Manifold vs Carb dis port vacuum

[Mr Comet]

Here's my ignition setup

Pertronix flame thrower coil
GM module
DS II dist.

At 850 idle carb dis 0" and manifold is 15-17 inches vacuum.

I do get vacuum on carb dis as I increase RPMs

1500 rpm yields 10"
2000 rpm yields 16-17 inches

idle timing no vacuum hookup is roughly 14 btdc.

I have been running the carb dis for vacuum but have been experiencing rough exhilaration from time to time. Would it be better to use vacuum?

Thanks Denny

 

[CZLN6]

Howdy Denny:

Best way to find out is to try it out. several factors enter in to which is best, and every case is different. With manifold vacuum you may have to reduce initial timing setting to 10 degrees or so, again, depending on several factors.

Adios, David

 

[Mr Comet]

What are those factors?

 

[JackFish]

The DS2 expects to see ported vacuum.
No vacuum at idle.

 

[Mr Comet]

Thanks Jack do my figures sound good?

 

[CZLN6]

Howdy Back:

Other factors- What carb, cam, compression, elevation, Transmission, Rear gearing, and others. In stock form, a DS II used a ported vacuum source, which will give the engine no vacuum advance at idle speeds so you can set the initial advance somewhere between 10 and 16 degrees of advance, depending. in an engine with a longer duration cam with loss of low speed power, some gain can be had by switching to manifold vacuum and decreasing the initial advance setting. Increaseing the engines CR can also help.

Again, It's easy to switch vacuum sources and experiment to determine which works best for your engine. The rest is just guessing.

Adios, David

 

[Mr Comet]

David

I have a 74 Carter RBS carb
compression is approx. 9 to 1
Tranny T5 with 3.35 1st gear .68 5th gear
I am not sure of exact elevation other than to say I don't live in a high mountain thin air area of the country
rear gear is 3.2 to 1

Denny

 

[wsa111]

First you have to get the correct mechanical advance with the correct total necessary & as much initial advance as possible.
Then you can concentrate on the amount of vacuum advance necessary. As David said with some engines with a lot of camshaft it may of benefit to use vacuum advance using manifold vacuum. Do not use the vacuum advance as a Band-Aid for a incorrect mechanical advance.
Always get an adjustable vacuum chamber to fine tune the vacuum settings. There are many vacuum chambers available with different totals of advance.

 

[Mr Comet]

Oops I forgot to post I have a 264/264/110 clay smith cam which already has a 4 degree btdc setting via the timing set I used. So how do I get the correct mechanical advance?

 

[CZLN6]

Howdy Denny:

Aah, that explains the vacuum readings you posted. With that cam you will have lost idle vacuum as well as low end power. See Bill (AKA wsa111) for a distributor recurve to match the cam. He will also want to know what your engines compression ratio is. Once the mechanical advance is optimized that is the time to begin experimenting with vacuum advance.

Good luck and keep us posted on your progress.

Adios, David

 

[First Fox]

Hey Denny,

The factory timing settings were for a stock engine and once it is modified, those numbers will likely be quite different. You may need to run much more initial timing to get her to idle nicely and that is what you want it to do is to idle well and not to stick to some arbitrary number on a sticker. The absolute best way to do this is with a vaccum gauge. Here is the procedure: Install a vaccum gauge on the manifold, start her up and let her get up to operating temp. DISCONNECT the vaccum advance entirely and advance the timing at the distributor until the vacuum gauge is at its highest reading. Check the ignition timing at that point with a timing light and write it down. It may be nearly 20 degrees advanced or more and that is fine. If you drive it that way it will be super responsive and make good power and will likely feel like a different car but it will have too much TOTAL timing at that point and will need a recurve so don't drive it far or hard. Remove the distributor and make arrangements to send it out for a recurve and let them know you need 20 degrees of initial timing or whatever your number was that gave you your highest vaccum. They will then set the total timing and curve for you so it is safe and sound under load and at speed and ship it back to you.

THEN you can install it in the car and experiment with the vaccum advance. Manifold vs ported vaccum is like religion with people and should NEVER be discussed because fists will fly, but it boils down to whatever works for an individual application and has to be experimented with but again AFTER the recurve process. You can't put the cart before the horse here.

Look me up if you need help man, I am not far.

 

[wsa111]

Hey Denny,

The factory timing settings were for a stock engine and once it is modified, those numbers will likely be quite different. You may need to run much more initial timing to get her to idle nicely and that is what you want it to do is to idle well and not to stick to some arbitrary number on a sticker. The absolute best way to do this is with a vaccum gauge. Here is the procedure: Install a vaccum gauge on the manifold, start her up and let her get up to operating temp. DISCONNECT the vaccum advance entirely and advance the timing at the distributor until the vacuum gauge is at its highest reading. Check the ignition timing at that point with a timing light and write it down. It may be nearly 20 degrees advanced or more and that is fine. If you drive it that way it will be super responsive and make good power and will likely feel like a different car but it will have too much TOTAL timing at that point and will need a recurve so don't drive it far or hard. Remove the distributor and make arrangements to send it out for a recurve and let them know you need 20 degrees of initial timing or whatever your number was that gave you your highest vaccum. They will then set the total timing and curve for you so it is safe and sound under load and at speed and ship it back to you.

THEN you can install it in the car and experiment with the vaccum advance. Manifold vs ported vaccum is like religion with people and should NEVER be discussed because fists will fly, but it boils down to whatever works for an individual application and has to be experimented with but again AFTER the recurve process. You can't put the cart before the horse here.

Look me up if you need help man, I am not far.

Just remember that initial is in a non load situation. Be cautious under a full throttle condition. The amount of centrifugal advance may have to be very slow at lower engine speeds & progress from there.
Static compression & cranking compression #s will have to be factored in.

 

[First Fox]

Soooooo? When will you have your paws on that fresh new distributor? 8)

 

[Mr Comet]

That is a good question? I am at the mercy of the vendor and since I want hi quality work I am not going to bug him. I could call you when I get it. If you could PM me your number. Drove the car the other day and it seemed to be even more jerky then before. Let's hope the marriage to the new dizzy helps.