Picking a good vacuum advance pod choice

[Douglas C.]

Posting here even though this is for my 1968 SS 350 Nova. Figuring a good number of experienced people here (Duke,etc?). My engine is fairly close to a L-46 if you added Edelbrock intake, Holley 650 DP, and headers.

Ok, just finished engine for the 68 Nova, 4 speed, 3:73 posi.

350 (30 over) 10.25:1, 462 heads, Comp Cam very close to a GM 350 350 HP cam, Edelbrock intake, Holley 650 DP, Headers.

Manifold vacuum at idle (vac. adv. plugged) reads about 13.5 " hg. So, the old rule 2" less would be a unit to reach full vacuum pull at about 11.5 " hg.

My all in mechanical advance is 36 degrees (around 2,800), so I need 16 degrees at full vacuum advance to reach 52 degrees at cruise.

I see a VC1765 / B20 vacuum advance pod with specs of 8 @ 11-13".

So, any thoughts on this choice, any other suggestions?

Thanks, Doug

[Duke W.]

Re: Picking a good vacuum advance pod choice

The VC-1765 or equivalent in another brand would be appropriate for full time vacuum advance, but the part in the box may be stamped either B20 or B26. They are both effectively the same.

Check it at the store with a Mity-Vac to be sure it's reasonably within spec.

Specifying idle vacuum without specifying the idle speed is MEANINGLESS!

Increasing idle speed increases vacuum and vice versa.

Duke

[Douglas C.]

Re: Picking a good vacuum advance pod choice

The VC-1765 or equivalent in another brand would be appropriate for full time vacuum advance, but the part in the box may be stamped either B20 or B26. They are both effectively the same.

Check it at the store with a Mity-Vac to be sure it's reasonably within spec.

Specifying idle vacuum without specifying the idle speed is MEANINGLESS!

Increasing idle speed increases vacuum and vice versa.

Duke

Ok, got your point, the 13.5 " hg was @ 850 rpm's. That is about where I will leave the idle speed once all is fine tuned.

My final target idle numbers will be around 12 initial + 16 vacuum (full manifold source) for 28 idle advance. Set distributor springs to be 24 @ around 2,800 rpm's, that + the initial of 12 will be my all in of 36 @ 2,800 mechanical. Then with 16 vacuum advance at cruising speed would be my total of 52.

If that sounds like a reasonable plan. And I do appreciate you opinion.

Tks. Doug

[Duke W.]

Re: Picking a good vacuum advance pod choice

Best total WOT advance is usually at the upper end of the 36-40 range. I usually set 'em up at 38-39, and the increased cruise advance from more initial timing will improve fuel economy.

Your spark advance map looks pretty good for you configuration, but you should try lighter springs for a more aggressive centrifugal to get it all in in the 2000-2500 range.

The basic rule of thumb is to go as aggressive as possible, but detonation can be a limiting factor.

Duke

[Douglas C.]

Re: Picking a good vacuum advance pod choice

Best total WOT advance is usually at the upper end of the 36-40 range. I usually set 'em up at 38-39, and the increased cruise advance from more initial timing will improve fuel economy.

Your spark advance map looks pretty good for you configuration, but you should try lighter springs for a more aggressive centrifugal to get it all in in the 2000-2500 range.

The basic rule of thumb is to go as aggressive as possible, but detonation can be a limiting factor.

Duke

Thanks a lot Duke. I'll try the quicker centrifugal all in and push the all in WOT a little more. Running with a Holley 650 DP and trying to maintain a smooth idle and clean running. So far I'm close with getting that on the money. Have the secondary's open just a bit (transfer slot) to be just about the same as the primary's. Have manual choke cable which works fine. Currently have a slight surge while cruising, not sure what vacuum advance is on the distributor now, may be advancing in total too far at cruise. That is why I want to get the right vacuum pod, set the timing right, then find tune the carb from that point. With 3:73's and stock height 14" wheels / tires the rpm's are always up in general.

Thanks again for your help & thoughts.

Doug

[Duke W.]

Re: Picking a good vacuum advance pod choice

Manifold vacuum at idle (vac. adv. plugged) reads about 13.5 " hg. So, the old rule 2" less would be a unit to reach full vacuum pull at about 11.5 " hg.

Manifold vacuum @ idle speed is properly measured with the engine in its normal operating condition. That means the VAC must be connected, and with the VAC connected the engine will pull more vacuum than disconnected/line plugged at the same idle speed.

The VAC is disconnected and the line plugged either to set initial advance or total WOT advance. Total idle and cruise advance are measured with the VAC connected.

Duke

[Douglas C.]

Re: Picking a good vacuum advance pod choice

Manifold vacuum @ idle speed is properly measured with the engine in its normal operating condition. That means the VAC must be connected, and with the VAC connected the engine will pull more vacuum than disconnected/line plugged at the same idle speed.

The VAC is disconnected and the line plugged either to set initial advance or total WOT advance. Total idle and cruise advance are measured with the VAC connected.

Duke

Right now the engine is on ported vacuum to I don't know what range VA pod, and runs quite well. I checked the vacuum with the ported hooked up for now and at 850 idle speed, hooked my vacuum gauge to the port on the carb base (manifold vacuum). 13.5 " @ 850 rpm's.

I understand your point. As I intend to run full manifold VA maybe I should plug the ported, hook VA pod to the carb base (manifold vacuum) and hook my vacuum gauge to the manifold fitting being used for the power brakes. Good point, I may have missed the obvious.

Thanks. Doug

[Douglas C.]

Re: Picking a good vacuum advance pod choice

Right now the engine is on ported vacuum to I don't know what range VA pod, and runs quite well. I checked the vacuum with the ported hooked up for now and at 850 idle speed, hooked my vacuum gauge to the port on the carb base (manifold vacuum). 13.5 " @ 850 rpm's.

I understand your point. As I intend to run full manifold VA maybe I should plug the ported, hook VA pod to the carb base (manifold vacuum) and hook my vacuum gauge to the manifold fitting being used for the power brakes. Good point, I may have missed the obvious.

Thanks. Doug

Checked again tonight. Full manifold vac connected to VA pod, vacuum slightly higher reading, 14.5 ", close to 15.0". I would think the B20 or B26 as mentioned before is still a fairly good choice.