Using higher octane gas than suggested

[dkm777]

Hi guys,

My car has a high compression NA engine (11.5:1) so I thought maybe it would benefit from higher octane gas. The tank cover says 95 octane (Euro rating - not the same as American octane rating) but I tried 98 octane for a few weeks. What I noticed was a not insignificant drop of low end torque but definitely better mileage. I went back to 95 octane now, because it's cheaper and I get better torque which helps a lot when barely crawling in traffic jams. Mileage is worse, but since the fuel is cheaper it works out the same. I have a question to experienced engine builders/tuners - what happens when you have an ECU tuned for specific octane gas and start using higher octane gas? I find the drop in torque baffling.

 

[LTC8K6]

If it likes 95, it ought to like 98.

Is there an ethanol % involved?

 

[dkm777]

In Europe ethanol is always involved nowadays. Can't say how much exactly, because there are notes only when it's 15% or more (e.g. 98E15). Manual clearly states NOT to use E15 so I only use normal gasoline, but there's probably ~5-8% ethanol in it anyway.

 

[_Rick_]

I almost always put 98 into mine, and when I once had to resort to 95, I didn't notice any difference in low end torque - but then we both know, that below 3krpm there is no torque either way....
Around here 98 ought to be <5% Ethanol, while most of the 95 is E10.

 

[Ferzerp]

Usually won't do squat other then costing you more (and lowering mileage by a nearly unnoticeable amount). There is the rare car that has mappings for octane higher than the suggested amount where it can actually help though. That's by no means common though.

 

[smackababy]

Usually won't do squat other then costing you more (and lowering mileage by a nearly unnoticeable amount). There is the rare car that has mappings for octane higher than the suggested amount where it can actually help though. That's by no means common though.

I was under the impression the higher octane than you're rated offers nothing except higher costs per gallon in like 99.9% of cars. I've always been told that as such. I've tried higher octane on long road trips (1200 miles or so) and noticed no real difference in miles per gallon or how the car ran. That was in a Gen 4 Eclipse.

 

[Ferzerp]

The higher octane have *slightly* less energy density per volume. As I said, you'll not notice it, but there is less energy in your tank than with the lower octane. This is assuming same ethanol content in each.

 

[Throckmorton]

What car? Some will automatically change spark timing to adapt to higher octane and you'll gain power.



But here's what I want to know... Does using a higher octane in a car that can't adjust for it result in a less complete burn? That would be bad because it would deposit carbon on the catalytic convertors I think...

 

[Eureka]

I was under the impression the higher octane than you're rated offers nothing except higher costs per gallon in like 99.9% of cars. I've always been told that as such. I've tried higher octane on long road trips (1200 miles or so) and noticed no real difference in miles per gallon or how the car ran. That was in a Gen 4 Eclipse.

Read the manual for the car. I believe a lot of newer cars will have variable maps... mine does.

What car? Some will automatically change spark timing to adapt to higher octane and you'll gain power.



But here's what I want to know... Does using a higher octane in a car that can't adjust for it result in a less complete burn? That would be bad because it would deposit carbon on the catalytic convertors I think...

The way I've had octane explained (howstuffworks) is that the octane level just relates to how much the fuel can be compressed before it spontaneously ignites (which causes knocking). Higher octanes compress better and so you use it with high compression engines. But if you use a higher octane fuel in an engine with lower compression, you'll still be able to reach maximum compression and the spark plug should burn the fuel completely.

 

[Phanuel]

FI cars will typically have multiple maps for different octane ratings. Power figures for these kinds of cars are usually quoted at 91 octane (US) with the ECU having maps for 89 and 87 as well. But these will typically reduce power and mileage a little, you may or may not notice.

NA I have no idea, most of them are just quoted at 87 octane and do nothing with higher quality fuel?

 

[Eureka]

FI cars will typically have multiple maps for different octane ratings. Power figures for these kinds of cars are usually quoted at 91 octane (US) with the ECU having maps for 89 and 87 as well. But these will typically reduce power and mileage a little, you may or may not notice.

