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Re: '71 &LT1 PCV Valve
Ray------
The GM part number for the AC CV-746C PCV valve was GM #6484525. It was that GM part number from the time it was released in late 1967 (for the Camaro Z-28 application) until it was discontinued without supersession a few years ago.
While the GM part number and the AC part number remained the same over all of those years, there were some differences in configuration. However, as I say, it is not possible to determine configuration differences by GM or AC part numbers. Keep in mind that GM part numbers for a particular part may change over the years while the AC number may not. In this case, though, that never happened: the GM and AC part numbers remained the same over the entire life of the valve. Also, in the case of a PCV valve, even if the GM part number were to change, it could not be discerned by the stamping on the valve, itself, since PCV valves typically have only the AC part number.
I have seen this GM #6484699 bantered about previously, particularly in Camaro Z-28 circles. However, I can find no GM reference to it. It's possible it was a PRODUCTION-only part number for the valve (or something related to the valve). I can tell you that I cannot find that it was ever available in SERVICE under that part number. And, as I mentioned, GM part numbers usually do not appear on PCV valves. So, it's really a moot point if it ever existed. The 1970-72 LT-1 PCV valve was AC 746C.In Appreciation of John Hinckley -
Re: '71 &LT1 PCV Valve
Joe,
Got it and thanks for the info on the PCV part numbers, usage and the cross referencing between AC and GM.
Since the AC CV-746C is no longer available, I guess I'll just have to pick a reproduction one (for my originally owned '71 LT1).
Ray KComment
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Re: '71 &LT1 PCV Valve
Ray------
The original configuration of the GM #6484525 AC 746C PCV valve which MAY have been used through 1971 is significantly different than later variations. That earlier version, which I do not believe is reproduced, usually sells for HUGE $$$$$$. The reproductions mimic the configuration of the later style CV 746C which came into production sometime in the early 70's.
The reproductions are probably those manufactured by Standard Motor Products which I think are still available and are very similar to the "2nd design" 746C.
I would NEVER pay the kind of money that these "1st design" valves seem to command. NEVER. EVER.In Appreciation of John HinckleyComment
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Re: '71 &LT1 PCV Valve
Ray,
One available on fleabay for $55.
Leif
'67 Coupe L79, M21, C60, N14, N40, J50, A31, U69, A01, QB1
Top Flight 2017 Lone Star RegionalComment
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Re: '71 &LT1 PCV Valve
Leif------Ray,
One available on fleabay for $55.
This is the "2nd design" 746C.
This is an example of the "1st design" 746C (which may or may not have been used as late as 1971)
In Appreciation of John HinckleyComment
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Re: '71 &LT1 PCV Valve
Joe & Leif,
Thanks for the info on the AC first & second design and the Standard Motor Product PCV valves.
I went out on the Standard Motor Product website and found that their "performance PCV" repro is V158 (see below).
And went out and looked at both of the original AC CV746C valves and agree that the first design PCV is ridiculously priced at $1,499.95
Looking at the second design PCV (with the purple coating) priced at $55.00, leads me to ask the following question:
- Does anyone "possibly" know when they changed from the first design to the second design?
My '71 LT1 was built in January of '71 with the last five digits of the VIN being: 07164
Ray KComment
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Re: '71 &LT1 PCV Valve
My 70, Vin 14544, has the second design 746
Gary BComment
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Re: '71 &LT1 PCV Valve
Gary,
Thanks for your PCVs design level. Since yours is a '70 and built much earlier than mine, I'm assuming that my '71 (built in January 1971) is of the second design also.
I noticed in the 1970-72 Technical Information and Judging Guide, that some LT1s were built with the CV-736C valve. . . . .
Since my LT1 was one of these, I'm also trying to understand the "functional" differences between the CV-746C and the CV-736C.
Was it because of the hydraulic lifters (736 on the base and LS5 engines) and the mechanical lifters (LT1 and LS6 engines)?
Or is there "more to it" than lifter differences?
Ray KComment
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Re: '71 &LT1 PCV Valve
My 71 LT1 #16798 Bowtie car had a 736 PCV. The guys on the line got it wrong.Comment
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Re: '71 &LT1 PCV Valve
The difference with the PCV valves is related to the amount of vacuum the engine pulls. That, in turn, is related to the camshaft configuration. However, for all practical purposes, in your case a 736C will not perform noticeably different from a 746C.In Appreciation of John HinckleyComment
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Re: '71 &LT1 PCV Valve
Ray. I was in error. I apologize. I went and looked at my 70 today, to get a picture. It is a 736CV. See the Pictures below.
2023-03-23 11.56.20.2.jpg
I looked at my 68, L71 (an April1st, 68 car) and it uses the same valve. looks the same, see below.
2023-03-23 11.55.29.2.jpg
I have Terry's 70 LT-1 close by, so I also looked at his car. It's vin 2161. It's the same as my 68 and my 70, uses a 736.
During judging over the last several years, I have seen both used, 736 and 746. Doesn't seem to be any rhyme or reason to it. Hence, the wording in the judging manual.
Sorry for the confusuion, I apologize.
Gary BosselmanComment
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Re: '71 &LT1 PCV Valve
I did a quick experiment with my LT-1 using both a CV-736 and a CV-746 last summer. While the results were not scientifically conclusive, the 736 valve seemed to provide a more consistent and smoother idle, and my cruise 2,200 rpm intake manifold vacuum appeared to be about a half inch higher in reading. (it was difficult to obtain exact data under controlled conditions) That being said, however, I did not go back and readjust the carburetor idle mixture screws with the change, so the idle quality data is suspect.
I believe the 746 valve was designed and utilized for lower manifold vacuum engines.
Interested if anyone may have further insight into this topic. I find it hard to believe that so many original manufactured engines could have been the subject of factory installer mistake. Perhaps an engineering change to switch from 746 to 736 (or vice versa) or simply a supply shortage on the line?Bill Bertelli
Northeast and Carolinas Chapters Member
'70 Resto Mod LT-1 w/ partial '70 ZR-1 drivetrainComment
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Re: '71 &LT1 PCV Valve
Bill,
Thanks for the info PCV comparison, as you said although it's "not scientifically conclusive" it does provide some level of comparison between the two PCV valves. I do have my original CV-736C, and a AC Delco CV-746C (sitting on my parts shelf and bought many years ago from Paragon), maybe I'll run a similar test as you did and see what I might find out.
I went back and looked thru my '71 AIM and unfortunately, I do not have the sheet that contains the LT1 PCV part number information.
However, I did look at my '72 AIM (UPC: <1, Sheet: B2) and found that the LT1s PCV was part of the Vapir Hose Assembly (GM #3991444), more than likely the same as the for the '71s, as indicated in the note (4-8-71, c/o LT1 -B2, ECS #39633). Interestingly enough, this same Vapir Hose Assembly was also used on the LS6 (ref: UPC: &LS6, Sheet: B1)
Since the LT1s PCV was part of the hose assembly it couldn't have been an assembly plant issue, it would have been a supplier assembly issue, unless as you said there was an Engineering Change that indicated both the CV-736C and CV-746C could have been used interchangeably (without changing the part number that was shipped to the plant). One would have to have the design drawing and Engineering Change Summary to validate this scenario though . . . . . . .
Ray KComment
'71 <1 PCV Valve
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