Learn me something=Octane+timing

[10musketeer]

ok, after reading numerous threads arguing about the octane of fuel, I'm still confused about something.

If octane changes ignition (gets rid of pinging), why wouldn't you use a lower octane and then adjust your timing? I think the answer has to do with compression, but I have to admit I'm at a loss.

O.K. all you guys with the knowledge, start teaching! /ubbthreads/images/graemlins/grouphug.gif

 

[aeronca65t]

First, you should use the lowest octane rating that will run in your car with timing and other adjustments set properly. Many of these cars will run fine on "regular" or "mid-grade". Putting "high test" in a car that only needs 87 octane is a waste of money.

And please keep in mind that some of these cars had specs that were written years ago, when octane ratings were measured differently. Modern "89 octane" fuel may be equal to older fuels rated at around 93 or so (in the '60s).

But some of our cars need high octane fuel or, like mine, have been modified to require higher octane fuel.
My car has around 10.25:1 compression ratio. I run 100 octane race-gas so that I can run the timing up around 32 total degrees BTDC (at over 5000 rpm....I don't really care where it runs down at lower rpms).
If I run 93 octane street gas, I have to back the timing down to around 26 degrees (at 5000+ rpm). This means the spark occurs when the fuel mix is less compressed...in fact the fuel mix "thinks" it is only being compressed to about 9:1. This is an OK compromise, but it essentially "wastes" my high-compression pistons and reduces efficiency (meaning that I burn more fuel per hour).
I can get better mileage and much more power if I use the correct fuel (for my compression ratio).
Same on an LBC street car: If a car is supposed to use 93 octane and you back the timing down so that it will not ping on 87 octane, it'll run fine but make less power and burn more fuel (because it fires when less compressed, so you get less energy out of the fuel).

Modern fuel-injected cars with knock sensors and better combustion chamber design can throw all this logic out the window....some modern cars can run super lean fuel ratios with low-octane gas and 11:1 compression ratios. They can make great power, pollute less and get excellent mileage to boot. This is the beauty of computerized systems....but you can't "futz" around with adjustments like our old cars, so they are less fun.

 

[10musketeer]

Thanks! That's actually one of the best descriptions that I've heard. That really helps me understand this a lot better.

So what is the best way to tune the engine for overall performance? I know how to do each one (carb. mix, timing, etc.) individually, but what is the best method to use to know if they are working together at their peak performance? Do you just "feel" it out, or use a product like colortune?

 

[jlaird]

Yep, back and forth, back and forth. And of course the color of your plugs and exhaust tells the tail.

 

[spritenut]

I'm running a nicely modified street 1275.
I always ran high octane (Super at the pump) gas.
But last summer with super running 3.50 a gallon or more, I dropped to the mid grade. My car doesn't like that.
So I retarded the timing but I miss the snappy feel of what used to be. So back to high test gas and the timing where it likes to be.
You will have to drive and feel your own engine wants and needs. But a stock 1275 runs fine at factory specs on mid grade gas.

 

[jlaird]

And a stock 948 always ran well on regular. That was under the old rateing system, no clue as of today.

 

[DrEntropy]

Compressing a gas (air/fuel mix) to the highest potential explosion at the time of ignition is what it's all about. Less volitility at pressure peak is less efficient. As is lighting the mix off too early or too late. "Pinking" is pre- or post- ignition detonation, not efficient and can be downright destrutive. The fuel managed engines in the 'modern' cars can optimize burn with what ~some people~ think is a good idea: mixing computers with cars. The idea is to monitor and adjust at the "speed-o-light" to continuously "peak" the end-result... My butt is all the 'pooter I want in my cars...

IT knows if the thing ain't right. /ubbthreads/images/graemlins/jester.gif

...and don't all you OBDII guys go jumpin' on me all at once. I know sliced bread has its place. /ubbthreads/images/graemlins/devilgrin.gif

 

[swift6]

And why you run the lowest octane you can get away with is because the lower the octane the more volatile the fuel. Contrary to popular opinion high octane fuel is not high power fuel, though high power engines tend to need higher octane fuels(which is where most of the confusion comes from). Lower octane fuels actually contains more energy. The higher the octane, the more stable the fuel. The added stability is what helps prevent pre-ignition. The higher the compression, the more control is needed for the flame front of the 'explosion'.

The increased stability of high octane fuels is also what adds to the increased shelf life.

There are also two different methods used to determine a fuels octane rating. I believe England uses only one of these methods while the US uses an average of the two.

/ubbthreads/images/graemlins/cheers.gif

 

[10musketeer]

great info guys. I suppose I'll just keep tweaking things until I think I got it right. That's why I have this car in the first place- to tinker and learn with. My engine's data plate is gone, so I think I can safely assume it has been rebuilt, and therefore probably new valves-so unleaded should be o.k. What modern octane is closest to the original intention?

 

[drooartz]

So I have a related octane question. Here in Utah our gas is a lower octane generally (regular =85, mid = 87 or 88, premium = 90 or 91). Up here (5550') mid grade is 87--this is what my daily (Hyundai) calls for, so it's what I use.

My question is: do cars run okay on lower octane at higher altitude?

 

[10musketeer]

I think they would require a lower octane (more volitile) fuel so it will compensate for a lower air density. I know that electric motors will fail sooner due to arcing from brushes because of less resistance (air density).

Of course auto ignition characteristics are pretty new to me so this is a shot in the dark. /ubbthreads/images/graemlins/wink.gif

 

[swift6]

Drew, I'm nearly the same altitude here in Northern Colorado as you are and our Octane ratings are the same as yours. At our altitude you have to remember that we essentially lose about a full point of compression due to the lower air density. 9:1 becomes 8:1 etc... One reason why supercharged and turbocharged cars are so popular at altitudes. They help counter the lower air density by shoving more air in. That essentially lower compression means that even lower octane is necessary. John is spot on with his response here.