Back in my hot rod days, we studied the small block Chevy with a religious
fervor. For the street, the hot ticket was something called pocket porting
or short porting. The idea was that, in the RPM ranges seen by typical
performance street engines, you could get 90% of the benifit of a full race
porting job at 30% or less of the price by cleaning up the airflow around
the valve head. This involves working on the "short" wall where the tight
radius is, working around the valve stem guide, and working in the pocket
directly behind the valve. I would think that the MG head would respond
favorably to the same type of treatment. For a street engine smooth
transitions and gentle radii are much more important than increasing cross
section area.
Polishing ports is a subject of great debate. It is generally agreed (2
out of three people who own a die grinder agree) that polishing the intake
port to a fine finish is a bad idea because the turbulence caused by the
rough walls helps mix the fuel with the air. The exhaust, on the other
hand, can be polished within an inch of its life although the cost vs
benefit of this is questionable for a street engine.
Does anyone have any personal experience with the Extrude Hone process on
MG heads? I have read some articles that make this sound like a really
good idea. I have never met anyone who has done it but it does sound
interesting.
One area of an engine that it does make sense to polish is the combustion
chamber. Careful polishing and removal of sharp edges will reduce an
engine's tendancy to ping. This allows higher CR and better timing with
available gasoline. I believe that other benefits are claimed but I can't
remember what they were.
Someone mentioned that their engine idled smoother and faster when the
timing was advanced. Most engines idle more retarded than optimal. This
is to allow slower idle for a given air flow. It also, supposedly, makes
the idle smoother since it reduces the intensity of the pressure pulses and
smooths fuel delivery. A typical carburated engine will sound less labored
at idle if the timing is advanced but, for a given rpm, it should not run
any smoother. It may be interesting to first advance the timing until the
engine idles smoother and note the engine speed. I would bet that if you
retarded the timing to the original setting and increase idle speed to the
advanced level by adjusting the throttle opening, the engine would run just
as smooth or smoother. The siamesed intake ports on MG engines are not
great for even airflow at low speed and very low idle speeds can cause them
to get rough, in my experience. Another possibility is a rich mixture at
idle which the advanced timing can more effectively ignite (higher pressure
at the moment of ignition results in easier ignition). Does it idle
smoother before warmup? This could indicat a rich idle mixture. In fact,
very low idle speed makes any little problem with the carbs more obvious.
I let the A idle at 600-700 rpm. This is too slow really but I think it
sound neat. I guess this is a carryover from my thumper / Harley days
where I like to hear each cylinder hit. It does idle a lot smoother at
900-1000 rpm though and it also hold more oil pressure at that speed. I am
also running a little rich but this helps keep the engine cool. Life is a
compromise.
Regards,
Bill Eastman
61 MGA
Bob Allen certified Motorized Fossil ;-) (and you know what? For some
reason this certification makes me feel rather smug)
|