MS 360 12 Vacuum Advance pod

[Douglas C.]

Hi. On my 69 L68 I find a MS 360 12 advance pod. Am I correct this is for 12 degrees advance @ under 12" of vacuum?

And, as my L68 is stock except for a Comp Cam 280 Magnum cam is this a good choice? Meaning lets say 34 degrees all in mech. at around 2,800 rpm's, then 12 degrees more under cruising situation? Plus, I will use vac. adv. manifold vacuum to get a smooth idle.

Also, should the vac. adv. arm have a small rubber bushing or is it just the plain metal bent rod?

[Joe L.]

Re: MS 360 12 Vacuum Advance pod

Hi. On my 69 L68 I find a MS 360 12 advance pod. Am I correct this is for 12 degrees advance @ under 12" of vacuum?

And, as my L68 is stock except for a Comp Cam 280 Magnum cam is this a good choice? Meaning lets say 34 degrees all in mech. at around 2,800 rpm's, then 12 degrees more under cruising situation? Plus, I will use vac. adv. manifold vacuum to get a smooth idle.

Also, should the vac. adv. arm have a small rubber bushing or is it just the plain metal bent rod?

Douglas------


Your spec on the '360' vacuum advance is correct.

[Duke W.]

Re: MS 360 12 Vacuum Advance pod

The 360 12 vacuum advance control (VAC) specs are start @ 6", 12* @ 12", so it provides 12 degrees advance at manifold vacuum of 12" or GREATER!

Whether it is suitable for your aftermarket cam with full time vacuum advance can only be determined by measuring idle vacuum and applying the "Two-Inch Rule", which you can find in the archives. My hunch is that your aftermarket camshaft, which has considerably more overlap than the OE cam will need the next most aggressive VAC, which is the 8" B28, but you don't know for sure until you measure idle vacuum. If the OE VAC marginally fails the Two-Inch rule, increasing the idle speed may allow it to pass. I recommend you set the idle speed at no less than 900.

I have heard about rubber bushings on VACs, but I don't know if this is how the ...360 was configured.

The optimum WOT spark advance - the sum of initial and full centrifugal - for big blocks is in the range of 36-40. If you can run it at the top of this range without detonation, do so.

You should use a Mighty Vac and timing light to verify that the VAC meets spec. If it does not the closest substitute is the B20 or B26, which is NAPA VC1765, and the same part ID (B20 or B26) is available under different brand names/part numbers. The specs are start @ 6", 16 @ 12". However, you may need a B28.

All replacement VACs are manufactured by Standard Motor Products regardless of the brand name or part number on the box. The "B-numbers" are Standard's manufacturing ID numbers.

Duke

[Douglas C.]

Re: MS 360 12 Vacuum Advance pod

The 360 12 vacuum advance control (VAC) specs are start @ 6", 12* @ 12", so it provides 12 degrees advance at manifold vacuum of 12" or GREATER!

Whether it is suitable for your aftermarket cam with full time vacuum advance can only be determined by measuring idle vacuum and applying the "Two-Inch Rule", which you can find in the archives. My hunch is that your aftermarket camshaft, which has considerably more overlap than the OE cam will need the next most aggressive VAC, which is the 8" B28, but you don't know for sure until you measure idle vacuum. If the OE VAC marginally fails the Two-Inch rule, increasing the idle speed may allow it to pass. I recommend you set the idle speed at no less than 900.

I have heard about rubber bushings on VACs, but I don't know if this is how the ...360 was configured.

The optimum WOT spark advance - the sum of initial and full centrifugal - for big blocks is in the range of 36-40. If you can run it at the top of this range without detonation, do so.

You should use a Mighty Vac and timing light to verify that the VAC meets spec. If it does not the closest substitute is the B20 or B26, which is NAPA VC1765, and the same part ID (B20 or B26) is available under different brand names/part numbers. The specs are start @ 6", 16 @ 12". However, you may need a B28.

All replacement VACs are manufactured by Standard Motor Products regardless of the brand name or part number on the box. The "B-numbers" are Standard's manufacturing ID numbers.

Duke

Hi Duke,

Yes, you are correct. I intended to get it started and check my manifold vacuum at idle, then determine the correct vac. adv. pod. I'll also use manifold vacuum, not ported like factory, to get the best idle.

With cast iron heads and today's fuel, 93 octane at best, I thought to start with a setting of around 34 degrees all in at about 2,800 rpm's. I have the MSD spring / bushing kit so I figured maybe a 21 degree mech. stop bushing stop and 13 degrees initial for the all in 34.

