schardine

I recently experienced an anomoly with my '90 C4 (88K miles). It has seemed sulgish latelty with poor acceleration and flat spots. I have been running either 92 or 93 octain gas.

At the same time, but not linking the two, I was reading this thread. I decided to try some lower octain gas because of the price alone. I put in a tank of 87 octain expecting to get some pings and lower performance. To my surprise the performance is better. The idle and acceleration is smoother with a noticable incrase in response and torque. I can't hear any pings but that may be because of engine and exhaust noise.

This anomoly may be an indication of what is wrong with my car that made it run poorly on the higher octain. Any ideas?

springer3

It sounds like you got some really poor 93 octane, or else really good 87 (I guess it is legal to sell the higher grade on the low grade pumps). Was there any difference in temperature or humidity? That would make a big difference.

Arjan B.

I never did understand why the US guys have such a low octane fuel??

If we converse it, we can drive only on as low as 95 Octane [european standards] Higher is always better, lower is dangerous.

springer3

Arjan:

I think part of the problem is that are at least three methods for reporting octane ratings. The "motor" method is a direct measurement using a variable compression ratio test engine. The "research" method is done based on chemical analysis, and typically gives a higher number than the motor method. In the US, we tried to eliminate confusion by adding a third method, which is average of the motor octane number (MON), and the research octane number (RON). This method is called (R+M)/2. That is what is displayed on the gas pumps in the US.

I don't know which number the Europeans use, but it sounds like the research number. If that is the case, your 95 octane is exactly the same as our 93 octane.

A little more octane trivia for anyone interested:

The octane rating is based on the anti-knock properties of iso-octane, the hydrocarbon with 8 carbon atoms. Pure iso-octane has an octane rating of 100. In the motor method, the research engine is run on iso-octane to determine the compression ratio when knocking starts. Then the test fuel is run to determine its knock properties. The motor octane number is measured by direct comparison with the anti-knock properties of iso-octane.

In the early part of the last century, iso-octane was a by-product from lubricant manufacturing . It was sold as a cleaning fluid, and it was cheap and abundant. Then came the automobile, which runs great on iso-octane. In just a few years, there was not enough iso-octane compared to lubricant production. The heavier hydrocarbon chains in raw petroleum were "cracked" to provide more light fuels. The resulting mix of hydrocarbons has less knock resistance. Octane boosters are added to to raise the octane rating for cracked fuels. One reason old fuel gets "gummy" is that the cracked molecules are chemically active, and manage to rejoin, forming heavier oil and grease-like substances.

garrett376

That was interesting... learn something neat every day here!!!

Arjan B.

Springer,

Thanks for the coversion.

So, that means that a 964 can only drive in the US with a different DME programming for ignition. W may drive only as low as 95 European octane, which is 93 for US habits. Anything lower would cause pinging.
In my visits to the US I never noticed 93 octane at a gas station as far as I can remember........

I saw 87 [which I fill the rental car with, tipical Dutch habit ] 89 and 91/92 octane. Probably the 964 DME has a different programming then.

95 Is the lowest we can get overhere, over in Germany they have still 91.

springer3

I think one reason the European fuel has higher octane ratings is that fuel economy is better with higher octane in engines with knock controlled ignition timing (most engines these days). In the US, there is less concern over fuel economy. Perhaps now that fuel prices are rising, the octane ratings will improve.

Arjan B.

Octane level higher.........prices up probably.......

Syd B.

I was told that lower octane fuel burns cleaner than higher octane fuel. (less pollution). Any truth to this?

springer3

IC engine pollution is a complicated subject. Measures to reduce one type of pollution generally raise levels of other types. The answer to your question depends on which pollutants you are talking about. Assuming you have a modern engine (964s are modern), you will burn less fuel, and therefore generate less emissions, with higher octane gas (but only up to the level that the car can use). If you run premium in a car with mechanical spark advance, and you don't change the engine controls to take advantage of the higher octane, you will burn extra octane booster with no benefit. In that case, your emissions would be slightly higher.