just bought an adjustable one on e-bay for $10.50 that has an adjustment range from 2-27 inhg. Even after buying two different vacuum switches, I spent less than $100 and now my power brakes actually work.


Pete

 

[philly]

hell yea, you compiled that data fast grumpy! let me paste this in here from the other thread (where i guess it was only mildly off topic) as it relates to hobbs switchery

using a hobbs switch on a turbo motor can do some interesing power enhancing things too. lets say i had a stock turbo drivetrain but wanted to increase the boost past the realm of capabilities for the stock fuel system but didnt want to lose my stock style driveability or my cruising comforts while in vacuum. well, i could tap the manifold and install a hobbs switch rigged to an extra injector to come on at say, 10 psi. and "tune" the combination by playing with injector size and number of injectors, for example.or staging them with multiple hobbs switches so one injector comes in at 7 psi, another at 12, another at 17, and so on.

once upon a time before the wisespread use of in car lambda (wide band and narrow band afr gauges) we would just make sure the turbo cars stayed huffing black diesel type smoke throughout the powerband at WOT so we knew we were running rich and safe.

msd used to sell a "engine knock alert system" http://www.summitracing.com/parts/msd-8964

that we could use that has a simple bar graph type (green bars, yellow bars, red bars!) to tell you when you have a dangerous amount of knock. this was a grea ignition tuning tool back in the days because it allowed you to move your timing to the threshold of detonation and have a good idea what was going on inside your combustion chamber.

when combined with another indispensable msd product, the BTM or boost timing master http://www.summitracing.com/parts/msd-6462/overview/

you could set up your initial timing to really wake the dead "off boost" and come into boost quicker with less lag, knowing that msd BTM would pull one, two or three degrees of timing for every pound of boost it detected (up to a total of 15* retardation) just by twisting that fancy little knob that it comes with.

let me tell you seeing a turbo six cylider eighties super car with digital dash instruments and crazy msd gizmos all over the center console and the "oh my god, why does it have twelve fuel injectors if its only six cylinders?!" made quite an impression on a younger version of me once upon a time.