What is the correct part number for the vacuum advance cannister for 70 LT1

[Alfred W.]

I have a 70 LT1 with a MS 201 15 number on the vacuum advance canister. Is that correct for this application ?
The TCS does not operate since the vacuum required to start movement in the distributor is 9-10 mm hg and the vacuum
produced by the engine is 9-10 mm hg with 11-12 mm hg with revs. I cannot get operational TCS for NCRS judging. What gives?

[Joe L.]

Re: What is the correct part number for the vacuum advance cannister for 70 LT1

I have a 70 LT1 with a MS 201 15 number on the vacuum advance canister. Is that correct for this application ?
The TCS does not operate since the vacuum required to start movement in the distributor is 9-10 mm hg and the vacuum
produced by the engine is 9-10 mm hg with 11-12 mm hg with revs. I cannot get operational TCS for NCRS judging. What gives?

Alfred------


The correct vacuum advance for your application, whether or not you also have K-66, is GM #1115360.

[Duke W.]

Re: What is the correct part number for the vacuum advance cannister for 70 LT1

The VAC specs for the '70 LT-1 are identical to all '63 engines, so it's always been my belief that the '70 LT-1 OE VAC is 201 16. Other owners may be able to verify. What does the JG say?

I recall the ...360 is used on L-72 and the specs are O @ 6", 12 @ 12" (crankshaft degrees).

...can't help you with the TCS as I have no technical documentation on that system, but at idle and in the lower gears there should be no vacuum signal to the VAC unless the engine temperature exceeds something like 220 to 230. The CSM should have a good explanation of how it works and how to troubleshoot.

Your vacuum measurements don't mean anything because you didn't state the speeds at which they were measured. Typical LT-1 is about 12" @ 900.

The only other thing I can add is that if a LT-1 is converted to full time vacuum advance, a 8" B28 VAC needs to be installed. The OE VAC does not meet the Two-Inch Rule with full time vacuum advance.

Duke

[Joe L.]

Re: What is the correct part number for the vacuum advance cannister for 70 LT1

The VAC specs for the '70 LT-1 are identical to all '63 engines, so it's always been my belief that the '70 LT-1 OE VAC is 201 16. Other owners may be able to verify. What does the JG say?

I recall the ...360 is used on L-72 and the specs are O @ 6", 12 @ 12" (crankshaft degrees).

...can't help you with the TCS as I have no technical documentation on that system, but at idle and in the lower gears there should be no vacuum signal to the VAC unless the engine temperature exceeds something like 220 to 230. The CSM should have a good explanation of how it works and how to troubleshoot.

Your vacuum measurements don't mean anything because you didn't state the speeds at which they were measured. Typical LT-1 is about 12" @ 900.

The only other thing I can add is that if a LT-1 is converted to full time vacuum advance, a 8" B28 VAC needs to be installed. The OE VAC does not meet the Two-Inch Rule with full time vacuum advance.

Duke

Duke------


The GM blueprints for the 1970 LT-1 distributor specifies the vacuum control is GM #1115360. The P&A catalog specifies the same.

[Duke W.]

Re: What is the correct part number for the vacuum advance cannister for 70 LT1

There's a lot of conflicting information. The shop manual specs for the OE 201 15 VAC on my '63 327/340 are 0 @ 8", 15 @ 15.5". (This was a very poor choice for that engine, but it's another story that I've discussed many times.)

My 9-77 P & A catalog has the following listings:

70 Y W/H/PER (350) 1115360 (I believe this is the L-46)

70 Y W/SP. H/PER (350) 1116163 (I believe this is the LT-1)

I believe it's been well established that the ...163 replaced the ...201 for service parts even after the ...201 continued to be used on some production engines. The ...163 is currently available as the B1 and I refer to it as a "boat anchor" because it takes up to 18" to pull to the limit, which doesn't even meet the Two-Inch Rule for a base small block.

I believe the specs for the ...360 are 0 @ 6 (or 7), and 12 @ 12. This jibes with the '70 AMA specs for the two available big blocks.

The AMA specs for the L-46 are 0 @ 7" 19" @ 17, which doesn't jibe with what I understand to be the ...360.

The 71 and 72 model years are confusing. There is only one listing for small blocks:

71-72 Y(350) 1973437,

but the '71 LT-1 VAC specs are the same as '70. The '72 LT-1 is 0 @ 8, 20 @ 17", but the base engine is the same as the ...201.

GM left us with a real mess regarding VAC utilization due to poor specification for some original selections with full time vacuum advance, and then the proliferation of different parts during the emission control added to the mess. On top of that was subsequent parts consolidation.

Duke

[Mike F.]

Re: What is the correct part number for the vacuum advance cannister for 70 LT1

I hope the OP (Alfred) doesn't mind me chiming in.


I spoke with Alfred over the phone, trying to help him with his TCS issue. One of the troubleshooting procedures was connecting a vacuum hose from the carb T-fitting directly to the distributor (by-passing the TCS solenoid/system) to see if he was getting any vacuum advance. (He did not.)

I suggested using a Mity-Vac to see if the distributor vacuum advance canister was moving at all. He indicated that it needed quite a bit of vacuum to start moving the canister (12"hg or so). I suggested that it could be either the vacuum canister or the points plate is not moving as freely as it should.

The 1970 Corvette manual indicates that LT-1 vacuum advance should begin at 8.00 hg and max at 15.5 hg.

[Duke W.]

Re: What is the correct part number for the vacuum advance cannister for 70 LT1

IMO that 201 15 is likely original, so at 46 years it's provided a lot of life considering that the elastomeric diaphragm inside is subject to thermal degradation and fatigue. The current equivalent replacement is the B22, but a B28 would be necessary if the TCS is bypassed and direct manifold vacuum is used as the VAC signal.

Any VAC should start to pull the link at about +/- one inch of the start spec and pull smoothly with increasing vacuum until the link reaches maximum travel at about +/- about one inch of the spec. They typically fail by seizing or leaking.

The VAC is only worth three total points in judging - two for originality and one for condition, but it's really important to achieve maximum fuel economy, idle quality, and part throttle performance and response, especially when a full time vacuum source is used.

Duke