Here's a couple of responses that I posted to the Healey site that may be of
some interest.
GM
----- Original Message -----
>
> > So since I have a low compression engine (stock BN2), I should run as
low
> > octane fuel as I can without causing pinging and advance the timing
until
> the
> > cows come home ?
> >
> > Buy why does it seem to run so good. Could it be a placebo effect?
> >
> > How long would you guess, I have to keep this up to build up lead on the
> > valve seats?
> >
> > The local speed shops sell 115 octane leaded gas.
> >
> > Jim L.
> > 56 BN2
>
>
> As consumers, we use the pump octane and manufacturers recommendation to
> determine which gasoline to buy. Octane is a general term used to indicate
a
> gasoline's ability to resist engine knock. The pump octane is also
referred
> to as the Anti-Knock Index (AKI). AKI is determined based on an average of
> the Research Octane Number (RON) and the Motor Octane Number (MON). The
> formula is RON+MON/2 normally abbreviated as R+M/2 on the pump.
>
> Octane is tested in a single cylinder octane test engine. The MON is a
> measure of the gasoline's ability to resist knock under sever operating
> conditions. MON affects high speed, part throttle and performance (under
> load such as in passing). The RON on the other hand, is a measure of
> gasoline's ability to resist knock under less sever conditions. RON
affects
> low to medium speed knock and engine run-on (dieseling). For a given AKI,
> RON is typically 8-10 points higher than the MON. As an example, 87 AKI
> (pump octane) fuel would have a MON of 82 and a RON of 92.
>
> What your engine requires to operate knock-free, is referred to as the
> Octane Number Requirement (ONR). The ONR for an engine is affected by
design
> factors and real world conditions. Engineers must balance performance,
> economy and environmental concerns in their design. Compression ratio,
> ignition timing, air/fuel ratios, temperatures and combustion chamber
design
> all have an affect on the ONR. Compression ratio has the most significant
> impact on the ONR and engine efficiency. The higher the compression ratio,
> the higher the ONR and combustion efficiency. Retarded timing, rich or
lean
> air/fuel ratios, lower combustion temperatures and high swirl combustion
> chambers all work to reduce an engine's ONR.
>
> In the real world, there are other factors that affect these designs
> including barometric pressure, temperature and humidity. Increases in
> barometric pressure or temperature, increase the ONR. Increases in
humidity
> or altitude (lower barometric pressure), reduces the ONR. Combustion
chamber
> deposits increase temperature and compression thereby increasing the ONR.
>
> My preference in all cases is Chevron due to the Techron additive in their
> fuels. This additive is also available under different names, however, I
> don't have to guess at the Chevron station. Note also that some brands do
> not include cleaners in their mid and low grade fuels.
>
> You should use the minimum pump octane (AKI) fuel that will run in your
> engine without knocking. You're wasting your money on higher octane fuels
if
> they aren't needed to control knock. The two most common myths regarding
> pump octane (AKI) are that it will increase performance, and result in
> better fuel mileage. You may see improvements in your vehicles due to the
> cleaners in the higher grade, higher quality fuels. However octane by
itself
> will not have any effect. Also, motor and exhaust temperatures are lower
> with lower octane fuel, prolonging exhaust valve and exhaust system life.
> Increasing timing will further lower temperature, etc.
>
> There seems to be a perception that all gasoline's of one octane grade are
> chemically similar, and thus general rules can be promulgated about
"energy
> content ", "flame speed", "combustion temperature" etc. etc.. Nothing is
> further from the truth. The behavior of manufactured gasoline's in octane
> rating engines can be predicted, using previous octane ratings of special
> blends intended to determine how a particular refinery stream responds to
an
> octane-enhancing additive. Refiners can design and reconfigure refineries
to
> efficiently produce a wide range of gasoline's feedstocks, depending on
> market and regulatory requirements. There is a worldwide trend to move to
> unleaded gasoline's, followed by the introduction of exhaust catalysts and
> sophisticated engine management systems.