NA I have no idea, most of them are just quoted at 87 octane and do nothing with higher quality fuel?

My NA V6 has maps for 87 and 91.

 

[dkm777]

What car? Some will automatically change spark timing to adapt to higher octane and you'll gain power.



But here's what I want to know... Does using a higher octane in a car that can't adjust for it result in a less complete burn? That would be bad because it would deposit carbon on the catalytic convertors I think...

Suzuki Swift Sport. I got this idea of trying higher octane after having driven my mother's '06 Mitsubishi Colt 1.3 liter for a while and trying higher octane in it (says 95+ on the tank cover). That car really came alive on higher octane, so I assume the ECU uses very aggressive ignition timing when possible. I can only think that my car's ECU always uses the same ignition timing all the time so no use filling up with expensive gas.

 

[LTC8K6]

Well, I think your 95 is our 91, so that's already high octane fuel to me...

Modern engines all adjust to the octane level as far as I know.

Does it have knock sensors?

 

[dkm777]

It should have knock sensors since it's a rather new engine. I'm beginning to suspect there's more ethanol in that particular brand of 98 that I used. But then again when there's more than 10% the mileage gets totally crap. I'm confused. I'll try some other brands now. If performance is the same then my engine just dislikes 98 and I'll stick to 95 for good.

 

[hanoverphist]

all my cars are old, so this is subjective...

i have one car (95) that calls for 87 octane, and runs great on it. when i put 91 oct in it, it runs fine but the mileage drops. one of my other cars (96) calls for 91 oct, but if i put 87 in, it pings and the mileage drops. my 01 truck will burn pretty much anything the same.

 

[desura]

Um, IIRC, octane is about resistance to combustion.

High octane is used to obtain higher compression ratios in engines. Basically, the cylinder can compress more before ignition.

So it is specific to the engine.

Now, if you put a high octane fuel into an engine designed for low-octane fuel.

What will happen is a portion of the fuel per stroke will be unburned.

So basically, it's not a good idea.

 

[exdeath]

Placebo effect. No way 3 points of octane rating had any effect on perceivable torque or fuel economy on a stock NA engine in a commuter car tuned for the lower octane. Likely something else like fuel blend in the event there was a measurable difference.

You need boost, aggressive tuning, and an appreciable jump like 87 octane to 100+ octane to genuinely notice.

On a car tuned for 98, going to 95 MIGHT produce a perceivable difference dropping 2-3 degrees timing, but a factory tune for 95 isn't going to have a spark map to take advantage of 98. On a ROM based factory ECU, knock sensor reduces timing from the max defined in the spark map as a safety feature, it doesnt accommodate adding more timing than the target in the map.

 

[JCH13]

Here is what I found on Exxon's website:

Why should I use your premium gasoline (91-93 octane) instead of regular (87) or midgrade (89)?

To find out what octane your engine needs, first check your owner's manual. The recommended level is often 87 octane. Some models have high compression engines which are designed to utilize the octane levels of 89, 91 or higher.

Ordinarily, your vehicle will not benefit from using a higher octane than is recommended in the owner's manual. But if your engine knocks or pings at the recommended octane level, you may need a higher octane gasoline to prevent the knock. Knocking may occur under certain conditions. A small percentage of vehicles may knock because of variations in engines of the same model due to manufacturing tolerances, or because of an unusual build-up of engine deposits. Other factors such as extremely hot weather, changes in altitude or hard driving conditions (like towing a heavy load) may also cause knocking. Many modern vehicles are equipped with an electronic device that detects and eliminates light knocking before you hear it.

 

[Zenmervolt]

Modern engines all adjust to the octane level as far as I know.

Modern engines with knock sensors will retard timing and enrich the fuel mixture (which reduces power) to combat knock, but outside of very limited applications they will not advance the timing and lean out the mixture beyond the "default" point, which is what the factory establishes will work with the octane specified in the owner's manual.

That is to say, the engine will adjust down for lower octane than required but it will not adjust up for higher octane than required.

