1969 L36 auto transmission distributor vacuum can choice ? Help Duke !

[Mark M.]

Duke, I can use help with a 69 L36 auto trans ,3:08 rear end, points distributor with all original parts, no ac, all original drive train car. Good compression. I replaced the original 360 leaking can with another original 360 working can. New points, shimmed the drive gear tighter, The car feels pretty strong, but out of the hole I suspect it should pull better. What would your suggestion be for the vacuum can choice with ported vacuum and also if I change it to manifold vacuum? I have many original and replacement (B-_) cans, NOS and used to help choice from. I wonder if the diaphragm on aging cans get stiffer and perform differently through time !

[Duke W.]

As you probably know the spark advance map for your emission controlled engine was set up to meet emission standards, not optimized for performance and fuel economy. The OE 360 12 VAC starts at 6" and is 12 (crank degrees) @ 12" max. VACs can fail because the diaphragm ruptures and leaks, or they can become "sticky" and not respond properly to vacuum signals.

As long as your car doesn't require field emission testing I recommend converting to full time vacuum advance. This can be done with a 1/8" plastic tee and a foot or two of 1/8" vacuum tubing by "teeing" into the choke vacuum break. Remove the original hose and use the new hose and tee to make a new harness that connects the manifold vacuum source to both the choke vacuum break and VAC. You'll also need a 1/8" vacuum cap to cover the original ported vacuum source to the VAC.

Assuming an OE cam you should be able to get it to idle at 600 in DRIVE to go through the idle speed mixture adjusting procedure. Manuals are done neutral and automatics must be done in DRIVE.

The '69 L-36 OE max centrifugal is short, offering only 26 degrees. The recommended initial timing is only four, so this yields only 30 max at WOT, which is well short of the optimum 36-40 range.

So start by picking a good OE VAC. Test it with a vacuum pump to ensure than it pulls and retracts smoothly with vacuum changes. To swap it out pump down the currently installed part, which will allow access to the two screws. With the two screws removed carefully wiggle the pin out of the breaker plate. Then pump down the replacement and install.

I recommend you set initial at 10. The OE centrifugal starts at 900, but verify because it may no longer be OE. Set initial in neutral below the centrifugal start point. Then connect the VAC and set idle at about 800.

To do the idle speed adjustment, which MUST be done in DRIVE, double chock at least one wheel and set the parking brake as tight as possible. Since the disk brake system parking brake is not known for being very robust, I recommend you have a helper keep a foot on the brake pedal in addition to double chocking at least one wheel.


With the engine at normal operating temperature set the idle mixture screws at the recommended initial setting in the service manual. Start the engine place in Drive and set idle speed at 600. Verify that manifold vacuum is at least more than what it takes to pump down the VAC all the way, and check timing. It should be about 22 deg. (10 initial and 12 from the VAC). Adjust idle mixture screws equally to get maximum vacuum/idle speed, then reset idle speed back to 600 and go through the procedure again until there is no increase in idle speed.

Take if for a test drive and evaluate for drivability and detonation. Once you have it dialed in reset the fast idle per the service manual because changing from ported to full time vacuum advance will raise the fast idle speed several hundred revs.

In my experience early closed chamber head big blocks don't like as much low speed/load spark advance as small block of similar tune (like valve overlap). I recently worked on a '66 L-36 (same 360 12 VAC, 30@ 5000 centrifugal). We swapped in lighter springs to bring it all in at 3500 but could only run 4 initial because any more caused low speed/load "trailer hitching". So we swapped in a 32 degree cam assembly (standard on the '67 L-36) with the same initial, which yielded 36 total WOT advance with excellent throttle response, low speed/load driveability and no detonation on California 91 PON "premium".

So depending on your maximum available octane and compression ratio you may have to do a little experimenting to find an optimum setup.

BTW if your car has C-60 the idle speed/mixture adjustment procedure should be done with the compressor engaged.

The VAC has no real effect on initial acceleration from a dead stop because when you open the throttle, even part way, vacuum goes to near zero, so even with full time vacuum advance it should fully retard from idle in a faction of a second. The more initial + centrifugal advance you have from off idle to higher revs the better the engine should pull, but the downside is that the combination of initial plus centrifugal plus vacuum advance may cause drivability issues at low speed/light load.

The optimum spark advance map for L-36/68 would be a properly functioning OE 360 12 VAC connected to a full time vacuum source, A 32 degree cam assembly (maybe even filing it out to give 34 degrees), 6-10 initial and light springs to bring centrifugal in as quickly as possible, the limiting factor being detonation or low speed/load driveability issues.

Duke

[Mark M.]

