Larry Unger wrote:
<<... the increase in power from 1588 to 1622 is out of proportion
to the increase in capacity ... why? ... IMHO, because the 1622 was
fitted with flat top pistons (increasing the CR from 8.3:1 to 8.9:1) and
the
head with larger valves and improved porting. IOW, the 1489/1588
engines...>>
Larry, as you can see:
HP Liter HP/Liter % change hp/liter
68 1.489 45.7
80 1.588 50.4 10.3
93 1.622 57.3 13.7
94 1.789 52.5 -8.4 (yuck!)
HP / liter has increased both times although, as you say, the 1622 had a
greater increase than the 1588. I believe that the same cam was used for
all MGA's as well as most early MGB's so something else must be at work.
In general, I believe that compression ration is overemphasized in
horsepower changes. A change from 8.3 to 8.9 would not make a lot of
difference. I would say that the increased engine efficiency is could
almost all be attributed to better breathing. Other books that I have read
mention valve shrouding as the main limitation of B series engines
although, I would expect that sooner or later port geometry limitations
would take over. As Kirk mentioned, this is probably why the MGB engine
doesn't make much more horsepower than the 1622.
You always hear how crossflow heads make more power. In actuality, the
engine doesn't care a bit which way the ports are facing. Putting the
ports on opposite sides does allow more room for each individual port,
however so that would allow better port design. Another thing that may
have helped was to run the cam and pushrods on the opposite side of the
engine but, for reasons unknown to me, MOWOG chose not to do that.
Horsepower is only part of the performane equation. I expect that the 18
series engine produces significantly more low end torque that either A
engine and I am also sure that it runs a lighter flywheel. Low end torque
is what make a car feel responsive in day to day driving and a lighter
flywheel will also make the car feel more responsive. When street engine
builders talk, they talk about increasing the "area under the torque curve"
as the ultimate goal of performance modification. This means increasing
the torque output of the engine over a wide operating range rather than
going for peak horsepower. This usually means using a fairly mild camshaft
and improving breathing by other means. If I remember correctly, B series
modification books don't reccomend a camshaft change until well down the
list of performance modifications.
Regards,
Bill Eastman
61 MGA with a flywheel off a Mack truck
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