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Swap a mechanical advance distributor for a vacuum one?


02Coastie

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The PO put an 002 Mech Advance distributor on the car, but with no obvious performance additions (other than a 32/36). If I had to guess, he probably ditched the CA vacuum adv/retard distributor because it broke, and figured he'd "upgrade" while at it. Everything I've recently read says that for a stockish motor (mainly with respect to compression) it's actually better to have a vacuum advance distributor. Any thoughts? What would be the best model year to source one from if I wanted to try the swap back?

Thanks!

-Lloyd

"Sabine" 1976 Polaris / Navy - 12+ year Inop Ebay Find. Now with Microsquirt and Cold A/C. 

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...as do I. They are harder to find than you'd think, though.

I'd look for the earliest one in good shape... problem is,

the late dizzies are built much better, but have more 'emission'

than performance curves.

Oh, hey- I'd get one in good mechanical shape, then have it

recurved to the early spec. Which is SOMEWHERE in this bb...

t

"I learn best through painful, expensive experience, so I feel like I've gotten my money's worth." MattL

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Macartney's BMW '02 Restoration Guide states that on a mostly stock motor, a mech dizzy can actually reduce performance vs. a vac advance dizzy (pp193-194).

Another issue with the mech. dizzy is that overall, it offers less total advance and under load (which slows the dizzy down) advance can actually be retarded for a short bit until the engine re-establishes it's rotational speed, allowing the weights to swing out again (and increasing advance).

In some cases, the mech dizzy is a compromise to running DCOEs which do not have a vacuum port to run a vac dizzy.

Cheers!

1976 BMW 2002

1990 BMW 325is (newest addition)

1990 Porsche 964 C4 Cabriolet

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On a '76 Cali car, don't you have to pass smog testing? OEM the '76 cali car had a vac advance/retard dizzy.

Can you pass without it or are the inspectors just not so knowlegeable or picky so long as the emissions printout is within spec?

Cheers!

1976 BMW 2002

1990 BMW 325is (newest addition)

1990 Porsche 964 C4 Cabriolet

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On a '76 Cali car, don't you have to pass smog testing? OEM the '76 cali car had a vac advance/retard dizzy.

Can you pass without it or are the inspectors just not so knowlegeable or picky so long as the emissions printout is within spec?

Cheers!

If it's registered in California you do =o) God bless the military and my FL residency.

"Sabine" 1976 Polaris / Navy - 12+ year Inop Ebay Find. Now with Microsquirt and Cold A/C. 

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Looks like I gotta get my advance put on then! My distributor was originally working in a vacuum retard setup (apparently), so I had it rebuilt to mechanical advance specs.

It's not that a mech dizzy won't work, but it isn't simply plug & play. You need to have it recurved to bring the advance in sooner and also extend the total advance available.

This is done by changing the springs, weights and possibly even the breaker plate. Companies such as Advanced Distributors specialize in this kind of work. Advanced Distributors even has a Distributor Curve Worksheet which you fill out (it even wants your compression numbers, the type of ignition, fuel, mods etc.) so they can set the dizzy to your individual engine

Cheers!

1976 BMW 2002

1990 BMW 325is (newest addition)

1990 Porsche 964 C4 Cabriolet

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I have a vacuum advance distributor if you would want to try it out to see if there is a difference. I can bring it to work tomorrow.

Steve T.

That'd be awesome! I've got to leave early for a reception at the quarters, but I can swing by in the AM.

"Sabine" 1976 Polaris / Navy - 12+ year Inop Ebay Find. Now with Microsquirt and Cold A/C. 

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Update: Thanks to Steve loaning me his Vac Advance dizzy, I was able to swap out my 002 mech this afternoon.

Initial impressions:

Early to part throttle action feels noticeablly peppier. In some ways it feels like the secondary is opening up sooner. Top end has a healthy surge past 4.5k that wasn't there previously. Overall it's not earth shattering, but it does feel "better." The real question is going to be what the fuel mileage gain (if any) there is. I've been getting around 20mpg pretty consistently in mixed city driving, with 26-28 on the highway. I'll report back in few weeks and let you know what I find.

"Sabine" 1976 Polaris / Navy - 12+ year Inop Ebay Find. Now with Microsquirt and Cold A/C. 

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