Correct timing and which vacuum port

[texasmike]

I just finished reading a 24 page report by Lars Grimsrud, a GM engineer, titled 'Timing & Vacuum Advance 101'. It's mostly over my head and it left me with a few questions that some of you gear heads may be able to answer. My setup is mostly original with about 56k miles. The emission system has been removed except for the canister. It has a re-manufactured QJet from National and a new HEI distributor to replace the original ESC unit. Currently, the dist. gets vacuum from a port at the base of the carb and measures 0" at idle and around 10" at 3000 rpm. Manifold vacuum is 19.5" at idle. Timing is 4 btdc with vac line removed and plugged. The truck starts well and runs okay except for dieseling every now and then.

This GM engineer says that for max performance, distributors should ALWAYS use manifold vacuum. What do you guys think and would it be worth the time and effort to make the change? What should the timing be set to and what will happen when the vac line is connected with manifold vacuum? Thanks for your valuable time.

 

[AyWoSch Motors]

I just finished reading a 24 page report by Lars Grimsrud, a GM engineer, titled 'Timing & Vacuum Advance 101'. It's mostly over my head and it left me with a few questions that some of you gear heads may be able to answer. My setup is mostly original with about 56k miles. The emission system has been removed except for the canister. It has a re-manufactured QJet from National and a new HEI distributor to replace the original ESC unit. Currently, the dist. gets vacuum from a port at the base of the carb and measures 0" at idle and around 10" at 3000 rpm. Manifold vacuum is 19.5" at idle. Timing is 4 btdc with vac line removed and plugged. The truck starts well and runs okay except for dieseling every now and then.

This GM engineer says that for max performance, distributors should ALWAYS use manifold vacuum. What do you guys think and would it be worth the time and effort to make the change? What should the timing be set to and what will happen when the vac line is connected with manifold vacuum? Thanks for your valuable time.

Well, I not sure about manifold Vs. Carb. I've always used the port on the bottom of the carb. Should check out UncleTony'sGarage on YouTube. He did a video on that I believe. Wss going to watch it, but havent had time.

As far as advance, with the vacuum unplugged and capped, I'd say you should advance it to about 8-10 BTDC at a medium idle.
Should be about 8 at low idle, 10-12 at fast idle/low rev.
I run my 350 at between 10 and 12 BTDC at idle. Just about running out of timing mark on the timing chain cover. Seems to like it. Very snappy.

 

[Matt69olds]

Manifold vacuum. That will bump up the idle speed, allowing you to close the idle screw. It also bumps up ignition timing at idle, helping to clean up stinky exhaust. It also adds ignition timing when the engine is under a slight load, leaning on the throttle will reduce manifold vacuum, resulting in the timing backing off accordingly. That gives you the best balance of economy and power.

If you find the engine pings and rattles with the advance hooked up, don’t back off the ignition timing. Remove the vaccum advance canister and either replace it with on that takes more vacuum to start advancing, or braze the slot as outlined in the article. Always recheck the timing, the vacuum advance canister controls the starting point of the advance curve. Anytime that chances, the initial timing also changes.

Rule of thumb, give the engine what it wants, not what you think it needs!!!

 

[Shawn Watson]

I agree: 10* at idle with the advance line removed/plugged (initial timing.) Then manifold vacuum to the distributor. Hopefully the canister isn't one of the funky ones. If so, you may need an adjustable one that provides an additional 12* and have it adjusted to pull max advance about an inch below your current manifold vacuum.

Think about this, pretty much all of the EFI stuff is idling in the mid 20s.


Shawn

 

[texasmike]

Is the vacuum advance canister that black ball thing on the drivers side firewall? Thanks everyone.

 

[Snoots]

NO!
That's the vacuum reservoir for your HVAC system. The vacuum advance cannister is located at the outer edge of the distributor base.

 

[85K304SPD]

That is the vacuum accumulator. The vacuum advance canister is the cone shaped thing on the bottom of the distributor.

 

[texasmike]

Is that the small galvanized looking part that the vac line attaches to? I was expecting something larger.

 

[Snoots]

You got it!

 

[texasmike]

I feel really dumb but I guess we are never too old to learn.

 

[85K304SPD]

So, hook the vacuum line to the port that has vacuum at idle. The one that increases the idle speed when you plug it in, whatever carburetor you have. That is what is factory on all the stock GM carbureted motors that I have had.

 

[Bextreme04]

The problem here is that you will have two completely different methods to end up in generally the same area. You need to do what is required for the distributor you have, or get a new distributor that is adjustable. For maximum power on a SBC, you will need to have 34-36 degrees total timing at WOT once it is all-in. This means you need to rev the motor until timing stops increasing, with the vacuum line disconnected from the distributor and plugged at the carb(usually 3k RPM or so) and then set the distributor to 34-36 degrees. Let it go back to idle and see what the initial timing is now at. It should be at around 12 degrees if you have a distributor set up for manifold vacuum. If it is more than that, you likely have a distributor that is set up for ported vacuum and it is expecting the vacuum canister to be putting timing in at WOT instead of at idle and low throttle conditions.

Basically you need to find out how much mechanical and vacuum advance the distributor is set up for, then connect to the proper vacuum connection on the carb to support having 34-36 degrees of advance at WOT and 12+ degrees of advance at idle.

