So last night I finally found the time to start cleaning up
the head that I took off The Green Car (my '71 MGB) a week
or so ago with Chris Kantarjiev's help. Basically, what I
found was a tremendously filthy head, with some carbon gunk
in the pushrod holes adjacent to the #3 cylinder (where the
head gasket had failed).
Lots of soaking and brushing and brushing and soaking in
the parts washer finally got me down to something like bare
cast iron in a couple places. So I did something that I'd
been going over with my conscience.
I stuck a grinder into the Dremel and attacked the ports.
The castings on the intake ports are very rough in the stock
head. The B series engine has a round entry to a port that
turns square about 3/4" in; about 3/8" in from the gasket
surface it's been machined smooth, but after that it's bare
cast iron with a rough finish. So I decided to be gentle
and try not to screw anything up.
I just concentrated on smoothing out the high spots rather
than on changing the shape; I know enough to know that I don't
know what shape would work best. So I got the first inch
or so of the port walls to look smooth and shiny. (Actually,
I lied a little. I do know that on the A head, the floors
of the port have next to no effect on gas flow, so I spent
most of my time and energy getting the roofs smooth, and
since I had a clear indication of where the gasket had been,
I didn't mind opening the ports up a teeny bit -- less than
a sixteenth of an inch -- in that direction.)
Since then I've refreshed my memory by referring to Vizard's
book and to the Abingdon Special Tuning guide. ST says that
you're supposed to do just what I've set out to do: get rid
of the bumps and make it smooth, not trying for a mirror
finish but making it ground fairly evenly. They claim a 3 bhp
increase from this, called Stage 1, but it's the key to all
subsequent stages.
So tomorrow I'll be getting a valve spring compressor and
taking out the valves so that I can clean up the inside of
the port. It turns out that the part I've worked on already
is the part that flows best in factory configuration; the
most restrictive part of the port (at least on A Series
engines) is the part just above the valve seat, followed
by the section where the port makes the 90-degree bend to
shoot down the port. For that I might just use sandpaper,
or a coarse emery mop. Anybody done this to a cast iron head?
My next step will be to have the head just resurfaced by my
machine shop, something that is a good idea to do any time
the head comes off. It ensures a good smooth surface for the
gasket. I might have them take off 1/32", which should raise
the compression to about 9.2 or 9.3:1 from its stock 8.8:1
ratio. (1/16" raises it to 9.6:1, which ought to be livable
with 92-octane gas but I'm not doing enough to the rest of
the engine to warrant that high a bump. And I can always take
off the other 32nd when I do the bottom end in a year or two.)
I don't know whether I'll really be able to feel the difference;
at 4% per whole point of CR, I'll add about 2 bhp from the bump
in compression and about 3 bhp from the porting, and I'm not sure
I can tell a 5 bhp difference. (Of course, it should just run
better with a complete head gasket, too...) But it'll be nice
knowing that it's my own work in the motor.
But anyway, I guess I've started down the road to hot-rod mania.
What's next, putting in a blown Chevy engine? :-)
--Scott
|