Vacuum question (Demon carb) - Team Camaro Tech
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post #1 of 10 (permalink) Old Oct 5th, 09, 12:23 PM Thread Starter
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Derek
 
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Vacuum question (Demon carb)

Hello, I just bought a 383 crate engine and it came with a Demon 750 vac. sec. carburetor and an MSD hei ignition 4 pin with vacuum canister. Ive never dealt with a barry grant before and I noticed there are 2 vacuum hook up's on the passenger side of the carb. I am assuming it would be ok to hook the vacuum hose up to either one of them but I am not certain. Is this correct?
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post #2 of 10 (permalink) Old Oct 5th, 09, 01:46 PM
 
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Re: Vacuum question (Demon carb)

front one is ported(only vacuum after throttle blades open a bit) and rear one is maniflold(always has vacuum)
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post #3 of 10 (permalink) Old Oct 6th, 09, 09:16 AM Thread Starter
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Derek
 
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Re: Vacuum question (Demon carb)

ok thank you...so I am assuming my vacuum line from distributor should hook up to the rear one (The one closest to distributor)? Thanks.
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post #4 of 10 (permalink) Old Oct 6th, 09, 02:25 PM
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Re: Vacuum question (Demon carb)

I have mine on the front, the one closest to the rad.

Steve
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post #5 of 10 (permalink) Old Oct 6th, 09, 04:55 PM Thread Starter
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Re: Vacuum question (Demon carb)

so is it supposed to go on the front one or the back one?
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post #6 of 10 (permalink) Old Oct 6th, 09, 09:07 PM
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Re: Vacuum question (Demon carb)

Quote:
Originally Posted by pro67street View Post
so is it supposed to go on the front one or the back one?

That'll start a whole new discussion....

I personally use the ported vacuum for my vacuum advance but have not tried full manifold vacuum. That's the way MSD and Holley both said to do it so I did.

Ported will only give you vacuum on the can for advance with the throttle open. Full manifold port will give you vacuum advance at all times.

You might be better suited to search the discussions on ported vs. full vacuum for vacuum advance. There are arguments for both sides and you may like one better than the other.

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post #7 of 10 (permalink) Old Oct 7th, 09, 05:45 PM
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Re: Vacuum question (Demon carb)

Quote:
Originally Posted by Steiner View Post
That'll start a whole new discussion....



You might be better suited to search the discussions on ported vs. full vacuum for vacuum advance. There are arguments for both sides and you may like one better than the other.
X2^.Excellant advice from Steiner imo.

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post #8 of 10 (permalink) Old Oct 7th, 09, 06:08 PM
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Re: Vacuum question (Demon carb)

Full article is on this site in many posts. My files contain the long and short versions.

Excerpt from John Hinckley's "Timing 101" (John Z).

Now, to the widely-misunderstood manifold-vs.-ported vacuum aberration. After 30-40 years of controlling vacuum advance with full manifold vacuum, along came emissions requirements, years before catalytic converter technology had been developed, and all manner of crude band-aid systems were developed to try and reduce hydrocarbons and oxides of nitrogen in the exhaust stream. One of these band-aids was "ported spark", which moved the vacuum pickup orifice in the carburetor venturi from below the throttle plate (where it was exposed to full manifold vacuum at idle) to above the throttle plate, where it saw no manifold vacuum at all at idle. This meant the vacuum advance was inoperative at idle (retarding spark timing from its optimum value), and these applications also had VERY low initial static timing (usually 4 degrees or less, and some actually were set at 2 degrees AFTER TDC). This was done in order to increase exhaust gas temperature (due to "lighting the fire late") to improve the effectiveness of the "afterburning" of hydrocarbons by the air injected into the exhaust manifolds by the A.I.R. system; as a result, these engines ran like crap, and an enormous amount of wasted heat energy was transferred through the exhaust port walls into the coolant, causing them to run hot at idle - cylinder pressure fell off, engine temperatures went up, combustion efficiency went down the drain, and fuel economy went down with it.

If you look at the centrifugal advance calibrations for these "ported spark, late-timed" engines, you'll see that instead of having 20 degrees of advance, they had up to 34 degrees of advance in the distributor, in order to get back to the 34-36 degrees "total timing" at high rpm wide-open throttle to get some of the performance back. The vacuum advance still worked at steady-state highway cruise (lean mixture = low emissions), but it was inoperative at idle, which caused all manner of problems - "ported vacuum" was strictly an early, pre-converter crude emissions strategy, and nothing more.

What about the Harry high-school non-vacuum advance polished billet "whizbang" distributors you see in the Summit and Jeg's catalogs? They're JUNK on a street-driven car, but some people keep buying them because they're "race car" parts, so they must be "good for my car" - they're NOT. "Race cars" run at wide-open throttle, rich mixture, full load, and high rpm all the time, so they don't need a system (vacuum advance) to deal with the full range of driving conditions encountered in street operation. Anyone driving a street-driven car without manifold-connected vacuum advance is sacrificing idle cooling, throttle response, engine efficiency, and fuel economy, probably because they don't understand what vacuum advance is, how it works, and what it's for - there are lots of long-time experienced "mechanics" who don't understand the principles and operation of vacuum advance either, so they're not alone.
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post #9 of 10 (permalink) Old Oct 8th, 09, 01:08 PM
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Re: Vacuum question (Demon carb)

We have had customers have success with it in either location depending on how their ignition was set up but our preference is the full time port. As noted though in a lot of cases the distributor will need to be recurved and you may need to change the vacuum advance out for one with a different range or an adjustable one.

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post #10 of 10 (permalink) Old Oct 15th, 09, 10:14 AM Thread Starter
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Re: Vacuum question (Demon carb)

Thank you for all the help and advise. I think I will try it on both locations and see which one seems to work best...The engine builder recommended that I put it on the port that only has vacuum when the throttle plate opens.
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