EGR was designed to lower the 3rd emission: Oxides of Nitrogen or NOx. 1st and 2nd are CO and HC.
http://www.epa.qld.gov.au/environme...itrogen_oxides/
Oxides of Nitrogen are formed when the combustion temperature exceeds 2500F. By adding a % of inert material, in this case spent exhaust, the combustion temps drop.
This is a benefit to air cooled engines: It keeps the head temps lower.
And I love what people say about emission control equipment, does not matter if it is an EGR valve or a catalytic converter: "just take it off and throw it away".
Wrong! Even if you live in a non-smog check state or county, you are still violating the Federal Clean Air Act in the USA.
Here is Section 27156 of the California Vehicle Code:
27156. (a) No person shall operate or leave standing upon any
highway any motor vehicle which is a gross polluter, as defined in
Section 39032.5 of the Health and Safety Code.
(b) No person shall operate or leave standing upon any highway any
motor vehicle which is required to be equipped with a motor vehicle
pollution control device under Part 5 (commencing with Section 43000)
of Division 26 of the Health and Safety Code or any other certified
motor vehicle pollution control device required by any other state
law or any rule or regulation adopted pursuant to that law, or
required to be equipped with a motor vehicle pollution control device
pursuant to the National Emission Standards Act (42 U.S.C. Secs.
1857f-1 to 1857f-7, inclusive) and the standards and regulations
adopted pursuant to that federal act, unless the motor vehicle is
equipped with the required motor vehicle pollution control device
which is correctly installed and in operating condition. No person
shall disconnect, modify, or alter any such required device.
(c) No person shall install, sell, offer for sale, or advertise
any device, apparatus, or mechanism intended for use with, or as a
part of, any required motor vehicle pollution control device or
system which alters or modifies the original design or performance of
any such motor vehicle pollution control device or system.