Distributor Vacuum Advance Part #

[Duke W.]

Re: Distributor Vacuum Advance Part #

Duke-----


For 1969-1970 300 hp engines they must have forgot what they learned earlier about the vacuum controls because for these engines they used the GM #1115357 vacuum control (which replaced the 1116238 and has virtually identical specs).

But remember that these were exhaust emission controlled engines with ported vacuum advance. When the emission era began the whole spark advance map had to be set up for emission control, so spark advance maps began to look very different beginning in the late sixties.

For engines of similar configuration like the 327/300s and later base engines, a spark advance map set up for emission control will look completely different than one optimized for performance, throttle response, and fuel ecomony.

In general, emission control spark advance maps have slower centrifugal advance with less total centrifugal advance and less intial advance and more vacuum advance at a higher vacuum level compared to a map optimized for a non-exhaust emission controlled engine.

Duke

[Joe L.]

Re: Distributor Vacuum Advance Part #

But remember that these were exhaust emission controlled engines with ported vacuum advance. When the emission era began the whole spark advance map had to be set up for emission control, so spark advance maps began to look very different beginning in the late sixties.

For engines of similar configuration like the 327/300s and later base engines, a spark advance map set up for emission control will look completely different than one optimized for performance, throttle response, and fuel ecomony.

Duke

Duke-----


I believe that all 1965 and later engines used ported vacuum advance regardless of whether they were K-19-equipped, or not.

Also, 1968 300 hp engine, all of which were equipped with AIR, used the GM #1115355 vacuum control as earlier 66-67 300 hp had used. For 1969 and 1970 300 hp the vacuum control reverted to the GM #11115357 as, essentially, used for 1965 (1116238 and 1115357 are equivalent). The only significant difference between 66-68 300 small blocks and 69-70 is an additional 23 cid difference in displacement.

1966-70 300 hp centrifugal advance specs were all virtually the same, too.

[Duke W.]

Re: Distributor Vacuum Advance Part #

The only '65-'67 engines I know of that used ported vacuum advance were L-72 and L-71 and those equipped with K-19.

The '63 L-84 also had ported vacuum advance, but it changed to full time in '64.

By 1968 most engines had ported vacuum advance to meet federal (and CA) exhaust emission standards, and ignition maps changed virtually every year as GM stuggled to achieve emissions bogeys with the best possible fuel economy and driveability, and it was a stuggle. California adopted NOx standards in 1971 and the feds in 1973, which threw another variable into the equation.

The worst emission controlled engines (from a driveability and fuel economy standpoint) were 1974. The use of catalysts on most engines in '75 allowed the spark advance maps to revert back to something that would have been considered close to "normal" in the sixties except for the ported vacuum advance, but power suffered due to the added exhaust back pressure. The adoption of EGR helped in this regard, too. Without EGR the spark advance has to be retarded considerably from what is ideal to reduce peak combustion temperature, which is the source of NOx. For a given engine of that era the spark advance map would look very different depending on whether EGR was used or not.

Duke

[Larry M.]

Re: Distributor Vacuum Advance Part #

Agree with Duke regarding ported versus regular (non-ported) vacuum advance. I have seen it on the 1966-67 big block cars, but not on the small blocks from that time period.

My 1967, 327/350 HP (4-speed) car has regular (non-ported) vacuum advance. The 300 HP cars were the same. The only exception was K-19 California cars.

Larry

[Barry K.]

Re: Distributor Vacuum Advance Part #

It would help if someone with the '65 AMA specs could state what they list as VAC specs for the '65 250 and 300 HP engines.

Duke

I have the '65 AMA

for the 250hp and 300hp motors:
vacuum advance begins (In Hg) = 6
Max Degrees @ In. Hg = 22 @ 12

for the 350hp, 365hp, and 375hp motors:
vacuum advance begins (In Hg) = 4
Max Degrees @ In. Hg = 16.5 @ 8.2

for the 396/425 motor:
vacuum advance begins (In Hg) = 8
Max Degrees @ In. Hg = 15 @ 15.5

hope this helps



Barry

[Barry K.]

Re: Distributor Vacuum Advance Part #

I believe that all 1965 and later engines used ported vacuum advance regardless of whether they were K-19-equipped, or not.

Joe
my '65 L76 is correct with full manifold vacuum. In fact, I don't believe the 2818 carb even has a connection for ported vacuum although I never had a reason to look for one.
The correct nipple on the carb to connect for vac adv is a full manifold vacuum port

Barry

[Joe L.]

Re: Distributor Vacuum Advance Part #

I have the '65 AMA

for the 250hp and 300hp motors:
vacuum advance begins (In Hg) = 6
Max Degrees @ In. Hg = 22 @ 12

for the 350hp, 365hp, and 375hp motors:
vacuum advance begins (In Hg) = 4
Max Degrees @ In. Hg = 16.5 @ 8.2

for the 396/425 motor:
vacuum advance begins (In Hg) = 8
Max Degrees @ In. Hg = 15 @ 15.5

hope this helps



Barry

Barry-----


I have found quite a few errors in the AMA specifications in the past. I believe this is another case of the same. To wit:

The specifications shown for the 1965 base and L-75 vacuum advance do not match ANY vacuum controls in the GM parts bin at the time. There are only 2 vacuum controls that even come close to the specs listed. One is the GM #1116212, aka Delco #D1308A. I don't think this vacuum control was ever used for any Chevrolet.The other was Delco #D1331C which was used for 1973 Cadillacs.

[Barry K.]

Re: Distributor Vacuum Advance Part #

Joe

since the shop manual is obviously wrong I guess the AMA specs being incorrect shouldn't surprise me either.

