Vacuum Advance, Why
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Junior User
Joined: Jul 2001
Posts: 61
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From: Rockwall, Just East of Da
Vacuum Advance, Why
Why do some distributors have Vacuum advance and others do not. My 76 F-250 with 390fe has it, but my 1988 American Skier Boat powered by 351w marinized by Pleasure Craft Marine does not.
Please tell me. I am always wanting to learn more.
Thanks
Please tell me. I am always wanting to learn more.
Thanks
Lead Driver
Joined: Apr 2001
Posts: 6,175
Likes: 21
From: Texas
I'm not the biggest proponent of vacuum advance, but I'll give you my take anyway.
1. First off- vacuum is free and non-power robbing as are other forms of powering devices.
2. Intake vacuum is a good indicator of what the engine is trying to, probably better in some cases than engine RPM. As is seen in using a vacuum secondary carburetor. For example, and engine running 2500 rpm with your foot in the floor is asking for somthing different than the same engine at 2500 rpm on deceleration.
Don't know if that makes any sense, and I may be wrong altogether. I run an MSD all mechanical distributor, because it's seems easier for me to adjust the timing advance ramp and max settings. I like to refer to the stock setup as using Vacuum VooDoo, but I'm starting to get a better understanding and appreciation for that setup. Although, I believe there are substantial changes in the vacuum profiles of a modified engine, that make it difficult to accurately setup the vacuum advance. Again, these are all opinions of a novice engine builder.
1. First off- vacuum is free and non-power robbing as are other forms of powering devices.
2. Intake vacuum is a good indicator of what the engine is trying to, probably better in some cases than engine RPM. As is seen in using a vacuum secondary carburetor. For example, and engine running 2500 rpm with your foot in the floor is asking for somthing different than the same engine at 2500 rpm on deceleration.
Don't know if that makes any sense, and I may be wrong altogether. I run an MSD all mechanical distributor, because it's seems easier for me to adjust the timing advance ramp and max settings. I like to refer to the stock setup as using Vacuum VooDoo, but I'm starting to get a better understanding and appreciation for that setup. Although, I believe there are substantial changes in the vacuum profiles of a modified engine, that make it difficult to accurately setup the vacuum advance. Again, these are all opinions of a novice engine builder.
Post Fiend
Joined: Feb 2000
Posts: 5,130
Likes: 8
From: Brea CA
Gtex is right. An engine needs ignition advance for two reasons, speed and load. As an engine revs higher, the ignition needs to fire sooner t keep up. Also, when an engine is under a light load, it needs more advance because of the partial fill from the mostly closed throttle plates burns slower.
A boat dosen't need vacuum advance for the same reason a hot rod or heavy truck does not need one. It's because the engine is mostly under load and a vacuum advance would not add anything significiant since the vacuum is low anyway when you got your foot into it.
Barry.
A boat dosen't need vacuum advance for the same reason a hot rod or heavy truck does not need one. It's because the engine is mostly under load and a vacuum advance would not add anything significiant since the vacuum is low anyway when you got your foot into it.
Barry.
Posting Guru
Joined: Jul 2003
Posts: 2,232
Likes: 1
From: Colville, Washington
Originally posted by BB
As an engine revs higher, the ignition needs to fire sooner t keep up. Also, when an engine is under a light load, it needs more advance because of the partial fill from the mostly closed throttle plates burns slower.
Barry.
As an engine revs higher, the ignition needs to fire sooner t keep up. Also, when an engine is under a light load, it needs more advance because of the partial fill from the mostly closed throttle plates burns slower.
Barry.
That is the very reason that vacuum advance is needed- under partial throttle, light load(cruising down the highway) more advance is needed. The mech. advance doesn't know that the engine is under light load, so it advances to whatever the weights allow. Which if set for adequate part throttle response will be TOO MUCH for full throttle acceleration. The vacuum advance does nothing under full throttle because the engine has zero vacuum. Hope this helps.
