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Another distributor question

3

300jk

Well-known Member
Along the same lines sort of. My 300 Farmall does use vacuum advance. Our 1650 Oliver gas used to use vacuum advance. Why do some have it and others dont ? Or do some tractors use another method to advance timing ? Is it really necessary on a tractor ?
 
Timing near TDC starts easier, less kick back. Advancing the timing as engine speed increases, keeps the firing at a better time to make the best use of the power stroke, the fire is sooner so pressure in the cylinder builds higher while the piston is nearer the top of the down stroke, and the gas expansion is used for a longer duration of the stroke.

An engine can run without advance but will not develop its potential and there would be a lot of wasted heat out the exhaust.

Some engines use just centrifugal, some use both centrifugal and vacuum. Using both can allow finer tuning of the optimum time to induce the firing as intake system vacuum, not just engine speed is taken into account. There is a lot more to it if you want to study it, but this is some basic info.
 
It's unusual to see a tractor, industrial, marine, or heavy truck engine with a vacuum advance.

The biggest reason, it's not really needed, and second, it's a source of trouble as they are prone to fail.

The centrifugal advance is really more of a starting retard on a tractor engine. Unlike an automotive engine that is capable of high RPM and uses a wide power band, a tractor engine is limited to a very low torque range, so it doesn't need the timing advanced to compensate for high RPM performance.
 
IH does not use Vacuum advance if you have it it is a non stock replacement it must have a hose and diaphragm. as shown. Jim
cvphoto133132.jpg
 
So a question then, and I do understand what your
saying. So does my Farmall 300 have the centrifugal
type advance ? What about tractors with mags ? Do
they also have some type of advance ?
 
Even on a tractor that may run from idle to say 1800 rpm ? I know it will effect performance some , but how much on a tractor engine ? You have a Farmall or 2 ! Are those centrifugal advance?
 
They have a spring loaded retard locking mechanism. It helps with hand cranking and the spring helps give it a spin to generate current.
 
(quoted from post at 21:53:33 08/13/22) So a question then, and I do understand what your
saying. So does my Farmall 300 have the centrifugal
type advance ? What about tractors with mags ? Do
they also have some type of advance ?
Click to expand...
ll distributors and magnetos have centrifugal advance. Since most tractors are used at or near wide open throttle I can't see where vacuum advance would do much. Maybe save a little fuel during part-throttle operation.
 

The parts books say show the distributors for the 300 with just centrifugal advance. If yours has vacuum advance, as you posted, I agree with Janicholson, it's not stock.
 
Ye most use the weight system type advance. Has to do with as RPM increase the time the spark needs to fire if a tad bit more advanced or it will spark after the piston has hit TDC
 
I have no idea what tractor has or does not have vacuum advance.

But vacuum advance only works at high vacuum that comes with cruising at highway speeds.
Any slight touch on the accelerator asking for more power will reduce vacuum turning off vacuum advance.

A tractor never reaches cruising speed.
It is always lugging looking for more power.
 
With respect, IH magnetos do not have advance mechanisms. when starting with the impulse coupling they are set to spark at TDC
the impulse coupling then sets the advance at full advance, no in between. early mags had driver adjustable timing, and it had to
be set to TDC before cranking (or broken arms). Jim
 
(quoted from post at 17:07:44 08/13/22) With respect, IH magnetos do not have advance mechanisms. when starting with the impulse coupling they are set to spark at TDC
the impulse coupling then sets the advance at full advance, no in between. early mags had driver adjustable timing, and it had to
be set to TDC before cranking (or broken arms). Jim
Click to expand...

Advance is still advance, they advance from firing @ TDC at slow cranking speed to ''running timing'' all at once, call it want you want, but it's still a form of timing advance.
 
You generally don't find vacuum advance on tractor and industrial engines for the same reason you don't find it on racing engines: It isn't needed.

There are two reasons you need spark advance: First, the faster the engine turns, the shorter the time for the flame in the cylinder to propagate, and you want the maximum cylinder pressure to occur roughly when the cylinder velocity is greatest, in the middle of the stroke. So the faster the engine turns the earlier the spark needs to occur. Centrifugal advance handles that nicely.

The second reason to advance the spark is that under low-load conditions the spark can be further advanced for better economy without risking detonation. That's important with passenger vehicles that typically are under low load most of the time, so vacuum advance is added to give extra advance under low-load conditions. But in the case of tractors and industrial engines, they typically are under fairly high load. And when they're not, the fuel economy gained by vacuum advance isn't that significant. So they often don't have vacuum advance.

Of course, modern automotive engines with computerized spark and fuel control consider a large number of conditions, including engine speed and manifold air pressure (the reciprocal of vacuum) to determine spark timing.
 
I guess I wasnt too clear in my original post. My 300 does not have vacuum advance. We have a 1650 Oliver gas which did. It no longer does.
 
Only tractors I've seen with vacuum advance are the 4 digit Oliver series - 1550 thru 1855. There may be others but I've not seen one.

My understanding is vacuum advance is provided primarily to eliminate backfiring when the throttle is suddenly closed on an unloaded engine running at governed RPM.

This was demonstrated when the vacuum advance failed on my son's Oliver 1850. Snapping the governor closed on an unloaded engine would fire 2 or 3 shotgun-like blasts out the exhaust as the engine coasted down. Otherwise the tractor ran fine even with an inoperative vacuum advance.
 
I believe the aspect he was addressing was whether magnetos had centrifugal advance, which the majority of mainstream magnetos do not have. Just because ,-advance is advance-, doesnt mean the ignition timing has the same characteristics... centrifugal advance is infinitely variable throughout the rpm range. With impulse coupling, you are stuck with x BTC timing (unless mag has manual control) the entire time the engine is in operation.
 
In reply to MarkB, another reason that vacuum advance was use on road vehicles was a bit different. Many cars supplied vacuum from a ..ported.. source on the carb base. In this case at an idle the opening to the port would be just above the throttle plate so it would not be subject to manifold vacuum. As you open the throttle to start the car in motion the throttle plate edge would move above the port supplying vac to the advance. This quick advance of the timing helped with off idle stumble or bog. The main jetting of a car carburetor is somewhat leaner than that of a tractor, this helps vehicle fuel economy. The ported vacuum and accelerator pump in the carb worked together to overcome the stumble that would otherwise take place coming off idle. And no this does not apply to tractors, but is very inline with the question asked. For the original poster I would suggest he go into Modern view and edit his first post to read ..his 300 does not use a vacuum advance.. Just a suggestion.
 
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