mechanical or vacuum advance? - Team Camaro Tech
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post #1 of 3 (permalink) Old Jul 2nd, 00, 07:51 AM
Rich253
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Can anyone tell me if there is a disadvantage to running a distributor that does not have vacuum advance, such as the Mallory Comp 9000, on a street driven 67? This is my daily driver with a stock 327. Is the lack of a vacuum advance hurting my gas mileage? Will the car be more driveable at low speed in traffic if I used a distributor with a vacuum advance? This distributor seems to work very well with the throttle wide open. I guess my question is, why do some distributors have vacuum advance while others don't?
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post #2 of 3 (permalink) Old Jul 2nd, 00, 08:39 AM
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Vacuum advance was used mainly for fuel economy. Yes, using a mechanical advance will hurt fuel economy, but mechanical is much more adjustable for street use.
As far as a daily driver, it's a toss up as to which is better. An engine with performance mods would benefit from mechanical over vacuum.

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post #3 of 3 (permalink) Old Jul 2nd, 00, 07:44 PM
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David Pozzi
 
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Your engine needs more advance under part throttle but the mechanical advance can't provide it. You can get by with mech only advance if you have an agressive advance curve and jet the engine lean in cruse mode.

If not, it will load up when cruzing and when you punch the throttle it will smoke a bit and then "clean out".

When crusing along the plugs will run cooler because of the retarded spark. This will form deposits on the plugs. When you punch it the plugs will warm up and burn off the deposits, or most of them.

With todays fuels maybe it's not such a problem anymore.

I had mine a little off and when driving on the freeway for about a hundred miles. I pulled in for gas and my engine almost quit! I revved it up and it belched smoke.

I re did the advance and leaned out the primary jetting and it worked very well. I could cruze all night and when I punched the throttle it accelerated cleanly.
The problem with running a vaccum advance is that the stock ones have too many degrees of advance to work with a re worked mechanical advance.
They tend to ping just off idle. Once the vaccum drops and the vaccum advance moves to retard, it stops pinging.
In this instance you need a vaccum advance can that retards sooner under fairly high vaccum and does not have too many degrees of advance. (stiff spring, limited travel)
The L88 Corvette distributor uses a good one.
I think it's about 12 degrees advance.
David


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[This message has been edited by davidpozzi (edited 07-02-2000).]
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