Fuel Octane vs. speed of burn

[goose]

The above posting on Detonation is a good article, the only thing I have to add to it is a quick note on fuel octane. I also for a long time thought that a high-octane fuel was very slow burning; it makes perfect sense. A few months ago I read an article in Hot Rod magazine on fuel octane rating that changed my perception... the article's authors did some tests with various gasolines and additives in one engine on a dyno, ranging from pure pump gas to gas with octane additives to race gas. The results were as one might expect, except for this fact: the higher the octane of the fuel, the LESS ignition advance was required to obtain maximum power! This meant that the high octane fuel was burning more quickly, not slower as according to conventional wisdom. The article prompted me to do some further research.

Apparently, knocking is caused by the fuel burning too slowly. A small part of this is if the flame front is traveling too slowly the ignition timing must be advanced to gain maximum power; advancing timing causes more back-pressure on the piston which is what knocking is. The main this is that for some reason if the fuel has a burn rate that is too slow, it is more likely to explode all at once rather than burn smoothly. This is how a diesel engine works; the fuel octane (and hence fuel burn rate) is extremely low and the fuel all explodes at once, which causes the familiar knocking from a diesel engine. This knocking leads to hot spots on the piston. In a diesel engine, the hot spots cannot cause pre-ignition since ignition timing is controlled by the injection of the fuel. In a gasoline engine, the fuel is already present in the cylinder because it has been mixed with the intake charge before the time of ignition; this makes it possible for any hot spots on the piston (from the knock on the previous ignition cycle) to ignite the fuel/air mixture before the proper time, causing an extreme back-pressure on the piston. And since the fuel is low in octane and tends to explode rather than burn controllably, the pressure spikes in the combustion chamber are even worse than one might think. This really heats up the piston, which causes more pre-ignition, which heats the piston more, and so on. Eventually the end result is a BOOM!! which is usually followed by some adult language or an incoherent scream from the engine's owner.

[sbuc72]

If anyone is interested in this article, im sure i have it, and i could scan it up, and post it somewhere.
(it was a very good article, and also changed my opinion on HIGH octane fuel).

The short of the long is, Aslong as you have Octane your good, Too much octane is a bad thing.

.sbuc72

[motorhead440]

I'll take you up on that offer, you can email it to motorhead446@hotmail.com. Thanks

[440cj]

Email me at rey440cj@msn.com if you please...Cuda66273...whaddya think? 440cj

[ehostler]

If you are going to scan it in, just post it here and anyone who wants it can download it.

[sbuc72]

They May be kinda big, but i prefer to have the text still readable
If you want to save these, dl them here

http://whacked.technine.net/junk/octane1.jpg

http://whacked.technine.net/junk/octane2.jpg

http://whacked.technine.net/junk/octane3.jpg

http://whacked.technine.net/junk/octane4.jpg










enjoy
.sbuc72

[6 packin]

This is not true! If so they need to rewrite many many years of books being Organic Chemistry! Have you ever herd of Reid vapor pressure Test! Thats the test that chemistist Like my self find out how fast things burn! The higher the octane level the slower burning the gas is! This holds true when compared even to 5 year old gas drained from a lawn mower compared to fresh gas from the pump! This test is done with equipment that costs more than most people make in 1 year! The people who have said or wrote and artical may be good engine builders but there no chemists! Why is higher octane needed in a race engine? Because of the high compression of race engines the gas & oxygen burn so quickly before the piston reaches TDC and actually causes it to rattle in the bores! Thats the sound you here (knock,ping,etc)! The slower the gas the better flame travel you have! The psiton gets a better push with higher octane fuel. In complete cobustion = CO & H20, Carbon monoxide and water! I am sure you have started your car up in the mornings and seen water drizzling out the tail pipe? This holds true but after the pipes heat up it vaporizes before it can escape! Just get me some hard facts more $10,000 test and we will change the way people think. I work at formaly Ashland Oil in Notheastrn Kentucky which is the largest oil refinery inland in the Northern Hemisphere!

