Truxtun Southworth wrote:
>
> I Œm about to do a valve job on my 69 B and have a few questions before I
>begin.
> First, what gains should I expect from polishing the valve ports?
Small gains, but it's fun to do. Most of the gains are at the higher
RPM's.
What I mainly do is 'clean up' the
ports, cut out any boogers or casting flash, generally smooth the parts
that are conveniant to get at. Try to keep the modifications equal
for the ports.
The gains are as follows; Intake; the
smoother the flow the better the engine breathes or the easier it sucks
the air, the less turbulence the less the gas will seperate out or
condense on the port walls (especially when cold, although some
theorize that minor turbulence is good to keep the fuel mixed), you
can get more of a 'ram air' effect as the air charge is not slowed
down, so it can keep its momentum better. Exhaust; breathing- like the
intake, the head will be cooler around the exhaust port as there will
be less heat disapation to the head via the rough surface, a smoother
surface has less surface area so less area to collect heat, and once
again you can match the exhaust port size and shape between head and
manifold.
The easist way to match ports is to match both sides to the gasket, but
watch out with the gasket sealer on the intake port, you don't want to
match the ports and then have gobs of goo sticking out into the air
stream.
> If this is a
> good idea, should I polish both the intake and exhaust ports?
Yes.
> I know I¹m not
> suppose to touch the valve seats when I do this but what about the area on the
> manifold side, does this get polished out also?
If possible you should polish completely through, but anything is way
better than the way it came from the factory. I would mainly match the
size and shape of the manifold port to the head port.
Try to make the radius (where it turns down into the valve seat area)
of the in/ex ports as gentle as possible, which
usually means taking more off the top of the ports than the bottom.
While you're at it you should cc the head too, in my opinion it is
worth it for the smoothness, and doesn't take that much investment in
tools.
Save cutting the valve seats untill after your port and polishing, but
there are different theorys as weather to cut the seats before or after
cc'ing the head.
Since the MGB is raced, there should be a book or article with a good
cross section picture of the head, showing where the best benefits are
and where the danger areas are. You don't want to cut too close to a
coolant passage or an area known to be crack sensitive, for example.
As always- be careful, it's better to cut too little than too much.
-Aron Travis-
"always in a automotive frenzy"
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