Vacuum Advance. Where does it go?!
Thread Starter
Melting Slicks
Joined: Sep 2004
Posts: 2,412
Likes: 3
Vacuum Advance. Where does it go?!
I've been told different things about this and I don't know what to believe. Does the vacuum advace hose go to manifold vacuum or timed vacuum?
Does anyone have a pic of where the vacuum line plugs into the carb? (Stock 78 HEI/ Q-Jet)
Does anyone have a pic of where the vacuum line plugs into the carb? (Stock 78 HEI/ Q-Jet)
Race Director
Joined: Sep 2004
Posts: 11,874
Likes: 263
From: Arlington Va Current ride 04 vert, previous vettes: 69 vert, 77 resto mod
Originally Posted by torqvette
I've been told different things about this and I don't know what to believe. Does the vacuum advace hose go to manifold vacuum or timed vacuum?
Does anyone have a pic of where the vacuum line plugs into the carb? (Stock 78 HEI/ Q-Jet)
Does anyone have a pic of where the vacuum line plugs into the carb? (Stock 78 HEI/ Q-Jet)
Thread Starter
Melting Slicks
Joined: Sep 2004
Posts: 2,412
Likes: 3
OK, just found this.
So from the sound of it, I'll get better performance from manifold vacuum but the car will work fine either way. I have been running ported vacuum and that's where I timed and tuned the car. Once I swapped it to manifold vacuum the idle speed jumped up to 1600rpm. So until I get that new carb and dist, I'll stick with ported vacuum.
Originally Posted by lars
Not to be argumentative, but VETTEVICs information is factually incorrect. Too much to type, and no point in arguing this - the correct info is readily available from many sources. But to summarize:
Ported vacuum acts simply as an on/off switch to eliminate vacuum signal at idle. The throttle blades block the port at idle, and expose it once cracked open - as zwede stated. Once the throttles are cracked open beyond a given amount, ported vacuum is the same as manifold vacuum. Exactly the same. There is no venturi effect or lower pressure area for the ported signal.
Ported vacuum is used solely for emissions purposes to reduce timing at idle, producing a sharp reduction in hydrocarbon emissions. Pre-emission GM cars (high compression engines) used straight manifold vacuum for the vacuum advance - there is no issue with detonation if the correct vacuum advance control unit is used. EGR-equipped vehicles use ported vacuum to trigger the EGR: Since ported vacuum produces no signal at idle and normal (manifold vacuum) signal at cruise, it acts as an on/off switch for the EGR to prevent it from opening at idle. EGR-equipped cars (70s-era Vettes) use ported vacuum for the EGR. Pre-EGR cars use ported vacuum to the distributor. Pre-emissions cars use manifold vacuum to the vacuum advance, and most cars will benefit from this if emissions are not a concern.
Ported vacuum is not proportional to engine rpm. Ported vacuum is the same as manifold vacuum when the throttle blade exposes the port. It is relative to engine load - not proportional to rpm or rpm change.
Ported vacuum acts simply as an on/off switch to eliminate vacuum signal at idle. The throttle blades block the port at idle, and expose it once cracked open - as zwede stated. Once the throttles are cracked open beyond a given amount, ported vacuum is the same as manifold vacuum. Exactly the same. There is no venturi effect or lower pressure area for the ported signal.
Ported vacuum is used solely for emissions purposes to reduce timing at idle, producing a sharp reduction in hydrocarbon emissions. Pre-emission GM cars (high compression engines) used straight manifold vacuum for the vacuum advance - there is no issue with detonation if the correct vacuum advance control unit is used. EGR-equipped vehicles use ported vacuum to trigger the EGR: Since ported vacuum produces no signal at idle and normal (manifold vacuum) signal at cruise, it acts as an on/off switch for the EGR to prevent it from opening at idle. EGR-equipped cars (70s-era Vettes) use ported vacuum for the EGR. Pre-EGR cars use ported vacuum to the distributor. Pre-emissions cars use manifold vacuum to the vacuum advance, and most cars will benefit from this if emissions are not a concern.
Ported vacuum is not proportional to engine rpm. Ported vacuum is the same as manifold vacuum when the throttle blade exposes the port. It is relative to engine load - not proportional to rpm or rpm change.
Drifting
Joined: May 2003
Posts: 1,634
Likes: 2
From: Melbourne, Victoria, Australia, the World
No need to wait for a new carb and dizzy. Because you connected the manifold vac to the dizzy you now have more advance at idle. Your motor likes that and it speeds up. Wind down the idle speed on the carb to what you had before and maybe reset the idle mixture. It will run better but be a little dirtier if you have to have it smogged.
Race Director
Joined: Sep 2004
Posts: 11,874
Likes: 263
From: Arlington Va Current ride 04 vert, previous vettes: 69 vert, 77 resto mod
Originally Posted by torqvette
.........So from the sound of it, I'll get better performance from manifold vacuum but the car will work fine either way. I have been running ported vacuum and that's where I timed and tuned the car. Once I swapped it to manifold vacuum the idle speed jumped up to 1600rpm. So until I get that new carb and dist, I'll stick with ported vacuum.
