Vacuum Advance ?
Thread Starter
Racer
Joined: Nov 2004
Posts: 422
Likes: 0
From: Owensboro Ky
Vacuum Advance ?
What exactly is the purpose of vacuum advance on a distributor.
What is the difference (Pro and Cons) between a HEI distributor and a standard distributor.
I'm looking to buy the ignition system for my 69 vert BBC and was curious about distributors with or without vacuum advance and whether I should use HEI.
What is the difference (Pro and Cons) between a HEI distributor and a standard distributor.
I'm looking to buy the ignition system for my 69 vert BBC and was curious about distributors with or without vacuum advance and whether I should use HEI.
Melting Slicks
Joined: Oct 2001
Posts: 3,025
Likes: 12
From: Pleasant Grove AL
Vacuum advance give you the opportunity to have an extra 16* or so of advance under moderate load when the engine can tolerate it. When you open the throttle, vacuum drops and retards the spark. As Martha would say, "it's a good thing".
An HEI distributor with a mechanical tach drive will set you back about 500 bucks. Your existing distributor will do just fine if in good shape and properly tuned with quality components. You can always convert it to electronic if you don't want to fool with points, but you won't really gain anything.
An HEI distributor with a mechanical tach drive will set you back about 500 bucks. Your existing distributor will do just fine if in good shape and properly tuned with quality components. You can always convert it to electronic if you don't want to fool with points, but you won't really gain anything.
Le Mans Master
Joined: Oct 2002
Posts: 7,098
Likes: 378
From: Las Vegas - Just stop perpetuating myths please.
Major question. U need major help.
Originally Posted by BDVT69
What exactly is the purpose of vacuum advance on a distributor.
What is the difference (Pro and Cons) between a HEI distributor and a standard distributor.
I'm looking to buy the ignition system for my 69 vert BBC and was curious about distributors with or without vacuum advance and whether I should use HEI.
What is the difference (Pro and Cons) between a HEI distributor and a standard distributor.
I'm looking to buy the ignition system for my 69 vert BBC and was curious about distributors with or without vacuum advance and whether I should use HEI.
). And yes the ignition info will apply to BB's also. Too much to xplane here but u want/need a vac advance for street use. A HEI with vacuum cannistor and a new performance module installed but without mech tach drive for $150 (Summit) will give u all the performance u need under 8,000rpm. Since the sb and BB distributors are the same u could find 1 in the boneyard for dirt cheap and overhaul yourself to try 1 (and i won't tell anyone u r using a sb part :o ). Great experience and not much to them - 1 wire hookup. Play with different weights and springs to customize your advance curve. Just a warning, don't listen to anything about a dimple on dist shaft for alignment.
Happy hunting.
cardo0
Tech Contributor
Joined: Aug 1999
Posts: 14,377
Likes: 6,376
From: At my Bar drinking and wrenching in Lafayette Colorado
Originally Posted by cardo0
Just a warning, don't listen to anything about a dimple on dist shaft for alignment.
Are you kidding? The distributor gear is not symmetrical. If you don't correctly align the dimple with the rotor, you will never be able to correctly adjust the timing on a Vette equipped with the distributor shielding without the distruibutor hitting the shielding and running out of timing adjustment. On high performance C2's with the aluminum square-bore intake, the distributor will hit the rear runner on the manifold if the dimple is not correctly aligned. It is critical that the dimple be correctly aligned - that's why it's there. No BS.
Race Director
Joined: Apr 2000
Posts: 17,676
Likes: 201
From: Who says "Nothing is impossible" ? I've been doing nothing for years.
Many here are fans of vaccum cans, I am not one of them however. In a daily driver looking for good gas mileage etc. it is mandatory. I like running no can because I can dial in my advance curve and know exactly what it is doing and I have never had any pinging problems that can happens when the vacuum advances you timing to over 50 degees 
Team Owner
Joined: Aug 1999
Posts: 30,995
Likes: 99
From: Ontario
I can't let the comment on the dimple go by without comment. It was a source of frustration for me last summer. With a Sears advance timing light I could not get things set up properly. I came close but the vacuum can was hitting the intake runner.
I knew about the dimple alignment issue so I pulled the distributor. It was indeed 180 degrees out. I re-installed and achieved what is generally accepted as the norm for timing when correctly done: vacuum can tip pointing to rear/top valve cover bolt. Never looked back.
I knew about the dimple alignment issue so I pulled the distributor. It was indeed 180 degrees out. I re-installed and achieved what is generally accepted as the norm for timing when correctly done: vacuum can tip pointing to rear/top valve cover bolt. Never looked back.
Last edited by Paul L; Feb 14, 2005 at 01:30 PM.
Le Mans Master
Joined: Oct 2002
Posts: 7,098
Likes: 378
From: Las Vegas - Just stop perpetuating myths please.
We don't use no stinking ignition shields.
