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Octane believer now!

Old 03-27-2015, 05:35 AM
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Default Octane believer now!

There was another thread that touched on it but,

I have always been a cheapskate when it comes to premium gas. I have had a lot of German cars and always felt comfortable (still do) that they adjust timing to correct for knock, etc.

It is a fact that after driving 100's of thousand miles on Mercedes, audis, passat, and most recently a run of 3 different 3-series BMW; I have not had one fuel related issue.

After the other THT thread I decided to fill up with premium and see what the difference is.

results after one tank and I will continue to record finding (for myself at least) after further premium fill ups.

Little more pep. No way to quantify it, but especially over 3k I feel more pep- like when passing.

Drove 1/2 tank so far on 93 octane premium in mixed city and 2 days of commuting 20 miles and my mileage computer is recording 3.5 better mileage. I reset one mpg computer each fill up as a habit. The other I leave alone as it acts like a mpg recorder for the life of the car.

Car is bmw330 which has a 3.0 liter straight 6 and is high compression.

color me a believer.

now 3.5 mpg better isn't curing cancer or going to save me enough $ to buy another boat. But it is enough to keep me filling up with premium.
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Old 03-27-2015, 06:03 AM
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I have vehicles that require it and I notice the difference. Even in an SUV. Seems like one of those debates that could go in circles forever. I did read an interesting article on octane rating and how the octane numbers are assigned are different in different countries. That's why in Europe they have super high octane ratings when the fuel is not much different than ours. You would have to be a petroleum engineer to understand it, but interesting reading for sure.
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Old 03-27-2015, 06:14 AM
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my 2006 Passat 2L calls for minimum 91. We usually use the mid-grade 89 here with no problem. This week we were at Costco and filled up with 93. Normal highway mpg is 31-32, and yesterday we drove the GSP and got... 35+. I'm convinced it works, but I think it's still cheaper to stick with the 89. 87? I won't try it in the Passat.
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Old 03-27-2015, 09:45 AM
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My understanding of it (I could be mistaken) is that all the octane level does is to allow a high compression engine work as designed without pre-ignition. If the knock sensors/computer detect pre-ignition (related to the lower octane) they retard the timing and that reduces power. That's why high octane gas does nothing to add power in an engine designed for 87.
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Old 03-27-2015, 10:27 AM
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Originally Posted by Cousin Eddie View Post
My understanding of it (I could be mistaken) is that all the octane level does is to allow a high compression engine work as designed without pre-ignition. If the knock sensors/computer detect pre-ignition (related to the lower octane) they retard the timing and that reduces power. That's why high octane gas does nothing to add power in an engine designed for 87.
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Old 03-27-2015, 10:52 AM
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I have a Dodge Hemi. With the cheap gas you get poor mileage and poor performance. With the mid-grade it gets better mileage and runs better. I save no money going with the cheap stuff
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Old 03-27-2015, 11:18 AM
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The lower compression motors like 6.0 gm running high octane will make the computer go crazey
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Old 03-27-2015, 11:21 AM
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Originally Posted by ladyjane View Post
The lower compression motors like 6.0 gm running high octane will make the computer go crazey
How?
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Old 03-27-2015, 11:21 AM
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Originally Posted by Cousin Eddie View Post
My understanding of it (I could be mistaken) is that all the octane level does is to allow a high compression engine work as designed without pre-ignition. If the knock sensors/computer detect pre-ignition (related to the lower octane) they retard the timing and that reduces power. That's why high octane gas does nothing to add power in an engine designed for 87.
This is correct. Only one minor bit, and that is it usually is not preignition, but detonation. Preignition is where fuel starts burning before the spark, detonation is when burn starts normally with spark, but near the end of the burn the remaining fuel mix ignites spontaneously from heat and pressure. Both are bad in that pressure rises too fast and makes the knock. Sensors pick up on this and retard timing and possibly richen mixture. I think modern sensors are sensitive enough to pick up the beginning of detonation before we can hear it.

It really depends on the car/engine. Some don't get a mileage hit on low grade, some do. Just need to experiment and see what it likes best.

High octane means it spontaneously ignites at a higher temp than low octane. That's it. And usually high octane has a slightly lower BTU/gallon content.
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Old 03-27-2015, 11:28 AM
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I can also tell you premium fuel contains more detergents, keeping things a bit cleaner.

Last edited by Eastport205; 03-27-2015 at 12:06 PM.
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Old 03-27-2015, 11:33 AM
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Originally Posted by Corndog38 View Post
This is correct. Only one minor bit, and that is it usually is not preignition, but detonation. Preignition is where fuel starts burning before the spark, detonation is when burn starts normally with spark, but near the end of the burn the remaining fuel mix ignites spontaneously from heat and pressure. Both are bad in that pressure rises too fast and makes the knock. ....

This is almost correct, but for one slight thing.

The actual "knock" is from multiple flame fronts hitting each other (like high pressure waves that smack). If you could look into the cylinder during a knock event, you can get different modes of knock. Think of it like different ways to cut a pie (3 big pieces, one round piece in the middle and sections in the outside, 7 pieces, etc). Each of those modes will create a different frequency of knock. The loudest and easiest frequency to pick up is the lowest (about 6Khz). The problem with that is that there are often other noises in the engine that create noise at that frequency making the sensor deaf to slight knock. Some systems have band pass filters to window/look for higher frequencies (higher knock modes) and avoid primary engine noise.

