[Gmecm] TBI Swap - Iron Duke Tech IV

[Gary]

Bruce, if I understand you correctly, I agree, as this would be 
similar to what was done to distributors prior to any form of 
electronic ignition.  IOW, to get proper rotor phasing with just
mechanical advance, it was necessary to lock the breaker plate
in a full vacuum advance position after rendering the vacuum
advance inoperative, then reset the timing.  Most did not do
this procedure when operating with just the mechanical advance
and didn't realize the gap the voltage needed to bridge to reach
terminal in the cap.  Did I understand you correctly?

Gary

----- Original Message ----- 
From: <bcroe at juno.com>
To: <gmecm at diy-efi.org>
Sent: Monday, February 20, 2006 5:33 PM
Subject: [Gmecm] TBI Swap - Iron Duke Tech IV


> Just to see what's possible, I did some measurements 
> on a V8 HEI, mechanical advance.  I believe these would 
> apply also to a 4 cylinder/electronic advance.  
> 
> The rotor overlaps the plug wire post by about 19 degrees 
> of rotation.  That's 38 degrees at the crank.  So if the 
> maximum crank advance minus the minimum crank 
> does not exceed 38 degrees, its possible (by rotor 
> phasing) to have the rotor overlap the post for any 
> advance.  The typical way to achieve the desired 
> phasing if needed, is to move the pickup coil angle.  
> Then of course the HEI has to be rotated the other 
> way to get timing back on the mark.  
> 
> The posts are pretty far apart on a 4 cylinder.  It would 
> also be possible to widen the rotor tip or the posts to 
> increase total overlap angle.  Phasing is less of a 
> problem with a mechanical HEI, because the rpm 
> advance moves the rotor without changing the phase; 
> only the vacuum changes it.  Bruce Roe