Originally Posted by lars
Cardo -
Are you kidding? The distributor gear is not symmetrical. If you don't correctly align the dimple with the rotor, you will never be able to correctly adjust the timing on a Vette equipped with the distributor shielding without the distruibutor hitting the shielding and running out of timing adjustment. On high performance C2's with the aluminum square-bore intake, the distributor will hit the rear runner on the manifold if the dimple is not correctly aligned. It is critical that the dimple be correctly aligned - that's why it's there. No BS.
Are you kidding? The distributor gear is not symmetrical. If you don't correctly align the dimple with the rotor, you will never be able to correctly adjust the timing on a Vette equipped with the distributor shielding without the distruibutor hitting the shielding and running out of timing adjustment. On high performance C2's with the aluminum square-bore intake, the distributor will hit the rear runner on the manifold if the dimple is not correctly aligned. It is critical that the dimple be correctly aligned - that's why it's there. No BS.
Ignition shields? Those are C2 poo poo. But out'a respect for the master i'll plead ignorant. The only shields i use are for the first couple dates with a new girlfriend.
And now from Pauls pix i can c little room for that canister (or actualy don't c much room) between intk runner and coil - and with shield on would even be worse.
I just don't have a problem clocking the dist with the canister where ever there's room - but maybe i don't want to go there now. So i'll qualify it better if ever to mention it again.
Thx for the explaination & pix gents.
cardo0 
Le Mans Master
Joined: Jul 1999
Posts: 9,206
Likes: 17
From: Sewell NJ
Originally Posted by lars
Cardo -
Are you kidding? The distributor gear is not symmetrical. If you don't correctly align the dimple with the rotor, you will never be able to correctly adjust the timing on a Vette equipped with the distributor shielding without the distruibutor hitting the shielding and running out of timing adjustment. On high performance C2's with the aluminum square-bore intake, the distributor will hit the rear runner on the manifold if the dimple is not correctly aligned. It is critical that the dimple be correctly aligned - that's why it's there. No BS.
Are you kidding? The distributor gear is not symmetrical. If you don't correctly align the dimple with the rotor, you will never be able to correctly adjust the timing on a Vette equipped with the distributor shielding without the distruibutor hitting the shielding and running out of timing adjustment. On high performance C2's with the aluminum square-bore intake, the distributor will hit the rear runner on the manifold if the dimple is not correctly aligned. It is critical that the dimple be correctly aligned - that's why it's there. No BS.
The man knows his chit. 
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Tech Contributor
Joined: Aug 1999
Posts: 14,377
Likes: 6,376
From: At my Bar drinking and wrenching in Lafayette Colorado
Motor -
The roll pin hole that is drilled through the distributor gear is drilled in such a way that the hole is aligned with a "tooth" on one side of the gear, and aligned with a "groove" on the other side of the gear. Thus, the gear is not symmetrical (1/2 tooth difference between sides). On the gear, there is a small "dimple" on one side. To install the gear in the correct "clocking" related to the distributor mainshaft, the dimple needs to point in the same direction as the rotor firing tab. If you fail to do this, the distributor will be clocked incorrectly in the block when the timing is set. This incorrect clocking either makes the distributor hit the back of the intake manifold (see Paul's picts above), or hit the distributor shielding in such a way that the timing canot be set. Later model Vettes with HEI and no shielding do not have this problem, but the "correct" gear installation still aligns the dimple with the firing tab.
The roll pin hole that is drilled through the distributor gear is drilled in such a way that the hole is aligned with a "tooth" on one side of the gear, and aligned with a "groove" on the other side of the gear. Thus, the gear is not symmetrical (1/2 tooth difference between sides). On the gear, there is a small "dimple" on one side. To install the gear in the correct "clocking" related to the distributor mainshaft, the dimple needs to point in the same direction as the rotor firing tab. If you fail to do this, the distributor will be clocked incorrectly in the block when the timing is set. This incorrect clocking either makes the distributor hit the back of the intake manifold (see Paul's picts above), or hit the distributor shielding in such a way that the timing canot be set. Later model Vettes with HEI and no shielding do not have this problem, but the "correct" gear installation still aligns the dimple with the firing tab.
Team Owner
Joined: Aug 1999
Posts: 30,995
Likes: 99
From: Ontario
If I remember correctly, having the dimple in the wrong direction skews the distributor off by ~13 degrees. Hence timing becomes a nightmare in the little room available as I noted. (I do like the ignition shields.) That darn project almost drove me crazy as I had the new Sears advance timing light - proud as punch with a new toy - and followed Lars' instructions to a tee. Except the damn vacuum can nestled into the intake runner. This coincided with the failure of my Holley and the installation of a new Edelbrock. The project was not coming together! I do not use this type of language often but it was like p**ing into wind.
But all worked out eventually. And I learned a lot. The oil pump slot alignment for the distributor was a real treat....Finding compression on #1, getting the rotor tip aligned, etc.
But all worked out eventually. And I learned a lot. The oil pump slot alignment for the distributor was a real treat....Finding compression on #1, getting the rotor tip aligned, etc.
