Big Block...What vacuum advance can should I purchase? - Chevelle Tech
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post #1 of 24 (permalink) Old Jul 12th, 20, 12:18 PM Thread Starter
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Dan
 
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Big Block...What vacuum advance can should I purchase?

Posting here as not very many folks over in the ignition forum right now.
I have been using a mallory unilite with no vacuum advance canister for decades.

I am putting in a points distributor today hoping I can get a lower idle and better top end and better gas mileage...and so my car does not go into gear so hard. Have always had a bit of trouble with the unilite when playing with the timing but always set it where I thought it felt best.

My engine
71 CE-BB 402 With 063 Heads
Quadrajet and headers
No big thumping cam
Probably 350 to 370 HP just guessing.

Vacuum reading is pretty steady around 15 if my old gauge is close.

So what can should I purchase?
Is there a basic can I should get to start with?

A napa or autozone part# would be very helpfull if anyone has suggestions


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post #2 of 24 (permalink) Old Jul 12th, 20, 3:55 PM
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Re: Big Block...What vacuum advance can should I purchase?

DV1810 from autozone or equivalent should work fine.
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post #3 of 24 (permalink) Old Jul 12th, 20, 8:14 PM
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Re: Big Block...What vacuum advance can should I purchase?

There is no way anyone can tell you what you need until you measure manifold vacuum at idle

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post #4 of 24 (permalink) Old Jul 12th, 20, 10:22 PM
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Re: Big Block...What vacuum advance can should I purchase?

Here's quite a list.

http://outintheshop.com/faq/Vac%20Adv%20Spec.pdf

Jim

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post #5 of 24 (permalink) Old Jul 13th, 20, 9:25 AM
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Keith
 
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Re: Big Block...What vacuum advance can should I purchase?

read this
http://www.metroli.org/pdf/2012%20Na...ingseminar.pdf

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post #6 of 24 (permalink) Old Jul 13th, 20, 9:58 AM
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Re: Big Block...What vacuum advance can should I purchase?

Quote:
Originally Posted by keithb View Post
Page #17:
Regarding vacuum advance - The "2 inch rule".
I've never heard that.

Thanks for posting

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original paint, M20, & 3:31 posi.
Unrestored, but dreaming of doing so
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post #7 of 24 (permalink) Old Jul 13th, 20, 10:12 AM
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Keith
 
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Re: Big Block...What vacuum advance can should I purchase?

Quote:
Originally Posted by guitargeak99 View Post
Page #17:
Regarding vacuum advance - The "2 inch rule".
I've never heard that.

Thanks for posting
Most people including engine builders never have either and the engines build suffers from it. My engine was off after a rebuild and after years of guessing how to fix it. The author of that article helped me dial in my engine. In short after replacing the vacuum advance can and following the two inch rule my engine never ran any better.
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post #8 of 24 (permalink) Old Jul 13th, 20, 1:43 PM
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Re: Big Block...What vacuum advance can should I purchase?

"There is no way anyone can tell you what you need until you measure manifold vacuum at idle"

Answered: "Vacuum reading is pretty steady around 15 if my old gauge is close.
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post #9 of 24 (permalink) Old Jul 13th, 20, 2:57 PM Thread Starter
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Dan
 
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Re: Big Block...What vacuum advance can should I purchase?

The vacuum has stayed between 14 and 14 1/2 the couple times I checked it.
A bit low from what i have read about it?
Pretty much the same checked on the manifold where tranny mod fitting is
and the front bottom of carb where the pvc plugs in the bottom plate of carb.

I decided to just use the 201-15 can being it was not dead, that was already on it after
going to napa with only one choice available and autozone did not have
anything available in stock.

I was shocked that the initial timing had to bumped up to 21ish btc
for me to get the idle down enough to slow the idle to where it feels
lower than before and goes into gear smoother.
Hope I am going down the right path?

I always set the unilite at about 8 btc thought it was ok...i have not had this
car on the road in 20yrs since before I had the internet...WOW time flies!

Going to drive it later today...should be interesting to see the difference between the
no vacuum adv unilite and an old points setup on a BB...dog or no dog?

