After reading a bunch of "how do I add more power" posts on these boards, I decided to bypass the OSAC valve on my 1975 Dart. When I popped the hood, I only saw one vacuum hose -- running from the distributor vacuum directly to the carburetor. Did the previous owner already bypass the OSAC valve for me?

The chassis service manual indicated that the OSAC valve is located on the air cleaner, but all I see on the air cleaner is the temp sensor. Am I missing something?

Yep, if you've got one hose running directly from carb to dizzy, somebody's already bypassed the OSAC valve.

The OSAC valve is located on the outer wall of the round main housing of the air cleaner, not on the bottom (where the temp modulator is) and not on the snorkel (where the vacuum motor is). '73 and earlier air cleaners didn't have an OSAC valve on them, so if you don't find one, it's possible someone swapped on an earlier air cleaner for a "clean" bypass.

Not all the 1975's came with an OSAC valve. My 1975 never had one from the factory. No hole in air cleaner to mount it either. The underhood vacuum diagram confirmed that my car never had one.

Interesting, Craig. I've seen a lot of '75s, and never one without one, but they must've existed or you wouldn't've had one. Which emissions package did your '75 have? Federal 49-state, high altitude, California, Canada, Export...?

This may be a dumb question. But what is a OSAC? Out Side Air Conditioning? Only Send A Check? Old Seniors Are Cute?

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There is no problem that cannot be solved by the proper application of thermo nuclear weapons

Orifice Spark Advance Control. Above a certain ambient temperature (approx 58°F) it delays vacuum to the distributor by 17 seconds. This was to reduce part-throttle NOx emissions for the purpose of passing the primitive, unrealistic Federal new-car emissions certification tests of the day, and it ruins driveability and part-throttle economy.

This is a test. This is only a test. If this were an actual emergency, your Slant Six Mopar would have started itself up in preperation for evacuation of the area.

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Chuck Rivers, Webmaster

What year cars was that wonderful federal mandated scientific adance found in?

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There is no problem that cannot be solved by the proper application of thermo nuclear weapons

OSAC first appeared for 1973.

It replaced a '71-'72 (California) system tied in with the transmission, which only permitted vacuum advance in top gear above 58°F. OSAC does pretty much the same thing, but was much less costly and troublesome to implement.

Hey, at least the EGR valve appeared for '73. You don't wanna know how exhaust gas recirculation was handled in California in '72, but in case you do: A hole in the intake manifold. Oh, it was a "calibrated" hole, and it went by the fancy name of "floor jet", but that's all it was: A hole from the exhaust side of the intake to the intake side of the intake. Open all the time, including at startup and idle. It made engines run like poo.

According to the vacuum map in the engine bay, my car did have an OSAC valve originally in place.

My air cleaner does not seem to have a mounting place for this, so I think you're right SlantSixDan about my air cleaner being non-original. Still looks nice though

It was Federal emissions. At least that's what the engine emissions label under the hood said. I was the third owner of the car, bought it when I graduated from high school back in 1984 with 59,000 miles on it.

Here's a pic of the vacuum hose label, note the OSAC hoses are dashed to indicate "when equipped". Also see pic of card that was in glove box with owners manual packet. My hose lable does not match the numbers given on the card so there obviously is a misprint or a lot more options and variations floating around out there. My car did have the EGR Maintence Reminder Light as part of the dash wiring, BUT it did not have the counter switch in the speedometer cable so it was non functional. The speedometer cable was one piece from tranny to speedo.

Somewhere I should have a picture of the VIN label from the door jamb to show the manufacture date if that helps you figure out which cars did and did not have certain options.

I shrunk the pictues to fit the forum so lost some clarity.

Then if you buy a used manifold from that era. You should look out for this "calibrated hole" and plug it up?

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There is no problem that cannot be solved by the proper application of thermo nuclear weapons

Look at that hose routing label again and see if the hoses to OSAC valve are dashed like in my picture above.

No, no. It's not on the manifold. It's a device on the air cleaner housing ('74-up) on the passenger side of the firewall ('73 only) with two hoses running to it -- one from the carb, one from the distributor. If you find it, you simply run a hose directly from the carb to the distributor, that's all.

Thanks for the scans, Craig. Most of my underhood time with '75s was in Denver, and all of them had OSAC. I suspect it was part of the High Altitude emissions package in '75 on 225s.