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Old 03-17-2018, 05:02 AM   #1
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Premium Fuel in High Compression Engines.

There has been much debate both here in the UK but also in the US about whether a higher octane fuel will benefit an ordinary car.

My car is a 2017 1.3 Honda Jazz (Fit in the US) with an automatic transmission. The Compression Ration is 13.5:1 - much higher than in the previous Jazz (Fit) which was 11:1 (I think).

A lot of the debate suggests that high octane fuels are wasted on ordinary cars and especially on cars with average compression ratios. However the new Jazz has a high Compression Ratio by most standards and the manual says "95 RON or higher" - it's the "or higher" bit that gets me. Kind of suggests that 95 is a minimum as opposed to a recommended grade.

I know the octane grades are measured differently in the States but the general principle is the same. Would this car benefit from high octane fuel?
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Old 03-17-2018, 01:57 PM   #2
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Octane is a specific hydrocarbon molecule that makes gasoline (petrol) more resistant to detonation (AKA ping, knock), which is harmful to your engine. You want a gasoline engine to ignite the air/gas mixture.

Read the owner's manual. It'll tell you the fuel your specific vehicle requires.

In research I have seen -- using various brands and models of vehicles -- using gas with a higher octane rating than recommended by the manufacturer might deliver slightly better fuel economy and/or slightly more power. However, the solution us never cost-effective. You always get the lowest cost per mile by using the recommended fuel, and not by overpaying by using a higher octane fuel. In other words, you might pay 12% more for premium gas, and get a 1% increase in fuel economy.

Gasoline manufacturers will try to upsell you by telling you the higher octane (and more expensive) product has "added cleaning agents", thereby implying that over the long-run, you'll save money. You won't. The regular gas is not "horrible." It also has additives to reduce fouling and other build-up in your engine.

Bottom line: Use the fuel recommended in your vehicle's owner's manual.


Added Notes: Some people note that track cars use higher octane fuel, and they mistakenly believe the fuel is what gives the car more power. That's exactly backwards. The engine was race-designed or race-tuned. It has higher compression ratios and/or it has forced induction. It's also subject to much higher loads while racing. Therefore, the engine was designed to require higher-octane fuel. Without it, the engine will self-destruct while racing.
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Old 03-17-2018, 08:25 PM   #3
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It's not really a debate. Only if you make it. For high-compression engines all the technical data is at least possibly around 9 years old. That includes the octane and why. It's up to your preference, but the idiot-proof way to keep a GDI (or any other acronymn for a high-compression engine) clean is to stay on top of the fuel filter maintenance and add a fuel-cleaner at every oil change to the gas tank. If you do that religiously there's nothing to fear. Why? There's non-technical folks out there who know zero about their cars and these cars have 150-200k miles with no issues. That's the indicator. The information is a never-ending rabbit hole and not many dedicated sources help much but for shilling a brand of higher octane. Personally if it were up to me with a newer GDI and it was in my budget I would go with the highest octane because of previous experience (ownership of HFE vehicle) and the overall feel. The cost-to-benefit analyses is all up to you. Since I'm not driving a car with high compression at the moment the vehicle gets 87 and mostly no-ethanol gas.
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Old 03-18-2018, 12:59 AM   #4
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Quote:
Originally Posted by SteveMak View Post
Read the owner's manual. It'll tell you the fuel your specific vehicle requires.
The original post does state that the manual says "95 RON or higher", so he has read the manual.
Like you said, I found 97 RON gave smoother performance and marginally better mpg, but not at a viable increase in price.
My advice is to try it and see, then do the maths and see if it saves money.
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Old 03-18-2018, 03:27 AM   #5
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Thanks for the replies so far. I'm sure using regular fuel won't do any harm - the manual even says you can drop a grade if 95 RON is not available but only on a temporary basis - but I am intrigued by the "or higher" thing in the manual and inside the filler cap. I test drove a Toyota Hybrid recently and that uses the "or higher" phrase as well. Maybe a Japanese car thing? As I've only recently got this car, I'll do a comparison over a few tankfuls - I do a small mileage so it won't be a big extra cost.
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Old 03-18-2018, 11:26 AM   #6
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Experiment with various fuels, only you can decide if it's worth it. I always try to use premium, it's "more likely" to give better performance and/or better economy, for me that likelihood is enough of a reason to justify it as cost per mile is not a high priority personaly, however performance is.
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Old 03-18-2018, 01:28 PM   #7
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Yes - I put some 99 RON in not long after I got the car but there was almost certainly a few litres of 95 RON in before that so I'm probably on 97 at the moment.

One thing here in the UK is that the price differential is about 10 pence a litre more for high octane (about 8 or 9%) at all the big companies - Shell, BP, Esso etc but only 5 pence a litre at the 2 big supermarket chains that sell it (all the others only sell 95 octane). I think I can handle 4% extra on the limited mileage I do.

Interested to know what the price differential is in the US.
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Old 03-19-2018, 08:48 AM   #8
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By the way, when people conduct experiments on different fuels (to see which delivers the better fuel economy), make sure you do so under identical conditions... which is impossible, unless you're a scientist in lab or lab-like conditions.

Here's my fuel economy chart using the same fuel. All variances are due to "real world" factors, such as traffic, weather, outside air temperature, different routes, etc.
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Old 03-19-2018, 11:29 AM   #9
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Similar to my own. Only thing I can say, with any certainty, is is is better in the summer than in the winter.
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Old 03-19-2018, 12:10 PM   #10
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The Sonic/Aveo I had was equipped with the 1.4L port injected turbo. I read that it was designed to sue higher octane than the regular GM recommended, so I tried some tanks of premium and midgrade octanes through the years.

They did improve efficiency. To equal regular in the cost per mile measurement the midgrade would have to be 10 cents at most per gallon more than the regular price, and the premium would need 25 cent differential at most. A few times the price differences were such, but they are mostly twice that.

Mixing your own midgrade in the car tank would be cheaper than buying it, but involves the hassle of going through the fill up motions twice.

The "or higher" phrasing is likely for the ignorant. Letting them know it is safe to use higher octane fuel in the event they couldn't get the recommended octane.
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