Subject: Re: [6pack] Distributor Retard

From: Ed McGuirk <[email-masked]>
Date: Sun, 12 Jan 2014 23:15:05 -0500

On 1/12/2014 2:00 PM, [email-masked] wrote:
> This is not a racing question.  I'm sure the ignition retard is often
> disabled on 6 cylinder Triumphs.  Does anyone know why it was used in
> the first place?  I have always thought it was strictly for emissions,
> which became more restrictive in 1968.  Perhaps it improves combustion
> at low throttle such as when decelerating or when descending a hill.
> Just curious.
> - Larry
There are several types of vacuum that could in general be applied to a 
distributor.

Some american cars use something I believe is called port vacuum which 
has zero vacuum at idle and then more vacuum as the throttle opens. I 
believe something similar to this style of vacuum was used for vacuum 
retard on later TR6s. I believe the purpose of this was to lower 
combustion pressure and temperature to lower NOX. Later TR6s also had 
some kind of temperature switch which changed which port was supplied to 
the distributor. Unfortunately I do not understand the purpose of that 
switch but I suspect it allowed a little less retard on cold engines 
since cold engines need more power and also do not produce as much NOX 
anyway.

The manifold vacuum which should be high at idle and go to zero at WOT 
is what I believe that was used on early TR6s for vacuum advance. 
Traditional vacuum advance like this gives you no advance at WOT but as 
you close the throttle for cruise, it advances the ignition for more 
power and MPG. I don't think it has a large effect on pollutants but it 
does help MPG and it is a shame that it was dropped from the later TR6.

I think the middle years of the TR6 had both vacuum advance and retard.

This is all just my recollection off the top of my head, please correct 
me if I am wrong.

ed

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