... but I digress...
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Old 03-27-2015, 12:13 PM
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I'm pretty sure the 345hp 6.0 liter in my Silverado is not low compression. It's 10.08 to 1 I believe. 8.5 to 1 is low to me, 10 to 1 is on the higher side compared to most street engines.
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Old 03-27-2015, 12:19 PM
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i dont know the technical stats but my 6.2L GMC runs better on 93 than it does on 87. seems like more now than ever with 160,000 on it
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Old 03-27-2015, 12:25 PM
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Originally Posted by Cousin Eddie View Post
My understanding of it (I could be mistaken) is that all the octane level does is to allow a high compression engine work as designed without pre-ignition. If the knock sensors/computer detect pre-ignition (related to the lower octane) they retard the timing and that reduces power. That's why high octane gas does nothing to add power in an engine designed for 87.
Originally Posted by Corndog38 View Post
This is correct. Only one minor bit, and that is it usually is not preignition, but detonation. Preignition is where fuel starts burning before the spark, detonation is when burn starts normally with spark, but near the end of the burn the remaining fuel mix ignites spontaneously from heat and pressure. Both are bad in that pressure rises too fast and makes the knock. Sensors pick up on this and retard timing and possibly richen mixture. I think modern sensors are sensitive enough to pick up the beginning of detonation before we can hear it.

It really depends on the car/engine. Some don't get a mileage hit on low grade, some do. Just need to experiment and see what it likes best.

High octane means it spontaneously ignites at a higher temp than low octane. That's it. And usually high octane has a slightly lower BTU/gallon content.
yes these are correct

One addition low compression (mechanical compression ratio-wise) engines with superchargers like the Chevy 3.8supercharged that require hi test also get more power and fuel economy with the hi test.

But basically as said above if the manual does not require it there is a good chance you are just wasting money by paying more per gallon a (for less BTU's) and getting no better and worse gas mileage (again since you have less BTU's per gallon.
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Old 03-28-2015, 05:57 AM
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It's all about cylinder pressures. At part throttle, the intake charge is restricted by the throttle plate, and not as much air enters the cylinder. Less air/fuel/less pressure/less need for "octane"/less power. At cruise, any grade gas will be fine. Wide open throttle, you need higher octane, or the ECM backs up timing to delay the point at which maximum pressure occurs.
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Old 03-28-2015, 06:21 AM
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Originally Posted by ladyjane View Post
The lower compression motors like 6.0 gm running high octane will make the computer go crazey
I heard this too. I have an Escalade which has the 6.2 engine. I'm a member on the Cadillac forums and some people say when they put premium gas in their check engine light came on.

The truck has loads of power on the 87 octane, if it would have more power on premium I would certainly use it.

One thing I do notice, sometimes when I fill up the engine will idle a little rough. I assume some gas is poor quality and probably has more than the advertised 10% ethanol.

And that brings me to my next complaint. Ethanol is the real problem. It robs power and lower mileage. Find a gas station that sells "rec 90" which is 90 octane non ethanol and fill up with that and see what happens. I wish there was a gas station around me that sold it, I would never get ethanol gas again.
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Old 03-28-2015, 06:28 AM
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I drive a diesel pickup truck that will pull down a house with anything you want to burn in it from cooking oil to 40 Cetane.
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Old 03-28-2015, 06:37 AM
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[QUOTE=billinstuart;7704526.... At cruise, any grade gas will be fine. ....[/QUOTE]

Kind of, depending on what "at cruise" is defined as.

If you are driving on the highway, 75mpg, the engine might be about 2000rpm in 6th gear, locked. If you tip in to pass, or the cruise tips in to go up a hill, the throttle may only need to go to 25% to get high MAPs to where the engine is borderline limited. This means that there is more TQ available if the spark could be advanced but the engine will knock before you could advance it that far. Often EGR was introduced to slow down the burn and allow more spark advance. Also, driving the same road/hill with cool & high humidity lower octane fuel may not matter, but that same trip in hot & low humidity may result in some spark being pulled out.

I summary, especially on a vehicle that "requires" higher octane, there are times that octane matters even at cruise.

Edit: The other part is what is meant by "matters". So at cruise on a hill or accel a little spark is pulled back, a bit less fuel economy, does that really matter. The driver would never notice it.
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Old 03-28-2015, 06:41 AM
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For the ones getting better MPG, what does the cost per mile end up to be.
In other words, does, increased MPG + increased $ per gallon= $ savings per mile?



.
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Old 03-28-2015, 07:31 AM
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Originally Posted by ericinmich View Post
Kind of, depending on what "at cruise" is defined as.

If you are driving on the highway, 75mpg, the engine might be about 2000rpm in 6th gear, locked. If you tip in to pass, or the cruise tips in to go up a hill, the throttle may only need to go to 25% to get high MAPs to where the engine is borderline limited. This means that there is more TQ available if the spark could be advanced but the engine will knock before you could advance it that far. Often EGR was introduced to slow down the burn and allow more spark advance. Also, driving the same road/hill with cool & high humidity lower octane fuel may not matter, but that same trip in hot & low humidity may result in some spark being pulled out.

I summary, especially on a vehicle that "requires" higher octane, there are times that octane matters even at cruise.

Edit: The other part is what is meant by "matters". So at cruise on a hill or accel a little spark is pulled back, a bit less fuel economy, does that really matter. The driver would never notice it.
No argument. Definitely a balancing act. Maybe another example was "restrictor plate" racing in NASCAR. Could only run wide open at high rpms because the mechanical compression ratio was so high, the engine needed the restriction to keep cylinder pressures and temperatures in line.

An interesting analogy is aircraft engines. Only run about 24 degrees total advance, even with the huge piston diameter. But, they have dual plugs for 2 reasons..redundancy, and to initiate 2 flame fronts. A 'mag drop" works because the pilot eliminates one flame front, effectively retarding the timing. Too much initial timing, and you get no mag drop.
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