Leaded Fuel and your Valves

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May 29, 2013
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Tucson, AZ
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There seems to occasionally be a lot of talk on this subject and the stance on the issue seems to vary (much like anything) depending on who you ask. So I wanted to do some research and find out some answers for myself.

So, prior to 1974 virtually all gas sold in the US contained lead. Why was this? Primarily lead was added to gas to increase the octane rating of the fuel. This is what we commonly see on pumps today to describe the "grade" of gasoline. (89, 91, 93, etc) This number is a measurement of the fuels ability to resist pre-detonation.

To digress for a moment:

This number however is NOT a measurement of a fuels "energy content" The reason that modern day cars make more horsepower on the higher octane fuels is not because they contain more energy per unit, but because the resistance to knocking allows the engine to tune for more advance on the timing.

Here is a quote from Wikipedia (This is a good article, I should know I contributed to it)

"Octane ratings are not indicators of the energy content of fuels. (See Effects below and Heat of combustion). It is only a measure of the fuel's tendency to burn in a controlled manner, rather than exploding in an uncontrolled manner. Where the octane number is raised by blending in ethanol, energy content per volume is reduced. Ethanol BTUs can be compared with gasoline BTUs in heat of combustion tables."

Source: http://en.wikipedia.org/wiki/Octane_rating

It's a good article worth reading about what the Octane Rating is, how we determine it in the lab, and why it varies from country to country (mostly it's everywhere, and then the US and Canada)

Now back to lead!

So why was it decided to remove lead from the fuel? Well lead in small quantities was being released to the atmosphere from the combustion of the fuel. So what? Well lead is quite a harmful substance:

"Very high lead levels in children can cause severe neurologic problems such as coma, convulsion, and even death, although such levels are now rare in the United States.

• Lower levels cause adverse effects on the central nervous system, kidney, and hematopoietic system.

• Blood lead levels as low as 10µg/dL, which do not cause distinctive symptoms, are associated with decreased intelligence and impaired neurobehavioral development.

• Many other effects begin at these low blood lead levels, including decreased stature or growth, decreased hearing acuity, and decreased ability to maintain a steady posture or growth."

Source: http://www.cdc.gov/nceh/lead/Publications/RefugeeToolKit/pdfs/MedicalTechnicalBrief.pdf

Aside from the medical effects, there are a good number of mechanical reasons for wanting to remove lead from an engine:

"leaded vs unleaded

spark plug changes: every year / every other year

oil changes and filter: twice per year / one per year

muffler replacements: twice per 5 yrs / one per 5 yrs

exhaust pipe replacements: one per 5 yrs / None

Overall maintenance savings from unleaded fuel were estimate to average about $38 per year;

for a car averaging 10 liters per 100 kilometers fuel consumption, this is equivalent to 2.4c per

liter of gasoline."

Source: http://www.walshcarlines.com/pdf/mechanicalimplications.d4e.pdf

So now we know why lead was removed, but this doesn't answer the real question that everyone has, will the lack of leaded gasoline lead to premature valve seat failure in my engine?

This is a hard question to answer definitively for specific engines.

"the phasedown of lead in gasoline began in 1974 when, under the authority of the Clean Air Act Amendments of 1970, the U.S. Environmental Protection Agency (EPA) introduced rules requiring the use of unleaded gasoline in new cars equipped with catalytic converters"

Source: http://web.mit.edu/ckolstad/www/Newell.pdf

The 1971 mustang was in development as early as 1967, so it's possible to say that Ford didn't see the change coming and didn't incorporate the change needed into the Cleveland heads to handle the unleaded gasoline. There is some argument on this topic, saying that since Ford was a forward thinker, that all of their engines from 1970 and onward were set-up to run unleaded gasoline. I can't find any direct evidence of this that can confirm it as true.

There is also some statements about 351c heads, that if they wear a "M" stamping on the leading edge of the head that these are late 73 heads that had induction hardened valve seats and are therefore good for use without any kind of worry.