Thanks, Now I have info needed, I'll try it with full manifold vacuum and dial it in as suggested. I'll be getting to the 69 soon and report results when finished. The goal is to wake it up off the line, although when it gets going it keeps pulling hard with the 3:08. Cheers

[Mark E.]

The recipe for best performance "out of the hole" (from idle to WOT) is maximum static timing followed by immediate, smooth ignition advance when throttle is applied. This does not happen using manifold vacuum for VAC.

Using manifold vacuum: VAC contributes 12 degrees or more at idle, so the engine can only handle limited static timing without bucking- generally less than 10 degrees. When throttle is applied, VAC goes to zero which RETARDS timing to this limited static setting just when the engine wants more. Result: sluggishness if not stumbling.

Using ported vacuum: VAC contributes 0 advance at idle so more aggressive static timing is possible, generally 12-16 degrees depending on the engine. When throttle is applied, VAC remains zero while centrifugal advances smoothly from a more advanced starting point. Result: improved engine responsiveness from idle.

This is why cars set up for drag racing don't use VAC. For a street car, connecting VAC to ported vacuum provides responsiveness and good economy while cruising.

Be careful setting static + centrifugal more than 36-38 degrees. For street, 32-36 is safer and generally adequate. After static and centrifugal are set, use a variable VAC to dial-in what works best for your engine- usually 50-52 total degrees at cruise. Then if you want an original look, select a VAC with specs that match that setting.

[Duke W.]

Ported vacuum advance is an emission control tactic.

The most efficient idle state to minimize fuel consumption and EGT requires low twenties to low thirties total idle spark advance for typical wedge chamber automotive engines, and this is provided by the sum of initial plus full vacuum advance, and some engines might add a little centrifugal like the 365 and 375 HP 327s because centrifugal starts at 700 but they need to idle at about 900 for reasonable idle quality.

With very few exceptions, pre-emission controlled engines have full time vacuum advance, but it was found early on that EGT was too low for injected air to oxidize enough CO and HC to meet tailpipe standards. By eliminating vacuum advance at idle, EGT was raised considerably, enough to enable sufficient oxidation reactions to meet tailpipe standards. Of course this also increased fuel consumption in urban driving due to the less efficient idle state. Emission controlled engines continue to use ported vacuum advance, even if they don't have air injection, to maintain the catalyst bed hot enough to maximize oxidation and reduction reactions.

With full time vacuum advance, exhaust manifolds typically show no more than 500F temperature measured with an IR gun, but will be more like 900 with ported vacuum advance.

Valve timing has a lot to do with idle and part throttle spark advance requirements. High overlap cams create a lot of exhaust gas dilution at idle and part load, which slows combustion propagation, so the fire needs to be lit earlier than low overlap cam engines that don't create as much exhaust gas dilution at idle and part load.

The instant the throttle is opened from a dead stop, even just a little throttle, manifold vacuum drops in a faction of a second, which causes the vacuum advance to back off completely or at least mostly, so the engine is left with initial plus whatever centrifugal is provided, and it's best for this to be right at the detonation limit, but since we can't program vintage engines for every speed and load condition much less transient states, we have to experiment with what combination of initial and centrifugal curve offers the best detonation free results and no driveability issues.

It's an interesting exercise to rig up a vacuum gage in the cockpit that you can view while driving. You'll find that it's constantly on the move and you will see that even a light throttle application from a dead stop causes vacuum to briefly go to near zero for a faction of a second before revs increase and vacuum increases at a constant throttle setting.

In my experience, early closed chamber big blocks don't like as much low speed/load spark advance than small blocks, and this is probably the reason why they have up to 32 degrees total centrifugal with 12 degree VACs and recommended initial timing of usually less than ten degrees, and this is likely because of the dual quench zones and more optimally placed spark plug than single wedge chambers, which increase the rate of combustion propagation compared to a small block.

VACs should be selected using the Two-Inch Rule to ensure that it is pulled to the limit under idle conditions. Optimum WOT advance, the sum of initial and full centrifugal, is 36-40 degree for both big and small blocks according to the Chevrolet Power Manuals from the seventies, and it should be brought in as quickly as possible with detonation being the limiting factor.

If a detonation free centrifugal advance yields low/speed load driveability issues then centrifugal can be increased while reducing initial or max vacuum advance can be limited.

My tuning semlnar discusses this in more detail along with recommended starting points for optimizing the spark advance maps for the vast majority of OE engine configurations. You should study it if you want a greater understanding of the physics going on inside the combustion chamber of a spark ignition engine. It's included in the "restoration documents" thread near the end of the sticky section.

Duke