Vacuum advance

[Gary Schisler]

According to my 66 Chevy service manual, may vacuum advance should be 16 degrees at 8" 0f Hg. Centrifugal advance should be 15 degrees at 1500rpm and 30 degrees at 5100 rpm. Does anyone know if GM still services this vacuum advance cannister. If not, are there any companies out there who make an acceptable, quality unit, disregarding originality of appearance. I am looking here for opinions, good and bad, on replacement units.

In all of the various discussions on overheating and our beloved Corvette radiators, a lot of discussions include checking timing. Obviously, vacuum and centrifugal advance play a critical part in this. Are vacuum and centrifugal timing cumulative? Is there a good way to check this advance with the older style timing light, such as I have, that doesn't have the "dial a timing" feature of the new style light? I have used the "suck as hard as you can" (hold the jokes please) method on the vacuum hose and gotten good movement of the mechanism, but obviously this method gives you an idea that at least something is working but not how well. You can see where I am going with this. In my previous post, I had stated I was going to buy a Harrison repro radiator. Before I do, I thought I would do a quick check of the centrifugal and advance timing, just to be sure. Any input here is appreciated.

[Jack H.]

Re: Vacuum advance

Service of vac advance unit is by replacement. Service replacements are made offshore these days and, while perfectly functional, they MAY exact an originality deduction because markings are different.

Originals had the last three digits of PN embossed on top surface of distributor mounting bracket AND a two digit code indicating the mechanism's max advance in degrees. Some parts houses (Dr. Rebuild, as I remember) have limited supply of originals and they turn up at older auto parts stores, AC-United Delco dealers, and swap meets....

[Jack H.]

Re: Vacuum advance

Service of vac advance unit is by replacement. Service replacements are made offshore these days and, while perfectly functional, they MAY exact an originality deduction because markings are different.

Originals had the last three digits of PN embossed on top surface of distributor mounting bracket AND a two digit code indicating the mechanism's max advance in degrees. Some parts houses (Dr. Rebuild, as I remember) have limited supply of originals and they turn up at older auto parts stores, AC-United Delco dealers, and swap meets....

[Jack H.]

Re: Vacuum advance

Yes, the two advance methods (centrifical and vac) are cumulative. That's why tune up has you disconnect vac line from distributor to set timing at idle. This defines your 'static' timing. When engine revs and both centrifical and vac advance kick in, you have 'dynamic' advance.

About all you can tell with a conventional timing light is whether or not you're 'in the ballpark' for the overall system. You can't pinpoint very well the intermediate portion of the advance curve and with some setups you can go 'off scale' w.r.t. the engine's calibration index next to the balancer.

BUT, you CAN pull the distributor and walk it down to a shop equipped with a distributor tester (most of the local AC/United Delco shops have one). They can control RPM, vac and run the distributor through its paces for you checking for things you can't really see like wobble, Etc.

[Jack H.]

Re: Vacuum advance

Yes, the two advance methods (centrifical and vac) are cumulative. That's why tune up has you disconnect vac line from distributor to set timing at idle. This defines your 'static' timing. When engine revs and both centrifical and vac advance kick in, you have 'dynamic' advance.

About all you can tell with a conventional timing light is whether or not you're 'in the ballpark' for the overall system. You can't pinpoint very well the intermediate portion of the advance curve and with some setups you can go 'off scale' w.r.t. the engine's calibration index next to the balancer.

BUT, you CAN pull the distributor and walk it down to a shop equipped with a distributor tester (most of the local AC/United Delco shops have one). They can control RPM, vac and run the distributor through its paces for you checking for things you can't really see like wobble, Etc.

[Gary Schisler]

Thanks

Thank you for comfirming my question about the advance. I just called Chevy and they discontinued the units in 94. I now have access to a timing light with the advanced "dial a timing" feature. I intend to check that out.

[Gary Schisler]

Thanks

Thank you for comfirming my question about the advance. I just called Chevy and they discontinued the units in 94. I now have access to a timing light with the advanced "dial a timing" feature. I intend to check that out.

[Gary Schisler]

Vacuum advance - revisited

I just took a good look at my vacuum advance and found the numbers "360" on the left side of the stem going into the dist. housing and the numbers "12" on the right side of the stem. At the top of the stem, under the lip of the dist. cover are the letters "MS", centered over the two part numbers. They were well hidden by corrosion and a little dirt.

Would one of you kind folks with the old parts books see if that corresponds to any known application. I know that the GM parts numbering system should have a three digit number and a two digit number as mine does. I just would like to find out its application. Thanks.

[Gary Schisler]

Vacuum advance - revisited

I just took a good look at my vacuum advance and found the numbers "360" on the left side of the stem going into the dist. housing and the numbers "12" on the right side of the stem. At the top of the stem, under the lip of the dist. cover are the letters "MS", centered over the two part numbers. They were well hidden by corrosion and a little dirt.

Would one of you kind folks with the old parts books see if that corresponds to any known application. I know that the GM parts numbering system should have a three digit number and a two digit number as mine does. I just would like to find out its application. Thanks.

[Joe L.]