[6 packin]

Originally posted by 6 packin
This is not true! If so they need to rewrite many many years of books being Organic Chemistry! Have you ever herd of Reid vapor pressure Test! Thats the test that chemistist Like my self find out how fast things burn! The higher the octane level the slower burning the gas is! This holds true when compared even to 5 year old gas drained from a lawn mower compared to fresh gas from the pump! This test is done with equipment that costs more than most people make in 1 year! The people who have said or wrote and artical may be good engine builders but there no chemists! Why is higher octane needed in a race engine? Because of the high compression of race engines the gas & oxygen burn so quickly before the piston reaches TDC and actually causes it to rattle in the bores! Thats the sound you here (knock,ping,etc)! The slower the gas the better flame travel you have! The psiton gets a better push with higher octane fuel. In complete cobustion = CO & H20, Carbon monoxide and water! I am sure you have started your car up in the mornings and seen water drizzling out the tail pipe? This holds true but after the pipes heat up it vaporizes before it can escape! Just get me some hard facts more $10,000 test and we will change the way people think. I work at formaly Ashland Oil in Notheastrn Kentucky which is the largest oil refinery inland in the Northern Hemisphere!

oh yea just for refer.

A combustable = any gas which will produce enough vapor above 100 deg F to ignite!

Flamable = any gas which will produce enough vapor below 100 deg F to ignite!

examples

Kerosene - Combustable

gasoline- flamable

[mr_340]

This is a reply that my professor from college sent me on that subject. He does research in combustion processes and related subjects. He is responding to my question on ignition timing on gasoline vs. methanol.

" Although ignition timing and flame speed ARE always related,
Octane
Rating is NOT always related to flame speed.
Knock is the result of a horserace between the rate of
combustion
(which is a function of the flame speed) and the rate at which reactions
in
the unburned zone are progressing. If the mixture in the unburned zone
reaches an energy liberating stage of reaction before being consumed by
flame passage, knock occurs - the wrong horse won the race. Thus, a
fuel
may have a high Octane Rating either because it produces a higher flame
speed or because the preflame reactions involving this fuel are slower,
both of which promote the "good" horse winning the race. Addition of a
very small amount of tetra ethyl lead to gasoline produced a fuel with a
significantly higher Octane Rating but with no effect on the flame speed
-
TEL soaks up radicals produced by the preflame reactions; since these
radicals are required to accelerate the reaction, the preflame reactions
slow down, taking much longer to reach the energy liberating stage.
Although TEL also soaks up radicals within the flame, so many radicals
are
formed at the much higher temperatures within a flame that the TEL has
no
noticeable effect on the flame speed.
Fuels with a higher flame speed do not necessarily have a higher
Octane Rating, because these fuels may also produce more reactive
preflame
reactions. Conversely, fuels with a higher Octane Rating do not
necessarily have a higher flame speed. Methanol and natural gas
(predominately methane) are an interesting example. These are
essentially
the same molecule but methanol is methane with one hydrogen atom
replaced
by an OH group. Both have essentially the same flame speed but the
Octane
Rating for methane is off the top of the extended Octane scale - more
than
127. The OH group in methanol makes it much easier for the preflame
reactions to reach an energy liberating stage (compared to methane).
When comparing flame speeds, it is important to note that most
data
is for combustion at 1 atm. and room temperature reactants. However,
combustion occurs at much higher pressure and temperature within an
engine
- the former of whcih slows down the flame speed, but the latter of
which
accelerates the flame. Overall, a laminar flame at engine-like
conditions
will burn faster than one at room conditions, but the pressure and
temperature dependencies are different for EVERY fuel.
Ignition advance is dictated by the need to have peak pressure
at
about 15 crank angle degrees after top dead center. The rate of
combustion
is a function of the flow field (velocity and turbulence) and the
laminar
flame speed, the latter of which depends upon the fuel, the air/fuel
ratio,
the temperature of the unburned zone, the pressure, the exhaust residual
fraction, and the amount of EGR. If all of these factors are the same
except the fuel, the fuel that requires the least advance must have a
higher flame speed. That is, under engine-like conditions, methanol
MUST
have a higher flame speed than gasoline."

