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Colin,

The path is battery +>Master Relay>Starter Relay>thru Starter Selenoid>
Starter>engine case>ground wire>battery-.  Any resistance of just a few ohms will cause a large voltage drop with a high current draw (I forget the formula).  You must check all of these connections for tightness, corrosion, contact erosion, etc.  This is addition to the loss from the wire.  Furthermore, during the start you also have current requirements for the master relay, the starter relay, the starter selenoid, gear down lights, electric turn coordinator, alternator field, engine instrumentation, etc.  If the master switch has resistance, each low voltage start can cause the weak pull of the master relay to bounce the contacts (arcing) introducing more resistance which can cause a similar effect on the starter relay.  I have seen this happen. A bad master switch along with a corroded ground on a Skymaster, and a loose master relay-to-battery connection on my Lancair.  B&C sells an excellent starter relay.  I also use their starter motor because it still has turning strength at lower voltages.  And, like you, I use #4 wire with the battery located behind the co-pilot seat.

Scott Krueger
N92EX

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