Difference between 87 and 89 octane- worth it?

[baraff]

Hi everyone - We have a '05 Winnie Aspect with the Ford V10. The manual states that 87 octane gas should be used. With 87 I have noticed a slight engine ping on grades. My mileage averages about 8.9-9.1 on a good day.
I remember my Dad moved up to 92 octane in his Toyota Avalon and his mileage improved substantially. Now, realizing that his Toyota and my Winnie are apples and oranges, I'm not sure this is a good comparison however I was wondering has anyone done the experimentation and the math and determined whether its worth the extra cost?
At the very least I am hoping that 89 or 92 would eliminate the pinging. (probably due to ethanol addition).
I did a search on the forums here but did not find much on the topic. One guy claimed an 8.9% mileage increase using a higher grade of fuel, but I was wondering if anyone else had similar results?
Thanks,
-Burns

[Wanabee FTer]

Hi baraff,
Your MH is designed to run on 87 octane unleaded fuel, so theoretically you should not need to run 89 octane. That being said I suspect you are right about additives in the fuel may be causing the end product to have an actual octane rating that is less then specified. Have you tried changing brands of fuel yet?

Another possibility is a faulty sensor. On your engine there is a device that retards the spark timing to reduce pre-ignition (spark knock). If it is faulty it could be causing the ignition timing to stay too far advanced. This could cause the engine to ping under load with 87 octane fuel. I would suggest having the knock sensor and its wiring inspected.

Spark knock (pinging) is caused by fuel burning too fast or too soon. The rapid ignition of the fuel pushes back against the piston while it is still on the upward compression stroke and tries to kick the piston back down before it should causing a pinging sound.
The octane rating of the fuel is an indicator of how fast it burns. The higher the octane rating the slower the fuel burns. If this problem is left unchecked it can damage the pistons and cause pre-mature engine failure.

I think going to a higher octane fuel may either solve your issue or mask the symptom of another problem.

Edit: Your Dad's Toyota Avalon may or may not have a Knock sensor, OBDII vehicles began using this technology around 1995 to 1999

[baraff]

John,
thanks for the reply.
We actually just purchased the Aspect in October, and have only had it out for one 350 mile jaunt so far. The pinging is minor, and I have not been real picky about fuel brands, always thought gas was gas. I suspect I am wrong about that.
Next month we leave for Florida and I will try some "good" brand name fuel and see what happens. I'll try a tank of brand name 87 and if results don't improve maybe I'll try a tank of 89. If that doesn't work I'll check out the knock sensor. I would think that a faulty sensor would throw a code though... the scangauge isn't showing anything.
The engine has 50K on it and the previous owners took excellent care of the motorhome and they used it regularly on long trips.
-Burns.

[Wanabee FTer]

Originally Posted by baraff

John,
thanks for the reply.
We actually just purchased the Aspect in October, and have only had it out for one 350 mile jaunt so far. The pinging is minor, and I have not been real picky about fuel brands, always thought gas was gas. I suspect I am wrong about that.
Next month we leave for Florida and I will try some "good" brand name fuel and see what happens. I'll try a tank of brand name 87 and if results don't improve maybe I'll try a tank of 89. If that doesn't work I'll check out the knock sensor. I would think that a faulty sensor would throw a code though... the scangauge isn't showing anything.
The engine has 50K on it and the previous owners took excellent care of the motorhome and they used it regularly on long trips.
-Burns.

If there are no soft or hard codes in the pcm memory, I agree the knock sensor should be ok. I have gotten bad tanks of fuel in the past that will cause the type of problem you are having.
Have fun and good luck in your travels,
John

[Midniteoyl]

If you are getting slight pinging under heavy, prolonged load - like up a long hill - then by all means, use the 89 octane when you suspect you will be in hilly country. It will not, however, improve gas milage UNLESS there is a problem with the engine/systems as John noted..Also of note is that the lower octane fuel usually is the one with less additives. The other have more, to raise the octane The weapon of choice? An alcohol, like Ethanol...

Does this engine have an EGR? A faulty EGR could cause a rise in temps under heavy loads.

Spark knock (pinging) is caused by fuel burning too fast or too soon. The rapid ignition of the fuel pushes back against the piston while it is still on the upward compression stroke and tries to kick the piston back down before it should causing a pinging sound.

What you describe is actually called Pre-ignition, and is not the same as 'pinging', or Detonation .

