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Old 06-11-2012, 02:00 PM   #1
beatified
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Default Vacuum requirements for brake booster

I am only producing about 8 lbs of vacuum and was wondering if I need to consider hydroboost at this point. Right now its a stock brake booster on a 77 cdv.
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Old 06-11-2012, 04:51 PM   #2
Nashalac
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What idle speed?? higher idle will help vac
I saw the dyno sheets and timing should be 2* more. This will help a bit
Could also change the bushing in the dist from the 21* bushing to the 18* one. This would allpw another 3* initial advance

To answer your question above 12 should be OK for the brakes
The number is 16 but have found that 12 will do normally

I would go to a vac pump for the brakes before changing out the system
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Old 06-11-2012, 10:51 PM   #3
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Quote:
Originally Posted by Nashalac View Post
What idle speed?? higher idle will help vac
I saw the dyno sheets and timing should be 2* more. This will help a bit
Could also change the bushing in the dist from the 21* bushing to the 18* one. This would allpw another 3* initial advance

To answer your question above 12 should be OK for the brakes
The number is 16 but have found that 12 will do normally

I would go to a vac pump for the brakes before changing out the system
Thanks for the reply Marty. The reason I was unable to get it to 32 degrees is because it was detonating. We even lowered the temp to 160 and below and still could only get it to 30. I dont totally understand the bushing thing but I would love to be able to get another 3 to 5 degrees out of it.

Also I know at one point you had sent me a set of springs for the distributor that were to go in when I got the 500 together would there be any significant benifit to still doing that? By the way I think I lost them.

Idle was about 575 and was about 8 lbs. Although it seemed to stick right around 8 no matter what the rpm. I also believe this was at 33 degrees.

Last edited by beatified; 06-11-2012 at 11:33 PM.
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Old 06-25-2012, 10:15 AM   #4
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Quote:
Originally Posted by Nashalac View Post
What idle speed?? higher idle will help vac
I saw the dyno sheets and timing should be 2* more. This will help a bit
Could also change the bushing in the dist from the 21* bushing to the 18* one. This would allpw another 3* initial advance

To answer your question above 12 should be OK for the brakes
The number is 16 but have found that 12 will do normally

I would go to a vac pump for the brakes before changing out the system
Since I have an HEI Distributor it seems that the way you accomplish limiting mechanical advance is through a kit of weights and springs and swapping out the weights. I guess the idea behind this is that the other weights are designed a little different limiting the travel distance which allows you to set more initial timing and still attain the same amount of total advance.
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Old 06-25-2012, 08:33 PM   #5
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Quote:
Originally Posted by beatified View Post
Since I have an HEI Distributor it seems that the way you accomplish limiting mechanical advance is through a kit of weights and springs and swapping out the weights. I guess the idea behind this is that the other weights are designed a little different limiting the travel distance which allows you to set more initial timing and still attain the same amount of total advance.
No.

LIMITING the advance is best done by welding/brazing the slot provided. In factory form, the slot will allow enormous amounts of advance; therefore you close up the length of the slot to prevent over-advance. The weights will continue advancing if the RPM goes up high enough. You are best off to prevent the advance mechanism from advancing beyond whatever you determine to be your best maximum amount.

Note how the inner pins travel within the slots of the upper plate:



Another thing to watch is for wear to the pivot pins--the outer pins in this photo are trashed from wear, and the holes in the weights (not shown) are similarly oversize.

Changing the advance curve of an HEI will require weights, springs, and the center piece. The weights must match the pivot pins, there's at least three designs I'm aware of. One of them uses plastic bushings in the weights, the others don't. The center piece has at least as much effect as the weights. There are many part numbers for center pieces; but almost none available in the aftermarket. They're pretty-much Treasure Yard items now. An example of parts I'm talking about:



The stamped numbers go face-up or face-down depending on direction of distributor rotation.

Last edited by Schurkey; 06-25-2012 at 08:42 PM.
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