>
> Chevron demonstrated that a well-formulated package could improve fuel
> economy, reduce NOx emissions, and restore engine performance because, as
> well as the traditional liquid-phase deposit removal, some additives can
> work in the vapor phase to remove existing engine deposits without
adversely
> affecting performance ( as happens when water is poured into a running
> engine to remove carbon deposits :-)). Most suppliers of quality
gasoline's
> will formulate similar additives into their products, and cheaper product
> lines are less likely to have such additives added. As different brands of
> gasoline use different additives and oxygenates, it is probable that
> important fuel parameters, such as octane distribution, are slightly
> different, even though the pump octane ratings are the same.
>
> So, if you know your car is well-tuned, and in good condition, but the
> driveability is pathetic on the correct octane, try another brand.
Remember
> that the composition will change with the season, so if you lose
> driveability, try yet another brand. As various Clean Air Act changes are
> introduced over the next few years, gasoline will continue to change.
>
> The octane of aviation fuel is not measured in exactly the same way as is
> automobile fuel.
> You have to convert the motor fuel octane rating to the "Aviation Lean"
> rating using a conversion table. Below about 110 motor octane (a
performance
> number of 110), the aviation lean and motor octane numbers will differ by
> only about 1 or 2 points. Above 110 motor octane the differences can be
> significant.
>
> HTH,
>
> GM
>
----- Original Message -----
>
>
> > Greg--
> >
> > Thanks for the very readable and informative post.
> ======
> You're welcome.
> ======
> And without meaning to be
> > simplistic, I must ask a simple question, although I wonder if it is
> > susceptible to a simple answer :
> >
> > I have a BN1 modified to LeMans Specs, but it does not have the
> > high-compression pistons as far as I can tell from records, and I have
not
> > pulled the head. Is the consistent use of a 93-94 Octane fuel--Sunoco,
> > Chevron--sufficient without the use of any additives to protect the
valve
> > seats?
> ======
> It depends on whether you actually need this much octane. If you do, then
> yes. I question whether you need it though.
> ======
> (I have no pinging and the engine runs on only infrequently.)
> ======
> If you have pre-ignition (run on), what occurs when the fuel/air mixture
in
> a cylinder ignites before the spark plug fires, extreme overheating
occurs.
> This causes exhaust valve / seat / piston / bearing damage far more than
the
> lack of lead deposits. Pre-ignition will eventually reduce your motor to
> boat anchor status if allowed to continue unabated. Even if pre-ignition
is
> only occasionally obvious, it's going on to a lesser extent all the time.
>
> I don't remember what the stock CR is for your car, but it's bound to be
> <8.5:1, so 87 octane should be plenty sufficient.
> ======
>
> Or
> > should I be using a "lead substitute" on a regular basis, and if so is
> there
> > a preferred brand and dosage? Thanks to all for help--
> ======
> The object is to maintain the lowest combustion temperature without
> knocking. Lead increases the octane rating, requiring a higher temperature
> to ignite the fuel. If the CR isn't high enough or the spark hot enough,
> incomplete combustion occurs, leaving deposits that will eventually
increase
> the CR, which means hotter temperatures, etc.. An insidious cycle that
> eventually burns the top end out, etc.
>
> If you feel you must use lead, then run the lowest octane with no
cleansing
> additives (if such still exists), otherwise, for the most part, the
> additives will strip it away before it can bond. The info I have indicates
> that up until the late 1960s, alkyl leads were added to gasoline's in
> increasing concentrations to obtain octane. The limit was 1.14g Pb/l,
which
> is well above the diminishing returns part of the lead response curve for
> most refinery streams, thus it is unlikely that much fuel was ever made at
> that level. 1.05 was about the maximum, and articles suggest that 1970 100
> RON premiums were about 0.7-0.8 g Pb/l and 94 RON regulars 0.6-0.7 g Pb/l.
>
> If the budget can stand it, do yourself a favor and remove the head, clean
> it and the pistons, and do whatever is necessary to get the valves / seats
> right. Run 87 octane with lubricating and cleansing additives, such as
> Chevron and all the timing it can handle.
>
> If not, then run the lowest octane with additives that doesn't knock.
>
> GM
> >
> > Michael Oritt, St. Leonard, MD.
> > 1955 BN1
>
>
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