Distributor Vacuum Advanced

[Bruce Hart]

Trying to find the correct vacuum control can for my 1976 455. The parts interchange lists a VC1835 (as2).
Specs for the vc1835 are:

VC1835 AS2 5.5"-7.5" maximum 12 degrees advance @ 15"-17"
I live at 5,000 feet above sea level and my vacuum readings are:
Idle 14" to 15" @ 900 rpm
Cruise 13" to 14"

After researching articles on vacuum advance the VC 1835 would be the wrong vacuum control can to use.

Has anyone done any investigating on which vacuum control can to use at a higher elevation?

A very good article that I ran across published by a GM engineer:
https://www.460ford.com/forum/42-general-tech/117504-timing-vacuum-advance-101-good-article-gm-engineer.html

--
Bruce Hart
1976 Palm Beach
Milliken, Co

 

[chris tyler]

Have you considered an adjustable canister? THat way you can optimise it yourself for the altitude.
--
76 Glenbrook

 

[Bruce Hart]

Wasn't aware of an adjustable one.
Do you have a part number?

On Wed, Aug 15, 2018 at 4:25 AM, Chris Tyler
wrote:

> Have you considered an adjustable canister? THat way you can optimise it
> yourself for the altitude.
> --
> 76 Glenbrook
>
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--
Bruce Hart
1976 Palm Beach
Milliken, Co
GMC=Got More Class

 

[powwerjon]

The correct person to comment on this is Dick Paterson (Springfield Ignition 7zero5-3two5-4five5four) who builds the distributors that are the final word for the GMC ignition system. I have had several and always worked right out of the box. The “Vacuum Advance CAN” that you are looking for is an 10 degree “CAN”. This “VAC” can be difficult to find. Also very important for those reading is to check and see if your centrifugal advance weights are not seized or stuck or worn out, you should do this every few years even if you don’t drive the coach often.

Researched the part numbers and found this in the GMCMI Newsletter, Issue 48, Page 1, Col 2, Para 2.

Paraphrase here:
“NAPA# VC1836 is NOT the correct item for either the 403 or the 455.”
“Correct PN for 1973 thru early 1975, prior to Chassis #5V100472 455 is VC1755. This is a direct replacement for GM PN”

“Found that NAPA PN# VC1835 fits later 403 (HEI) and 455 (HEI) engines. This part# does not have identical start and and end vacuum specs , but it comes fairly close. California members should further check specs as vacuum can affect emissions.”

Another very good source would be Jim Bounds and Jim K for parts if you want the correct parts for your ignition.

JR Wright
GMC Great Laker MHC
GMC Eastern States Charter Member
GMCGL Tech Editor
GMCMI
78 GMC Buskirk 30’ Stretch
1975 GMC Avion (Under Reconstruction)
Michigan

>
> Wasn't aware of an adjustable one.
> Do you have a part number?
>
> On Wed, Aug 15, 2018 at 4:25 AM, Chris Tyler

>
>> Have you considered an adjustable canister? THat way you can optimise it
>> yourself for the altitude.
>> --
>> 76 Glenbrook
>>
>> _______________________________________________
>> GMCnet mailing list
>> Unsubscribe or Change List Options:
>> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>>
>
>
>
> --
> Bruce Hart
> 1976 Palm Beach
> Milliken, Co
> GMC=Got More Class
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org

 

[chris tyler]

Going with Dick Patersons setup certainly isnt a bad idea, I have one of his Dizzys myself

But if you would rather 'roll your own" and tune for altitude here is a link:
https://www.jegs.com/i/Pertronix/751/D9006/10002/-1
--
76 Glenbrook