#201 Vacuum Advance Can

[Peter S.]

Re: #201 Vacuum Advance Can

Noted. Thank you.

Peter

[Peter S.]

Re: #201 Vacuum Advance Can

I got my Mityvac and sure enough, the old unit works just fine. Infact, it matches the exact specifications that Duke mentioned in your first post: begins to move at 8 in. Hg, fully open at 15 in. Hg.

[Richard M.]

Re: #201 Vacuum Advance Can

Peter, Ahh, good, it looks like "Corvette night" got "advanced". nyuk nyuk......

Glad you tested it before it got thrown out. My Mityvac is my friend when I test any automotive part with a rubber diaphragm.

Now I just went back and re-read this entire thread, and I don't see why you changed it in the first place. You say later you have a detonation issue. Obviously the vacuum advance change did nothing to alter that.

That I'll leave to the others more experienced than myself, but after you install your original MS201-15 back in, you may want to get out the dial back timing light in preparation for the next round of discussion.

Post #8 tells me you have a '69 350/350hp with TI, correct?

Rich

[Peter S.]

Re: #201 Vacuum Advance Can

Peter, Ahh, good, it looks like "Corvette night" got "advanced". nyuk nyuk......

Glad you tested it before it got thrown out. My Mityvac is my friend when I test any automotive part with a rubber diaphragm.

Now I just went back and re-read this entire thread, and I don't see why you changed it in the first place. You say later you have a detonation issue. Obviously the vacuum advance change did nothing to alter that.

That I'll leave to the others more experienced than myself, but after you install your original MS201-15 back in, you may want to get out the dial back timing light in preparation for the next round of discussion.

Post #8 tells me you have a '69 350/350hp with TI, correct?

Rich

It doesn't count as "corvette night" if I can do it in the house.

Yes, '69 350/350hp with TI. The old unit will get a gentle scrub and will go back in place. I experience detonation under cruise conditions with light throttle response. It's now clear (and makes sense) that this wouldn't be related to the vacuum advance, unless it was supplying too much advance. So now I need to look at other options:

Not enough vacuum to actuate advance can? On true "corvette night" I'll have to start it up and hook up my new Mityvac.
Too lean during cruise/light acceleration? Duke doesn't mention this in his presentation under this category, but it may be the cause for high exhaust gas dilution? The carburetor was rebuilt by Lars. I am less knowledgeable about quadrajet tuning than I am about ignition tuning.
Hot air entering the carburetor giving me a less dense mixture? I don't know where to begin that would cause this.

I'll also check the total timing (initial plus advance) at idle and 1500-2000 rpm.

At this point, I might as well map out my own distributor curve.

Peter

[Duke W.]

Re: #201 Vacuum Advance Can

Not enough vacuum to actuate advance can? On true "corvette night" I'll have to start it up and hook up my new Mityvac.
Too lean during cruise/light acceleration? Duke doesn't mention this in his presentation under this category, but it may be the cause for high exhaust gas dilution? The carburetor was rebuilt by Lars. I am less knowledgeable about quadrajet tuning than I am about ignition tuning.
Hot air entering the carburetor giving me a less dense mixture? I don't know where to begin that would cause this.

I'll also check the total timing (initial plus advance) at idle and 1500-2000 rpm.

At this point, I might as well map out my own distributor curve.

Peter

A lean mixture will cause the engine to run rough and uneven at cruise and not respond cleanly to small throttle increases. Lean detonation is WAAAAAAAAAAAY leaner than this.

You have to look at what your ACTUAL centrifugal curve is compared to OE spec, which is in the AMA specs and CSM. Use a dial back light to determine where it starts, where it ends, and the maximum advance at the end point. The OE end point is WAAAAAAAAAAAAY beyond 1500-2000, but a modified curve will likely end sooner than OE. You have to rev the engine until the the timing stops advancing. So your head is six inches from the fan at 6000 revs. Don't worry, it won't explode. I've been doing it for 50 years.

First you study the presentation. Then you get out the tools to TEST AND RECORD the actual spark advance map on your engine. Then you decide what changes are necessary to optimize it, and recommended starting points for your engine are in the presentation. One of them is to convert to full time advance, and then use the Two-Inch Rule to select a proper VAC for full time advance, which is not the one you have.

Before anyone speculates further, you need to absorb the presentation and run the requisite tests. Anything else is just a total waste of time speculating.

Your next post should:

- quote the spark advance map as specified in the AMA specs and/or shop manual including any differences between the two sources

- the results of your tests for the VAC and centrifugal curve.

- your apinion of what the ideal spark advance map should be, and what you think is needed to get there

Then we'll all have something to discuss.

Duke

[Joe L.]

Re: #201 Vacuum Advance Can

It doesn't count as "corvette night" if I can do it in the house.

Yes, '69 350/350hp with TI. The old unit will get a gentle scrub and will go back in place. I experience detonation under cruise conditions with light throttle response. It's now clear (and makes sense) that this wouldn't be related to the vacuum advance, unless it was supplying too much advance. So now I need to look at other options:

Not enough vacuum to actuate advance can? On true "corvette night" I'll have to start it up and hook up my new Mityvac.
Too lean during cruise/light acceleration? Duke doesn't mention this in his presentation under this category, but it may be the cause for high exhaust gas dilution? The carburetor was rebuilt by Lars. I am less knowledgeable about quadrajet tuning than I am about ignition tuning.
Hot air entering the carburetor giving me a less dense mixture? I don't know where to begin that would cause this.

I'll also check the total timing (initial plus advance) at idle and 1500-2000 rpm.

At this point, I might as well map out my own distributor curve.

