1974 vette smog pump

[Billy Harris]

Hi--new to this--but wondered if anyone knew exactly what kind of smog pump my mostly restored 74 vette would take? I have seen 2 kinds--one with a little release valve on top, and one without--I have read some on them, and thought the one with the release valve was only used on the 68-69 big blocks--but have since talked to a guy who has one with the release valve and says it has matching code numbers on it that would match my 74--could anyone help me?

[Tom R.]

Re: 1974 vette smog pump

billy:

You don't engine RPO but all those equipped with AIR used the 7801149 unit that was used from 1970-77. However, not all 74's were equipped with AIR...depends on engine RPO, tranny and whether CA certified.

You're correct...relief valve was used among selected BB cars but I believe primarily 68 models. Someone will jump in with specifics.

[Joe L.]

Re: 1974 vette smog pump

billy----

No 1974 Corvette ever originally used an AIR pump with a relief valve on the body of the pump. Pumps with relief valves on the body of the pump were used for 1966 and 1967 with K-19, all 1968, and 1969 with L-88/ZL-1. That's it. The 1968 and 1969 with L-88/ZL-1 pumps were the same; the 66-67 pumps were of a different overall configuration.

1969-71 Corvettes (except 1969 with L-88/ZL-1) used a pump with no relief valve. These pumps can be easily identified inasmuch as they have a slightly raised, circular "protrusion" in the center of the "flat" boss on the top of the pump housing.

The 1972-78 pump was similar to the 1969-71 except that there was no slightly raised, circular "protrusion" on the top of the "flat" boss on the top of the pump housing. In addition, the flat boss was otherwise configured differently and was larger.

There is no part number for the complete pump assembly found on any Corvette AIR pump. The number found on the backing plate is the casting number for the backing plate. It is generally the same for all 1968-78 pumps and provides no information as to the exact "identity" of the pump in question.

While all 1968-69 Corvettes used AIR pumps, after 1969 the first year that used AIR for all Corvette applications was 1973. 1974 was an "almost all engines used AIR". For 1974, Corvettes with L-48, auto transmission, and federal emissions did not use AIR. All other 1974 Corvette applications did use AIR.

[Billy Harris]

Re: 1974 vette smog pump

Joe,
Thanks fo all your input,it helps me understand the pump system better.but why would a pump with a relief valve have a date code 24131S?(the one I was looking to buy)Unless it had been tampered with?And another one with 065 day of 73 with slightly rsised boss,no relief valve.And if I understand you correctly,the raised area should not be there,only a flat area. Thanks Bill

[Billy Harris]

Re: 1974 vette smog pump

I am sorry, but this smog pump stuff is all new to me--what is RPO?

[Joe L.]

Re: 1974 vette smog pump

Billy-----

Very simple. The pump with the relief valve on the body of the pump was manufactured for SERVICE LONG AFTER it was ever used in PRODUCTION. In fact, the pump with the relief valve in the body was manufactured for SERVICE until, at least, 1984. So, the "3" could mean 1973 or 1983.

The 69-71 style pump was also manufactured for SERVICE for quite some time after it was last used in PRODUCTION, although it wasn't manufactured for as long as the pump with the relief valve (that's because the relief valve pump can be used for ANY 1968-71 application whereas the 69-71 style pump can only be used for 69-71 applications that originally used this pump). I'm sure that it was still being manufactured in 1973, though.

As far as the basic configuration for easy ID purposes of the Corvette pumps go, it's as simple as this:

1) pump with relief valve and "T" fitting on body for outlets= 66-67 with K-19;

2) pump with relief valve on body and NO "T" fitting (and, no outlets on back plate of pump)= ALL 1968 and 1969 only with L-88/ZL-1;

3) pump with no relief valve but "round protrusion on top of flat boss" = 69-71, except 1969 L-88/ZL-1

4) pump with larger, flatter boss= 72-78 pump

After 1978, the "flat boss" on top of the pump was machined as a flange for SOME applications and the pump changed in some other ways.

[Joe L.]

Important Correction and Addendum

Billy-----

I did some more resarch on this one and came up with some more accurate information. As a result of the research, I learned something that I didn't know previously, AND I corrected some mis-information that had plagued me for a long time. As it turns out, this situation with 68-78 AIR pumps is WAY more complicated than I once believed. I'm not going to cover it all here, but I will hit the "high points" and the issue of concern to you. Here's the deal:

In PRODUCTION, all 1968 Corvettes used AIR pump GM #7801512. In SERVICE, this pump was known as GM #7803947. One of these pumps was also used in PRODUCTION for 1969 Corvettes with L-88/ZL-1. These pumps have the external relief valve on the upper aluminum body of the pump.