I also assumed I would need more than the 12 degrees vac. adv. pod for cruising mileage. My thought with the MS 360 12 was maybe that was the original / correct one for my car, is that correct?

So, thanks so much for your comments and I hope to have a really nice running L68.

Doug

[Wayne M.]

Re: MS 360 12 Vacuum Advance pod

.... My thought with the MS 360 12 was maybe that was the original / correct one for my car, is that correct? ...

Yes, the 360 is correct for either 1111926 (points) or 1111954 (T.I.) per Delco test spec book DR-324S-2 (1964-1976 Supplement), although the advance specs are slightly different for the two.

The 926: centrifugal starts @ 500 distr. RPM (1000 crankshaft) with a zone of 0 to 2.7 distr. degrees of advance (0 to 5.4 deg. crank). At 1180 crank RPM, centrifugal advance spec zone is 4.4-9.8 crank degrees; at 1240 crank RPM, zone is 6.4 to 10.4 crank degrees; at 2000 crank RPM, 15.0 to 19.0 crank degrees; at maximum distrib limit of 5000 crank RPM, spec zone is 30 to 34 crank degrees.

The K66 '1111954' has a few more data points (all crank degrees): 1000 = 0 to 5.2; 1150 = 3.6 to 9.4; 1240 is same; 2000 is same; 3800 is 24 to 28 deg; max at 6000 RPM =21 to 28.

The latter has the note that after max centrifugal advance is reached and engine speed is increased above that needed to obtain maximum centrrifugal advance, it is the characteristic of this [T.I.] unit for centrifugal advance to decrease approximately 1 degree (crank) for each increase of 1000 (crank) RPM.

[Timothy B.]

Re: MS 360 12 Vacuum Advance pod

Douglas,

My best guess is that that camshaft is going to idle with about 8-9" vacuum. Depending on the compression you are running be careful giving the engine to much idle timing. The heat from compression is very important to vaporize (gas) the fuel load so you don't want to lite the fire to soon. Engines with stock cams can run with more idle timing because the higher manifold vacuum will help boil the fuel before it gets to the cylinder.

The engine may run best with about 20* idle timing, if you have a ported vacuum port on the carburetor just as a TEST you may try 20* inital timing then 18* centrifugal all in at approx 2500rpm. You will get that vacuum advance back when the throttle blades are opened just a little so it's there for cruise conditions. By doing this you can tune the carburetor idle mixture richer which should give better combustion and when put under load the engine will have more power.

I know manifold vacuum advance works great, I am not suggesting run with the book settings of 10-12* inital, what I am trying to state is with too much advance the engine gets to efficient and lean and when put under load it costs you power. After all, that is why you changed the camshaft isn't it...

[Duke W.]

Re: MS 360 12 Vacuum Advance pod

High overlap cams cause considerable exhaust gas dilution at idle and low speed driving, which slows down combustion propagation. Therefore, they need lots of advance under these conditions to operate efficiently. On a fully warmed up engine low idle vacuum doesn't necessarily result in poorer fuel vaporization. Assuming the engine still has an open heat riser passage, idle vacuum is a minor issue.

The higherst OE overlap cam is the 30-30. It produces about 10" manifold vacuum at 900 and idles best with low thirties degrees total idle timing. This is achieved with the OE VAC which provides 16* at 8" or more plus 14-16 initial, and this amount of initial combined with 24* centrifugal at 2350 yields 38-40 total WOT advance.

As long as there is no detonation, the above is the ideal setup for 30-30 cam engines.

Duke

[Duke W.]

Re: MS 360 12 Vacuum Advance pod

Hi Duke,

Yes, you are correct. I intended to get it started and check my manifold vacuum at idle, then determine the correct vac. adv. pod. I'll also use manifold vacuum, not ported like factory, to get the best idle.

With cast iron heads and today's fuel, 93 octane at best, I thought to start with a setting of around 34 degrees all in at about 2,800 rpm's. I have the MSD spring / bushing kit so I figured maybe a 21 degree mech. stop bushing stop and 13 degrees initial for the all in 34.

I also assumed I would need more than the 12 degrees vac. adv. pod for cruising mileage. My thought with the MS 360 12 was maybe that was the original / correct one for my car, is that correct?

So, thanks so much for your comments and I hope to have a really nice running L68.