Peter

Peter-----

One thing to keep in mind: all 1970 Corvettes except LT-1 use the Controlled Combustion System (CCS) for exhaust emissions control. This system often produces driveability problems even if the engine is at-new specifications and condition.

[Richard M.]

Re: #201 Vacuum Advance Can

Peter,

What Duke and the guys here helped me with on that '67 L71 2 years ago made for total success. Converting to full time vacuum, using the 2 inch rule to select the correct vacuum advance, and using that dial back timing light, etc.

You will likely be putting that nice original MS201-15 on the shelf for posterity, but your engine will love you for it. Let us know when Corvette Night continues. I'm looking forward to it.

Rich

[Terry M.]

Re: #201 Vacuum Advance Can

Peter-----

One thing to keep in mind: all 1970 Corvettes except LT-1 use the Controlled Combustion System (CCS) for exhaust emissions control. This system often produces driveability problems even if the engine is at-new specifications and condition.

Joe,
This information may not change your opinion of early 1970s emissions systems, but 1970 has a TCS (Transmission Controlled Spark) system, as does 1972. 1971 Has IIRC a CES, but I thought it was called Combined Emissions System. I am at the Deep South Regional and don't have my reference materials at hand, so I will rely on you, and others for the names, of these three systems. Regardless of what they were called and what makes up the components they all functioned in a similar fashion. What no one has addressed so far is the temperature component of these systems. Do not ignore that function. It is worth having additional vacuum advance on cold start and engine overheat.

Additionally moving the vacuum from ported to full time will necessitate adjusting the idle stop to a different than designed position. This could result in completely exposing or closing off the idle slot in the carburetor. Do that and you will have difficulty in establishing a smooth idle. I recognize we like to think we know more than the engineers who designed the system, but count me among the skeptics. I am humble enough to admit I don't know more than those guys.

[Joe L.]

Re: #201 Vacuum Advance Can

Joe,
This information may not change your opinion of early 1970s emissions systems, but 1970 has a TCS (Transmission Controlled Spark) system, as does 1972. 1971 Has IIRC a CES, but I thought it was called Combined Emissions System. I am at the Deep South Regional and don't have my reference materials at hand, so I will rely on you, and others for the names, of these three systems. Regardless of what they were called and what makes up the components they all functioned in a similar fashion. What no one has addressed so far is the temperature component of these systems. Do not ignore that function. It is worth having additional vacuum advance on cold start and engine overheat.

Additionally moving the vacuum from ported to full time will necessitate adjusting the idle stop to a different than designed position. This could result in completely exposing or closing off the idle slot in the carburetor. Do that and you will have difficulty in establishing a smooth idle. I recognize we like to think we know more than the engineers who designed the system, but count me among the skeptics. I am humble enough to admit I don't know more than those guys.

Terry------


The TCS and CES are component systems of the CCS exhaust emissions control systems. However, TCS and CES will also be found on LT-1's which used AIR instead of CCS. Basically, CCS uses "distributor advance and carburetor modifications" to achieve exhaust emissions control while AIR achieves a similar end by injecting air into the exhaust to "afterburn" hydrocarbons. As I've said repeatedly, AIR is a much more performance-friendly means of exhaust emissions control than CCS. That's why it was used on special high performance engines like LT-1 and LS-6. However, it's also a lot more expensive than CCS.

[Peter S.]

Re: #201 Vacuum Advance Can

Hi Joe,

This is a 1969, so I only have the A.I.R. system to worry about.

Peter

[Joe L.]

Re: #201 Vacuum Advance Can

Hi Joe,

This is a 1969, so I only have the A.I.R. system to worry about.

Peter

Peter-----

I guess this thread got so long I somehow lost track of the fact the car was a 1969 and not a 1970.

[Terry M.]

Re: #201 Vacuum Advance Can

Terry------


The TCS and CES are component systems of the CCS exhaust emissions control systems. However, TCS and CES will also be found on LT-1's which used AIR instead of CCS. Basically, CCS uses "distributor advance and carburetor modifications" to achieve exhaust emissions control while AIR achieves a similar end by injecting air into the exhaust to "afterburn" hydrocarbons. As I've said repeatedly, AIR is a much more performance-friendly means of exhaust emissions control than CCS. That's why it was used on special high performance engines like LT-1 and LS-6. However, it's also a lot more expensive than CCS.

Joe, I didn't know that CCS was the umbrella name for ALL three years (and perhaps more, since the temperature part of this system went on for some years). Any chance you can provide me a Chevrolet reference for that term? Where did you find it?

[Joe L.]

Re: #201 Vacuum Advance Can

Joe, I didn't know that CCS was the umbrella name for ALL three years (and perhaps more, since the temperature part of this system went on for some years). Any chance you can provide me a Chevrolet reference for that term? Where did you find it?

Terry------


CCS was first used for the 1968 model year but not on Corvettes. It was used for most Chevrolets with automatic transmissions. Those with manual transmission used AIR. Starting in 1968 "it was one or the other". The same thing continued for 1969. For 1970 many applications, both manual and automatic transmission, used CCS, Corvette included.

Most applications used a thermostatic air cleaner as part of the CCS system. However, early C3 Corvette was an exception. Corvette did not get a thermostatic air cleaner until 1976.

Below are excerpts from the 1970 Chevrolet Factory Service Manual and the 1969 Corvette Owner's Manual.

By the way, CCS was used on Corvettes throughout the 1970-82 period. Sometimes, it was used exclusive of AIR and sometimes it was used in conjunction with AIR (later C3).


CCSdescription(b) 001.jpgCCSdescription(a) 001.jpg