Ford Focus Forum banner

1 - 6 of 6 Posts

231 Posts
Discussion Starter #1
I'm just gonna post this here since I own an SVT and maybe somebody can help. But I was curious, I've been hearing that you can actually gain HP by putting 93 and above octane in our engines instead of just 91. But than i've also been hearing that you won't gain hp, but the car will just run a lot smoother...which one is true? Do you actually gain hp?!

885 Posts
The octane issue is all about the computer controlling the engine perameters.
i.e. the timing. the ecu (computer) adjusts timing of the spark, as well as other variable, accourding to the imput from sensors.

if a sensor sends a message of ignition detonation from too little octane, the engine performance is reduced to avoid "knock" or pre-ignition or toooo fast burning of the fuel/air mixture.

SOOOOOO the theory is if there is "super" duper gas in there the computer will adjust for more power.

NO, there are set perameters that are not exceeded for the engine, given that it has the 91 octane that is planned for use.

certainly using lower octane reduces the performance, and may lead to damage if the computer can't accomodate the low octane.

Keep in mind that the octane number/rating used to day is NOT what was historically used. there is research octane and motor octane. the feds declared years ago that an average of the two would be the standard for advertisements.

Now everyone refers to that "averaged" number.

12,749 Posts

Some of my quotes from that thread:
The reason I stated that is because Octane does not provide more power in and of itself. Octane is nothing more than the gasolines ability to resist knock. So 100 octane will be more resistant to knock than 93. That's IT!

It does not burn hotter, it does not burn more smoothly, the burn properties are no different. The difference is solely the resistence to knock. So while you believe you've experienced an increase in power, I would wager that what you are experiencing is mental and not physically happening. It's typical for anyone really. You change something, you want it to make a difference, so you just take more notice and "believe" you feel something.

But octane cannot give you more power (unless your car is detonating and the knock sensor is pulling timing, in that case your tune is jacked and yes octane would help). Octane is nothing more than resistance to knock. That's it. No power gains, not cleaner burning, not smoother burning, not hotter, nothing... it just resists knock better.

If you want to read up on it, read this thread on timing

The guy BillW is an expert and explains everything very well. If you do a search, you'll find other threads. Sorry, I sincerely doubt you gained any power or added peppiness.

Unfortunately I have to disgree yet again. There IS a point to 100 octane and it is to AVOID detonation which will destroy your pistons and possibly your engine. It is necessary for some engines because of the power they are putting out. If we raised the compression in our engine to say 13 or 14:1 then the ONLY way we'd be able to do it without killing our engine, is to increase the octane used.

I don't mean to keep harping on this, but it's important that people get the correct facts concerning octane, and not what you "think" happens. I'd feel bad telling someone to go spend 5.50 a gallon on fuel that will do jack-squat for their cars.

The best octane for your car is that number listed on the inside of your tank. Any more is your perogative, but don't expect anything from it. IT does not give you more power. Octane is a safety net. It saves engine, it has a HUGE benefit for those that need it. For a NA SVT Focus engine with bolt-ons, it is nothing more than an expensive fuel that one can brag to his or her friends about. Kind of like stickers.

Here's some more "detail" for you, since you obviously won't believe me.

* Octane has something to do with the flame propagation speed (what I
think this guy meant by "speed of burning" (sic)).
* Octane rating has something to do with the heat content of the fuel.
* Octane rating has refers to anything other than knock resistance.
The fact is, octane rating has NOTHING to do with flame propagation,
heat content or anything else other than the fuel's ability to resist
knock in a spark-ignited engine. Period. Octane is an arbitrary
scale based on the ability of iso-octane to resist knock. Iso-octane
is arbitrarily defined to have an octane of 100. The other end of the
scale is arbitrarily anchored at zero octane by N-heptane. Numbers
between 0 and 100 represent the knock resistance of a mixture of
the two substances. For example, 80 octane is represented by a mix
of 80% iso-octane and 20% n-heptane. Numbers greater than 100
are simply extrapolated. Fuels are compared to these mixes for the
purpose of determining the octane rating of the fuel in the Waukesha
Knock engine using either the Research method (considered obsolete for
modern high speed engines) or the Motor method (closer to realistic

Since the Waukesha engine, which turns under 900 rpm
under all conditions, is pretty remote from modern engine conditions,
the concern is how the arbitrary scale relates to real-world conditions.
One measure is the severity index. The severity index of a fuel
is simply a measure of how the fuel as the severity of the conditions
in the combustion chamber are changed. A common indicator is the
fuel's sensititivity. The sensitivity is simply the span between the
research and motor octane number. If there is a wide spread (the motor
number invariably being lower), the fuel is likely to perform poorly
in a high severity environment compared to a fuel with the same
average octane number but with a smaller span between R and M.
For more information on the testing methods, see:

Motor ASTM-D2700
Research ASTM-D2699

As to the claim that high octane fuels contain less heat energy
per unit volume, consider this. Looking at "Automotive Fuels Handbook",
Fig 6.6, the octane of pure straight run naptha is about 73. It
jumps to about 90 with the addition of 1 gram/gallon of lead in the form
of TEL. Using the most common form of TEL, Ethyl corp's compound
CR-50, one gram equiv amounts to .62 cc of CR-50. As one can easily
see, the addition of this tiny amount of CR-50 materally alters
nothing about the fuel except its octane rating. How a target octane
is achieved in lead-free fuel is more complex but the BTU/gal and
density is fairly closely regulated by ATSM and EPA specs. The reason
being, of course, that it is highly undesirable for fuel to vary much
which would affect the calibration of engine management systems, the
intentional oxygenation of fuel in certain cities in the winter being the

In summary, any claim that octane has anything to do with any property
of a fuel other than its ability to resist knock in a Waukesha test
engine is completely false.

