vacuum advance.....or not.

cuzncletus

cuzncletus

Senior Member
I just hooked up the vacuum advance on my 46 59A, basically a stock motor .040 over with two 94's, crab distributor by Bubba's. But something has been haunting me. Do I remember reading somewhere that vacuum advance on a mild motor in a light car (29 AV8) is counterproductive or a waste at best?
 
I don't ever remember reading that. Vac advance in an out and out race car is probably unnecessary, but very useful in any street driven car.

Mart.
 
vacuum advance at idle means that the ignition timing is advanced which means the mixture is getting all burned up which helps the engine run cooler at idle.
 
Vacuum advance is to compensate for running a leaner mixture in non full power operation.
For full power at a narrow rpm range you don´t need any kind of advance.
For a wider range of rpm still only concearned about full power you need mechanical advance.
For better fuel economy and keeping you engine running good at cruise you need vacuum advance.
The beauty of the crab is that you can tune it to your cars need with the turn of a screwdriver.
 
Crab Vacuum Brake Details:

The most misunderstood part of a 1942-1948 Crab style distributor is the "vacuum brake". Often called by many a vacuum advance , with that not really being a correct name.
The vacuum piston is hooked to manifold vacuum and has a piston with leather wear washer glued to the end. This leather washer drags on a advance ring inside the distributor causing the timing to slightly retard when vacuum is lower than spring setting.
This should always be used even when the vacuum is not hooked up as it steadies the spark advance.

When removed the advance jerk or jumps around quite a bit.

Engine off= no vacuum causing the piston to push against the advance ring causing a slight retard when cranking.
Engine running st idle , once the engine starts then the manifold vacuum will be strong causing the piston to pull away from advance ring causing a slight spark advance.
Driving at highway speed with low loads this vacuum will keep the advance ring pulled away causing slight advance plus mechanical advance against spring tension of advance weights.
Under loaded conditions ( low vacuum) the spring pushes the piston inward to advance ring causing a slight retard of ignition timing to prevent spark knock on hills etc...

Note : Typically we ship the crab set at three turns inward on the spring for a light tension on advance ring. Ford suggests driving the vehicle with vacuum hooked up and turn outward until spark knock occurs on a slight load with throttle. Then turn adjustment in until it stops.
 
>>>Ford suggests driving the vehicle with vacuum hooked up and turn outward until spark knock occurs on a slight load >>>

Well that's easy enough to say when the knock sensor works. But what if it doesn't work any more? Sure would be nice to have an inexpensive visual knock detector. Any suggestions? Jack E/NJ
 
Jack E/NJ said:
>>>Ford suggests driving the vehicle with vacuum hooked up and turn outward until spark knock occurs on a slight load >>>

Well that's easy enough to say when the knock sensor works. But what if it doesn't work any more? Sure would be nice to have an inexpensive visual knock detector. Any suggestions? Jack E/NJ
Click to expand...

Jack,

I think these 42 Fords didnt use a knock sensor. Those were on engines late 70s or so. Ford needed you to hear the kncok on acceleration with your ears. Most flatheads with todays fuel and low compression dont knock.
In my case i wouldnt hear it if it did..:D
 
BUBBAS IGNITION said:
Jack,

I think these 42 Fords didnt use a knock sensor. Those were on engines late 70s or so. Ford needed you to hear the kncok on acceleration with your ears. Most flatheads with todays fuel and low compression dont knock.
In my case i wouldnt hear it if it did..:D
Click to expand...
I have the same problem..... deaf in one ear and I don't much in the other one.... Mark
 
Jim>>>Most flatheads with todays fuel and low compression dont knock. In my case i wouldnt hear it if it did>>>

Well my 38 allegedly does, according to some of my much younger passengers. They said it sounds like a bunch of marbles being dropped on a sidewalk. So any suggestions on a cheap visual detector to see if they're telling me the truth? Jack E/NJ
 
The knocksense doesn't look too bad. Does this mean I'd have to first choose a knock sensor from Amazon? Then order the knocksense box, cable and plug that would fit the sensor receptacle?

Jack E/NJ
 
Jack, it would be easier for you to round up a few kids with good hearing and make a contest to see who hears knock/pinging and reports first.
 
Maybe. But I've always liked the crisper response & better mileage that always seemed to come from setting a few degrees more advance than recommended even on 60s/70s engines/distributors. Enough so that it seemed worth using higher-test *and* driving more cautiously on hearing pings.

A car full of kids that *might* do the same thing today? I don't think so. That seems about as easy & practical as a new pair of super high-tech noise-cancelling digital hearing appliances from your local hearing-appliance dealer.

However, seeing pings might work even better and cost less than hearing them once did for me in the old days.

Jack E/NJ
 
I Personally don;t think that a flathead on today's grade of fuels is very susceptible to spark knock.
 
35fordtn said:
I Personally don;t think that a flathead on today's grade of fuels is very susceptible to spark knock.
Click to expand...

I tend to agree. I'm chasing a surging in my engine at low rpm and wanted to eliminate the distributor advance as a cause. I put the load-a-matic distributor in with no vacuum connected. I inched up on the timing until I was at 20 degrees. It ran great, no kick back on starts, no pinging, winds up great through the gears. The engine has 130-140# of cranking compression. That setting will probably sacrifice some economy, but I couldn't tell it. Running regular 0 ethanol.
 
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