Distributor vacuum
Thread Starter
Burning Brakes
Joined: Mar 2004
Posts: 816
Likes: 1
From: Finland
Distributor vacuum
Yesterday measured my intake manifold vacuum at idle - 6HG not too much...Then put some 1000rpm more and it steadily rose to 16HG.
I have no vacuum canister today in my distributor but I have ordered a MSD 8572 distributor. It will, according to some if I found from MSD web site, give 10degrees advance with 15lbs of vacuum; and is not adjustable. Anybody knowing what is the ration to convert lds to HG?
I´m afraid that my engine advance would fluctuate, if my 6HG is too little to pull all the vacuum out with the canister, that might end to poor idle quality???
Anybody knowing if the MSD 8572 distributor vacuum canister can be easily removed and replaced with Crane´s adjustable canister - that might pull all out then with less vacuum...
I have no vacuum canister today in my distributor but I have ordered a MSD 8572 distributor. It will, according to some if I found from MSD web site, give 10degrees advance with 15lbs of vacuum; and is not adjustable. Anybody knowing what is the ration to convert lds to HG?
I´m afraid that my engine advance would fluctuate, if my 6HG is too little to pull all the vacuum out with the canister, that might end to poor idle quality???
Anybody knowing if the MSD 8572 distributor vacuum canister can be easily removed and replaced with Crane´s adjustable canister - that might pull all out then with less vacuum...
Race Director
Joined: Dec 1999
Posts: 19,610
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From: Forked River NJ
You really don't want any vacuum advance at idle. 14 to 16 mechanical advance degrees at idle should be enough to keep the idle steady. The vacuum is really just there for added gas mieage under part throttle cruise conditions.
Le Mans Master
Joined: Mar 2004
Posts: 7,424
Likes: 1,458
From: Little Rock AR
The measurement you took of your car's vacuum at idle seems pretty low. I suspect the gauge is not reading right. That number sounds like it would not even run.
If the new vacuum can is not adjustable and your engine does not provide enough vacuum to pull the can all the way in at idle, you need to run ported vacuum to the can. The ported vacuum will not provide a signal at idle but it will provide a signal when you crack the throttle. This is the emissions style hookup that all of our C-3's came with stock.
-Mark.
If the new vacuum can is not adjustable and your engine does not provide enough vacuum to pull the can all the way in at idle, you need to run ported vacuum to the can. The ported vacuum will not provide a signal at idle but it will provide a signal when you crack the throttle. This is the emissions style hookup that all of our C-3's came with stock.
-Mark.
Thread Starter
Burning Brakes
Joined: Mar 2004
Posts: 816
Likes: 1
From: Finland
Originally Posted by Gordonm
You really don't want any vacuum advance at idle. 14 to 16 mechanical advance degrees at idle should be enough to keep the idle steady. The vacuum is really just there for added gas mieage under part throttle cruise conditions.
Thread Starter
Burning Brakes
Joined: Mar 2004
Posts: 816
Likes: 1
From: Finland
Originally Posted by stingr69
The measurement you took of your car's vacuum at idle seems pretty low. I suspect the gauge is not reading right. That number sounds like it would not even run.
If the new vacuum can is not adjustable and your engine does not provide enough vacuum to pull the can all the way in at idle, you need to run ported vacuum to the can. The ported vacuum will not provide a signal at idle but it will provide a signal when you crack the throttle. This is the emissions style hookup that all of our C-3's came with stock.
-Mark.
If the new vacuum can is not adjustable and your engine does not provide enough vacuum to pull the can all the way in at idle, you need to run ported vacuum to the can. The ported vacuum will not provide a signal at idle but it will provide a signal when you crack the throttle. This is the emissions style hookup that all of our C-3's came with stock.
-Mark.
Maybe my cam just is too radical etc...
I must measure the ported vacuum readings today (today some local vetteguys helped me out and showed where the port is in my 850dp).
Thread Starter
Burning Brakes
Joined: Mar 2004
Posts: 816
Likes: 1
From: Finland
Originally Posted by Gordonm
I only get about 8 inches of vacuum at 900 rpm. It is because the cam has so much overlap. If you have a mild cam and 6 inches there is probably a leak somewhere.
I have tried to locate all possible leaks - but have found zero yet...
Last edited by Vesa; Sep 24, 2004 at 08:40 AM.
Thread Starter
Burning Brakes
Joined: Mar 2004
Posts: 816
Likes: 1
From: Finland
Originally Posted by Gordonm
You really don't want any vacuum advance at idle. 14 to 16 mechanical advance degrees at idle should be enough to keep the idle steady. The vacuum is really just there for added gas mieage under part throttle cruise conditions.
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Pro
Joined: Sep 2002
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From: near Memphis TN
Originally Posted by Vesa
I do not know the specs of my cam but it pulls like a beast over 6500rpm - I use the 6500 on dragstrip as redline/shift point. I have a 396BB.
I have tried to locate all possible leaks - but have found zero yet...
I have tried to locate all possible leaks - but have found zero yet...
At what RPM at idle do you get a reading of 6 lbs (and I think HG and LB is interchangeable in this discussion)? You may be able to tweak a bit more if you raise the idle RPM speed by tweaking the carb or the initial timing. Also like the other guy said don't worry about getting advance vacuum at idle, unless you have serious idle problems just set the initial timing at 14-16 at idle and hook the vacuum to ported.
Thread Starter
Burning Brakes
Joined: Mar 2004
Posts: 816
Likes: 1
From: Finland
Originally Posted by SpyderD
Yeah, sounds like you got a healthy cam in there all right....
At what RPM at idle do you get a reading of 6 lbs (and I think HG and LB is interchangeable in this discussion)? You may be able to tweak a bit more if you raise the idle RPM speed by tweaking the carb or the initial timing. Also like the other guy said don't worry about getting advance vacuum at idle, unless you have serious idle problems just set the initial timing at 14-16 at idle and hook the vacuum to ported.
At what RPM at idle do you get a reading of 6 lbs (and I think HG and LB is interchangeable in this discussion)? You may be able to tweak a bit more if you raise the idle RPM speed by tweaking the carb or the initial timing. Also like the other guy said don't worry about getting advance vacuum at idle, unless you have serious idle problems just set the initial timing at 14-16 at idle and hook the vacuum to ported.
Thanks for the conversion, now I know what MSD means!
I have nowadays no problems with idle at all, it is rather solid (even though it is rough due to the cam) - I have set the initial to 16 now.
Thanks!