[gumshoestu]

I have a question regarding my 440 engine, which is slightly above stock: The engine has about 5,000 miles on a total overhaul and I am not satisfied with the performance. I have to blend 110-octane race gas with 93-octane pump gas (a 50/50 blend) to get the engine to run well. I was hoping to run premium pump gas. As it is now, the engine knocks under duress (even moderate acceleration) unless I mix the fuel — or I have set the timing much lower. But then the engine hesitates slightly from the start and is robbed of power is all r.p.m. ranges. No after-market fuel additive has worked so far. The engine has a 270 Comp Cam (270 duration, a .470 lift and 110 lobe); has an Edelbrock Performer manifold and matching 750cc carb.; is bored .30 over with forged, domed 10.34:1 TRW pistons; has electronic ignition, and 2225 stall in the transmission (if that's relevant). If I hook up the vacuum advance the engine backfires when accelerated in park, and stalls/stammers when accelerating in drive. I have had advice ranging from gas/air mixture adjustments, to replacing the pistons with 9:1 flat-tops, to simply accepting the fact that the engine is so stout it needs high octane fuel. I'm about ready to pull my hair out over the matter. Any thoughts would be appreciated

[451Mopar]

I am not sure if there is a good way to compare octane rating and burn rate? Octane rating is the fuels resistance to knock or pre-ignition and with all the additives (and blends) in the fuel these days, you could end up with fuels that have the same octane rating, but different burn rates.

I'm not a chemist, so I'll leave that up to the other posters to explaing the chemical reactions.

[PaulS]

Gumshoestu, I'd personally go the 9:1 flattops to cure the problem you have with pump fuel. You could also switch to alloy heads which I understand "forgive" about a full point in compression.

A friend runs a 340 with 10.5:1 compression & original (iron) heads and he is limited to using the "premium" 98 octane unleaded fuel we have available here at the pump.

The article is very interesting, but it should be noted that their test engine used alloy heads with 10.4:1 compression; to me this reads as 9.4:1 with iron heads.

Each type and brand of fuel has it's own blend specs and therefore different burn rates, so in the real world all we can do is pick the fuel that works best for our particular engine.

Another factor to consider is that most petrol companies use the RON octane number to advertise their product (Research Octane Number), whilst the MON number (Motor Octane Number)is a "truer" indication of the fuel's anti-knock capabilities because it reflects such factors as burn rate in a more dynamic test procedure.The MON can vary widely between fuels which have the same RON.....

Forgive my syntax,

Cheers, Paul.S

[mtrv8n]

The octane ratings here in the states use an average - mon+ron/2, to rate their fuels. The mon rating is more useful to us, but we don't get a breakdown at the pump.

[cuda66273]

I'm working on a laymans explaination...."I'll be Bach"

[John Kunkel]

Just because it's written in a magazine (or the internet) doesn't make it true. All you need to be published in a monthly magazine is a word processor and an editor willing to print your stuff.

Without a definitive study on several different combustion chamber designs, the information in the Hot Rod article is questionable.

[gumshoestu]

Paul S. and 451 Mopar:
I did a mechanical advance timing check today with the following results: the engine idles at 800 R.P.M. and the timing was 26-degrees; it was also the 26-degrees at 1,000 R.P.M.; at 1,500 R.P.M. it read 30-degrees; at 2,000 it was 38-degrees; and finally at 2,500 R.P.M. it was 42-degree and remained that up to 4,000. I assume that meant it had peaked out. My mechanic felt that was a good "curve' and I concur.
We also checked the numbers in a service manual and found that the 440 high-performance engine, stock, came from the factory between 1968 and 1971 with compression ratios between 9.5:1 and around 10.75:1, depending on which year it was produced. So we feel there is no reason to think that the 10.34:1 ratio my engine has should have any bearing on my performance problems. Being comfortable with the timing curve and the compression ratio, that leads us to conclude there is a fuel problem, with this being our logic and my experience so far:
1.) If I set the timing to, say, 10-degrees advanced and run premium pump gas, I get the hesitation from a dead stop that I find so annoying. But no spark knock/pinging.
2.) When I advance the timing and get the spark-knock, I have to run higher octane fuel and the car runs strong. But not like I expect it to...
So we think it's time to re-jett the carburetor. I am going to try that next. Any thoughts, guys?