Since we can't know for sure (short of pulling a bunch of stock heads and having the valve seats tested for hardness with a Rockwell test, if anyone wants to donate some heads I have access to these machines!) what does that mean? Do we all need to run additives to prevent our engines from eventually wearing themselves out?


"Some auto enthusiasts own vintage or antique vehicles. These vehicles are unlikely to be operated under conditions that would induce excessive valve seat wear, so operating them with unleaded gasoline is generally not a problem if the octane rating is sufficient. In the event that someone has a concern, several options are available: (1) Operate the vehicle only at speeds lower than 60 miles/hour (96 kilometres/hour); (2) Install redesigned valve seats when the engine is rebuilt; (3) Use fuel designed to replace leaded gasoline, if available; and (4) Use one of several anti-valve wear additives that are commercially available at filling stations or automobile parts and supply stores (note discussion in Section 3.5 below)."

Source: http://www.unep.org/transport/pcfv/pdf/vsr-finaldraft.pdf

"Mitigating factors offsetting the potential for excessive valve seat wear:

1. Older light-duty vehicles with obsolete valve systems are declining in number due to fleet turnover or have been rebuilt (with valve seat inserts).

2. Older vehicles are typically used less and for shorter trips.

3. Older vehicles do not typically engage in continuous high speed driving or other driving modes that would result in continuous periods of operation at high engine speeds.

4. In normal everyday use, virtually all light-duty vehicles operate in mixed-mode driving, which has been shown not to cause high valve wear problems.

5. Older vehicles have an established layer of lead deposits on valve closing surfaces when they switch to unleaded gasoline.

6. Older vehicles have relatively high lubricating oil consumption and thus lubricating oil ash deposits (similar to lead deposits and AVWAs) provide a cushion between valve closing surfaces, preventing the first step of the generally accepted excessive valve wear process.

7. Some refiners voluntarily use AVWAs"

Source: http://www.unep.org/transport/pcfv/pdf/vsr-finaldraft.pdf

So what we find pretty overwhelmingly in the research is that the casual tooling around town that most of us will do in our vintage mustang, isn't going to cause the valves to eat themselves to pieces.

However, if you intend to race, drag or street, use the car for daily long hauls on the freeway or other similar type activities, you are putting yourself at risk for valve wear. Though, in most of these situation you end up either building the engine, or at least refreshing it, giving you an opportunity to take steps preventing even the possibility of valve seat wear.

The two main methods are either induction hardening, which is used in many cast iron heads, or hardened valve seat inserts, which are commonly found in aluminum heads (aluminum by nature is soft when compared to iron and steel and needs these inserts)

Also, I believe that all aftermarket heads already account for this in their valve seat design. However, if you have induction hardened heads, and you perform a valve job on the heads, you can actually blow through the hardened layer with the cutting needed for the valve job. So you should either have the seats re-heat treated, or look at inserts.

I hope this helps those, who like myself, were worried about ruining their engine with modern day gas!

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Jan 13, 2012
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1971 Mach1, 351 Cleveland, Ram Air (not factory), C6 Automatic, AM/8 Track, Bright Red.
Excellent right up on the subject. The 4V heads on my 71 were produced in Oct 70, according to machine shop I had them rebuilt at they were designed for use with lead, so I had hardened valve seat inserts installed. I'm not sure when our cars went to heads designed for unleaded but my early ones weren't.

After reading your right up, as little as I drive it, I probably didn't need to add that extra expense during the rebuild...but, at least now I don't have to wonder if the unleaded is doing damages to my heads.



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Apr 17, 2013
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1971 Mach 1
I rarely run lead additive in my Mustang. Been driving it like that for years with no ill affects. But I am also just tooling around in it. No racing and very rare long distance driving. When I pull my motor apart here shortly I will get some pictures of the valve seat. But I am guessing they will also show no wear issues.



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Jun 13, 2012
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1973 coupe
1998 mustang
1994 Nissan Sentra SE-R
I have a 80s block so I'd image I never have to worry about it.

However on my 65 289 I used to use lead additive and then slowly forgeting to add it, then stopped using it all together. Only used highest grade gas (93) as it would ping on anything less.

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