No AIR on 1974 L-48 TH400 (non-California)

[Paul L.]

My engine block is NOM but pretty much stock nonetheless. It does not have an AIR pump or the corresponding exhaust manifolds with injection tubes. My research and understanding leads me to the conclusion that a non-California L-48 with automatic transmission did not have an AIR pump. The four-speed cars did. Any comments.

If I am correct, why would a manual-transmission car not pass emissions and an automatic would?

[Joe L.]

Re: No AIR on 1974 L-48 TH400 (non-California)

My engine block is NOM but pretty much stock nonetheless. It does not have an AIR pump or the corresponding exhaust manifolds with injection tubes. My research and understanding leads me to the conclusion that a non-California L-48 with automatic transmission did not have an AIR pump. The four-speed cars did. Any comments.

If I am correct, why would a manual-transmission car not pass emissions and an automatic would?

Paul------


That's correct. For 1974, L-48 with THM-400 and federal emissions was the only Corvette application that did not come equipped with AIR.

The vagaries and intricacies of emissions requirements compliance would be pretty much impossible to speculate on.

[Terry M.]

Re: No AIR on 1974 L-48 TH400 (non-California)

The vagaries and intricacies of emissions requirements compliance would be pretty much impossible to speculate on.

Joe, you are a genius.

[Duke W.]

Re: No AIR on 1974 L-48 TH400 (non-California)

Paul------


The vagaries and intricacies of emissions requirements compliance would be pretty much impossible to speculate on.

No speculation! Here's a high level explanation of the engineering reasons for no air pump on some autos while the same engine configuration with a manual may have a pump.

Manual transmission cars are harder to get past emissions standards than automatics. The reason is that everytime you lift to shift, there is a HC spike that drives up the grams per mile measurment, which can bust the certification test. This is why manual transmissions were not available in California in some years.

Back in the seventies catalysts were oxidizing only. This usually required air injection to provide the necessary O2 for oxidation, but with an auto trans and fairly lean carburetor calibration, there can be enough O2 in the exhaust to oxidize HC and CO sufficiently to meet the stardards of the day.

Nowadays all cars have "three-way" catalysts that support both oxidation and reduction of NOx. Three-way cats require a stoichiometric mixture, which both minimizes overall "engine out" emissions and provides the proper exhaust chemistry (including a fraction of a percent O2) to maximize both oxidation and reduction, and no supplemental air injection is required.

Many modern cars do have air pumps, but they are only active during warmup. Once the O2 sensor is hot enough to send an accurate signal, supplemental air injection is shut off and the engine operates in "closed loop" to maintain an average stoichiometric mixture for minimum engine-out emissions and maximum effectiveness of the three-way catalyst.

Modern Corvettes have electrically powered air pumps to provide warm up oxygen. This starts the oxidation reaction as early as possible and also helps get the cat bed up to temperature as quickly as possible. The bed has to be about 300 deg C to achieve near maximum potential reactions. During the 45 minute emission certification test (which includes some engine off/heat soak time) about 80 percent of total emissions are generated during the first couple of minutes of operation.

Three-way cats/O2 sensors to maintain stoichiometry is the technology that allowed high output with good fuel economy while supporting ever tightening emission standards.

Bosch developed the zirconimum dioxide O2 sensor technology, which enabled this, and IIRC the first O2 sensor car in the US was the 1979 Saab. Bosch called it The Lamda System because in Germany the Greek letter lamda is used by engineers to denote the "equivalence ratio", which is the ratio of the actual fuel air ratio to stoichiometric. Lamda equals one is stoichiometric, greater than one, rich and less than one, lean.

In the US engineers use the Greek letter phi to represent equivalence ratio.

In a prior life, early in the C3 era, I was an emission research engineer.

Duke

[Paul L.]

Re: No AIR on 1974 L-48 TH400 (non-California)

Duke,

Wow! Thank you. I "almost" understand. But there is no cat con on a 1974. It was the last year of "true" dual exhaust.

[Michael W.]

Re: No AIR on 1974 L-48 TH400 (non-California)

Thanks for posting that info Duke, I sure learned something.

[Paul L.]

Re: No AIR on 1974 L-48 TH400 (non-California)

Thanks for posting that info Duke, I sure learned something.

Yes, but he did not answer the question for a non-cat-con car. Or was that the hint in the 2nd paragraph.

[Paul L.]

Re: No AIR on 1974 L-48 TH400 (non-California)

Paul------


That's correct. For 1974, L-48 with THM-400 and federal emissions was the only Corvette application that did not come equipped with AIR.

The vagaries and intricacies of emissions requirements compliance would be pretty much impossible to speculate on.

Thanks Joe,

I have a report from GM Canada that hints at that.

[Duke W.]

Re: No AIR on 1974 L-48 TH400 (non-California)

The same argument applies to both catalyst and non-cataylist.