Doug

If you have 93 PON fuel available, which is equavalent to 98 RON, you should be able to run an aggressive centrifugal curve with 38-42* total WOT advance. It depends on the actual CR of your engine, which is probably not as high as you think. The actual Tonawand as-built CR could be up to 0.5 less than the advertised 10.25:1, and that was with the OE shim gasket, which is think is .028" thick. You can measure the gasket thickness at the corners of the head/block interface. If the heads have ever been off, it's likely that a composition gasket of around .039" was used when the heads were reinstalled, and this would knock down the CR another several tenths.

Whether OE or rebuilt, most engines have a lower CR than the owners think. Reports of detonation are fairly rare and are usually traced to excessive block and head milling or pistons that reduce combustion chamber volume relative to the OE pistons.

The best approach to optimizing the spark advance map is to start out aggressive in terms of the centrifugal curve and total WOT advance. If detonation is detected at low revs slowing the centrifugal curve will usually eliminate it. If it detonates at high revs then total WOT advance must be reduced to below the detonation threshold.

A good intial starting point is 26-28 centrifugal, which would require 10-16 initial to achieve 38-42 total WOT advance, and with 16 degrees vacuum advance the total idle advance would be in the range of 26-32, which is where it needs to be with a high overlap cam.

Duke

[Douglas C.]

Re: MS 360 12 Vacuum Advance pod

I first will have to start it and see what my manifold vacuum is at idle. I am thinking the Comp Cam 280 Magnum is somewhat similar to a stock L72 cam, so I expect my L68 manifold vacuum to be fairly low compared to a stock L68. Although the Comp Cam has a LSA of 110 degrees and I think the stock L72 cam had a LSA of 115. I'll update here once I get to the point of starting the engine as the car has been in a shop for some time having bottom cleaned, new universals, frame clean up / paint, etc.

[Timothy B.]

Re: MS 360 12 Vacuum Advance pod

Duke,

I agree with your statement that more idle timing makes the engine idle best but idle is only one engine speed condition. The more idle timing the leaner you can run a engine but when it's put under load it can go lean without good power. What I am stating is there is a limit for idle timing depending on camshaft selection and overlap and today's fuel plays a role because the parts vaporize at different temperatures then the fuels used when these cars were made.

I have advanced the idle timing on my 300hp cars and noticed that little shake or lean miss because the fuel is still vaporizing/burning in the hot exhaust manifold (inefficient combustion). The carburetors can be set lean and the engine runs much cooler that's true but the combustion efficiency is poor because the fuel load is not given enough time with compression to vaporize which results in poor combustion. Retard the timing a bit like plugging the V/A hose and the motor becomes smoother without that occasional lean shake. Now I am not saying 12-14* idle timing either, I realize the importance of vacuum advance.

[Douglas C.]

Re: MS 360 12 Vacuum Advance pod

If you have 93 PON fuel available, which is equavalent to 98 RON, you should be able to run an aggressive centrifugal curve with 38-42* total WOT advance. It depends on the actual CR of your engine, which is probably not as high as you think. The actual Tonawand as-built CR could be up to 0.5 less than the advertised 10.25:1, and that was with the OE shim gasket, which is think is .028" thick. You can measure the gasket thickness at the corners of the head/block interface. If the heads have ever been off, it's likely that a composition gasket of around .039" was used when the heads were reinstalled, and this would knock down the CR another several tenths.

Whether OE or rebuilt, most engines have a lower CR than the owners think. Reports of detonation are fairly rare and are usually traced to excessive block and head milling or pistons that reduce combustion chamber volume relative to the OE pistons.

The best approach to optimizing the spark advance map is to start out aggressive in terms of the centrifugal curve and total WOT advance. If detonation is detected at low revs slowing the centrifugal curve will usually eliminate it. If it detonates at high revs then total WOT advance must be reduced to below the detonation threshold.

A good intial starting point is 26-28 centrifugal, which would require 10-16 initial to achieve 38-42 total WOT advance, and with 16 degrees vacuum advance the total idle advance would be in the range of 26-32, which is where it needs to be with a high overlap cam.

Duke

Good points Duke. The shop my car is in for underside work is owned by a buddy of mine and I'll probably do the timing / tuning there with him. Aggressive approach won't be a problem if he is helping me, very knowledgable guy, but as he has an 8.50 sec. Camaro he is pumped up to help me tune my L68.

I assume it should make good power. The person I bought the car from gave my some info from the previous owner who rebuilt the engine. 30 over, mild head work, 280 Magnum Cam, and with my car's 3:70 rear it should fly. But, as a matching numbers / buildsheet L68 I don't want to go Ka-Boom with it.

So, thanks again for the thoughts.

Doug

[Duke W.]

