Octane and ECU Questions

[Jimbo]

At least the"Top Tier" gasoline brands have the same amount of detergents, regardless of octane rating. A decade ago you may have gotten more detergent in premium, not anymore.

In Ford factory service manuals they specifically mention using too high an octane rating fuel as a possible cause of rough running during warmup. My BMW stumbles a little when cold on 91. I run 89 even in our mild winters, 91 in summer.

 

[brianl703]

I was under the impression that most ECUs are programmed to advance the timing to the point just before knocking starts.

 

[1 FMF]

... can easily retard engine timing to help put off detonation to a certain extent. It's sometimes called knock retard. So for say 3 deg of knock retard, then at a given rpm if ignition timing should've been advanced to say 28 deg BTDC, with 3 deg KR it's 25 deg BTDC, the result being less heat in the cylinder causing detonation but at the expense of power and fuel economy. Hotter climates will increase the likelihood of detonation with intake air temp being higher and engine coolant temps running hotter, so a higher octane fuel even in an engine that recommends against premium gas can benefit from more power and economy under certain but unfavorable conditions.

Independent of octane, certain gasoline brands meet different detergent standards. They are called top tier brands, I know shell is one and amoco is probably another. It's the amount of detergents that results in keeping the engine cleaner, so for an engine that recommends against premium gas it is probably better to run amoco 93 vs 87 from a ghetto station selling old stale gas who never change their filters and probably have water in their tanks. Premium almost always won't hurt an engine, nor fuel economy or power, only your pocket.

 

[mechtech]

The ECU has pre programmed spark maps. It can't give any more advance than what is there.
It will, however, retard the spark if it senses knocks . If these knocks are regular in occurance, it will then relearn different parameters and run with less advance.
So you may not knock with 87 because of relearned parameters. 89-91 may provide better performance after the computer relearns for the new fuel, and advances the timing back to where it should be.

 

[jmacmaster]

On page 210 of my 03 Ford Ranger Owners' Manual it states:

"If you are experiencing starting, rough idle or hesitation driveability problems during a cold start, try a different brand of "Regular" unleaded gasoline. "Premium" unleaded gasoline is not recommended (particularly in the United States) because it may cause these problems to become more pronounced."

 

[brianl703]

It would seem to me that the presence of a knock sensor would allow the preprogrammed spark maps to call for a bit more advance than they otherwise would, since there's now feedback about how the advance is affecting the engine provided via the knock sensor.

Without a knock sensor, I would expect the preprogrammed spark maps to be more conservative since there's no way to know if the engine is knocking.

 

[Patman]

quote:

---

I was under the impression that most ECUs are programmed to advance the timing to the point just before knocking starts.

---

This usually isn't the case. In the case of my Corvette's PCM, it has two preset timing curves, one is the high octane table and one is the low octane table. It starts out in the high octane table, which has a maximum timing advance of 28 degrees in the upper rpms. If the knock sensors pick up knock it will retard the timing as needed, but it will never advance the timing beyond 28 no matter what octane you run. If you get a lot of knock for a certain length of time, it kicks into the low octane table which has a less agressive timing curve. It won't go back into the high octane table until you've filled up with fuel again (at least 5 gallons)

When I had my car dyno tuned last week, with the stock computer program in the car (I got a new PCM so I lost my previous custom tune) we ended up getting 7 degrees of knock retard on the first run. So we ended up modifying the stock timing curve so it was less aggressive, with only 26 degrees of timing. We immediately ended up with no knock retard, and picked up 8rwhp.

FWIW, I was running 91 octane and the temperature in the dyno room was 40F. I could've run 94 octane since there are a lot of Sunocos around here which sell it, but since I prefer to use Esso or Shell and they sell only 91 octane, I figured it was best to tune for 91. Then in the hot summer I can run 94 octane when racing and I shouldn't see any knock retard.

 

[1 FMF]

compression ratio while a good indication does not alone dictate the octane rating of the fuel that needs or should be used. A lot also has to do with cylinder head design, piston shape, and valve timing.
Because everyone has the mindset of gotta have more power they say the computer can take advantage of higher octane fuel to produce more power. This isn't true. It's better said that the computer will allow the use of lower octane fuel without engine damage. The correct way to look at it is simply- the engine runs on gasoline which it compresses and ignites with spark at a certain time. Max capable power (or torque) is produced by sparking at the optimum time which varies with rpm from 10 deg to as much as 40-50 deg BTDC. This is all mapped out in the computer. With the use of a knock sensor, the computer can detect detonation. When it happens it

 

[grbr95]

I have friends who use Amoco Ultimate (93 oct) in their cars which recommend 87 octane. They feel that continual use of Amoco ULtiamte will keep their engine cleaner. Bascially, they don't use it for the higher octane, but for the extra cleaning agents. Is there any truth to this? Will it really keep your engine running cleaner?

