I just googled this to try and better understand what they mean.
I'm still a bit confused however...
I know putting a higher octane is required to prevent most engines from knocking... But my question is why is it that old cars don't knock when you put the lower octane in them?

 

I think you might be confusing what knocking is with reference to older cars.

Knocking is when the air/fuel mixture ignites due to compression rather than the spark. This is bad because it makes the ignition timing unpredictable and can cause major damage to your engine. Octane in fuel is actually an ignition inhibitor. It makes the fuel harder to ignite. Therefore, if you engine is knocking, if you use a higher octane fuel, it should theoretically stop the fuel from igniting prematurely. Usually though, when your engine begins to knock, it's an indication that something else is wrong. Compression ratio too high most likely due to maybe an incorrect gasket thickness, carbon build up, wrong piston valve relief, etc.

I've never really heard of a situation where putting in a lower octane fuel would help an older engine from knocking. Where did you hear that from?

 

The higher the compression ratio of an engine, the greater the chance of an engine having pre-detonation. Which is why premium gas (higher octane rating) is required and/or recommended.

More aggresive timing + high compression engines + low octane (87,89) = Poor engine

Someone correct me if im wrong.

 

I think you're misunderstansing his question. He's asking why older cars can get away with lower octane gas as opposed to some of the newer cars out there. Or atleast why some older cars don't require premium gas.

If that's what he's really asking, I believe its due to the fact that a lot of newer engines are pushing higher compression ratios and have been tuned specifically for the characteristics of higher octane to squeeze out as much performance as possible.

Again, someone correct me if im wrong.

 

Skater, you hit my question on the head. Thanks!
And itzcool, thank you for the summary.
Much appreciated.

 

Ahh. I missed the "don't knock" part. My mistake.

 

The problem is often the way octane is measure at the refinery and what it ends up in your tank after traversing pipelines and blending with others in terminal storage long before it get to service station. In ATL it takes 16 days for gasoline to get from refinery to stations.

The point is the marketing numbers on the pump may have little truth and only that premium is higher than regular and midgrade is somewhere inbetween. Few state labs are accurate enough to prove +- 2 octane points...........all states care about is that higher priced premium actually is some higher in octane than regular due to price differences.

Due to US requirements to improve MPG..........current compression ratios are higher than those used on average in 80's and 1990's primarily because they can be due to OBDII computer control.

 

This becomes a bigger issue when talking about E10 (Ethanol blended gas). The longer it is stored, the higher the risk is for water absorption and phase separation.

 

on a side note, a local station with an E85 pump has an octane rating of either 101 or 105. I'm getting old so i forgot which value.