You may recall that 1974 was probably the worst year for driveablility, and it was also the one and only year for the infamous seat-belt ignition interlock, which caused such an outrage that Congress dropped the requirement for a light/buzzer for 1975.

In order to meet '74 emission standards fuel calibrations were very lean including starting and warmup, and most '74 models had particularly bad cold driveoff characteristics. Stalling was not uncommon, something that would never be acceptable, today.

Also, spark advance maps were severely retarded in order to heat up the exhaust to enable sufficient oxidation reaction without a cat.

Dave McClellan, in his book, spoke of how close they came to the wire of not even having a product to sell, due to failure to meet standards in the seventies. I'm not sure how they did it, but they were able to get away with no air pump on the '74 49-state spec L-48 auto. That's actually pretty amazing.

Emission standards were considerably tightened in 1975. They were proposed about 1968. Everyone in the industry thought that a catalyst would be required, and there was considerable concern that there was not enough platinum and palladium in the world to build 10 million catalysts per year, but somehow supply caught up with demand.

Basic catalytic converter technology was around as they were used on propane powered forklifts and other gasoline powered equipment that are used inside plants, but only a relative handful relative to the number required for nearly all 1975 cars. There were only one or two GM engine configurations that did not use catalysts in 1975, and I think one was the base 1-bbl. engine used in the Vega, but the optional 2-bbl. engine with a more aggressive camshaft had a cat.

Catalysts allowed richer fuel calibration for better driveability, and more optimized spark advance maps for better fuel economy, but in the case of Corvette the use of just one cat to save cost really choked down the exhaust increasing back pressure according to McClellan by "half and atmosphere", which is why 1975 represented the low point in rated power.

More development got some power back, but the single cat fundamentally placed a severe limit on installed power.

I don't think dual cats were used until the Beginning of the C4 model, and this allowed steady increases in power as the years progressed.

Duke

[Don G.]

Re: No AIR on 1974 L-48 TH400 (non-California)

Duke

Thank you for your description of the function and develoment of the catalytic converter. In my previous life, I was involved in sourcing materials. The reason that platinum and palladium was available to support converter production is that long term supply contracts resulted in opening new mines in South Africa whose output was 100% dedicated to automotive use.

As an added note, most new automotive products start with low production and as the product gains acceptance, volume increases. In the case of the catalytic converter, the industry went from zero in the 1974 model to 100% in the 1975 model. This was the result of a multi-year effort of product develoment, product testing, production process development and ,ultimately mass production with quality control processes that were state of the art.

[Duke W.]

Re: No AIR on 1974 L-48 TH400 (non-California)

Thanks for the explanation. The mining industry had a good half decade to gear up, but I always wondered if the industry figured out how to get away with less precious metals in the mass produced automotive catalysts relative to the prior low volume devices for industrial uses.

Do you know?

By 1971 I was in the aerospace business and lost visibility into what was going on in Detroit.

Duke

[Paul L.]

Re: No AIR on 1974 L-48 TH400 (non-California)

Thanks for the replies. Very informative. And yes the car does stumble at warm-up and start off. Guess I have to live with that.

Strangely, all 1973s had the AIR pump. But it was dropped for the 1974 with TH-400. And as noted the cat con with AIR came back in 1975.

I will have a hard time time explaining this to the local Smog Police. But we shall see.

[Don G.]

Re: No AIR on 1974 L-48 TH400 (non-California)

Duke
The original converters used platinum and palladium in a 5/2 ratio which was the same ratio as these metals were found in the Merensky reef. I don't recall what the concentration was. As noted, these were oxidizing converters only. Later, rhodium was added. Throughout the development and production cycle, many experiments were run at both higher and lower concentrations to determine performance and cost optimum levels. Along the way the ceramic bead design was replaced by the ceramic monolith design. Also, reclamation processes were studied to reclaim noble metals from salvage yards. These are in place today but I have no idea of how much reclaimed metals find its way back into production.

[Terry M.]

Re: No AIR on 1974 L-48 TH400 (non-California)

Duke

Thank you Duke.

It is one thing to understand the engineering behind the internal combustion engine. It is quite another to be able to effectively communicate it in understandable terms.

[Paul B.]

Re: No AIR on 1974 L-48 TH400 (non-California)

Strangely, all 1973s had the AIR pump. But it was dropped for the 1974 with TH-400. And as noted the cat con with AIR came back in 1975.

I will have a hard time time explaining this to the local Smog Police. But we shall see.

Paul, all you really need is the underhood emission label and it clearly tells what came on the car. Got my '74 down from it's 18 month perch up on the BackYard Buddy finally. Will get a couple decent(I hope)TCS pics. Also, there is a page in the 1974 Chevrolet Service Manual that specifies which engines had an A.I.R. pump and which didn't. It shows specifically that a 1974 Federal Emission L-48 automatic Corvette did not have an A.I.R. pump. Need to have a copy of that for your smog ****s. Thought you lived in Canada, not Kalifornication.