Re: MS 360 12 Vacuum Advance pod

Duke,

I agree with your statement that more idle timing makes the engine idle best but idle is only one engine speed condition. The more idle timing the leaner you can run a engine but when it's put under load it can go lean without good power. What I am stating is there is a limit for idle timing depending on camshaft selection and overlap and today's fuel plays a role because the parts vaporize at different temperatures then the fuels used when these cars were made.

I have advanced the idle timing on my 300hp cars and noticed that little shake or lean miss because the fuel is still vaporizing/burning in the hot exhaust manifold (inefficient combustion). The carburetors can be set lean and the engine runs much cooler that's true but the combustion efficiency is poor because the fuel load is not given enough time with compression to vaporize which results in poor combustion. Retard the timing a bit like plugging the V/A hose and the motor becomes smoother without that occasional lean shake. Now I am not saying 12-14* idle timing either, I realize the importance of vacuum advance.

The 327/300 cannot tolerate as much idle and low speed/low load advance because it has a low overlap cam that reduces exhaust gas dilution relative to high overlap cams, which means combustion propagation is faster. Valve timing has a lot to do with what the ideal spark advance map looks like, and it's something like this:

327/300 - 30 total centrifugal, 6-10 initial, 16 @ 15" vacuum. This yields 22-26 total idle advance and 36-40 total WOT advance

SB w/ high overlap cam - 24 centrifugal, 12-16 initial, 16 @ 8 or 12" VAC depending on idle vacuum (must meet Two-Inch rule) This yields 28-32 total idle timing, and the same 36-40 total WOT advance.

For either configuration, the rate that centrifugal can be bought in may be limited by detonation, but as a general rule, a high overlap cam with tolerate a faster rate than a low overlap cam.

A too aggressive spark advance map in a low overlap cam engine can cause a phenomenon like "lean surge" because some cylinders may want to run backwards.

There is no way fuel would survive combustion in the liquid state unless there was something terribly wrong with the mixture or the spark advance map - assuming there is adequate ignition energy from the spark plug. Too rich a mixture will produce very high hydrocarbon emissions, but they are in the vapor state. All vintage engines require a fairly rich mixture to idle smoothly - on the order of about 12.5:1, which is 20 percent excess fuel, and this results in several hundred ppm HC emissions.

Duke

[Ken A.]

Re: MS 360 12 Vacuum Advance pod

The 360 12 vacuum advance control (VAC) specs are start @ 6", 12* @ 12", so it provides 12 degrees advance at manifold vacuum of 12" or GREATER!

Whether it is suitable for your aftermarket cam with full time vacuum advance can only be determined by measuring idle vacuum and applying the "Two-Inch Rule", which you can find in the archives. My hunch is that your aftermarket camshaft, which has considerably more overlap than the OE cam will need the next most aggressive VAC, which is the 8" B28, but you don't know for sure until you measure idle vacuum. If the OE VAC marginally fails the Two-Inch rule, increasing the idle speed may allow it to pass. I recommend you set the idle speed at no less than 900.

I have heard about rubber bushings on VACs, but I don't know if this is how the ...360 was configured.

The optimum WOT spark advance - the sum of initial and full centrifugal - for big blocks is in the range of 36-40. If you can run it at the top of this range without detonation, do so.

You should use a Mighty Vac and timing light to verify that the VAC meets spec. If it does not the closest substitute is the B20 or B26, which is NAPA VC1765, and the same part ID (B20 or B26) is available under different brand names/part numbers. The specs are start @ 6", 16 @ 12". However, you may need a B28.

All replacement VACs are manufactured by Standard Motor Products regardless of the brand name or part number on the box. The "B-numbers" are Standard's manufacturing ID numbers.

Duke

AC DELCO still sells VA's under part # 19110657 and they are stamped B1. What does that make them fit?

[Ken A.]

Re: MS 360 12 Vacuum Advance pod

AC DELCO still sells VA's under part # 19110657 and they are stamped B1. What does that make them fit?

Never mind, I see the answer elsewhere.

[Duke W.]

Re: MS 360 12 Vacuum Advance pod

One of the following three VACs is functionally correct for any vintage Corvette engine with full time vacuum advance or later emission controlled engines converted from ported vacuum advance.

8" B28
12" B20 or B26
15" B22

If you merely guess you have a 33 percent change of getting the right one, which are better than typical Vegas odds, but if you measure idle vacuum (in neutral for manuals and Drive for autos) and apply the Two-Inch rule, your odds increase to about 100 percent.

Duke