Re: Vacuum advance - revisited

Gary----

The "360" distributor vacuum control is GM # 1115360. This control was used on the following:

65-66 (396, 427) w/sp hi perf

67-68 (427)

69 (350, 427) w/ hi perf

70 (350) w/hi perf

70 (454) w/hi perf

As far as I know, this part number is still available from GM/Delco, although its configuration may be slightly different from the originals.

[Joe L.]

Re: Vacuum advance - revisited

Gary----

The "360" distributor vacuum control is GM # 1115360. This control was used on the following:

65-66 (396, 427) w/sp hi perf

67-68 (427)

69 (350, 427) w/ hi perf

70 (350) w/hi perf

70 (454) w/hi perf

As far as I know, this part number is still available from GM/Delco, although its configuration may be slightly different from the originals.

[Bill Clupper]

Re: Vacuum advance - revisited

In addition to joe's explanation, the 12 ia the number of degrees of advance that the vacuum can is spec'd to provide at max vacuum. In addition, I'll try to touch on a couple of other issues raised as I read through the posts. First, vacuum advance and centrifugal advance (and initial advance for that matter) are all accumulative, but only under rare engine operating conditions do they all reach maximum advance together. the target for a Chevrolet engine UNDER HEAVY LOAD is 36-38 degrees generally. This means that at wide open throttle (no vacuum going to the Vac advance) the centrifugal advance and the initial (idle with vacuum not connected) advance should reach 36-38 degrees. there are a couple of hookers in this though because centrifugal advance is generally stated in spec books as "distributor degrees", which must be multiplied X2 to equal the crank degrees you time the engine with using your timing light. In the case of engine with 8 degrees of initial timing, and 15 degrees of distributor advance in the distributor, the actual timing the engine would see under full power at 5000 rpm + would be 8 + (15 X 2) = 38 degrees, about right. The vacuum advance is used to add advance when the engine is not running under full load nor at high rpm levels, when the centrifugal advance in the distributor is providing less than full advance. Under these conditions the engine will actually run most efficiently under more advance than would be desirable under heavy loads. So cruising down the highway at 3000 rpm say, with your 3.70 rear end and 8 degrees of initial advance, the distributor would not be fully advanced internally, so it may provide 10 degrees or so of it's 15 degree capacity. The vacuum advance yu are using would add another 12 degrees under these conditions giving you a total of 8 + (10 X 2) + 12= 40 degrees of advance for lightly loaded cruising. The actual target figure for cruising is dependent on the particular engine parameters, I've seen engines setup to carry anywhere from 40 degrees to as much as 48 degrees from the factory. It's determined at the factory with a lot of dyno time (in the old days) probably be computer simulation now, but if you have a stock rebuild configuration, you will be close using the stock spec parts per the service manual for your car. I would have any used distributor checked on a strobe machine and reconfigured to the stock configuration. There are parts that limit the advance that can wear or fall out with age and this can seriously affect your results.

[Bill Clupper]

Re: Vacuum advance - revisited

In addition to joe's explanation, the 12 ia the number of degrees of advance that the vacuum can is spec'd to provide at max vacuum. In addition, I'll try to touch on a couple of other issues raised as I read through the posts. First, vacuum advance and centrifugal advance (and initial advance for that matter) are all accumulative, but only under rare engine operating conditions do they all reach maximum advance together. the target for a Chevrolet engine UNDER HEAVY LOAD is 36-38 degrees generally. This means that at wide open throttle (no vacuum going to the Vac advance) the centrifugal advance and the initial (idle with vacuum not connected) advance should reach 36-38 degrees. there are a couple of hookers in this though because centrifugal advance is generally stated in spec books as "distributor degrees", which must be multiplied X2 to equal the crank degrees you time the engine with using your timing light. In the case of engine with 8 degrees of initial timing, and 15 degrees of distributor advance in the distributor, the actual timing the engine would see under full power at 5000 rpm + would be 8 + (15 X 2) = 38 degrees, about right. The vacuum advance is used to add advance when the engine is not running under full load nor at high rpm levels, when the centrifugal advance in the distributor is providing less than full advance. Under these conditions the engine will actually run most efficiently under more advance than would be desirable under heavy loads. So cruising down the highway at 3000 rpm say, with your 3.70 rear end and 8 degrees of initial advance, the distributor would not be fully advanced internally, so it may provide 10 degrees or so of it's 15 degree capacity. The vacuum advance yu are using would add another 12 degrees under these conditions giving you a total of 8 + (10 X 2) + 12= 40 degrees of advance for lightly loaded cruising. The actual target figure for cruising is dependent on the particular engine parameters, I've seen engines setup to carry anywhere from 40 degrees to as much as 48 degrees from the factory. It's determined at the factory with a lot of dyno time (in the old days) probably be computer simulation now, but if you have a stock rebuild configuration, you will be close using the stock spec parts per the service manual for your car. I would have any used distributor checked on a strobe machine and reconfigured to the stock configuration. There are parts that limit the advance that can wear or fall out with age and this can seriously affect your results.

[Gary Schisler]

Wow!

Thank you both! This is the first time that I have seen such a good description of timing.

[Gary Schisler]

Wow!

Thank you both! This is the first time that I have seen such a good description of timing.