Hope I am not disappointed...


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post #10 of 24 (permalink) Old Jul 13th, 20, 3:33 PM
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Keith
 
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Re: Big Block...What vacuum advance can should I purchase?

If you read that article I posted and followed it your engine should be fine.

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post #11 of 24 (permalink) Old Jul 13th, 20, 3:40 PM
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Steve
 
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Re: Big Block...What vacuum advance can should I purchase?

Dan,
Give it some more initial timing - 8 degrees is not that much.
Usually when you give more initial, it allows you to lower the idle - which closes the primary carb butterflies and ensures you are only on the idle circuit.
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post #12 of 24 (permalink) Old Jul 13th, 20, 3:52 PM Thread Starter
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Re: Big Block...What vacuum advance can should I purchase?

Quote:
Originally Posted by guitargeak99 View Post
Dan,
Give it some more initial timing - 8 degrees is not that much.
Usually when you give more initial, it allows you to lower the idle - which closes the primary carb butterflies and ensures you are only on the idle circuit.
I'm sorry... that 8 degrees was on my unilite without vacuum adv can...
that I just tookout.
I put a points distributor back in it for now.


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post #13 of 24 (permalink) Old Jul 13th, 20, 4:15 PM
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Re: Big Block...What vacuum advance can should I purchase?

Sorry Dan, I mis-read your post.
I'm looking forward to hearing how it runs for you.

Years ago I had a fairly hot SBC 350 in a 69 Camaro. I wasn't using the VA, mainly because I just didn't get the concept, and I heard horror stories of too much timing.
A buddy of mine set the total at 36, which put my initial at 14. Hooked up the VA and that gave me an initial of 24 - it started and turned off no problem, which blew me away. We then set up the total all in at 2600 rpm, re-adjusted the carb and bingo! A whole different car, clean burning spark plugs and throttle response up the whazoo.

I still didn't completely get the concept of VA until I started to frequent this forum.
Good luck
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post #14 of 24 (permalink) Old Jul 14th, 20, 4:05 PM
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Re: Big Block...What vacuum advance can should I purchase?

"Years ago I had a fairly hot SBC 350 in a 69 Camaro. I wasn't using the VA, mainly because I just didn't get the concept, and I heard horror stories of too much timing.
A buddy of mine set the total at 36, which put my initial at 14. Hooked up the VA and that gave me an initial of 24 - it started and turned off no problem, which blew me away. We then set up the total all in at 2600 rpm, re-adjusted the carb and bingo! A whole different car, clean burning spark plugs and throttle response up the whazoo.

In your explanation, you have 14 deg/BTDC INITIAL, and 10 deg/Added by full manifold vacuum to the vacuum advance, for a total IDLE timing of 24 deg/BTDC.

Yup, that's exactly what I have been saying for over 45 years, but nobody listens, they all know more than anyone else.
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post #15 of 24 (permalink) Old Jul 14th, 20, 4:53 PM
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Re: Big Block...What vacuum advance can should I purchase?

Quote:
Originally Posted by Dave Ray View Post
"Years ago I had a fairly hot SBC 350 in a 69 Camaro. I wasn't using the VA, mainly because I just didn't get the concept, and I heard horror stories of too much timing.
A buddy of mine set the total at 36, which put my initial at 14. Hooked up the VA and that gave me an initial of 24 - it started and turned off no problem, which blew me away. We then set up the total all in at 2600 rpm, re-adjusted the carb and bingo! A whole different car, clean burning spark plugs and throttle response up the whazoo.

In your explanation, you have 14 deg/BTDC INITIAL, and 10 deg/Added by full manifold vacuum to the vacuum advance, for a total IDLE timing of 24 deg/BTDC.

Yup, that's exactly what I have been saying for over 45 years, but nobody listens, they all know more than anyone else.
Yep Dave,
I've read your posts over the years and it had made me realize that this friend of mine, who helped me tune my car in early 90's, really understood ignition timing.
BTW about 5 years ago, on my request, you emailed me your write up on VA.
Regards

67 Chevelle SS:
original paint, M20, & 3:31 posi.
Unrestored, but dreaming of doing so
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