vacuum advance

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boilermaster

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Feb 19, 2015
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Location
michigan
My Car
71 fastback
Hello,

Currently running a Mallory unilite distributor with mechanical advance only.

Been looking at a DUI duraspark distributor and MSD box.

I have a very good idea of the mechanical aspects of what for a curve, but what about the vacuum curve .

I am running 351 c 4v with open chambers, 9.6 cr and a toploader.  .370 rear axle w/255/60 r 15 rubber

am pulling 17'' vacuum @16 degrees and 38degrees all in.

Camshaft is 223/223@ .050 on 114 lobe centers.

Engine runs quite well for what it is, but I am feeling that I could do better with a little vacuum advance .

Problem is the stock vacuum advance back in the day was for 8 degrees btdc ? or so.

Almost no one uses those specs today.

So what do we have out there fellas ? full or ported and how much ?

                                                         boilermaster

 
I am running a DUI Duraspark distributor and MSD box on my 460.

When ordering the distributor I gave them my car model, CID, cam specs and type of driving. They did the rest.

Arrived with 24 degrees mechanical @ 3000 RPM, no specs on the vac canister. I dropped it in, connected it to ported vacuum, set the initial at 12 BTDC and never touched it again.

I love it. Best distributor I have ever run.

 
16 initial, 20 mechanical all in by 2600 RPM, vacuum advance limited to 6 degrees, ported vacuum. Dura spark type trigger in a Rich Porter (now Spectra Premium) distributor, MSD digital 6AL amplifier, MSD Blaster coil. Engine is 408C, 4V CC heads. 9.6:1 CR.  Chuck

 
c9zx,

figuring that the vacuum advance would have to be a relatively low number compared to stock.

Not 100 % sure on the ported or full vacuum.

Been trying to scheme up some sort of electronic vacuum switch (s) that I could control the upper and lower limits of the vacuum signal.

may just be a lot less hassle to go ported.

might try some spare Harley voes switches.

are you totally satisfied with your advance rates ?

                                               boilermaster

 
I use 14* initial and I start applying centrifugal advance at 950 rpm for a total of 36* (22* mech adv max) at 2500rpm. For vacuum I use ported and 10* max. 351C, 10.5:1 cr, 4v cc heads with MSD distributor, 6AL box, MSD Blaster coil and Atomic EFI set up to control timing.

 
c9zx,

figuring that the vacuum advance would have to be a relatively low number compared to stock.

Not 100 % sure on the ported or full vacuum.

Been trying to scheme up some sort of electronic vacuum switch (s) that I could control the upper and lower limits of the vacuum signal.

may just be a lot less hassle to go ported.

might try some spare Harley voes switches.

are you totally satisfied with your advance rates ?

                                               boilermaster
It is a lot less than it was originally (about 14).  I'm less concerned about gas milage and more concerned about holding potential detonation at bay. I am satisfied with the rates, no hesitation and no detonation. But with a 3.89 gear and a 2800 stall convertor, 2600 rpm arrives very quickly. Chuck

 
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