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Can someone explain the operating theory of vacuum advance so i can apply to my situation?
I have a Fast 2.0 setup on a nearly brand new 350.
The EFI is controlling timing through a Fast locked distributor. Mechanical advance is set 18-34 all in by 2800 (almost new engine and expect to advance timing to 36 almost immediately). However, im completely confused by what the vacuum advance curve should look like. Here are excerpts from the timing articals:
VACUUM ADVANCE: The vacuum advance system is a vacuum canister that runs off vacuum from the motor that adds more advance to the spark timing. While the centrifugal advance is solely based on engine speed, the vacuum advance is based on engine LOAD. A typical vacuum advance canister adds approx 15º of advance over and above what the centrifugal advance adds into the timing. Although the two systems are completely different they work together to provide the proper amount of timing for the motor at any given time.
How does the vacuum advance system work by engine load? At idle or steady cruising the motor is under very light load and produces high vacuum. That vacuum causes the vacuum canister to pull against the distributor’s breaker plate and add more advance to the timing. Under acceleration the motor loses vacuum and at WOT throttle basically has no vacuum at all so there is nothing to cause the vacuum canister to pull on the breaker plate to add advance. The vacuum advance system adds advance to the timing under light loads and very little to no advance under heavier loads when it’s not needed. Remember, under heavier loads the motor has a rich fuel/air mixture burn burns more quickly so it needs to get “lit” later in the compression stroke, not sooner.
From this, i know the initial advance timing should be 16 degrees, and i presume at high rpm it goes to 0 degrees. However, the bold section below indicates the timing at high RPM is still accounting for the vacuum advance 16 degrees. This would imply a completely flat curve.
4. VACUUM ADVANCE:
Now that the total timing is set, unplug and reconnect the vacuum hose to the vacuum can on the distributor. You should immediately see an increase in idle speed so now is the time to readjust the idle speed back to it’s normal level, Depending on the motor, transmission, etc different vacuum cans give a different amount of advance but on average you will see about a 15º increase in timing advance once the can is reconnected. Check timing at idle now to confirm this. You should see a new timing level that equals: initial timing as set previously PLUS approx 15º advance from your vacuum advance. Check what can you have (they all have a number stamped on them on the bar) against the specs listed in Lars paper Vac Adv Spec.pdf to see just how much the can you have is giving you. (important, the specs list the vac, can advance amounts is DISTRIBUTOR degrees so you need to double that amount to get the correct number of crank degrees . If the can is listed at supplying 8º that means it’s really adding 16º of advance to the timing.
On my car the initial timing is 12º PLUS the vacuum can adds another 16º for a new initial timing of 28º at idle with the vacuum can connected.
Next check total timing with the vacuum can connected. It should be 36º PLUS whatever amount your vac. can supplies. On my car it would be 36º + 16º from the can = 52º.
No matter what, you do NOT want to exceed 52º (total plus vacuum advance) or again your timing will be too high and detonation becomes an issue.
Now that you see your vacuum advance is working take the car out for another test drive. Again make sure you aren't hearing knocking or pinging plus feel carefully that the car isn’t giving you a surging, chugging, or jerking feeling. If it is your vacuum advance adding too much in over and above the centrifugal advance and needs to be lowered.
All the other info i can find online is about what part number to use and what vacuum rating.
Can someone explain the operating theory of vacuum advance so i can apply to my situation?
You'll dial in 8-10 additional degrees of advance whenever you have more than 7" of vacuum. Fade the advance out so it is all gone a little before WOT (2-3" of vacuum).
In the simplest sense, a vacuum advance canister has a rated advance and a rated vacuum level. As the vacuum increases between no vacuum to it's rated vacuum, it will move the timing to it's rated advance. In the real world, they don't always work nice and linearly.
For the FAST, I think you set the degrees added and a load point. I would start with about 12 degrees and play with the load setting. Try adjusting the load point lower if you get pre-detonation or higher if you do not. If the load setting isn't helping with pre-detonation until you have it really low then try again with a lower degrees added.
A sad part of the FAST is that the vacuum advance works like using a ported vacuum source, as in you get no extra advance at idle. Yet, most performance engines work better with more idle advance.
Last edited by lionelhutz; 10-28-2016 at 11:38 AM.
If I'm understanding right, I need to measure my vacuum and figure out what rpms I have 7" or greater. Up to that rpm cross over I should have ~10 degrees of advance, then I should start a linear drop to zero vacuum advance near my redline?
Making progress! Drove the car yesterday with and without the vacuum advance. Engine likes the extra timing. However, at idle it smooths out a lot if I had a little timing (rotating distributor and returning to 18 degrees to match computer).
I'm going to boost initial to 20, keep the 8 deg vaccum and recheck proper idle screw adjustment one up to temp. The engine is running as rich as the computer will allow right now. I presume this is part of the learning process. Learning light is on when driving.
Try tweaking yours per those guidelines I posted...
For some reason I was obsessed with keeping my timing at 8° to 10° (not sure really why??? former traditional timing values??), and it was hard to start and would backfire when going downhill with the throttle released.
I have the mechanical set per the installation guidelines at 20°. I adjusted the curve to go from 15° up to 30°-- all in by 3200 rpm. Anyway, after I did moved that to 15° the car runs a lot better and starts easier. Fairly easy to make the adjustments and I would recommend that you give it a try.
Here you go.... went and took some pictures:
Last edited by carriljc; 10-30-2016 at 10:44 AM.
Reason: add pics
If I'm understanding right, I need to measure my vacuum and figure out what rpms I have 7" or greater. Up to that rpm cross over I should have ~10 degrees of advance, then I should start a linear drop to zero vacuum advance near my redline?
Nothing to do with RPM. The vacuum advance is only load based. So, off idle with little load the full vacuum advance gets added. As you load the engine (more gas pedal basically) the vacuum advance starts to be removed. The threshold sets the load where the vacuum advance starts to be removed.
Only work with the vacuum advance after the base timing is completely set correctly. I would even add a little more than the suggested 8* and see what happens. If you only get a bit of pinging at part throttle loads then drop the threshold a bit.
The idle trim is used to quickly add or remove timing as the idle rpm varies to try and make the idle rpm more stable. It's not even necessary unless your idle rpm isn't stable enough for you, but on the other hand it doesn't hurt to turn it on anyways.
10-28-2016, 08:59 AM
Timing curves Fast EZ-EFI 2.0?
