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Discussion Starter #1


It's nice to think that you're giving your car a "treat" when you pull up to the pumps and select a Premium grade of gasoline. Unfortunately, the fact is that you're better saving your money and treating it to a nice detailing. At least you can see the results from that.

While Old Wives didn't really know anything about octane, they seem to get blamed for a number of incorrect tales involving gasoline. Let's look at some of them.

High octane gasoline increases power

According to the Minnesota Department of Commerce, "If your car is designed to run on 87 octane gasoline, you shouldn’t notice any more power on high octane gasoline. If it does make a noticeable difference, your engine, or the engine’s electronic control systems, may need repair."

An Octane rating is an indicator of a fuel's ability to resist premature ignition, or "ping." Other factors can also impact the likelihood that an engine may ping, including ambient temperature and humidity, engine timing, cooling system effectiveness and more.

A requirement for the use of higher octane fuel may come from engine modifications such as higher compression ratio and advanced ignition timing. When an engine pings, power is lost because ignition usually doesn't happen at the right part of the cycle.

The flame front initiated by premature ignition can actually work against that from the spark plug in two ways. First, it consumes fuel that would have been available to the primary flame front. Second, because it is "premature" the increased cylinder pressures work against the rotation of the engine, slowing it down somewhat.

High octane gasoline improves mileage

The mileage you experience is mainly determined by the energy content of the gasoline, which has nothing to do with its octane rating, and the efficiency with which it is burned by the engine. Ethanol, for example, has a significantly higher octane rating than regular gas, but a lower energy content per gallon. As a result overall gas mileage when running E85 is lower than with E0 or normal gas.



High octane gasoline gives quicker starting

As it turns out, there is no strong correlation between octane rating and starting ability.

My engine has a knock sensor, so I don't have to worry.

Modern engines equipped with knock sensors that can detect detonation and modify engine timing to eliminate knock. If the condition is persistent, running the engine on retarded timing will have an undesired impact on both power and fuel economy.

Knock sensors work by listening to engine vibrations, particularly in the range of six to eight kilohertz. When detected, the sensor raises a signal to the control computer which reacts as described. Certainly, knock sensors can fail, so if you do hear pinging in your modern engine, something requires fairly immediate attention.

At the End of the Day

Buy the gasoline grade that your ride was designed for. Buy good quality product in that grade and you'll be saving money in the long run.

 

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Good point, what are the SRT8 running? I have lots of access to 93Octane and exclusively run that. Could I run 91Octane and get away with it? What would i give up?
 

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i dunno because i have a 65 thunderbird, it runs okay on reg

when i put premium in it, it feels like it wants to take off as soon as i put it in gear

i can also burn out with premium, no luck on reg
 

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Great info here. Thx.
 

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I suppose you could de-tune (retard the timing advance with a Predator) and run a SRT on 91 octane. You will loose performance. Remember why you purchased the SRT instead of other cars??

I exclusively run BP 93 octane with Johan's CMT tune and could not be happier. Got a patch of dry interstate yesterday morning very early going to work, while it was about -5 outside, did the 55 mph "clear the throat" to about 80....WOW! All I got to say is it did not take very long. What a wonderful machine.
 

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Discussion Starter #8
While the article was primarily intended for fuel injected cars, the answer remains the same. If your car was originally set up to run regular gas, say 87-octane, and you feel a significant difference when burning premium, then something is "wrong" with the current setup.

By wrong, I really mean changed here. It could be a different cam, ignition timing, compression ratio, carbon deposits on the piston and a few more things. You may have made some changes to try and get more power out of the original configuration, but they really only come into effect with a higher octane gas.

Given the age of the car, it may well be that you're actually getting less power from the changes and the use of premium gas is simply restoring it to where it was without the changes.


i dunno because i have a 65 thunderbird, it runs okay on reg

when i put premium in it, it feels like it wants to take off as soon as i put it in gear

i can also burn out with premium, no luck on reg
 

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Good point, what are the SRT8 running? I have lots of access to 93Octane and exclusively run that. Could I run 91Octane and get away with it? What would i give up?
91 is premium, like the article is stating 93 would be a waste unless you are using a Predator (or something of the like) to take 'advantage' of the extra octane. Although even that is up for debate. I ran the Predator 93 CAI tune in my car (with 93 gas), but actually got better results using the 91 CAI tune and 91 gas. IMHO I would stick to 91 and save 20c a gallon.
 
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