Octane

Here is a bit of a switch that might be of interest to some...

The octane rating is a measure of the autoignition resistance of gasoline (petrol) and other fuels used in spark-ignition internal combustion engines. It is a measure of anti-detonation of a gasoline or fuel.

Anti-Detontaion means to me something that does not bun as fast. Low grade/octane gasoline burns faster and in some cases too fast that causes a bang in the combustion space of our motors that sounds like a small hammer is hitting the metal producing a ringing or ping sound...so far I understand that.

Using a fuel that has a higher octane rating slows down that bang and lets the fuel expand as it burns in a more controled way...I understand that.

If we lower the CR and or both timing we can stop that from happening...but the amount of energy of the lower octane is actually greater hence it burns faster...so we add some ingredients... iso-octane and heptane...By definition, iso-octane is assigned an octane rating of 100 and heptane is assigned an octane rating of zero...thus added to gasoline in cetain prportions one can regulate the octane.

At the track I can buy 107 or 114 octane leaded fuel ...mind you rateher expensive at $17 a gallon.

This is all leading up to my question so hang in there...

So saying all that...there must be a crossover point where a 8 or 9:1 compression running on 87 or 91 octane can produce more spririted running than a high compression engine running on a less volitile higher octane fuel? So running a low octane fuel in my car down to the point of pinging would/could make more power as you don't get somethig for nothing...to me it makes sense that it it is harder for the engine to produce 230psi compression than an engine with 165 psi compression or is my thinking way to far off the wall?

Here is something else...if you have not nodded off yet.

Sometimes called the Anti-Knock Index (AKI), Road Octane Number (RdON), Pump Octane Number (PON), or (R+M)/2.The octane in the United States will be about 4 to 5 points lower than the same fuel elsewhere: 87 octane fuel, the "regular" gasoline in the US would be 91-92 in Europe. However most European pumps deliver 95 (RON) as "regular", equivalent to 90-91 US (R+M)/2, and even deliver 98 (RON) or 100 (RON).

Something like our gallon of gas in Canada contains 160 fluid oz but in the USA the same 1 gallon is 128 fluid oz.

You are correct... we used to run an SCCA Spec Racer, with rigid rules and sealed engines... about 9:1 compression and computer controlled timing (anti-knock). Power was worse on race gas than on 94, and worse on 94 than a blend of 87 and 94.
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We eventually found an octane mix that produced max power for this configuration, and it was a LOT lower than I would have predicted.
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Fortunately, we can tune our LBC's for max power and take advantage of the available octane level of the fuel we choose to pour in it.

When I worked at the local airport as flight instructor, I sometimes was called on to do fueling. Certain race car drivers would come to the airport for the 100 octane LL Avgas. I think they put some additives in before they used it.

200mph Wrote:

auto ignition and dentonation are two different things with different causes.

golf Wrote:

i know the old V12 Ferraris are supposed to be run on pure aviation fuel because they are set up for lead gas. regular mg work the same??

So I have the options of 87, 89, 91 at the gas stations here... If I used 89 instead of 87 or 91 would my power and mpg results could be better?

I just filled up today will all 91, next tank I'll use 89 and see what happens.

With this weber setup my mpg has been all over the place. I got 23mpg one tank, then 20 the next, down to 18, and now back to almost 20 while I make the same drive every week and I dont romp on it all the time either.

The SUs gave me a consistent 20-21 mpg every tank.

At the track (where my throttle control is generally from nil throttle to flat-to-the-floor) my engine would destroy itself with anything less than 105+ octane. I must use AVGAS (or BP Racing Fuel here at 110 octane) which sells for about twice and a bit more than the cost of Super (ULP).

I have spent some money trying to "tune" the car (retarded) for power on Super (92/95 octane) but it ended up a failure with a significant loss of horsepower.

I do run on the roads with Super but I really have to drive carefully to reduce the load on the engine - I have to think carefully how I accelerate, especially up gradients, and I am soon reminded of a lack of concentration by the familiar "rattling the chain" pinging.

My engine is running at an estimated 12:1 compression with shaved head/deck and flat-top, forged JE pistons (just 0.40 over).

I love it when the car has a belly full of AVGAS as she is responsive (and of course, smells great), revving freely up to 7000 revs - and I don't have to worry about detonation.

I am due to consider a new head and an engine refresh before to much longer and I will be looking at a lower compression combination if I can do it and get the same or better horsepower.

As an aside and FWIW...

As Tony points out, there are different methods of measuring octane...

Lots of younger folks here might not remember but there are actually two different methods of arriving at a published octane rating.

In the olden days the methods were known under different monikers...

One is the "research" method and the other is the "motor" method. I don't recall how each is testing is done but that doesn't really matter.

The "motor" method results in a numerical result on average of about ten or eleven points higher than the "research" method.

The octane rating that you see today on gas pumps is an average of the two. Notice the disclosure sticker saying something like R + M divided by two. Obviously, this results in a figure lower than what you might have seen on an old Sunoco 260 pump.

I bring this up here only to alert some folks that when they are reading old tomes on ignition timing, fuel delivery, compression and the like they might think of this before they start twirling their distributors or whacking lots of material off of their cylinder heads searching for more ponys.