But........ Duke asked if anyone had the '65 AMA if they could post what it lists so I was just trying to respond to that and help.

[Duke W.]

Re: Distributor Vacuum Advance Part #

The AMA specs are pretty close to what was published in Corvette News - 24 @ 13. Published specs for each VAC can vary depending on whether the nominal specs are quoted or something between the min. and max., inclusive, which is generally about +/-1" Hg and about +/- 2 degrees from nominal.

I tend to trust the AMA specs because they were revised through the model year as specs changed or errors were discovered. It's not uncommon to see revised data or pages on the photocopies of the final versions we now get from the GM restoration package.

Once shop manuals were published, errors were rarely corrected - even in later printings.

Duke

[Barry K.]

Re: Distributor Vacuum Advance Part #

The AMA specs are pretty close to what was published in Corvette News - 24 @ 13. Published specs for each VAC can vary depending on whether the nominal specs are quoted or something between the min. and max., inclusive, which is generally about +/-1" Hg and about +/- 2 degrees from nominal.

I tend to trust the AMA specs because they were revised through the model year as specs changed or errors were discovered. It's not uncommon to see revised data or pages on the photocopies of the final versions we now get from the GM restoration package.

Once shop manuals were published, errors were rarely corrected - even in later printings.

Duke

Duke
if it makes a difference, that's where I got my '65 AMA specs - from the GM Restoration package

[John D.]

Re: Distributor Vacuum Advance Part #

Can anyone tell me the correct original GM part number for the distributor advance vacuum for a 1965 327/300 hp? The car has automatic and air conditioning. Can't seem to find a page with the part listed in the '65 AIM.
Thanks in advance.

Frank

Frank, Boy you have a lot of reading to do for this post. I have collected old VA's for many years and I can't remember seeing a 238 though. This is one time you might want to just go with the manual whether it is correct or not. Use the 236 one for a start. VC 1802 are discontinued.
If you want some more info and are going to Corvette Carlisle stop over and see me in the L90's. John D.

[Joe L.]

Re: Distributor Vacuum Advance Part #

The AMA specs are pretty close to what was published in Corvette News - 24 @ 13. Published specs for each VAC can vary depending on whether the nominal specs are quoted or something between the min. and max., inclusive, which is generally about +/-1" Hg and about +/- 2 degrees from nominal.

I tend to trust the AMA specs because they were revised through the model year as specs changed or errors were discovered. It's not uncommon to see revised data or pages on the photocopies of the final versions we now get from the GM restoration package.

Once shop manuals were published, errors were rarely corrected - even in later printings.

Duke

Duke-----


The only vacuum control I can find that had specs even close to 24 crankshaft degrees @ 13" is the GM #1973421. There's no way that piece was used on any 1965 Corvette, though.

[Joe L.]

Re: Distributor Vacuum Advance Part #

Joe

since the shop manual is obviously wrong I guess the AMA specs being incorrect shouldn't surprise me either.

But........ Duke asked if anyone had the '65 AMA if they could post what it lists so I was just trying to respond to that and help.

Barry----


I was not being critical of your posting the AMA specs; I'm glad you did. If nothing else, they demonstrate that there was SOME difference between the vacuum control for base/L-75 versus the SHP engines. Also, they point to what I regard as another example of how there can be inaccuracies in the AMA specs.

[Duke W.]

Re: Distributor Vacuum Advance Part #

It's the VC-1820 8" VAC that is discontinued, not the VC-1802 (0@6", 16@15") that I recommend for 250/300 HP with manual trans.

I STRONGLY recommend AGAINST the use of a 236/B28 VAC on a 327/300 or any engine with the base engine cam. It is WAAAAAAY too aggressive for the manifold vacuum characteristics of these engines and is very likely to cause part throttle detonation.

I recommend the VC-1765 B22 VAC, which matches the specs of the final 327/300 HP engine in 1967 and in my engine system engineering opinion it is the best choice for your 300 HP/PG configuration.

The shop manual is WRONG! The JG is WRONG!

FWIW the Corvette News and AMA published specs basically jibe, and this is likely what your car originally had, but good luck finding one at this point in time, and it's not well matched to the engine/transmission, anyway, which is why it was replaced both in production and service. It is also getting very difficult to find original or NOS 236 VACs with the correct OE stamped data. So that leaves you with a current "B-number" replacement that is readily available and costs about ten bucks.

Sometimes it's just better to take the minor points hit on an OE replacement part rather than "going by the book" when "the book" is clearly wrong and there are potential significant operational consequences. There are a lot of easier deductions to work on than this one.

Spend some effort to get the book corrected.

Though the VAC is a simple an innocuous looking device, suitable specs are critical to proper engine performance, and a bad choice can cause serious problems. This goes WAAAAY beyond markings on the bolt head. The VAC is a critcal part of the engine control system!

Maybe it's time to close this thread before anyone adds more confusion!

Duke

[Joe L.]

Re: Distributor Vacuum Advance Part #

FWIW the Corvette News and AMA published specs basically jibe, and this is likely what your car originally had, but good luck finding one at this point in time, and it's not well matched to the engine/transmission, anyway, which is why it was replaced both in production and service.
Duke[/quote]


Duke-----

The AMA and shop manual specs may be basically consistent with each other. However, they are not consistent with the specs of the GM #1116238 (or, it's near identically spec'd replacement, the GM #1115357) which was used on the 1965 base and L-75. The specs for these controls are as follows:

7"-9" Hg to start the plunger; 20-21 crankshaft degrees @ 16-18" Hg

As I've previously mentioned, I can find NO GM vacuum control with specs that are anywhere close to those in the shop manual or AMA specs.