<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<HTML><HEAD>
<META content="text/html; charset=utf-8" http-equiv=Content-Type>
<META name=GENERATOR content="MSHTML 8.00.6001.23588">
<STYLE></STYLE>
</HEAD>
<BODY bgColor=#ffffff>
<DIV><FONT size=2>"</FONT><FONT size=3>until you notice preignition" Is a
bit drastic. Haynes says to drive at full throttle in 4th from 30 to 50
and advance until faint pinking can be heard. However I've noticed that I
can get pinking at part throttle and opening the throttle further stops it,
either because the bigger charge cools the combustion chamber more, or the
vacuum advance backs off. So I use the criteria 'at any combination of
throttle, revs and load' and keep it just short of that. </FONT></DIV>
<DIV><FONT size=3></FONT> </DIV>
<DIV><FONT size=3>The reason Haynes says that you can benefit from advancing it
over book values is that with the level of engineering used in our engines some
will be more prone to pinking than others,and the book caters for the worst
case, and maybe with a bit of a safety factor. That was definitely the
case with an A-series in the 70s, which as a new car and dealer serviced always
came back running like a dog (comparatively) as they always put it back to
book. But on my 73 MGB I couldn't run more than book, even on leaded, and
have to retard slightly from that on unleaded even 99 octane.</FONT></DIV>
<DIV><FONT size=3></FONT> </DIV>
<DIV><FONT size=3>Whether it pinks or not also depends on terrain. Living
in a relatively flat part of the world I don't get pinking, but when touring in
Wales and other mountainous areas it can. The 25D4 distributor is a great
benefit here as it allows one to retard the timing a degree or two very easily,
also if one can't get higher octane fuel in rural areas.</FONT></DIV>
<DIV> </DIV>
<DIV><FONT size=2>Completely agree with your reasoning behind centrifugal and
vacuum advance, but:</FONT></DIV>
<DIV><FONT size=2></FONT> </DIV>
<DIV><FONT size=2>"</FONT><FONT size=3>The other is pressure operated, offering
advance when the twin turbos raise the plenum pressure above
ambient."</FONT></DIV>
<DIV><FONT size=2></FONT> </DIV>
<DIV><FONT size=2>Are you sure that isn't vacuum retard? If the turbos
were increasing the pressure I'd expect that to make spontaneous combustion more
likely, needing retarding to control.</FONT></DIV>
<DIV><FONT size=2></FONT> </DIV>
<DIV><FONT size=2>PaulH.</FONT></DIV>
<BLOCKQUOTE
style="BORDER-LEFT: #000000 2px solid; PADDING-LEFT: 5px; PADDING-RIGHT: 0px; MARGIN-LEFT: 5px; MARGIN-RIGHT: 0px">
<DIV style="FONT: 10pt arial">----- Original Message ----- </DIV>
<DIV
style="FONT: 10pt arial; BACKGROUND: #e4e4e4; font-color: black"> I
write to open a discussion about ignition timing. Over on the Triumph forum (
Apologies for using profanity. ) a member wrote, "Just advance the timing
until you notice preignition." Now I recognize that Triumphs use farm
implement engines, but that advice seems a bit sketchy. So let me write a
preface then ask a question.</DIV>
<DIV dir=auto><FONT size=2></FONT> </DIV></BLOCKQUOTE></BODY></HTML>