Jack

Pretty close on the auto ignition...

As I understand it, autoignition and detonation differ in that in an autoignition situation the charge is ignited from one or perhaps two heat sources neither being the spark from the plug, like a glowing bit of carbon or a hot edge of some metal in the combustion chamber. This results in an untimely but nearly predictable flame front and a combustion pressure that is not too far from what would be produced by the spark plug.

Detonation on the other hand is the ignition of the charge purely by the heat generated from the compression pressures as it relates to the charge itself and the shape of the combustion chamber. Diesels work on a similar principal but they use carefully controlled fuels and their piston crowns and combustion chamber shapes are designed to accommodate and control the combustion process. Both direct and port injection in Diesels are also designed to help in the control. The danger with detonation is that several origins of ignition can occur simultaneously. One spot over here, another over there and still more in other places. This unpredictability coupled with the extreme shock loads spread those loads out at random points. The heat generated is obviously a source for melting things like alloy pistons and the shock loads being applied to odd points of the chamber rattle the fool out of the metal and can either cause fractures or sudden extreme wear.

I am no thermodynamics engineer but that is at least how I understand it.

Simply put, higher octane fuels have more additives that allow the designs of the combustion chamber and other factors to control the burning process at higher pressures and engine speeds resulting in the extraction of more of the potential engery that is available from a given volume of fuel. Lower octane rated fuels are used in applications where, because of lower pressures, lower engine speeds and less sophisticated chamber designs present less of a likelihood of problems.

Try..

http://www.mech-eng.leeds.ac.uk/res-group/combustion/activities/SI_engines.html

another, perhaps better...

http://www.stormingmedia.us/43/4318/A431892.html

Jack

It is nice to see someone talking sense about octane for a change. I repeat this to people that should know better and most of them don't believe that giving their cars a 'treat' by filling up with higher octane is a total waste of money.

A normal MG running either 8.8 or 9.0 compression should run well on 89 octane (R+M/2). An MGA with 8.3 will run on 87.

A 10:1 compression engine requires around 93 and 12:1 (as Eric says) about 105.

Aside from normal ignorance about this subject shared by owners of all sorts of cars, the biggest thing that fuels (no pun intended) this misinformation is the combustion chamber shape of the MG engine.

They have a tendency to accumulate carbon from running with a manual choke on too long and have inherent hot spots (the tip of the heart shape plus any head that has been surfaced without rounding the sharp edges). When they start pinging, people fill up with premium, and say "See, my thoroughbred British car won't run on anything but top grade". Tell them that their thoroughbred British car will run just fine on regular if it weren't carbonned up and/or didn't have built in hot spots promoting pinging, normally falls on deaf ears.

> but the amount of energy of the lower octane is actually greater

I'm not sure that this statement is correct. The speed at which the fuel/air mixture burns is not related to the amount of energy that the fuel contains. If it burned so slowly that the piston was able to travel a significant difference down the bore than you might not be able to *extract* the energy - but I doubt that any grade of gas burns that slowly.

ingoldsb Wrote:

I don't have time to go to the site. Does it cover the effects of octane rating between Wild Turkey 101 and 80? LOL

Jack

Hey ERic - that old Torana of mine used to run AVGas with 1/2 gal methonal to prevent pre-ignition.. Compression was (caluclated) around 15:1. When methonal itself got hard or expensive I used model aeroplane fuel instead..



Arhhhh the good old days..

an old saying from the 50s; there will always be an england because when you shut the ignition off it just keeps running ! thats a good description of auto ignition.
jack austin explained dentonatio0n well.
maybe someone will design a knock sensor that can be retrofitted to old brit iron
at a reasonable price.

golf Wrote:

Bill, my mechanic told me to run my rebuilt 74 on 91. He said it was to control pinging. I have it timed at 15 btdc at 900 rpm with Jeff's dist. I get 28 mpg mixed driving, Am I understanding that it should run fine on 89? if I retard the timing? Thanks
Paul

paul74 Wrote:

By the way, we have to get a special licence/passbook to use AVGAS here and sign off/record every litre we buy and use. It is not to be used on public roads.

This is because it contains lead and leaded fuels have been banned.

I got my permit because I drive an old car on a racetrack (BUT in a loophole designed for classic rally cars you can drive using AVGAS on public roads to and from events and between events... designed for the transport stages of rallies).

i do the right thing and and drive to the track on Super (judging as close as possible to run the car to empty by the time I reach there) and take fuel in jerry cans. AVGAS to use at the track and ULP to drive home on.

They keep saying this permit system will come to an end - hence my plans to rebuild the engine to run on ULP - but it keeps getting extended.

I would add to Eric's observations that 100LL AVGAS has A LOT of lead in it. I believe Eric dilutes his with premium unleaded (clarification). Anyway, besides being illegal, if you try AVGAS you may be very disappointed - it can gum up sparkplugs and engines very quickly. I hate having to use it in my airplane (which has been certified for Auto gas) because it fouls the plugs and makes the oil black very quickly. The engine actually runs much better on regular unleaded - probably because it is designed to run on 80 AVGAS, which has no lead, but is also almost impossible to find!

If it fouls the plugs on an airplane engine (take a look at one sometime), you can only guess what it will do to automotive sparkplugs!