For additional reading, I suggest:

Automtoive Fuels Handbook by Keith Owen and Trevor Coley.
Available from SAE. Yes, it is expensive and yes it is
exhaustively complete.

The above referenced ASTM standards.

A recent series of articles in Circle Track Magazine, available
in reprint, in which Smokey Yunick did an exhaustive test of
high octane racing fuels which included lab analysis of the fuels.
What is illuminating about this test is the wide variety of
chemicals used by the different vendors to achieve nearly the
same result.

Performance Engineering Magazine
Here's a quote from Dave Lyons, a noted mustang engineer. (some people dont' like him)
So you're at the drag strip and you want to impress all your friends.
You know just the secret to make a little extra power. You just slip
a little 112 octane racing gas in the tank and blow everyone away.
Sounds great. Good plan, right?

Except that it just cost you 16 horsepower.

That's right. Unless your car has 11:1 compression, a 150 shot of
Nitrous Oxide, or 9 psi of boost from a supercharger, racing gas will
slow you down.

High octane fuel burns much slower than low octane fuel. The slow
burning process keeps detonation from occurring, and is a necessity
in high compression race engines. But with a 9:1 compression street
motor, that slow burning process means the flame front doesn't push
on the piston as hard, costing you horsepower. Completely stock cars,
with stock ignition timing and all, have even been shown to make more
power on the dyno with 89 octane gas instead of 93. Making matters
worse, racing gas which contains lead will ruin your catalytic
converters and hurt the performance of your engines oxygen sensors,
causing poor fuel economy and throttle response.

Okay, you've got all that. But what if your car has a supercharger?
Racing gas will help you, right?

Yes, but only if you take advantage of the higher octane by increasing
ignition timing and/or boost level. Just dumping a couple of gallons
of Cam 2 purple won't make your car faster unless you make some
adjustments. Once the fuel is in the tank, increase the timing by a
few degrees. Start slowly, and make a run down the track. Keep
increasing timing as long as your trap speed keeps increasing. As
soon as your speed stops increasing, go back to the last setting.
With 10 psi of boost it is possible to pick up 20 horsepower or more.
100 octane unleaded works best for street cars because it doesn't ruin
your O2 sensors like leaded fuel. Nitrous Oxide users can see the
same benefits. Consult your Nitrous Oxide supplier for their

And to think I went 14.484 with race gas and 6-8 degrees of initial
timing on worn street radials and lifting my foot at the end to avoid a

David A. Lyons
And here is a website with the Federal Trade Commission, which explains that going with fuel that is of a higher octane than what is recommended will get you nada, nothing, zippo, zilch etc... The reason being??? Because octane does nothing more than fight knock.
Here is the website:

I'd do more research to try to find out the differences in the refinement process, but I'm pretty tired. So with this I'm done. But IMO there should be no question here. However it's just my uneducated opinion.

3,212 Posts
I am not familiar with the SVT ECU, but I wouldn't be surprised if there were some minor gains from going with 93 instead of 91, if 91 is what Ford specs for the SVT(?).

For my ZX3 the ECU could advance up to 10 degrees in the absence of knock being reported by the knock sensor. At timing cells that represent normal cruising loads and RPM, it could advance up to 7 degrees above computed timing when there was an absence of knock. (I use past tense as I disabled KS advance with my software.) In the presence of knock it will retard up to 5 degrees.

Now both of my ZX3s pinged like hell on 87 and warm dry days at a moderate cruise, say 65 MPH going up a slight grade. They also would deto bad on hard acceleration, and particularly when WOT, shifting, and then going immediately to WOT again.

It seems to me that Ford set timing at, or even well into knock under some atmospheric conditions, then let the sensor sort it out.

I assume that when I could clearly hear knock going down the road, the KS had already pulled all 5 degrees.... though I don't know that for a fact.

In any case I am pretty confident that higher octane under those circumstances would have both stopped the deto, and allowed the ECU to advance timing considerably.

A few degrees of timing is noticeable to an astute driver, and 4 degrees or so is pretty darn noticeable.

So anyhow I think it is possible. That said, my stock Focus still got fed 87 unless I knew it was going to get thrashed, then it would get 89.

39 Posts
Well I have never ran anything except 93. All the gas stations around here have .... 86, 89, 93, 96 ..... I havent ever seen a station that had 91??
1 - 6 of 6 Posts