At 08:44 AM 6/13/97 -0400, you wrote:
> Is this "polishing and relieving" (or whatever the expression is)
> something that Morgan owners have done to their cast intake manifolds?
> I know about tuned exhaust headers (I guess) but don't know what the
> hot set up on the other side is.
I'm no expert on this but its my understanding that the polishing of intake
ports and headers is done to improve the efficiency of mixing and the rate
of flow of air and gas into the cylinder. The logic is the same as for
enlarged intake valves, super- and turbo-chargers. The more air/gas mix
you can stuff into the combustion chamber in the fraction of a second you
have to do it, the more pow you get on the power stroke when the air/gas
ignites. The notion is that the rough cast surfaces of the intake header
and the intake port on the head cause turbulence that slows down air/gas
flow into the engine resulting in less gas available for burn. This sort
of polishing should be done by experienced speed shops. I gather you can
polish yourself into cooling ports and cracked heads if you're not careful.
Tuning exhaust headers is the same, only different. Here the desire is to
smooth the flow of exhaust gasses away from the engine (i.e. improving the
efficiency of the exhaust stroke and the removal of burnt gas and CO2
(which if left around inhibit the burn of the up and coming power stroke).
Apart from simply enlarging the diameter of the exhaust pipe, tuning
exhaust headers involves making sure that the distance traveled by exhaust
gasses from each exhaust port to the muffler is the same, minimizing a
'backup' of exhaust gasses that have to be pushed into too small a header
pipe in too short of a period of time. My experience in this regard is
limited to the MGA/MGB engine. There are three exaust ports on that
engine. One for #1, one for #2 and #3 and one for #4. The trick is to
make sure that the center port flows as nicely as the two outer ones, even
tho its "servicing" two cylinders while the outer ones are servicing only
one.
This brings up the crossflow head, another effort at smooth and efficient
gas flow.
My sense is that these thing do improve engine performance but, as the
saying goes, really only ought to be done (i.e. you really only benefit)
when in conjucntion with many other engine mods, like a racier cam, roller
rockers, higher compression pistons, higher octane, etc. If the fellows
are blowing by in their +4s its probably more than just intake/exhaust
manifold polishing/tuning (i.e. its $$). Each contributes a very small
percentage, that alone is imperceptible. Together they add up to a
siginficant boost in horse power.
Apart from the "no substitute for cubic inches" rule, and the use of nitro
glycerin or other bizarre fuels (which eat engines), the best things to do
to improve power in an engine are to improve breating (both the inhale and
the exhale). With this in mind, a waggish car-guy friend of mine once
remarked that automobile motors are really nothing but highly complicated
and rather inefficient air pumps. I think he was drinking "Old Crow" that
evening.
Will "Make mine a George Dickel" Zehring
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