[SEL] Old style Alamo question (not Blue line)

Rob Skinner rskinner at rustyiron.com
Mon Nov 7 10:34:29 PST 2005


> bottom of the exhaust valve so that the exhaust valve closes right at 
> TDC or just the slightest bit past TDC. That's as it should 
> be. But as I continue to roll it thru the valve doesn't open until fully at
BTC or 
> even slightly PAST BTC on the exhaust cycle.

Hi Curt,
This is nothing to get upset about.  There were plenty of old engines that fail
to meet the criteria of the 21st century top-fueler.  Sometimes the cam just
doesn't have the duration that you're looking for.  

Two things to remember:

1.  Consider a mark on the flywheel that is rotating.  As that mark approaches a
point indicating tdc, the speed of the piston is approaching zero.  So if you're
a few degress early or late, the actual effect is negligable.

2.  Modern engines open the exhaust early to allow the expanding gasses
oportunity to expand out of the cylinder so the engine doesn't have to work as
hard to push them out.  On a slow running engine, the valve can be opened much
later, allowing more use of the expanding gasses without causing the engine to
work excessively to exhaust the gasses.  But the trig involved in Rule 1 also
applies, so as the piston approaches the bottom of its travel, the power derived
from the pressure in the cylinder approaches zero.  

Experiment:
1.  Set the exhaust timing to open as early as you can.  Listen to the exhaust
report.  
2.  Set the exhuast timing to open near bottom.  Listen to the exhuast report.

=-=-=-=-=-=
Rob Skinner
La Habra, California
mailto:rskinner at rustyiron.com
http://www.rustyiron.com 












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