[ehostler]

What they listed as compression ratio was rarely the TRUE compression ratio. Often times, that was the advertised compression ratio of the piston.

Your timing sounds about 10 degrees to far advanced. You should have a total of 34 degrees by 2,500 RPM and it should not rise beyond that point.

Is the combustion chamber, on your head, polished. A rough chamber will cause hot spots and uneven burn. A polished chamber will not have these problems.

[gumshoestu]

Ed,
The combustion chamber on these factory iron-type heads is about 80cc. The heads were cleaned, but not polished. The pistons are TRW forged and domed.
I am hoping you're wrong about the CR, because if that's true, then nothing short of new, lower ratio pistons is going to make a difference.
I will lower the timing to about 10-to 12-degrees and I plan to re-jett the carb., since I think that may be part of the problem, if not the entire problem.
I was told today that even 93-octane gasoline isn't truly that high, since the standard used to establish octane rating has changed since the late-60s/early-70s.

[ehostler]

From what I just read in your other thread, you say that you are running 1973 heads. If that is the case, they are about 88cc, not 80cc. That is unless you had them shaved.

I would look at the timing first. That will probably resolve alot.

[gumshoestu]

Sir:
I'll take your word for it on the size of the combustion chambers. I may have misunderstood my mechanic anyway.
I am thinking it's a timing issue too, and perhaps some carb. adjustments. Per suggestions from another Mopar man, I'm going to try some bigger metering rods/springs in the carb. and see if when the timing is lower, say around 10-to 16-degrees, if the stammering and hesitation I have been experiencing will stop. My goal is to get it to run strong on premium pump gas, without tearing into a fresh motor.
Thanks for the advice and I'll keep all of you posted. I really appreciate the help everyone has given me.
Stu Barnhill

[cuda66273]

Gumshoestu:

By looking over your post I'm going to assume that from the information provided that you have 10:1 compression or slighty lower....usinig this assumption and assumeing also that you have about 230-240 cam duration at 50....

I note the following.....your max timing should not exceed 34 for street driving, you may want to try 36 on the track with a higher octane fuel.

If your distributor is curved correctly you should have 15-18 degrees initial (Idle) and it should all be in by 2500 @34*, but the idle timing if dependant on the carb and duration of the cam.

I also see that you must be running some sort of Rottenchester or Eddy carb, which I doubt is giving you enough fuel/air for your motor specs. Note I didn't say lean mixture thats another topic.

Without knowing much about your application I would say that at the very minimum you should be running a 750 Speed Demon or Mighty Demon but to determine which one I need more info.

At the very least you need a M4268 Carter mechanical pump (120 GPH) and a fuel filter capable of handling that type of volume which would be at very minimum a HPG 1 90 GPH(Fram)

Ideally a BG HR220 continuous duty electric pump and a BG5000 filter would handle this job much better.

To handle the potential of that 440 I would also reccomend -08 fuel lines from the tank all the way to the carb and with the Barry Grant set-up there's another very specific schematic to follow.

These are only recomendations based on assuptions and before you make any changes you should consult with someone qualified to engineer a proper fuel delivery system and properly match your engine to a carb based on cam, compression, gear ratio, tire size, convertor stall speed, manifold type, combustion chamber design and engine vacuum at idle in gear.

In summary.... I would say that your detonation problem is in your distributor and a properly balanced fuel system will add power and longevity to your investment.

