Tapping single vac advance distributor for boost retard?

[david58]

I see that it is possible to tap the stock fuel pump to make it boost sensitive, just wondering if it is possible to do the same on a single vac advance distributor? I'm trying to do as close to a 100% stock turbo setup and I'm having trouble finding a dual vac distributor. So far I've got the boost regulatoed stock pump, a VW branded turbo off a Passat and going to blow through the stock carb. Basically the only non-factory parts will be the exhaust, various oil lines, an MSD 6AL box and possibly 1.4 ratio rockers but I might just adapt some 1.25 TIV's.

If you took the single vacuum advance canister and flipped it around it would retard instead of advance the dist plate. You would just have to modify the arm so it would fit the plate.

[my_medusa]

so I hadn't so much time but better late than never
here a picture and infos about my pressur-tin setup.

at first, this is a 6volt dizzy for my 36hp supercharged engine.
tin is from a saab 9000 turbo.
the lever of the tin is same length like the stock one from my oval engine but it need some modifications to fit well.
in stock the tin has a good early (feels like the same like the stock vw one) and late adjusting to 0,6 bar ( about 8-9 psi)
in my case I needed a adjusting to 1 bar (14-15 psi) so I modify the end spacer from the tin.
in my case I also had to modify the mechanical adjusting but thats another story and easy to doo.

the red line show the spacers vor late and early and the green lines are the modifications...so its not a hard work.



I think this setup works even better on turbos because the turbo starts at its point and to reach that point its better to have a early adjusting + the exact late adjusting with boost.

btw: my 1200 36HP engine has power like a 1600 with webers on it(stock clutch and pressure plate can't hold the power).
i will dyno it next week

[david58]

If you took the single vacuum advance canister and flipped it around it would retard instead of advance the dist plate. You would just have to modify the arm so it would fit the plate.

When I posted this I didn't explain it too well. I don't have a vacuum dizzy handy. If you took the vacuum advance out and flipped it 180 around is there room to mount the lever so it pulls the opposite way? Or can the distributor plate be installed where the points are on the opposite side?

[turkish]

So I just got my dual vac in the mail yesterday, and now I'm confused. It is my understanding that hooking boost pressure up to the advance side of the can will produce retard under boost but it doesn't move if I try to blow air into it. Both sides work properly and hold vacuum but neither will move when positive pressure is applied. Do I need to modify the distro or do I have to put more pressure to it to see movement, I was just blowing into the can myself but I could hook it up to a regulated air compressor.

[mike thompson]

ok, it up to a regulator,start at zero out of the reg. and go up- you can watch what happens.
By the way, while it is nice to be able to run more advance off boost, it isn't a life or death thing. Even if you just run the vac advance + your normal 24 degrees, it is still better than no vac advance. Since most of the time you will have some level of vac giving more advance than without it.
Saab unit looks interesting.

[turkish]

So to make sure I am understanding this, the dizzy is set to 24deg total mechanical advance and then the vac advance brings it to ~32 when not under boost?

[Piledriver]

So to make sure I am understanding this, the dizzy is set to 24deg total mechanical advance and then the vac advance brings it to ~32 when not under boost?

No, the timing is set per usual, you have vacuum advance that works normally, for good drivability/economy.

The dual vac cans simply have the ability to retard using the same vac advance hose, assuming blow through, the "vac" hose will blow, moving the diaphram the other way from its rest position.

The retard line is left vented to air for this app.

[mike thompson]

You set it at 24 and the vac pulls it way ahead during high vacuum modes- (past 32- to whatever the vac can has built in) and wnen vac diminishes (as when you give it some throttle), the timing goes back to 24-( the mechanically set portion of the dist).

[Piledriver]

You set it at 24 and the vac pulls it way ahead during high vacuum modes- (past 32- to whatever the vac can has built in) and wnen vac diminishes (as when you give it some throttle), the timing goes back to 24-( the mechanically set portion of the dist).

I thought we were discussing dual vac cans, not single vac cans.

Dual vac cans can advance OR retard from the static or mechanical advance, depending on if they see vac or boost.

Single vac cans can ONLY advance. The SAAB unit pictured above LOOKS like a dual, perhaps the "retard" side is vented to atmosphere?

Why anyone would CHOOSE to run a single vac can and 24 degrees total mech. advance baffles me...will crank your head temps, kill your spool, and kill your mileage and drivability, so I'm fuzzy why anyone would want to run that on the street, if ever.

[knobstar]

Single vac cans can ONLY advance. The SAAB unit pictured above LOOKS like a dual, perhaps the "retard" side is vented to atmosphere?

The saab unit is single, quite obviously, and has a diaphragm which allows it to advance and retard the ignition using just one pick up from the inlet. It seems the best solution to me and they are fairy commonplace. I think it's the old style 900 (in the UK) from which you'd want one of these distributors

Matt

[Piledriver]

Single vac cans can ONLY advance. The SAAB unit pictured above LOOKS like a dual, perhaps the "retard" side is vented to atmosphere?

The saab unit is single, quite obviously, and has a diaphragm which allows it to advance and retard the ignition using just one pick up from the inlet. It seems the best solution to me and they are fairy commonplace. I think it's the old style 900 (in the UK) from which you'd want one of these distributors

Matt

Absolutely, I was not arguing it was a dual per se, but that physically it is made exactly as a dual, looks wise and construction wise... The only thing missing is the "retard" fitting, I will venture a guess the retard side is vented to atmosphere on this unit, as has been recommended practice for use of the "usual" dual advance cans.

The normal "single" advance units look quite different (esp. much thinner) and can physically not retard under boost.