Explain EGR please.
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From: Glen Allen, VA
Explain EGR please.
OK I understand what the function the egr has and it's advantages and disadvantages of it, but where exactly does it come from? What I'm asking is where does the actual EGR gasses come from and how is it routed back into the engine to be burned again? What is the purpose of the valve? What does the EGR valve actually open and close?
Joined: Mar 2000
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From: Littleton, CO USA
Car: 82 Berlinetta/57 Bel Air
Engine: L92/LQ4 (both w/4" stroke)
Transmission: 4L80E/4L80E
Axle/Gears: 12B-3.73/9"-3.89
Each head has a passage in one exhaust port that goes over to the intake side in the middle of the head. That was originally put there to mate with a passage cast into the intake manifold that routed exhaust gases under the intake plenum to warm it up a little and keep gas from puddling in the intake manifold when the engine was cold. This is known as the cross-over passage. To ensure the flow of exhaust through this passage in the intake, there was a valve on the outlet of one exhaust manifold that would close and force exhaust through this passage.
When the emissions-reducing benefits of mixing a little exhaust into the intake charge under certain engine load conditions was identified, it was convient to tap this cross-over passage since it was already there. All the EGR valve does is open a hole between the exhaust cross over passage and the intake plenum to mix in a little exhaust and cool down the combustion temperature, thus reducing the generation of nitrogen oxides compounds.
In later years, the cross over ports have been eliminated in some factory SBC heads, such as Vette L98 aluminum and Vortec heads. Aftermarket performance castings have eliminated them for years because of the flow disruption they cause in those two ports (one in each head).
When the emissions-reducing benefits of mixing a little exhaust into the intake charge under certain engine load conditions was identified, it was convient to tap this cross-over passage since it was already there. All the EGR valve does is open a hole between the exhaust cross over passage and the intake plenum to mix in a little exhaust and cool down the combustion temperature, thus reducing the generation of nitrogen oxides compounds.
In later years, the cross over ports have been eliminated in some factory SBC heads, such as Vette L98 aluminum and Vortec heads. Aftermarket performance castings have eliminated them for years because of the flow disruption they cause in those two ports (one in each head).
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From: Glen Allen, VA
So what effect does putting a block off plate for the EGR have? Does it leave that hole that the valve once controlled open all the time? Thank you for all the info. It explains a lot.
Joined: Mar 2000
Posts: 43,187
Likes: 42
From: Littleton, CO USA
Car: 82 Berlinetta/57 Bel Air
Engine: L92/LQ4 (both w/4" stroke)
Transmission: 4L80E/4L80E
Axle/Gears: 12B-3.73/9"-3.89
With the valve off, there will be a hole to the exhaust passage and a hole to the intake passage open at the mounting flange. Putting a plate on there with a gasket will seal the two off from each other.
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From: Central NJ, USA
Car: 1986 Firebird
Engine: 2.8 V6
Transmission: 700R4
EGR isn't really a power robbing emissions feature; it should only kick on under low-rpm, high speed situations... the same situation you'd be in if you were cruising down a highway at 65 on a hot summer day with nowhere to go. It doesn't come on under WOT when you're flying down the racetrack, or taking off from a stoplight.
It also lowers the NOx count on an emissions test; NOx is created from high-pressure high-temperature situations (think combustion!). When the computer kicks the EGR on, it allows exhaust gas back into the cylinder, and gives the cylinder a cooling effect, which lowers NOx's, and makes your car pass the sniffer test -- uh, I mean, it helps keep the planet green and makes mother nature happy and all that tree hugging stuff.
But then again, EGR only activates under low-rpm, high speed situations, so if your emissions test isn't done on a chassis dyno (like in NJ), but is done while the car is standing still, EGR won't activate.
It also lowers the NOx count on an emissions test; NOx is created from high-pressure high-temperature situations (think combustion!). When the computer kicks the EGR on, it allows exhaust gas back into the cylinder, and gives the cylinder a cooling effect, which lowers NOx's, and makes your car pass the sniffer test -- uh, I mean, it helps keep the planet green and makes mother nature happy and all that tree hugging stuff.
But then again, EGR only activates under low-rpm, high speed situations, so if your emissions test isn't done on a chassis dyno (like in NJ), but is done while the car is standing still, EGR won't activate. 
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From: USA
Car: yy wife, crazy.
Engine: 350, Vortecs, 650DP
Transmission: TH-350
Axle/Gears: 8.5", 3.42
Not that these guys are wrong... 
But here's something I found that explains it quite well.
But here's something I found that explains it quite well.
The purpose of the exhaust gas recirculation process was to simply put a small, carefully metered amount of exhaust gas back into the combustion chamber. Exhaust gas has already spent its fuel, so to speak, and is considered an inert gas and will not support combustion. This precise amount of recirculated inert gas slows the burning process in the combustion chamber where NOx is produced. Slowing the burning process lowers the temperature during combustion, thereby reducing or eliminating nitrogen oxide production. This action is most needed during cruise conditions and not at idle or heavy throttle conditions. Some may still incorrectly believe that the EGR is supposed to recycle the exhaust gas - burning it twice must be enough to reduce emissions, right? Wrong.
We just said the exhaust gas won't support combustion - it's already been burned. So, the main purpose? Reduce NOx. The side benefits? Better fuel economy for one, and maybe performance as well. Consider some of the advanced strategies for EGR flow control on some of the later engine applications. The inert exhaust gas actually displaces a small amount of space in the combustion chamber, which in turn reduces a small amount of volume that the air/fuel charge occupies. This can have an effect on fuel economy, albeit slightly, if the EGR valve is malfunctioning.
We just said the exhaust gas won't support combustion - it's already been burned. So, the main purpose? Reduce NOx. The side benefits? Better fuel economy for one, and maybe performance as well. Consider some of the advanced strategies for EGR flow control on some of the later engine applications. The inert exhaust gas actually displaces a small amount of space in the combustion chamber, which in turn reduces a small amount of volume that the air/fuel charge occupies. This can have an effect on fuel economy, albeit slightly, if the EGR valve is malfunctioning.
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From: Glen Allen, VA
I understand fully the function the EGR plays and it's advantages and disadvantages, but I didn't completely understand how it mechanically accomplished this. Basically five7kid explained everything I wanted to know. Thank you very much.
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