EGR or not


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Sep 14, 2006
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Help! My son recently bought a 1974 Duster with a 318 and three speed. He decided to replace the two barrel with and Edelbrock Performer and small four barrel. When he went to order the manifold he asked me if he needed an EGR or Non-EGR part. I assumed that the car was non-egr as there is no air pump. Upon closer inspection I noticed that there are vacuum lines that run from a fitting on the radiator to a valve of some sort on the stock manifold. Does anyone know how all this works and can I eliminate it. Also what is the difference between an Edelbrock EGR vs non EGR manifold. Any of you California guys out there who have run across this and can help me out. (Or anybody else). Thanks a bunch.
Well first things first. The absence of an air pump has nothing to do with an EGR valve. The air pump pumps fresh oxygen into the upstream exhaust to help burn unburnt fuel when the engine is cold. It pumps air into the catalytic converter after warm-up to add oxygen to the catalyst effect. It turns CO into CO2 ammung other things. This is post combustion emissions control and has nothing to do with the EGR valve.

The thing in the radiator is a thermal vacuum valve. It opens and closes a vacuum port depending on coolant temperature. These were used to run a number of emissions components on the engine including the EGR, just not directly.

These TVV's were also used to operate the temperature door in the air filter canister and the exhaust manifold valve (you know, the one in the passenger manifold that closes when cold to force hot exhaust gases across the intake crossover), although use of the TVV for exhaust crossover was mostly a GM thing. It pulled the valve open as the temperature of the engine warmed up.

The TVV was used to control the vacuum signal to the EGR valve solenoid. It only allowed a vacuum signal after the engine reached operating temperature. This is because the EGR valve is of no use with a cold engine.

One of the vacuum lines should run to a solenoid mounted on a bracket near the backside of the intake manifold or it may be mounted on the firewall.

The solenoid is is controlled by a small orange box that looks kinda like a voltage regulator. This electronic controller operated the solenoid through pulse width modulation (varied on/off signal). The greater the pulse width, the farther it opened and the stronger the vacuum signal.

Now for the EGR valve. Stands for Exhaust Gas Recirculation. The valve is responsible for introducing inert (dead air/lacking oxygen) exhaust gas into the intake manifold and then into the combustion chamber. The inert air from the exhaust cannot burn and when introduced to the combustion chamber helps cool the combustion process. Why is this important? Because at approximatly 2500 degrees F, under the extreme pressure of combustion and detonation, NOx is formed. That's Oxides of Nitrogen, not nitrous oxide. NOx is a formation (bonding) of nitrogen and various amounts of oxygen and this NOx gas is poisonous. It makes people sick. It also forms smog.

The EGR valve is only turned on during light to moderate cruise. It's of no use during idle and will cause driveability problems if open at idle. It's also never turned on at WOT. This is because the fuel enrichment circuit of the carburetor cools the incoming air/fuel mixture, and the combustion chamber/piston, and this cooling effect deminishes the hot spots and the tendency for detonation and preignition which lead to the formation of NOx.

The EGR valve does one more thing. Because it's a controlled vacuum leak of inert air into the manifold, it reduces manifold vacuum (raises manifold pressure) and this reduction of vacuum makes it easier for the piston to travel downward during the intake stroke. This reduces what is called pumping losses. The result is an increase in fuel economy because the engine does not have to work as hard.

If the engine has an operationable EGR valve then it would be beneficial to keep it, both in fuel economy and cleaner air for all of use to breath. If the EGR valve is inoperable then finding new parts to restore it's function will be challenging to say the least and the non EGR manifold would be the better choice.

Just keep in mind that the EGR valve has no effect on how fast the engine is at WOT because it's not open at WOT so the argument that it slows you down is bunk. Also keep in mind the egr valve helps prevent detonation that leads to preignition. Preignition resulting from detonation will destroy an engine in a matter of seconds. A functional EGR valve may allow higher compression ratios without the higher octane.

When a car comes into a shop with the complaint of pinging and knocking, a faulty EGR valve is the usual suspect.

Hope this helped.

G. Weaver
CSN Automotive Technology
Thanks for all of the insight on how the EGR system was designed to work. Some of my components are questionable, so at the risk of contributing to air quality emissions, I have elected to eliminate them. I plan on blanking off the tvv port in the radiator, and am hoping that the if I disconnect the wiring to the solenoid that is mounted on the manifold it will not effect performance. I also am trying to determine if the stock distributor on EGR motors is different from those that came off of a non-EGR motor. If so I have a complete 1970 318 that I can take the distributor off of. Thanks again for the help.