hei vacuum advance stop plate

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mjck5

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i am replacing my intake manifold gaskets and i figured id put in new hei distributor while im at it, seeing as how i need to remove distributor anyways, im replacing it with an accell hei, when i opened the box it comes with a vacuum advance stop plate, my question is do i need this? and if so what would be the best setting or how do i determine that, its going in a k5 blazer with a rebuilt 400sb with a rv cam.
any help/advice would be appreciated
 

Matt69olds

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Read this:


Delco made a BUNCH of different advance plates and different weights, and dozens of different springs to calibrate their distributors for different applications. Obviously , a corvette or Camaro will have different timing needs than a Cadillac, the advance plates is what GM used to shape the timing curve. At one time I had a notebook with all the different combinations and part numbers. I think I found the info on a Pontiac performance forum. I loaned my notebook to a buddy so he could make copies, of course he lost it before he got around to making them.

Basically, you find a good initial timing number where the engine starts easily, idles well, and accelerates smoothly. Then you pick an advance plate to limit the centrifugal advance to the desired number, and then use different weights and springs to adjust the rate the advance comes in.
 

mjck5

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so is it just trying different ones until you like it? the plate it came with has 5 thru 8, 8 thru 11, 11 thru 14 and 14 thru 17, i havent the slightest clue which to start with or if the plate is even necessary
 

Ricko1966

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You use the limit plate after you've figured your best total and curve. Those are figured before you add in vacuum advance. After you have your total you add vacuum advance on top and use the limit plate so you don't go too far. After you have your total subtract it from 44, add that much vacuum advance if you don't have working egr subtract from 50 if you have working egr busy right now. Write better instructions latter.
 

mjck5

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ok, appreciate it, so sounds like i should just leave it off when i first install it, ive never dealt with setting the timing curve or any of this so im pretty well clueless at this point
 

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i am replacing my intake manifold gaskets and i figured id put in new hei distributor while im at it, seeing as how i need to remove distributor anyways, im replacing it with an accell hei, when i opened the box it comes with a vacuum advance stop plate, my question is do i need this? and if so what would be the best setting or how do i determine that, its going in a k5 blazer with a rebuilt 400sb with a rv cam.
any help/advice would be appreciated
I've got some more time now exactly which accell hei are you installing? The plate is probably very neccesary. Most sbc depending on heads like somewhere between 30 and 36 degrees total. Total is base timing plus centrifugal. So let's say you have 36 total and no egr subtract 36 from 44 and you get 8 so you want to limit your vacuum advance to around 8. If you had working egr I'd subtract 36 from 50 I'd get 14 so I'd add 14 degrees of vacuum advance. It's more complicated than thar but that is bit in simplest form. I can give you more and better advice when I know more about the distributor.
 

mjck5

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its the accell 59130 going into a 400sb no egr, nothing fancy with the heads just stock, the motor was bored .030over and put in an rv cam
 

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If you read what Matt put up for you,you know how to set total. If not call accel ask them how much centrifugal the distributor has. Let's say they say 22* set your timing 12* to give you 34*. Use position c 8-11* on the plate. I'll help you dial it in better later. But that will be really close. Another example if they say it's got 18* set your timing at 16* so on so forth. Your looking for 34 degrees. Then notch C
 

75gmck25

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Base timing on a stock SBC was usually set to about 4-8 degrees BTDC (measured at about 650 rpm idle, vacuum advance line disconnected and plugged). You can get more performance from a stock engine by setting it to about 12, and maybe even a little higher. I have used up to about 15 BTDC with aluminum heads and 9.4 compression, and it works fine.

Centrifugal or mechanical timing adds about 18-22 degrees. Stock distributors have a “lazy” curve that doesn’t add the full timing until 3000-3500 rpm. Performance distributors often have it “all-in” by 2000 rpm. You can check it with a timing tape on the balancer, but a setback timing light is easier. Just gradually rev up the engine, and watch the timing advance. Write down the rpm vs degrees.

Base plus mechanical timing is what determines your performance, and the total degrees BTDC should not exceed about 34-36 degrees.

Vacuum advance is there mostly to get better gas mileage at cruise. Race-only vehicles run fine with no vacuum advance. It advances the timing only during high engine vacuum. Stock distributors add up to 18-20 degrees, and when you add that to your base and mechanical your total timing is up in the mid 50’s. That might cause pinging during light throttle cruise/acceleration. Performance distributors use more like 12-15 degrees vacuum advance, and some can be adjusted to even lower.
 

Ricko1966

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This is total timing
"The centrifugal advance system in a distributor advances spark timing purely as a function of engine rpm (irrespective of engine load or operating conditions), with the amount of advance and the rate at which it comes in determined by the weights and springs on top of the autocam mechanism. The amount of advance added by the distributor, combined with initial static timing, is "total timing" (i.e., the 34-36 degrees at high rpm that most SBC's like). Vacuum advance has absolutely nothing to do with total timing or performance, as when the throttle is opened, manifold vacuum drops essentially to zero, and the vacuum advance drops out entirely; it has no part in the "total timing" equation."
Yes this is confusing,but 99 percent of people and manufacturers use this as the definition of total timing. Vacuum advance even though added,which raises timing at light loads (because it needs it for optimal burn because its lean) is considered a separate number in the equation. You are going to want around 45* at light throttle loads.
O.P. Google articles on setting total timing so you have a good understanding of how to set it.
While you're googling things, Google MBT ( minimum best timing) ( minimum best torque) because after you get basically set up I'm going to teach you how to find it and set it. Too much timing even without audible ping is not necessarily the best timing for power and mpg.
 
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mjck5

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couldnt find any info on what the centrifugal advance for this distributor is online, ill have to call accel in the morning and see if they can give me any more info.
one thing i didnt think of before, do i need to take into consideration the cam? it has an rv cam does that change anything?
 
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Ricko1966

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i am replacing my intake manifold gaskets and i figured id put in new hei distributor while im at it, seeing as how i need to remove distributor anyways, im replacing it with an accell hei, when i opened the box it comes with a vacuum advance stop plate, my question is do i need this? and if so what would be the best setting or how do i determine that, its going in a k5 blazer with a rebuilt 400sb with a rv cam.
any help/advice would be appreciated
After rereading this. I suggest you put it back together. As it was, old distributor. Make sure it runs right and as it use too before you start throwing,different parts into the equation. After you're sure it's back together right hits on all 8 etc. etc. Then swap distributors if you want. You could be chasing timing issues like crazy and after pulling your hair figure out it was something with the intake install.
 

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I'm thinking on some aftermarket distributors they come with a lock out plate that keeps you from running any vacuum advance. A lot of drag race cars are run with no vac. advance. Some guys weld stop on the stock plates.
 

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Really good info and advice above! Way too many aftermarket distributors still come with a vacuum can that will easily add 20 degrees or more. That's what GM used back in the 70s to compensate for zero or less initial advance at idle, and only 25 or so mechanical. So, 0 + 25 + 20 = 45.

What I've done over the years is found a couple vacuum cans with anywhere from 10-12 degrees max. Or you can make your own stop plate for a 20 degree can. As I recall, something around .120" pin travel gets you 10-12 degrees max. There's an article on the web with old part numbers for various canisters, including some for use with high performance (low vacuum) engines.

Check out the attached. If you can't open it, I'll try to find it on the web.
 

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  • Vacuum_Advance_Specs.pdf
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shortshift

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Thank you Ricko1966 and mikeb. Good info.Have printed this for checking my timing on the 77
 

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