Unleaded Gas & Exhaust Valves

E

Ed Price

Guest
I recently came across a post, in rec.autos.tech, on the subject of using
unleaded gas and lead fuel additives in pre-1970 engines. This is one of the
best explanations I have read, and it's very applicable to Amphis. Here's the
posting, courtesy of Dan Stern.


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It is not actually a matter of "lubricating" anything. With an unhardened
exhaust valve and seat, the valve and seat can micro-weld to each other if they
get hot enough. Lead acts as a buffer to prevent this happening. The important
thing is that exhaust valve and seat recession ONLY takes place when the valve
gets hot enough to undergo localized welding. Then, when the valve opens next,
the metal pulls apart like taffy. This roughens the meeting surfaces, and they
become quite abrasive. The pounding/turning of a valve with such "pulled" metal
on it creates a nice grinding wheel effect on the seat. In addition, the
roughened surfaces no longer seal against each other properly, which eventually
allows still-burning combustion gases to flow through the "closed" valve,
causing a blowtorch effect on the poor valve, depriving it of any prayer of a
chance to cool while it's on the seat. The blowtorch effect rapidly
deteriorates the seal further, snowballing the seat recession.

The main thing to remember is that this bad stuff *cannot* happen if the valve
never reaches the crucial temperature. Whether the valve reaches the crucial
temperature depends mainly on how the car is driven and used. (Towing, drag
racing or pedal-on-the-floor hauling WILL heat the valves; driving down the
highway at a constant 70 won't, and neither will hopping from traffic light to
traffic light in the city or running down to the local grocery for a carton of
ice cream.) Other factors in the margin of safety include the size of the
exhaust valve, its material, and the efficiency of valve seat cooling in that
particular engine design.

Very *VERY* little lead is required to prevent the localized welding and "taffy
pull apart" effect that leads to the abrasive surface which, through incidental
or positive rotation of the valve, eventually grinds down an unhardened seat.
I'll emphasize that again: VERY LITTLE LEAD. The remainder was in the fuel as
an octane booster, that's all. It was widely used because it was a very cheap
and very effective octane booster. When unleaded fuels were first widely
introduced (by legislation) , there was generally only one grade of unleaded
available, and the octane was *quite* low; less than that of leaded regular. We
all know that when you use a fuel of insufficient octane, your engine pings
(detonation, pinging, spark knock; call it what you will.) This phenomenon
creates *tremendous* heat in the combustion chamber; certainly enough heat to
push the exhaust valves to the crucial temperature. Because for quite a while
only unleaded fuel of sub-regular octane was available, plenty of people
experienced these effects from using unleaded. While a fraction of those
engines that suffered under this low-octane unleaded really *DID* need the lead
(high load and/or high-RPM engines), the vast majority of the failures were due
to the low octane increasing combustion chamber temperatures. And so the myth
was born that old car engines "WILL DIE" if run on unleaded.

These days we have universal availability of higher-octane unleaded fuels, which
obviate the insufficient octane; cause of valve heating and subsequent localized
welding in all but the most extremely octane-hungry engines.

If you have an old car that is a low-stress application , used in daily-driver
service, then you generally need have no qualms about using whichever octane
grade of lead-free fuel your car runs well on and drive it for a long time with
nary a valve or seat problem. Many domestic 6 and 8 cylinder engines fall into
this category in normal daily driving service.

The way to eliminate even the *possibility* of valve heating causing localized
welding and subsequent seat recession is to install hardened exhaust valve seat
inserts and exhaust valves of upgraded material (typically 21-4N stainless
instead of 21-2N), if or when the engine eventually needs a head job. Hard seats
and valves are readily available for just about anything you want to put them
in. It's a very common operation and a competent machine shop can handle it,
and it adds very little to the cost of a cylinder head reconditioning job. But
the main thing here is that there's absolutely no reason to tear into the engine
solely to install hard seats. There is no collateral damage from seat recession.
Drive and enjoy! You likely won't experience any problems for a long time.

The additives available on shelves vary widely in what they do. Some of them
use a sodium salt and claim to duplicate the buffer effect of lead. Some of them
use "MMT" (methylcyclopentadienyl manganese tricarbonyl), which is not a very
nice substance at all and is of questionable benefit in buffering exhaust
valves. You may read about some of MMT's problems in a Scientific American
article at:

http://www.sciam.com/1998/0698issue/0698techbus2.html

Regardless of whether any human-health or environmental risk is posed by MMT,
the stuff causes hard red deposits on your spark plugs that will cause you to
need to replace them more often. Other additives are simply octane boosters of
varying effectiveness and varying side effects.

It's worth noting that on the East Coast of the US, Amoco marketed unleaded
high-octane gasoline for *decades* before the EPA decided to "unlead" the
country's fuel by regulation. That Amoco high-test unleaded was widely regarded
as quite a fine fuel indeed.

Daniel Stern
dastern@engin.umich.edu
University of Michigan Engineering


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Further correspondence with Dan yielded some specific comments & opinions from
him regarding the 1147cc Triumph engine in a "marine-duty" application.

1. It looks like Amphi owners regularly operate their Triumph engines in the
condition of possible valve erosion.

2. With that low of a compression ratio, insufficient octane is not likely to be
a concern. Do some testing to check for spark knock (ping) under loaded
conditions at the target RPM. Use the lowest-octane fuel that permits engine
operation without ping -- this may well be regular unleaded!

3. Personally, I don't feel the drawbacks (cost, combustion chamber fouling,
inconvenience, hazards in handling) are worth using even a small amount of lead
additive to your fuel. Run 'em till they recede, you won't damage anything else
in the engine, then...

4. When it becomes needed, replace the valves with hardened valves & seats.


Ed
El Cajon
67 Rust Guppy






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