HI

I am planning to sit a pair of sidedraft SUs to an a12 (in a 69 1000) and was told by a reputable performance company that I would lose all the vacum and thus no advance for the standard dizzy. He reccomended an electronic dizzy replacemnt (at $500!). Not keen to pay that sorta money, wondering if what he says is true and what options I have.
The block is shaved 3mm, headers, 2" straight thru exhast and agressive cam (wont idle below 1000rpm), and a downdraft 2 barrel progressive webber carb.

If you use a matched pair of SU's from one of the British cars [Austin, Morris, MG, Sprite etc] you should have a distributor vacuum port on at least one of these carbs, after all, the donor car had vacuum advance.
If you use an original set from a Datsun "A" series GX model, you will have a suitable vacuum port for sure.

Mind you, if the cam is agressive enough, & you keep ypur foot in it a lot, you may not have any vacuum to move the distributor advance anyway.

I ran the standard 1200GX twin Hitachi [SU type] carbs in my 1000 sedan for many years with great success. I was able to pull up to 18"Hg of vacuum on light cruise, but my cam was close to stock GX specs.

How does the reputable performance company explain that the electronic dizzy will fix the vaccum advance problem?

I got a New GX dizzy to use on a A12a (USA model 1237cc) I have a pair of Dellortos, will the GX dizzy provide the vaccume I need.
dog

The vacuum is not provided by the dizzy ... it comes from the intake manifold.

The guy who tried to sell the electronic distributor is either misinformed or is trying to sell something. SUs have little to do with creating vacuum ... which is generated by the cylinders.

It is true that some electronic distributor systems can provide spark advance without using a vacuum signal. But you can just use the vacuum signal from your intake manifold to a regular or GX distributor ... and save $500 dollars.

Remember, Vacuum advance is NOT used when the throttle is floored, so it's not much use for drag racing. But if you spend time driving at part throttle it can make a big positive benefit with better fuel economy and better throttle response (power).

Even with SU's and an aggressive cam you can still use Vacuum advance at cruise throttle settings. I modified my webers to pick up a vacuum advance point and it runs fine on my street/club race sunny. I dont have any probs running it to 8000 either.

The ignition remains stable without scatter if you have a strong damper on the vacuum system to slow its action. This is done by making the vacuum supply line hole to the diaphraghm very small. I soldered the line up and used a 0.8 mm number drill in a dremel to open the line.

Couldn't you use a vacuum restrictor from a late model EFI car? Like the restrictors in the Power Valve system for the CA18?

They'd slip straight into the vacuum line for the distributor.

Yep you could if you can get them small enough???

That would be a hell of a lot easier, good thinking Datto dude

not trying to deviate from the topic too muc but its about vacuum, a meachanic family freind has told me justto be safe when building s/c a15 that if i run brake booster that it'd be a good idea to use an alternator with a vacumm pump in it, just to make sure theres always vacuum to work the brakes. where do u get these from?

Nissan ld28s have them, as do most deisels. Go nissan and keepit in the family. something else you could try is a vacum tank. These are very good things to have when using a mega cam with sh1tloads of overlap.(bugger all vacum)