Chilton's 1984-1995 Civic/CRX/[blah blah] manual, Chapter 3, under "Engine
Mechanical, Engine Design":

"On the 1984-88 models, Honda decided to improve engine breathing by
replacing the single large main intake valve with a pair of smaller ones.
This allows a much greater total intake valve area than a single valve and
it also permits intake valve timing to be staggered slightly. This gave
Honda engineers a unique opportunity to design air swirl into the combustion
process. Such swirl (turbulence in the combustion chamber) not only tends to
reduce engine knock but improves combustion speed and therefore engine
efficiency, especially at low speeds. These engines retain the auxiliary
intake valve on the exhaust side of the head.

All of the 1988 and later Honda engines have substituted fuel injection for
carburetion, and the CVCC system is no more. In the interest of good fuel
efficiency, increased power and low emissions, the two intake valves were
retained."

This text is also available free at www.autozone.com .

"Steve" <estevew@hctc.com> wrote
> I'm new to these engines and don't understand the full function of the 4th
> valve in this head.
>
> Initially I thought it was an Aux. intake valve but notice that it is next
> to the exhaust valve.
>
> My Chilton manual doesn't provide any help..
>
> May seem like a dumb question but it's got me stumped. Once I have an
> understanding of this question, I'm sure I'm going to have a few more
before
> I get this nice little car ('85 CRX) running properly.
>
> Thanks,
>
> Steve
>
>