Detonation usually doesnt do harm unless its heavy enough to 'pit' the piston. In fact, cars today are designed to have some. Detonation is when there is a secondary combustion after the normal spark.. usually from a hot spot or too high compression. When this happens there are two flame fronts, and when the two collide, there is a pressure spike heard as a 'ping' by the ear.

Pre-ignition is something you would never hear, but you'll know it happened when the piston/rod leaves the engine Pre-ignition is a combustion event started before the normal spark, usually when the piston is on the way up the bore. The expanding gas pressure combined with the rising piston result in pressures ~100x normal, and really bad things happen really fast.

Engine Basics: Detonation and Pre-Ignition: Streetrod Stuff

[Rocky Larson]

I have found the middle grade to be cheaper, especially in the corn belt. While the octane maybe 89, it has a lot more ethanol than the 87. My 8.1 likes the straight stuff so I seek out the stations that post "NO Ethanol". Yep, few and far between.

[94-Newmar]

Higher octane fuels have a lower flashpoint and a longer burn time once lit in the cylinder. Lower octane fuels can pre-combust before it's time due to high cylinder temps -this causes the pinging/knocking sounds. The reduction in power is due to the computer removing a lot of ignition timing to keep the top end of the engine from grenading itself, which also results in loss of fuel mileage. Ethanol is used to raise the flashpoint of the mixture by increasing the octane to prevent pre-ignition, to clean-up the emissions, and to reduce the amount of fossil based fuel we use.

[jcthorne]

As was already stated, the engine was designed to run CORRECTLY on 87 octane. If its pinging on 87, something is WRONG and its not the fuel. Well, could be a bad tank of fuel with less than 87 effective octane.

First think I would suspect on a V10 with some miles on it pinging under load is a blocked or stuck EGR valve. The egr system lowers the combustion temps in order to lower emissions of oxides of nitrogen. When it gets dirty from exhaust soot, the valve can plug or stick. Resulting in higher combustion temps and pinging.

Many OBDII vehicles do not have knock sensors. You would have to look up your specific year engine controls to know for sure.

[Brazel's RV Performance]

It would be a great idea to have a scanner hooked up to it and monitor your Fuel Trims to see what kind of percentage they are showing. A dirty/contaminated/faulty Mass Air Flow sensor can definitely cause pinging.

[Nick-B]

Hi Burns,

Go ahead and try the 89 octane, but I doubt you'll see much if any mileage improvement. Not on a mototrhome.

Pinging can also be caused by a vacuum leak. Ford had a big issue with their 3.8 V6 engines in the late '90s and early '00s. The intake manifolds had poor seals on the bolts which caused a vacuum leak and pinging that nothing else would cure. New bolts with improved seals were issued.

[baraff]

Wow! So many things to consider. Thanks everyone for the replies. Probably start with the simplest and see what happens. Next tank will be name brand 87 and once I get on the road and really run some gas through it we'll see if the problem still exists.
As I said its minor pinging, I had much worse in other vehicles I've owned. ('66 Plymouth comes to mind)
Probably right about 89 not affecting mileage much.
Thanks again,
-Burns

[Arch Hoagland]

Let it ping.

I had a 79 GMC pickup with the 350 engine. It pinged and I decided I wanted a 454 engine, so if I could blow it up I could justify buying a new engine to my wife. I didn't know at the time you couldn't drop a 454 into a 1/2 ton pickup.

Anyhow I ran the cheapest gas and it pinged like crazy and would keep running when you shut it off, backfire, huff and puff before it would shut down. Sometimes you had to put it in gear and choke it to shut off.

I did that every day for three years. Never hurt the engine a bit. It always started and ran. So that made me a believer that pinging doesn't hurt anything, leastwise not enough to get a new engine.
On all the cars I've owned since then I ran the cheapest gas I could find and always the lowest octane.
And I don't believe the people who claim to get better gas mileage. Gas mileage has to be checked over thousands of miles in various conditions to have any accuracy.

[wa8yxm]

Though the standard preach is this: Use what the manual calls for

I know that there are occasions where an increase in Octane WILL mean an increase in MPG.. I also know why.. Something in the engine is out of adjustment or out of spec (Beyond adjustment)

I had a car once that acted just like that Toyota.. It got better MPG on "Mid grade" than on Regular.. Turns out the Carburetor (This was an older car by today's standards. New at the time) was defective. A new carb, properly adjusted, and it ran the same on both grades.

[mikf]

Burns, I would love to get 9 mpg, but with your rig it seems that you should get well over 10 mpg, so I am guessing something is amiss with your engine and needs to be checked out.