For 1969 (except L-88/ZL-1), the AIR pump used in PRODUCTION was changed to GM #7803948. This pump has no relief valve and, in its place, has the "roundish protrusion" in the top of the "flat surface". Otherwise, the configuration of this pump is the same as the 1968 pump. However, this pump was not then-available in SERVICE. For 1969 SERVICE, the only pump EVER catalogued by GM was pump GM #7806686. This pump has the external relief valve and is identical to the 1968 pump. It became the SERVICE pump for 1968, too, when the GM #7803947 was discontinued in October, 1969.

Now for the correction which is of importance to you: in PRODUCTION, all 1969 through 1974 Corvettes which used AIR pumps used the GM #7803948. This is the pump described above with the "roundish protrusion" on the flat surface atop the pump. For 1972-74 models this is contrary to the information I provided in my original post; this is the CORRECTED information.

In late 1972, the GM #7803948 pump became available in SERVICE. However, it was catalogued by GM as the SERVICE replacement for 72+ models ONLY. Even though it was the pump used in PRODUCTION for 69-71 models, too, it was NEVER catalogued by GM as the SERVICE replacement for those models. Instead, the GM #7806686 was the pump catalogued by GM as the SERVICE replacement for all 1968-71 Corvettes until it was discontinued in 1984.

In late 1976 a very strange thing happened. At that time, the GM #7803948 was discontinued from SERVICE and replaced, for SERVICE, by the GM #7817809 pump. This is the pump used in PRODUCTION from early 1975 through 1978. This is the pump with the larger, flat surface on the upper portion of the pump as installed in Corvettes. So, after that time, the SERVICE pump for 1972-74 Corvettes became the same as that used for 75+ Corvettes even though 1972-74 Corvettes had originally used in PRODUCTION the exact same pump as used for 69-71 Corvettes. Those applications continued to be SERVICED by the GM #7806686 pump which was never used on those applications in PRODUCTION but which, presumably, would have been the more correct SERVICE pump for 1972-74 applications, too. But, that's not how it happened and that's the thing that has caused my confusion all of these years. Now, the "record has been set straight".

Now, more strangeness: the GM #7806886 pump which SERVICED all 1968-71 Corvette applications was discontinued WITHOUT SUPERCESSION in October, 1984. However, the GM #7817809, which was used in PRODUCTION for 1975-78 Corvettes and became the SERVICE replacement for 72-74 Corvettes as well as 75-78 is still available in SERVICE to this very day (for $339.87 GM list). Since 1972-74 Corvettes originally used the same pump in PRODUCTION as 1969-71 Corvettes, then the GM #7817809 should be equally useful as a SERVICE replacement for them, too. Consequently, when the 7806686 was discontinued in 1984, there could have been a supercessive reference to the 7817809. I think that the reason may involve the fact that the 7806686 became the SERVICE replacement for 1968 models in 1969, as I described above. While the 7817809 pump would have been a perfectly acceptable SERVICE replacement for 1969-71 Corvettes, it CANNOT be used for 1968 models due to the lack of incorporation of a relief valve in the pump. So, 69-71 owners were "deprived" of an available and acceptable SERVICE replacement pump so that 1968 owners would not get an incorrect pump.

In any event, I've know for years that the 7817809 could be used as a 100% functional replacement for the 69-71. It's configuration is a little different than the original pumps used for 69-71 (and, for that matter, 72-74, too, as decribed above), but it's 100% functional.

What's the FUNCTIONAL difference between the above referenced pumps? I believe that the 68-74 pumps were of 17 cubic inch displacement and the 75-78 were of 19 cubic inch displacement. If not those exact numbers, then I'm virtually certain that there was some-such difference in displacement.

Why won't the 69-78 pumps work for a 1968 model? Well, the 1968 diverter valve does not incorporate a pressure relief function. For 1968, the diverter valve functioned ONLY as a diverter valve and the pressure relief function was performed by the pump's integral valve. So, if a 1969-78 pump with no pressure relief function is used on a 1968, then the 1968 AIR system will have no pressure relief capability, at all. That's not acceptable.

Why can a 1968 style pump with pressure relief valve be used on a 1969-74 Corvette? Well, 1969+ diverter valves incorporate BOTH diverter and pressure relief functions in the valve. So, if a 1968-style pump is used, these systems then effectively have 2 relief valves which just provides more system "redundency" and no problems, at all.

[Billy Harris]

Re: Important Correction and Addendum

Holy cow! where do you get all your info!--I never dreamed a simple smog pump project could be so detailed--I only had one car show judge even notice it was missing on my car, so wondering if all this time and effort trying to get the right one, is worth it Ha!--sure do appreciate all your info though,or any more you may have.

[Joe L.]

Re: Important Correction and Addendum

Billy------

It's ALWAYS worth it to get things right. Or, at the very least, to strive to get things right!