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Dual Can or Single Can Distributor???javascript:emoticon(':!:')

I can't get my head round the use of a dual can distributor to retard the ignition!!

If I use a Dual Can I would expect the following results:

Static Setting 10 deg BTDC
Mech advance say 14 deg = 24 deg at 3000 rpm approx
Ported Vacuum advance say 10 deg = total advance with no boost of 34 deg - sounds good
Now when under boost at 3000 rpm the dizzy would push back the vacuum advance by 10 deg plus another 10 deg (retard part) because the can will allow this. This would give advance under boost of appx 14 deg.
I know you can limit the retard by modifying the insides….

But why not just use a single can? As follows:
Static Setting 10 deg BTDC
Mech advance say 14 deg = 24 deg at 3000 rpm approx
Vacuum advance say 10 deg = total advance with no boost of 34 deg - sounds good
Now when under boost at 3000 rpm the dizzy would push back the vacuum advance by the 10 deg . This would give advance under boost of appx 24 deg. Ideal!!!!

Do have this round my neck??? Where am I going wrong??
Just about to order a new single vac + mech advance dizzy with electronic ignition. Currently running a 009 with max advance of 24 deg and 8 deg static – works ok.

Cheers,
Perry

i asume u want more than 24deg total. maybe 30-34 depending on your setup.
then add 10deg more att full vacum.so att full throttle u have 30-34 deg and at boost 20-24deg. u can easily retard 10 deg . but depends on boost level, compression, fuel etc.
i have setup one engine with the dual and limited the travel on the retard from 12 to 6 deg. on boost.

Has anybody any experience using a single can vacuum & mech advance distributor?

Don't know about the single can. I use the dual can but only one connection- the rearmost one.
By the way, with a vac can, the can ONLY advances when the engine is making vac. -never at full throttle.

The beauty of the dual can is what it can do for you at small throttle openings when not in boost. Here is a more likely scenario.
Initial timing: 15 (idle)
Mechanical advance, WOT, no boost yet: 29 total (2500 rpm)
Mechanical advance, vaccuum advance, light throttle, no boost: 39 total (max economy, improves torque and drivibility also)
Mechanical advance, WOT, boost retard in play: 22 to 25 (5000 rpm, boost retard varies with how much boost your wastegate is set for)
This is a good start for a light car on 93 octane. A bus or heavy car or 91 octane may require you to back off on the initial timing 3 or 4 degrees to scale everything back.
And for those that might scoff at that 39 total, some new cars run as much as 52 total under certain conditions. A cylinder that is only 20% filled has a very high resistance to ping, and that much advance helps build cylinder preasure which makes torque.

Buggsy wrote:And for those that might scoff at that 39 total, some new cars run as much as 52 total under certain conditions. A cylinder that is only 20% filled has a very high resistance to ping, and that much advance helps build cylinder preasure which makes torque.

No flame here, I run 40* at 16.5:1 AFR during cruise with my turbo 1915.

Buggsy wrote:The beauty of the dual can is what it can do for you at small throttle openings when not in boost. Here is a more likely scenario.
Initial timing: 15 (idle)
Mechanical advance, WOT, no boost yet: 29 total (2500 rpm)
Mechanical advance, vaccuum advance, light throttle, no boost: 39 total (max economy, improves torque and drivibility also)
Mechanical advance, WOT, boost retard in play: 22 to 25 (5000 rpm, boost retard varies with how much boost your wastegate is set for)
This is a good start for a light car on 93 octane. A bus or heavy car or 91 octane may require you to back off on the initial timing 3 or 4 degrees to scale everything back.
And for those that might scoff at that 39 total, some new cars run as much as 52 total under certain conditions. A cylinder that is only 20% filled has a very high resistance to ping, and that much advance helps build cylinder preasure which makes torque.

Would this not be the same for a single can (Vac & Mech)??

You will have to pressure test your can to see if it goes back with pressure- only use a regulator and hook it up then turn in the regulator screw and watch to see if it goes back.

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Buggsy wrote:
And for those that might scoff at that 39 total, some new cars run as much as 52 total under certain conditions. A cylinder that is only 20% filled has a very high resistance to ping, and that much advance helps build cylinder preasure which makes torque.



No flame here, I run 40* at 16.5:1 AFR during cruise with my turbo 1915.

AMEN

this makes all the difference in the world for a well behaved, efficient running and responsive machine.

slimesub5 wrote: Would this not be the same for a single can (Vac & Mech)??

?????

What does the second port on the dual can do?

With the dual canister, the boost moves the diaghram in a negative direction, retarding (subtracting) timing from the collective centrifigal and initial advance. Helps reduce detonation while boosted but maintains a good advance curve for normal driving.
It is very important to understand that the single vaccuum linefrom the outer nipple goes to ported vaccuum, not manifold vaccuum. The inside nipple on the canister is left open to atmosphere. Ported vaccuum is a small port that is uncovered by the throttle plate edge at about 5% to 10% throttle opening. At idle, the port should be just above the throttle plate, hence no vaccuum at idle.

What would be any different between a single can??

Where does the second port (closest to the body) that is left open to atmosphere usually go on a stock setup?

The way I see it, if you use ported vacuum on the front connection on a DVDA distributer and leave the rear open, when under boost you will loose the vac advance part & get more retard because it is possible for the distributor plate to carry on moving past it's static setting at idle. This will give you, under boost, retard of the total vacuum advance + the retard function of the dizzy.
If you use a SVDA dizzy you will just get, under boost, the vac advance amount retarded back to what what ever the mech advance is at. (you have no vacuum)
I have put a graph together of my expected results in xls, not sure how to upload....
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Here's the graph!!

I'll try one more time. The diaphram is at rest, centered, no advance or retard. Apply ported vaccuum to the outside nipple, and you will get vaccuum advance, as the diaphram moves away from the distributor. Apply pressure (boost) to the outside nipple and you get retard as the diaphram moves toward the distributor. The inside nipple is left open to atmosphere so it doesn't "air lock" the movement of the diphram either direction.
By the way, this set up is for blow through carbs and throttle bodies. It can be done, I think, with draw through also, but some one way check valves have to be used, and it gets pretty complicated.
By the way, at web site Old Volks Home, you can look up what your Bosch distributor has for mechanical and vaccuum advance. http://www.oldvolkshome.com/ovhignbo.htm