I'm reading the service manual to try to understand the emission controls on my '75, and it sounds like most of it is kind of useless and is sapping a lot of power. Now that my car is exempt, I want to get rid of useless stuff without making the air too dirty, so let's see if I understand this enough:

EGR: allows exhaust to enter intake, thereby displacing air into combustion chamber and lowering operating temperature -> less nox emissions.

AIR pump: supposed to pump air into exhaust to burn unused fuel vapors, but what I don't understand is it looks like it's only hooked up to one side.

Catlytic converter: By means of a catylst, converts harmful nox and co into less harmful nox and co2.

ASM valve: not a clue

If you guys know how effective this stuff really is and at what cost to performance, please comment. I intend to go to dual exhaust and splice in a pair of high flow cats, but I don't want to plug them up with junk if I remove other stuff from the car.

 

I am a bit OT here but think this is a point worth making. Your car is exempt from testing for the purpose of getting tags, i.e., you do not have to pass an emission test to renew the tags. It is not exempt from the emission standards. Those are federal and the state does not have the authority to exempt you from them. That being said, the Feds let the states control how they enforce the standards which allows for the states to give an age exemption if they so desire. Would you be tested in the future? Who knows? Was a thread here recently about roadside sniffers. If they detect what they sense to be too much pollution the snap a pic of your license plate and you get a notice in the mail to come in for emission testing. There is also the "roadblock" testing where the authorities test every car. Also you could be reported as a gross polluter and have to go in for a test. In Texas there is a law that you cannot sell a car unless it has all of its emission equipment in place and in working order. So be sure to weigh the pros and cons before you make major mods to the emission system.

tom...

 

Tom is 100% correct. But that didn't stop me from doing a few mods to get some HP back.

Just depends on if you want to keep the car stock or not and how much risk you are willing to take.

 

While I understand the point that I'm not exempt from the law, I want to understand what they did to make my car fit the standard. It seems most of the stuff they put on is supposed to choke the engine rather than resolve the true emissions problems. On the last test my car ran cleaner than most new cars, but that comes with a big hit in power output. Ideally, I want my car to run clean, but also perform well, so the hard part is finding a balance. I will be keeping some sort of upgraded(modern) cat system, but stuff like egr seems like a poor way to fix things, unless I'm not understanding it right. I'm hoping someone will jump in and clarify the purpose and efficiency of these emission control systems and whether or not there is a better alternative.

 

My 76 does not have an AIR pump so I cant be of much help with that, I have never seen an AIR pump hooked up to only one of the exhaust manifolds, also why does your 75 have an AIR pump and my 76 does not I wonder....
I do know that the EGR value if it is not working (stuck closed) you run the risk of detonation and you might have to back down (retard) the timing to stop it.

 

Keep in mind carb, distributor and engine is calibrated for those emission systems installed on your L48 THM 400 and in this configuration your performance loss is minimal. Obviously these systems aren't useless since the car was certified as emission compliant but not unlike many C3 owners, the itch to remove these devices persists and someone else down the road asks "who did all this?"

Your 75 came with only the passenger side hooked up to thei AIR pump which assists the complete combustion of the air/fuel mixture thus by GMs words, improves performance. In fact, many of these systems are intended to burn fuel efficiently, minimizing unburned fuel that gets emitted into the environment. So, without major engine changes, your performance gains will be minimal by removing these devices and you'll have to recalibrate to regain losses of efficiency.

 

The EGR system operates only under part throttle/high vacuum conditions like cruise, and does work to lower combustion temperatures to reduce NOx emmissions. Many cars experience spark knock at cruise when this system is disabled. Horsepower loss under WOT is zero, with the possible exception of the heat radiated to the intake manifold by having exhaust present.

The AIR system lowers emissions by causing an "afterburn" in the exhaust to reduce unburned hydrocarbons. The engineers discovered that they could pump sufficient air in through one side of the exhaust to convince the catalytic convertor that the engine was "lean", and were able to save some money by eliminating two inlets into the exhaust. Horsepower loss is equal to about 1 or 2, depending on pump effieciency.

Catalytic Convertor - You hit this one right on the nose. Horsepower loss can be significant, since this is a restriction in the exhaust. However, late model high-flow units do a pretty good job of reducing this loss up to a certain point.

ASM - This one doesn't ring a bell, my memory is slipping. Anyone?

All that having been said, my AIR pump and system is boxed up in storage along with my EGR valve. And my '74 was the last year without cats.


Steve