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  #16 (permalink)  
Old 01-14-2012, 10:09 AM
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Quote:
Originally Posted by Custom10
Power time it to 36 Deg BTDC @ 4000RPM without vacuum advance, then let it idle set up the carb to get 800 rpm and good vacuum. Check and post the base timing that you end up with. You can then do the math to see what you have for base and mechanical, you may not need to change mechanical maximum. If you base timing is less than 14 yes you need to get in there and change the mechanical limits but not yet,,,

Then add manifold vacuum advance at idle, readjust carb and check your idle timing, post it. you now know all your values. The three amigo's base + mech advance + vac advance + vacuum adv = total timing in my book, some just use base + mech when they talk total.

Most adjustable vacuum canisters DO NOT LIMIT THE VACUUM ADVANCE they only allow you to change the range of the diaphragm spring. Be careful when looking at the charts for these cans when it comes to engine hg" vs vacuum advance added. A limiter plate is widely use for limiting not a set screw.

Try to keep the mechanical advance out at idle, sometimes it can creep in above 800 rpm.

There are volumes of threads on this subject. Post some real numbers K
Good stuff!
I'll report back on my findings!
Thanks for that post!
Mark

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  #17 (permalink)  
Old 02-01-2012, 07:35 AM
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I recently went through this with a two Cardone rebuilt distributors. from AutoZone. (same thing with advanced autoparts, orielly - they all give the same distributors as far as paying attention to spec )

Let alone figuring out that the first distributor was missing the liming busing, ( that distributor gave me 36* fully advanced that had the car jerking like crazy at 70 miles an hour and Iím wondering whatís going on when I had the initial set as low as TDC trying retard it in attempts to resolve) as well, the cans that came with them had way too much advance. one of the cans gave me 26* and one gave me 20*.

It seems when buying a rebuilt distributor they don't put much thought, or at least as much thought as I was giving it to have them setup properly since they come up with the same part number for a 70 through 74! ( I had to also replace the crap points and condenser that came with them)

Neither of the vac cans had part numbers like in the article for me to compare to but I could basically figure which cans they were based on how much advance they gave me let alone when they actually started advancing. ( and these must have been cans that came in with minimum inches since I could easily move the pull rod on each of them compared to another can I have which is a B1 and that rod is hard to move compared to the others)

Having a dial back light is a must for this and removing springs on the mechanical as well. ( to get a general idea anyway )

Yesterday I finally had a B25 (vc1807, AutoZone part number - dv1807 )vac adv arrive at AutoZone.

My dist gives 25-26* of advance. (again by removing the springs to verify)

The vac can, per the spec listed, does in fact give 16* of advance.


You have to know how much vacuum you are getting at idle. I ended up with a can that goes to a 74 350 stock points. the one for a 72 was not right for my car because it needs more vacuum to be fully advanced. vc1802 ( stock new rebuilt 350)

These three articles were essential to getting this all together:

How to set you timing for peak performance
http://www.corvette-restoration.com/...ine_Timing.pdf

Timing and Vacuum Advance 101
http://chevellestuff.net/tech/articl.../timing101.htm

Distributor Vacuum Advance Control units
Specs and facts for GM Point-Style Distributors
http://www.corvette-restoration.com/...c_Adv_Spec.pdf

Before reading these I would have been none the wiser that my distributor and vac can were giving way too much advance and completely wrong for my car.

So now Iím running 10* + 26* = 36* total timing OR 10* initial + 26* mechanical + 16* of vacuum advance ( manifold vacuum) for a total of 52* total timing.

Now if anything I need a vacuum pump to make totally sure that I don't actually need a B26 can (vc 1808) instead of B25 (vc1807)since it takes less vacuum to fully advance but it is down to those two for my car but the car runs real good right now.

The last thing to play with would be the curve and I have a feeling again the springs with these rebuilds are way heavy.

I hope my journey here helps out!

Dial it in!

Mark

Last edited by 1972 El Camino; 02-01-2012 at 08:01 AM.
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Old 02-01-2012, 08:07 AM
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since i couldn't edit my post anymore i will add the amendment here to make it it clearer.

"You have to know how much vacuum you are getting at idle. I ended up with a can that goes to a 74 350 stock points. The one for a 72 (vc1802) was not right for my car because it needs more vacuum to be fully advanced at idle. (the engine in my car is a stock new rebuilt 350)"
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Old 02-01-2012, 11:57 AM
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I am happy to see/read when someone actually takes the time to figure out what he has and how to make it work, good job.

You may find that once you lighten up the springs on the mechanical advance it could cause surge at highway cruising again as the mechanical advance amount will be more at the same rpm. (unless it was already pinned in both cases).

If it were me I would at that point limit the vacuum advance to 12 with a limiter plate. 52 is kinda high for a total but the 10 + 26 = 36 is OK for base + mechanical on a stock motor so I would leave those be if you and your motor are happy with that and change the maximum amount of vacuum adv allowed only. You can use the same canister just add this

http://www.cranecams.com/uploads/instructions/254g.pdf
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Old 02-01-2012, 03:09 PM
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Quote:
Originally Posted by Custom10
I am happy to see/read when someone actually takes the time to figure out what he has and how to make it work, good job.

You may find that once you lighten up the springs on the mechanical advance it could cause surge at highway cruising again as the mechanical advance amount will be more at the same rpm. (unless it was already pinned in both cases).

If it were me I would at that point limit the vacuum advance to 12 with a limiter plate. 52 is kinda high for a total but the 10 + 26 = 36 is OK for base + mechanical on a stock motor so I would leave those be if you and your motor are happy with that and change the maximum amount of vacuum adv allowed only. You can use the same canister just add this

http://www.cranecams.com/uploads/instructions/254g.pdf
hey thanks for that! i figure i should learn something along the way!
i thought 52-54 was okay.

also keep in mind i'm using manifold vacuum and not ported so its not like 52 is all in at once ever. from how i see it it may get close but never will be completely 52 degrees all at once.

the thing with manifold vacuum, (or one of the many pluses) is that i have great throttle response at any rpm.

the car is happy with what it has at this point but i have thought about how to remedy if in fact that changes and the link to the limiter is just what i would think after the reading that i have done!
thanks very much!
Mark
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  #21 (permalink)  
Old 02-02-2012, 06:41 AM
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Quote:
Originally Posted by Custom10
I am happy to see/read when someone actually takes the time to figure out what he has and how to make it work, good job.

You may find that once you lighten up the springs on the mechanical advance it could cause surge at highway cruising again as the mechanical advance amount will be more at the same rpm. (unless it was already pinned in both cases).

If it were me I would at that point limit the vacuum advance to 12 with a limiter plate. 52 is kinda high for a total but the 10 + 26 = 36 is OK for base + mechanical on a stock motor so I would leave those be if you and your motor are happy with that and change the maximum amount of vacuum adv allowed only. You can use the same canister just add this

http://www.cranecams.com/uploads/instructions/254g.pdf
if 52 is kind of high for a stock motor what is considered good?
how does one now figure out why some would like more or less timing?

what about changing it up like instead of
26 centrif + 10 initial
to
20 centrifical + 16 initial
they both add up to 36

i guess the limitations would be how much could the car handle of initial?

i guess i could calculate this by seeing how much initial the engine can take before the starter kicks when starting it?

lets say i could get it to 14* before the starter would kick back.
i could then modify the dist to give 22* fully advanced for:
22 + 14 = 36 base
?
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