The easiest way to do this is to:
1. Check idle timing with vacuum advance canister disconnected. Write down the number as "Initial".
2. Get an adjustable timing light and see how much timing comes in at RPM when you rev it with the vacuum advance canister disconnected. It should be in the range of 10-22 degrees. Write this number down as "Mech Advance"
3. Then go back to idle and connect the vacuum advance can to manifold vacuum. Write down the timing value as "Total idle".
4. Now subtract "Initial" from "Total Idle" to get "Vac Advance"

You can get adjustable vacuum advance canisters and also kits to tune the mechanical advance total amount and also how quickly it comes in, but most people don't have the patience or interest in fine tuning their timing curves. Your best bet is to get an adjustable vacuum canister and a mechanical advance weight set that gives you around 22 degrees of mechanical advance. Set your initial timing for 12 degrees BTDC and then connect to manifold vacuum. This will let you use the vacuum canister to tune your lean cruise AFR and timing to get great mileage while avoiding knock. Basically you can lean out the cruise mixture using the APT screw by driving it and turning the screw in until you get a surging of the engine at cruise, then screwing it back out until it smooths back up. You make small adjustments when stopped and then go for a drive, repeat until you get it dialed in. Then you adjust the vacuum canister advance to give you more timing until you start getting a ping at cruise, then back it off a few degrees.

That cruise adjustment will also be there at idle and will have an effect on your idle speed, so you will probably have to readjust your idle mixture screws and idle speed screw also.

 

[Trucksareforwork]

What motor do you have? If you have a 305 then 12 BTDC is likely way too much initial timing. You are likely to get pinging. Stock is more like 4 degrees initial I think.

The answer to manifold vs ported vacuum is only a debate. Big cams idle better on manifold vacuum because it adds a lot of timing at idle. Stock motors tended to be run more on ported vacuum because lower timing at idle helped emissions by running hotter. Some earlier emissions systems actually had a vacuum switch that switched to manifold vacuum (and more timing) in the event of engine getting too hot.

If you have a 305 I would try something like 4-5 btdc initial with ported / timed vacuum and realize that without the esc you will probably still get some pinging as you crack the throttle under load.

Now, a 350 you can advance a lot (like the 12btdc mentioned) before you ping much. Totally different motor in some respects.

 

[texasmike]

The problem here is that you will have two completely different methods to end up in generally the same area. You need to do what is required for the distributor you have, or get a new distributor that is adjustable. For maximum power on a SBC, you will need to have 34-36 degrees total timing at WOT once it is all-in. This means you need to rev the motor until timing stops increasing, with the vacuum line disconnected from the distributor and plugged at the carb(usually 3k RPM or so) and then set the distributor to 34-36 degrees. Let it go back to idle and see what the initial timing is now at. It should be at around 12 degrees if you have a distributor set up for manifold vacuum. If it is more than that, you likely have a distributor that is set up for ported vacuum and it is expecting the vacuum canister to be putting timing in at WOT instead of at idle and low throttle conditions.

Basically you need to find out how much mechanical and vacuum advance the distributor is set up for, then connect to the proper vacuum connection on the carb to support having 34-36 degrees of advance at WOT and 12+ degrees of advance at idle.

The easiest way to do this is to:
1. Check idle timing with vacuum advance canister disconnected. Write down the number as "Initial".
2. Get an adjustable timing light and see how much timing comes in at RPM when you rev it with the vacuum advance canister disconnected. It should be in the range of 10-22 degrees. Write this number down as "Mech Advance"
3. Then go back to idle and connect the vacuum advance can to manifold vacuum. Write down the timing value as "Total idle".
4. Now subtract "Initial" from "Total Idle" to get "Vac Advance"

You can get adjustable vacuum advance canisters and also kits to tune the mechanical advance total amount and also how quickly it comes in, but most people don't have the patience or interest in fine tuning their timing curves. Your best bet is to get an adjustable vacuum canister and a mechanical advance weight set that gives you around 22 degrees of mechanical advance. Set your initial timing for 12 degrees BTDC and then connect to manifold vacuum. This will let you use the vacuum canister to tune your lean cruise AFR and timing to get great mileage while avoiding knock. Basically you can lean out the cruise mixture using the APT screw by driving it and turning the screw in until you get a surging of the engine at cruise, then screwing it back out until it smooths back up. You make small adjustments when stopped and then go for a drive, repeat until you get it dialed in. Then you adjust the vacuum canister advance to give you more timing until you start getting a ping at cruise, then back it off a few degrees.

That cruise adjustment will also be there at idle and will have an effect on your idle speed, so you will probably have to readjust your idle mixture screws and idle speed screw also.

Wow, this is great information and easy to understand. Thanks for taking the time to write it all down. I have switched over to manifold vac (20") and set the advance at ~11 degrees with dist line plugged. When I re-connected the line, I had to bring the idle screw down to ~730 rpm and can no longer see the timing mark as I only have an old Sears light. The distributor is a new HEI unit by Cardone. Everything appears to be alright at start, idle and revving it. I haven't gone for a test drive, yet. It's a stock 305 with 700R4 transmission and about 57.7k miles. In town mileage is about 10.5 mpg on no ethanol regular gas.

 

[Bextreme04]

Wow, this is great information and easy to understand. Thanks for taking the time to write it all down. I have switched over to manifold vac (20") and set the advance at ~11 degrees with dist line plugged. When I re-connected the line, I had to bring the idle screw down to ~730 rpm and can no longer see the timing mark as I only have an old Sears light. The distributor is a new HEI unit by Cardone. Everything appears to be alright at start, idle and revving it. I haven't gone for a test drive, yet. It's a stock 305 with 700R4 transmission and about 57.7k miles. In town mileage is about 10.5 mpg on no ethanol regular gas.

Since its a 305, you should have ESC and some other stuff on the engine. Removing all of the emissions stuff could cause other running issues on a 305, they can be touchy. If you don't have ESC anymore, you could find that you are having major issues with detonation under load. The only way I know to fix this is to pull initial timing until it stops detonating. The problem is that it will make it run like even more of a dog than a 305 normally does.