Tech Contributor
Joined: Aug 1999
Posts: 14,380
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From: At my Bar drinking and wrenching in Lafayette Colorado
If emissions are not a concern, almost all Chevy engines will benefit from running manifold vacuum. The engine runs cooler at idle, and throttle response is generally better. You don't have to re-time the engine to run manifold vacuum - the engine is not timed with vacuum advance consideration. Hook up the manifold vacuum and drop the idle speed - it's a 17-second operation.
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Verdad Gallardo
Racer
Joined: Apr 2005
Posts: 464
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From: Charleston South Carolina
Having full vaccum at idle means that you will lose advance when the throttle is abruptly brought to the wide open position because there will be a vaccum drop. I thought that you wanted to have more advance when taking off at hard acceleration. If you watch with a timing light when vaccum is full at idle the timing will retard for a split second before getting mechanically advanced. Thats why I have it setup on the port on the carb to get vaccum only when the throttle is applied. When the load is gone off of the engine the vaccum will go down and slowly retard by itself for higher rpms so that it dosen't detonate. This is the way I always thought it should be, but I could be wrong unless someone else could explain it differently.
Burning Brakes
Joined: Aug 2005
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From: Hudson Falls, N.Y. 76 Vette Modified L-48
OK...I think you have a good point, but wouldn't you have a brief delay/vac drop even on the ported side? Seems to me you're damned if ya do, damned if ya don't...maybe Lars will pop in here and further explain...
Tech Contributor
Joined: Aug 1999
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From: At my Bar drinking and wrenching in Lafayette Colorado
Originally Posted by pauvil
Lars, if your not running ported vac, aren't you advancing right at idle?? is this a good idea with a high compression motor coming off the line to throw that much advance at it under that kind of load??
Under light load, the manifold vacuum source keeps the fully advanced condition for better part-throttle response and lower engine operating temperature. As soon as you're under load, the vacuum advance drops off, reducing you down to your optimum 36-degree power setting. Prior to emissions requirements, all GM performance cars were set up this way from the factory for optimum performance. The ported source is only to reduce hydrocarbon emissions at idle by retarding the timing down to the base initial setting - it is not usually the best setup for a performance engine unless you're running over 20 degrees initial with a very short advance curve. If you run over 20 initial, the vacuum advance may apply too much additional timing at idle to obtain a good quality idle, and you may benefit from running ported vacuum - you just have to try it out and see what your engine likes for timing and give the engine what it wants.
Thread Starter
Melting Slicks
Joined: Sep 2004
Posts: 2,412
Likes: 3
Ahhh! Swapped the vacuum line back to a higher port on the carb, it looks like it's above the butterfly so I thought it was ported but I was wrong. Drove to work at 1600rpm idle. And funny thing, when the engine was warming up it would rev consistently from 600-1600rpm by itself...?
So does anyone have a pic for me of a good ported source on the Q-Jet?
So does anyone have a pic for me of a good ported source on the Q-Jet?
Drifting
Joined: Jul 2002
Posts: 1,944
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From: The only Corvettes in Highett Victoria
Holden V8 tune up
Lars,
I have just rebuilt a 308 Holden V8 for my Commodore and want to tune it properly. It comes standard with a 4.000" bore and 3.0625 stroke. I have had the valve seats hardened for unleaded petrol, a mild performance camshaft fitted, a 0.030" overbore and a full balance. It has a hybrid distributor, Holden bottom end and Chev HEI top with a presumably stock HEI coil, cap and rotor etc. It has the standard Quadrajet that was fitted to the old 253 cubic inch Holden V8. The standard timing is recommended as 6 degrees BTDC @ 850 rpm. The service manual recommends the vacuum advance going to the port at the lower front left of the Qjet which is where it is now fitted, but after reading this thread I am prepared to move it.
As the engine is new and has not had many miles on it, I haven't put the foot down yet, but I thought that I could probably set the timing up a bit better. Any thoughts on this?
Regards from Down Under
aussiejohn
I have just rebuilt a 308 Holden V8 for my Commodore and want to tune it properly. It comes standard with a 4.000" bore and 3.0625 stroke. I have had the valve seats hardened for unleaded petrol, a mild performance camshaft fitted, a 0.030" overbore and a full balance. It has a hybrid distributor, Holden bottom end and Chev HEI top with a presumably stock HEI coil, cap and rotor etc. It has the standard Quadrajet that was fitted to the old 253 cubic inch Holden V8. The standard timing is recommended as 6 degrees BTDC @ 850 rpm. The service manual recommends the vacuum advance going to the port at the lower front left of the Qjet which is where it is now fitted, but after reading this thread I am prepared to move it.
As the engine is new and has not had many miles on it, I haven't put the foot down yet, but I thought that I could probably set the timing up a bit better. Any thoughts on this?
Regards from Down Under
aussiejohn
Racer
Joined: Apr 2005
Posts: 464
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From: Charleston South Carolina
Originally Posted by kb2fzq
OK...I think you have a good point, but wouldn't you have a brief delay/vac drop even on the ported side? Seems to me you're damned if ya do, damned if ya don't...maybe Lars will pop in here and further explain...
