[MisterTea]

> Air pumps were for catalyst efficiency. The old ones needed extra oxygen molecules floating around for the big stuff (hydrocarbons) to oxidize with until the catalyst was up to operating temp and working at peak-ish efficiency.

My experience comes from driving and working on a 1988 GMC 6000 truck with an anemic 350 small block with a Muncie SM465 behind it. There was no catalytic converter, only a muffler. It featured not one, but two air pumps, each feeding a set of pipes that led to metal tubes which entered the exhaust manifold opposite each exhaust port. Another odd thing about that truck was it had a choke lever, something I thought was long gone by 1988, and was a pain to start in the winter.

Perhaps other vehicles had a cat but this truck certainly did not.

[Aurornis]

> My experience comes from driving and working on a 1988 GMC 6000 truck with an anemic 350 small block with a Muncie SM465 behind it. There was no catalytic converter,

Catalytic converters were required on most vehicles starting in 1975 in the US and the requirement was expanded to cover all vehicles in the early 80s.

Your truck was modified by someone.

[mulmen]

I don't think that's right. I believe this is the relevant EPA regulation: https://www.govinfo.gov/content/pkg/CFR-2004-title40-vol17/p.... This mentions emission limits but does not require a catalytic converter for medium duty (14,000lb-26,000lb GVWR) trucks like the GMC 6000.

[Aurornis]

> This mentions emission limits but does not require a catalytic converter

IIRC, technically if the vehicle meets emissions limits without a catalytic converter then it doesn't need one.

The emission limits are set such that a catalytic converter is required to meet them. They don't have to say "catalytic converter required" but the targets are chosen so that a catalytic converter can reasonably achieve them.

If the laws simply said "catalytic converter required" then manufacturers could put a tiny little square of catalyst in the exhaust and call it a day. Formula 1 isn't the only place where rules have to be written explicitly to avoid clever workarounds.

[mulmen]

> Catalytic converters were required

You claimed they were required by law, but they weren’t. Now you claim they had them anyway, but I can find no evidence of that either. Do you have some evidence that 1988 GMC 6000 trucks had a catalytic converter from the factory? I can find nothing online to support that claim.

[cucumber3732842]

350ish (or less) + 4spd trucks kinda fell out of favor over the course of the 70s for bigger engines and 5spds (usually with a 2spd rear end but I digress). I'm sure you could still get one, but who would when you could get something better on the lot for the same money.

Sounds like someone swapped a 70s-80s engine from a lighter application in.

I don't think that truck would've had manual choke from the factory. Lots of stuff could've happened over the years.

The amount of air your engine breathes is monumental compared to what the smog pump moves. The math of dilution just doesn't work. What does work is pissing a light stream of oxygen (remember, not much of that coming out of the engine, especially on warm up while it runs rich) to help the catalyst burn those hydrocarbons off of itself a wee bit faster.

I'm not sure if an 80s gas MDT would've had cats from the factory.

[mulmen]

> I'm sure you could still get one, but who would when you could get something better on the lot for the same money.

It was almost certainly cheaper to get the small block and if that's all you need why spend more to burn more gas? The 1988 GM Medium Duty Truck brochure lists the 5.7L (350CID) V8 and SM-465 as standard equipment. I have no idea what the sales numbers were but it's not far fetched for a 1988 truck to have been configured with a small block and a 4 speed from the factory.

https://xr793.com/wp-content/uploads/2020/02/1988-GMC-Medium...

[MisterTea]

Hey, thanks for finding that brochure. Same one I have.

[mulmen]

Neat! What do you do with it? I spent my summers in college driving C60 grain trucks. They were real workhorses! The 1979 with a 5 speed, split axle, and 427 was the best of the bunch but the 82 with a 366 did great work too. Sometimes I wish I could hop in one and split a few more gears but these days I shower before work.

[MisterTea]

> Sounds like someone swapped a 70s-80s engine from a lighter application in.

Nope, factory. I have the brochure to that truck and clearly remember the gasoline engine options: 350, 366 and 427 (tall deck big blocks) The 350 + 4spd was billed as a low cost drive line option for local work trucks that did not travel long distances. It had a rater tall rear end (tall is slang for deep reduction ratio) something like a 5:1 or better. On a flat road with your foot to the floor it could barley approach 50 MPH.

[alistairSH]

The 6000 was a commercial truck. It appears to have had a carburetor for most if not all of the 1980s.

[MisterTea]

Totally had a carb since it had a choke lever.

[to11mtm]

Medium duties are weird beasts indeed.

Well, FWIW the air pumps still can -help- with unburnt fuel...

It's not as good as a Cat for emissions but it's better than nothing, so they actually started being used before Cats; they just are used different now.

[bluGill]

In 1988 the factory put a cat in the exhaust. It also would have had fuel injection and a computer, and thus no choke. In short this truck was very much not stock (or possibly you are not in the US?) and so it is interesting but not helpful for the discussion.

[MisterTea]

Sigh. You didn't even try thinking and vomited out a bunch of nonsense. See the brochure this poster found, same one I have somewhere at home: https://news.ycombinator.com/item?id=48203115