In reference to the latest discussion on unleaded gasoline (petrol), lead
and other lead substitute fuel additives, a truly eye glazingly
comprehensive FAQ on gasoline, real heavy on the chemistry, can be found at
http://www.faqs.org/faqs/autos/gasoline-faq/. To save everyone from reams
of turgid rhapsody on the history of gasoline and lyric waxing on the
benzene ring, I have reproduced the relevant sections 8.8 and 8.9 from this
FAQ concerning unleaded fuel and additives below. Another tech-heavy
"Leadfree Gasoline FAQ" with a section titled "DO SOME CARS NEED LEADED
GASOLINE?" can be found at
http://www.princeton.edu/~vmthomas/pb_elim/pb-elimination.html#DO.
A little closer to the Amphicar port, you can read a more laymanly account
of the topic in the British MG Club's "The Lead-Free Petrol Question Part
II: Valve Seat Recession: The Size of The Problem and What To Do About It."
at
http://www.mgcars.org.uk/mgcc/sf/000302.htm. You'll want to scroll
straightaway down to its section about halfway down called "The situation
for late vintage and pre-War engines". Meanwhile, not to be outdone, the
British Jag-Lovers have similarly informative offerings in summary form at
http://www.jec.org.uk/fuel.htm and as a very slick (if slow loading)
thumbnailed page of image scans of the original article from their April '98
issue of Jaguar Enthusiast magazine: "Fuel and its effects on your engine"
at
http://www.jag-lovers.org/library/jecmag/fuel.html.
For those too weary to wade through all of these reports, the summary is
simple for Amphicar owners: Don't panic! Yes there will may be a little
something called "valve recession" now that the lead's gone, but you
shouldn't commence ripping off your head to have hardened valve seats put in
unless the head's already off for something else anyhow. The good news is
that certain caustic substances that were once actually produced because of
the lead in the gas will no longer chemically attack other even pricier
engine internals. It's a better than even trade off. By all accounts and
scientific reports, adding a squirt of lead substitute whenever you fill up
will help slow this "valve recession" to the point where it'll be the Quayle
Administration before you notice even the slightest lag in raw
cheek-twisting G-forces slamming you back into your seat to which we
Amphicar owners are accustomed when the light goes green.
Now here's that excerpt... ~Bilgemaster~
THE EXCERPT FROM THE GASOLINE-FAQ at
http://www.faqs.org/faqs/autos/gasoline-faq/:
8.8 Can I used unleaded on older vehicles?
Yes, providing the octane is appropriate. There are some older engines that
cut the valve seats directly into the cylinder head ( eg BMC minis ). The
absence of lead, which lubricated the valve seat, causes the very hard
oxidation products of the exhaust valve to wear down the seat. This valve
seat recession is usually corrected by installing seat inserts, hardening
the seats, or use of specific valve seat recession protection additives
( such as Valvemaster ). Most other problems arise because the fuels have
different volatility, or the reduction of combustion chamber deposits.
These can usually be cured by reference to the vehicle manufacturer, who
will probably have a publication with the changes. Some vehicles will
perform as well on unleaded with a slightly lower octane than recommended
leaded fuel, due to the significant reduction in deposits from modern
unleaded gasolines. If premium unleaded petrol containing relatively
high levels of aromatics is used, some carburetted engines from the 1960s
may experience spark plug fouling, however most vehicle manufacturers
have guides to ensure careful engine tuning will eliminate most of the
problem.
8.9 How serious is valve seat recession on older vehicles?
The amount of exhaust valve seat recession is very dependent on the load on
the engine. There have been several major studies on valve seat recession,
and they conclude that most damage occurs under high-speed, high-power
conditions. Engine load is not a primary factor in valve seat wear for
moderate operating conditions, and low to medium speed engines under
moderate loads do not suffer rapid recession, as has been demonstrated
on fuels such as CNG and LPG. Under severe conditions, damage occurs
rapidly,
however there are significant cylinder-to-cylinder variations on the same
engine. A 1970 engine operated at 70 mph conditions exhibited an average
1.5mm of seat recession in 12,000km. The difference between cylinders has
been attributed to different rates of valve rotation, and experiments have
confirmed that more rotation does increase the recession rate [29].
The mechanism of valve seat wear is a mixture of two major mechanisms. Iron
oxide from the combustion chamber surfaces adheres to the valve face and
becomes embedded. These hard particles then allow the valve act as a
grinding
wheel and cut into the valve seat [115]. The significance of valve seat
recession is that should it occur to the extent that the valve does not
seat,
serious engine damage can result from the localised hot spot.
There are a range of additives, usually based on potassium, sodium or
phosphorus that can be added to the gasoline to combat valve seat recession.
As phosphorus has adverse effects on exhaust catalysts, it is seldom used.
The best long term solution is to induction harden the seats or install
inserts, usually when the head is removed for other work, however additives
are routinely and successfully used during transition periods.