 

[Donald]

You need to look at the owners manual. Does it say "prefers" or "requires" high octane. If it says prefers then it has knock sensor and it adjusts the timing (and I think the valves).

 

[brianl703]

The Charles O. Probst book "How to tune/service/modify Ford Fuel Injection" states that the the timing is retarded when the knock sensor detects knock, and is slowly increased (presumably to a maximum programmed value) when knock is no longer detected.

I suspect it's a little more complex than that, though.

 

[JHZR2]

that is the explanation for my 04 saab too... upon knock sensor setoff, the timing is retarded to the equivalent of 87 octane, then increased... for my car, its increased to take advantage of 90 octane, max.

Im sure its more complex too...

JMH

 

[OriginHacker21]

Wow, it has been awhile since I've been here - quite a few postings.

Rickey - I believe the 04 Chavilier has the same 2.2L EcoTec just like mine. Yes, I've run several tanks of 91 octane (highest out here in California) and have noticed that 2+ mpg increase, especially on long trips. I'll be taking another 300mile trip tomorrow so I'll fill up tonight and get an accurate MPG reading.

I think JHZR2 hit the right tune. This is somewhat like what the GM Mechanic told me when I was getting my car repaired. The engine was built for 91 octane and what the knock sensor does is retard the timing to allow 87 to run without harming the engine.

The technical side? I have no papers to prove this so thats why I asked here...

Donald - I don't have an owners manual (bought my car used with 6k miles on it) but I am in the process of buying a manual through E-Bay as we speak...

About getting a scanner tool - They are like $200 so I don't plan on getting one soon but if finances allow it in the future I'll do a posting of my findings.

 

[brianl703]

The ScanGauge is $120, last time I checked.

http://www.scanguage.com

I have one. It's a good product.

 

[OriginHacker21]

So what you have do is find out if an extra 2-3+MPG and extra power is worth the higher price of Premium gasoline.

Here in California the difference between 87 and 91 is about 15-20 cents at most. Thats like an extra $160 dollars every 100,000 miles. To me that is not a lot.

What I am trying to find out is if the engine is going to run better/longer on the higher octane because it doesn't have to retard the spark timing...

Any comments?

 

[OriginHacker21]

Wow $120 for all that stuff? I'll order it later today or tomorrow. Once I get it I'll let you guys know how the octane test goes (will probably need help finding out how to do the test right).

Thanks brian703

 

[beanoil]

I'm surprised at the comments regarding 91 octane. Premium in my neck of the woods is all 93 octane. Do not overlook the fact that most pumps are blending units, giving pure regular, or pure premium, and a blend of the two to make midgrade from the two storage tanks. If you require premium, and the person before you filled with regular, the pump, piping and hose are all filled with it when the pump starts. Over a full tank, this amount is probably negligable, but in small quantities, it can have an effect on the octane rating. Around here, it is possible to manually make a 50/50 blend of regular and premium at stations with dedicated hoses (non blend pumps) and get a slightly higher octane than midgrade from a blend pump for the same 10 cent spread. For some, the 90 octane gets them close enough to the 91 than the 89 ( or possibly less from less scrupulous dealers) does for their octane requirements. Do you really need Premium?

 

[Matt_S]

OriginHacker21, the simple answer to your question is no, the engine won't run better/longer on higher octane fuel. That's why they include knock sensors these days. The fact that the computer is retarding timing should have no real effect on engine longevity. All that changes is when the plug fires.

You've noticed more power/better economy from the premium gas, and I believe you 100%, but that doesn't mean the engine wasn't running as designed beforehand. In your case, I'd say there is no real compelling reason to choose one octane grade over another. It's all personal preference. As suggested above, I would suggest you calculate cost per mile with regular vs. premium (vs. midgrade if you're willing) and use whatever returns the lowest number.

 

[stringsattached]

http://www.exxon.com/USA-English/GFM/Products_Services/Fuels/Gasoline_FAQ.asp
http://www.ftc.gov/bcp/conline/pubs/autos/octane.htm

Use the octane your manual calls for. You are basically throwing your money away buying higher grades. One caveat: if YOUR car knocks on the recommended grade (excessive carbon deposits, extreme weather, full moon), try the next level up.

If you want to keep your fuel system clean, buy your gas from a "Top Tier" vendor, and/or dump a fuel system cleaner in your tank once in a while.

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