Re: Unleaded Petrol

[Tim Buja <buja@domain.elided>]

Ian Baird wrote
> With regard to unleaded  petrol and our stags.
>
> It has now been confirmed in the S.O.C. magazine, that the stag 3.0L
> engine was designed to run on unleaded.
>
> This has been confirmed by Jim Parkinson, Chief Engineer for design
> and developement of all Triumph and Rover engines, upto 1976.
>
> The only thing he says is that the later MKII engines may need their
> timing retarded, but for the rest of us MKI drivers, it's safe for
> unleaded driving.
>
> So maybe this will put an end to the rumours of valve seat and valve
> deterioration, and expensive non-essential modifications.

Sorry, but I'm afraid we can't just ignore what Triumph had to say
about this.  The following quoted material is from the Emission Control
System Warranty (for 1973 models), and is bound in the center of my
Stag Driver's Handbook, publication part number 545106/73 (c) British
Leyland Motor Corporation Limited 1972, 1st edition 5M/11/72 (North
American Type Market):

"Use of Unleaded Fuels.  Regular use of unleaded or low-lead gasolines
may cause difficulties with the engine and will result in malfunctions
of the Emission Control System.  The anti-wear additives found in
leaded gasolines are necessary to avoid difficulties of this type.
While an occasional tankful of unleaded or low-lead fuel is unlikely
to cause such problems, conditions attributable to sustained use of
such fuels will not be considered 'defects'."

Also mentioned in the text preceding this paragraph is the statement
"engine valves must be regularly inspected, and replaced where
necessary, in order to make sure that emissions will not rise to
unacceptable levels."

The Emissions Warranty requirement for valve inspection may actually
be a check for recession, since this was written at a time when
low-lead fuels were about the only thing on the market.  Lead levels
in US petrol were significantly reduced starting in 1971. Unleaded
fuels began appearing about 1974 in preparation for the 1975 models
which could no longer meet emissions standards for the warranted 5
years/50,000 miles without the use of a catalytic converter, which
would be poisoned by the use of even low-lead fuels.  Leaded
automotive fuels seem to have disappeared in the early 1980s,
leaving 100 octane low lead fuel only available for aviation use.

Everything I've seen so far seems to indicate that the Stag heads
are suitable for unleaded fuel, but the OEM exhaust valves are not.
Replacement of the OEM exhaust valves with TR7 exhaust valves, part
number 155524, will make the engine mechanically compatible with
unleaded fuel.  You may have to retard the timing to avoid spark
knock under load.  My Stag (.040 over bore) knocks (pinks) at part
to full throttle with 87 octane (R+M/2) unleaded fuel when using
the 4 deg ATDC dynamic / 10 deg BTDC static timing settings per the
ROM.  Knocking is significantly reduced with the use of 89 octane
fuel and the same timing setting.

As I've previously posted here, TR7 exhaust valves have the part
number stamped on the valve stem just below the groove for the
valve spring retainers.
  _________  <---top of valve stem
 /         \
|           |
|___________|
 \_________/
 /_________\
|           |
|1 5 5 5 2 4|
|           |
~~~~~~~~~~~~~

Tim Buja - Rockford, Illinois, USA - 80 TR8, 73 Stag, 72 TR6