In some rare instances the owner's manual will say something like "Regular fuel required; Premium recommended for maximum performance." In those cases, the ECU does have maps for premium fuel and can adjust for it, but this is fairly rare and seems to have been more common in the mid-'90s than it is now.

ZV

 

[LTC8K6]

Modern engines with knock sensors will retard timing and enrich the fuel mixture (which reduces power) to combat knock, but outside of very limited applications they will not advance the timing and lean out the mixture beyond the "default" point, which is what the factory establishes will work with the octane specified in the owner's manual.

That is to say, the engine will adjust down for lower octane than required but it will not adjust up for higher octane than required.

In some rare instances the owner's manual will say something like "Regular fuel required; Premium recommended for maximum performance." In those cases, the ECU does have maps for premium fuel and can adjust for it, but this is fairly rare and seems to have been more common in the mid-'90s than it is now.

ZV

I think most of the cars I have owned have said something like that. My Jeep says that 87 is okay, but 89 will give the best performance, and anything higher is not recommended.

It would be interesting to be able to watch the ignition timing and see what goes on with different octane fuels. See if your ecu is actually adjusting and how much.

All of Ford's manuals these days seem to say 87 is okay but use higher octane for better performance.

I think many mfgs are just wary of saying their engine requires premium fuel.

 

[Zenmervolt]

I think most of the cars I have owned have said something like that. My Jeep says that 87 is okay, but 89 will give the best performance, and anything higher is not recommended.

It would be interesting to be able to watch the ignition timing and see what goes on with different octane fuels. See if your ecu is actually adjusting and how much.

All of Ford's manuals these days seem to say 87 is okay but use higher octane for better performance.

I think many mfgs are just wary of saying their engine requires premium fuel.

European manufacturers tend not to shy away from "requiring" premium fuel in the same way that American manufacturers do. At least in my experience.

It's also worth noting that the engine may not need to pull back timing if the car is lightly loaded and the weather is cold. The differences mainly come when the car is heavily loaded or when the temperatures climb as those are the situations where knock tends to be heard most often. This is one reason why Honda, for example, gets away with recommending regular fuel for the Pilot under normal circumstances but premium for towing over 3,500 pounds.

ZV

 

[_Rick_]

European manufacturers tend not to shy away from "requiring" premium fuel in the same way that American manufacturers do. At least in my experience.
ZV

You don't get anything below RON 95 anymore, in most of western Europe, so it doesn't really matter. I think RON 91 was phased out a decade or so ago. Especially with E10 now, it's "easy" to get RON 95 by blending the ethanol with a lower RON base fuel.

Edit: correction: RON 91 has been phased out since 2008 in Germany. I don't see it in France these days either, so at this point in time, regular fuel is pretty much only used as a base for 95-E10.

 

[tortillasoup]

European manufacturers tend not to shy away from "requiring" premium fuel in the same way that American manufacturers do. At least in my experience.

It's also worth noting that the engine may not need to pull back timing if the car is lightly loaded and the weather is cold. The differences mainly come when the car is heavily loaded or when the temperatures climb as those are the situations where knock tends to be heard most often. This is one reason why Honda, for example, gets away with recommending regular fuel for the Pilot under normal circumstances but premium for towing over 3,500 pounds.

ZV

THIS

 

[PeeluckyDuckee]

Using my buttmeter, I've recently switched from 91 Octane to 87 Octane on my rustbucket ('01 Prelude), aside from the extra $coins in my pocket I haven't really noticed any noticeable difference in power (not that it has any to begin with). Haven't heard any "knocking" either.

 

[CraigRT]

Using my buttmeter, I've recently switched from 91 Octane to 87 Octane on my rustbucket ('01 Prelude), aside from the extra $coins in my pocket I haven't really noticed any noticeable difference in power (not that it has any to begin with). Haven't heard any "knocking" either.

A 2001 Prelude should have plenty of power, I recently test drove an 01, Man what a great car!

But, does the H22 require premium? My Civic SI does, and I would be willing to bet running regular in it would result in poor performance.