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Performance- CR vs airflow

To: mgs@Autox.Team.Net
Subject: Performance- CR vs airflow
From: William Eastman <william.eastman@medtronic.com>
Date: Sun, 06 Jul 1997 11:38:31 -0500
Never one to let threads die gracefully, I will add one comment to this.

When you change the compression ratio, you do improve the power by some
amount, but you also increase pumping and frictional losses so CR increases
are self limiting with around 15:1 being the point where pumpins losses
equal power gains.  Having written that, I know some NASCAR fan is going to
say that Dale whats-his-face is running 17.5:1 on the superspeedways but
remember, actual CR is not the same as mechanical CR since you lose some of
the stroke because of valve timing.  Really big cams require a super high
mechanical CR to provide reasonable actual CR. 

Increasing airflow, on the other hand, increases the most important
variable to your engine, cylinder filling.  This also reduces pumping
losses so you get a double whammy- two effects going in the same direction
to increase available power.  

One of the reasons that many Americans consider CR to be so important for
performance is that two independent things happened at the same time.  In
1972(?), compression ratios were dropped to allow the use of unleaded gas
at the same time that the auto industry, added more polution control and,
this is important, changed  from gross to net horsepower.  As a result of
the change of CR, engines lost a little power but the change from gross to
net caused claimed horsepower to drop by as much as 150 hp on larger
engines.  In actuality, they probably lost 30-40 hp actually but the change
of measurment made everyone think that the lost CR was catastrophic.  

So, I doubt if half of the increase in power from the 1588 to the 1622 was
due to the CR change.  It helped but not as much as the increase airflow.

Regards,
Bill Eastman
61 MGA

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