[gumshoestu]

Sir:
I've got a 750cc Edelbrock with vacuum secondaries, which is why I am now suspecting — from what some on this chat have said — that it's being fuel starved when the timing is set at a proper range.
I will tell my mechanic what you have said about the fuel pump, becuase I'm using a standard mechanical and factory lines. Perhaps that's a good thing to change.
As fpor the cam, it's a Comp Cam with a 270 lift and a 224 duration at .50 and 110 lobe. It was recommneded by the local speed shop as an excellent replacement for my engine.
The engine is .30 over, and based on the combustion chambers being .88s — that's what several have said becuase the heads are factory '73 models — then the TRWs in this engine are probably 10.34:1. Thats what my mechanic felt was accurate.
I'm going to put some different metering rods and springs in this carb. to increase flow and add a spacer under the carb. to cool the fuel vaprs (also a suggestion from others on this site). I am definitely going to put a new 180-degree thermostat in the engine, I am hoping that the probems are too-much timing, poor-fuel flow and too-much engine-temperature related, which seems to be the general concensus of several men on this site.
These are cheap atemps , as opposed to a new carb., or tearing into the engine.
I'll let you know what develops.
Thanks for all the advice folks,

[6 packin]

I found that running a correct EGR setup helps pre ignition. Maybe up to 1/2 point in most cases! This will also increase fuel milage of a 2-3 miles/gallon. Without an adverse effect on performance! Just some thoughts!

[cuda66273]

First of all I'm no "Sir"
Cuda66 or Don will do just fine....

What your attacking is a lean condition not a fuel starvation condition...they are totally different.

Fuel starvation is when the air flow or quantity exceeds the fuel delivery capabilities of the system and the carb virtually runs out of gas, causing a lean condition...heat...and eventually detonation.

Quote:
"I'm going to put some different metering rods and springs in this carb. to increase flow and add a spacer under the carb. to cool the fuel vaprs "

You can never increase the flow of the carb, the Air/fuel mixture has 2 basic elements.. air volume...and Fuel volume...the air flow is determined by the CID of the engine and increases as rpm goes up, air volume per revolution never changes and the venturis in the carb are the determining factor as to the velocity that air goes into the motor.
Fuel ratio or fuel quantity is determined by the ability of the carb circuits to drop or dump fuel to maintain the approximate ratio of 14:1 through the various RPM levels or demand of the engines air flow.

I know it's all a bunch of Hocus Pocus ....but think of this. ..if a small carburetor wouldn't run a large engine then how do they get 440's to run with a 2 BBl?..easy...as long as the air/fuel ratio's are correct the engine will run fine, will it run to it's max potential? of course not but it will be smooth, no hesitation or stumbling.


So...before you change anything I want you to perform a simple test......

I will assume that you are fairly mechanically inclined so If I say something you don't understand just ask....

With the engine OFF (Good carb tuning always starts with the engine off)

OK....take a strip of paper about 1/4" by 4", your going to open the throttle a bit and place that paper under the idle speed stop on the side of the carb. Now with a screwdriver slowly start backing the idle speed screw out until you can slip the paper out easily.

How many turns did it take?

OK now turn it back in the same amount so we don't change anything...yet.....we're going to, but not yet.

While your doing that I want you to think about something you learned way back in High School physics class....so they say anyhow as I never got much High end schoolin' I'll have to take their word for it.....I wonder who "They" is anyhow?...must be a smart feller as everyone always refers to "They" as if "they" were Einstein?

Anyhow....we don't really burn fuel we burn Oxygen, when the O2 is used up the fire goes out no matter how much fuel we have without O2 there is no fire, so a rich condition is really a condition of not enough O2 to complete the combustion process...so think about that ...as you add more fuel by way of bigger jets and /or smaller metering rods whatever your case may be your not increasing the O2 as it's a fixed ingredient controlled by the CID of the engine.

So now through a mathematical formula we can determine how much air the motor can suck through the venturis measured in cubic feet per minute at a particular RPM and we know that fuel weighs 600 times more than gasoline so buy creating a venturi that multiply’s the vacuum created by the engine’s flow we can calibrate the carb to drop different amount’s fuel based on that velocity or flow.

Have you read the sticky’s at the top of this forum on tuning carbs…read both the 4-corner post and the Eddy carb posts?

That's all I have time for right now but do that tonight and I'll keep an eye out for your resp