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Using premium fuel (91oct) in a car designed for regular (87oct)

I know that if your car requires a higher octane gas and you use a lower one, it will hurt the performance due to issues with the compression among others. What about the reverse? In the U.S. the mazda6 only requires regular 87oct, I started using premium for the first time this week thinking it might help the performance, but I actually think I feel a slight decrease in power and acceleration. I could just be my mine playing tricks on me....any thoughts?
 

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Replying to Topic 'Using premium fuel (91oct) in a car designed for regular (87oct)'

It's definately possible. If there was gain or loss, it'd be negligable. It's usually within a few percent. Some magazine did a report on this last year.
 

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Replying to Topic 'Using premium fuel (91oct) in a car designed for regular (87oct)'

It should run slightly better...

NOT worse.
 

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Replying to Topic 'Using premium fuel (91oct) in a car designed for regular (87oct)'

Actually, no. A car designed for lower octane fuel will have less aggressive timing than a car designed for high octane. The result can and usually does become reduced output when you put high octane fuel into a car designed for low. Remember, octane rating is inversely proportional to the activation energy of the same amount of "fuel." High octane fuel is not "higher performance" because it is a hotter fuel... rather it's the opposite. It is a more thermally stable fuel, which allows for more aggressive timing, higher compression ratios, and more boost (in turbocharged applications).

The milder timing of low-octane engines may result in the A/F mixture of high octance fuel being too cool for complete combustion which can result in reduced output.

-Alt
 

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Replying to Topic 'Using premium fuel (91oct) in a car designed for regular (87oct)'

QUOTE
Originally posted by Altersys


            Actually, no.  A car designed for lower octane fuel will have less aggressive timing than a car designed for high octane.  The result can and usually does become reduced output when you put high octane fuel into a car designed for low.  Remember, octane rating is inversely proportional to the activation energy of the same amount of "fuel."  High octane fuel is not "higher performance" because it is a hotter fuel... rather it's the opposite.  It is a more thermally stable fuel, which allows for more aggressive timing, higher compression ratios, and more boost (in turbocharged applications).

The milder timing of low-octane engines may result in the A/F mixture of high octance fuel being too cool for complete combustion which can result in reduced output.

-Alt[/b]
I've heard similar things. Basically, these days, use exactly the octane rating designated for your car. I think it used to be that higher is better, but now a days, cars are so precisely tuned for what ever purpose (economy, performance, emissions, etc...) that any straying from that will reduce performance, mileage, or engine life.
 

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Replying to Topic 'Using premium fuel (91oct) in a car designed for regular (87oct)'

That said, when cars the run low octance fuel get older, their cylinders get carbon-coated. This can marginally increase compression ratio. Some owners of older car have found that their cars have mysteriously begun to detonate under high load conditions. That is one of the reasons why mid-grade fuel came out- to combat this problem without having to pay for premium fuel and also to avoid suffering a total loss of power.

-Alt
 

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Replying to Topic 'Using premium fuel (91oct) in a car designed for regular (87oct)'

In Australia our fuel is so crap that going up to a higher octane almost always results in gains in smoothness and fuel economy.
 

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Replying to Topic 'Using premium fuel (91oct) in a car designed for regular (87oct)'

Basically, these days, use exactly the octane rating designated for your car. I think it used to be that higher is better, but now a days, cars are so precisely tuned for what ever purpose (economy, performance, emissions, etc...) that any straying from that will reduce performance, mileage, or engine life.
*************

Actually, it has always been the case to use the correct octane whether it be these days or days gone by. :)

"Octane" is a measure of a fuels ability to resist auto-ignition under compression and that's been true any time.

One cool thing about the 6's V6 is that it is somewhat a "higher" than normal compression engine which has been tuned to run on standard octane rated fuel.
This means that this engine should respond nicely to ECU tuning with more aggressive timing when tuned to run on high octane fuel. Thus, the 3 litres could be put to use very easily with more power. I wouldn't be surprised if this engine produces 15-20 wheel HP with ECU and intake mods. I'm sure this engine is overbuilt and understressed given it's lower octane running ability.

T
 

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Replying to Topic 'Using premium fuel (91oct) in a car designed for regular (87oct)'

Modifying a Mazda ECUs is very difficult. There is no "pug in" chip, all components are hard wired into the board. So that usually means an external or piggyback Engine Management Unit. They can be very expensive. $1,000+ and the dyno time to set it up.

But who knows maybe somebody will come up with a cheap HP unit. But I'm not holding my breath.
 

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Replying to Topic 'Using premium fuel (91oct) in a car designed for regular (87oct)'

C&D did an article over the past year on this.

What it came down to was this:

If the car recommends 87, but you use 91, there is no noticable gain in performance nor fuel economy. Result: if the car recommends 87 use 87, you're just wasting money otherwise.

If car recommends 91/92(premium): Using 87 is possible now days, because the cars have knock sensors that automatically retard the timing. However, using 87 when 91/92 is recommended results in a slight, but noticable loss in both power and fuel economy due to the retarded timing. Result: The money you might save using 87 will be offset by the lower fuel economy, so use 91/92, the money saved on fuel is a wash, but you'll get the extra power.

Bottom line: if your car takes 87 using 91/92 doesn't do a darn thing except cost you more money. If car takes 91/92 using 87 hurts power, hurts fuel economy and thus saves you no money.

Hope that helps ;)
 

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Replying to Topic 'Using premium fuel (91oct) in a car designed for regular (87oct)'

QUOTE
Originally posted by Jerome81
If the car recommends 87, but you use 91, there is no noticable gain in performance nor fuel economy.  Result:  if the car recommends 87 use 87, you're just wasting money otherwise.[/b]
Though I know it doesnt help performance, I recently started using premium just as a test to see if it would help my mileage. Ive gone through 2 tanks of premium and both times I got over 420 miles before I got nervous and finally refilled. Before that, using regular, I could only manage ~375 at best.

I'll use premium the rest of the month to see if it's the cause or if I just got lucky.
 

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An easy way to test your mileage

1) run your tank nearly dry

2) Fill up the tank fully, until the gas pump "trips". Do not top off

3) Zero the trip meter

4) Run the tank nearly dry

5) Return to the SAME pump refill the same way

6) Note the gallons purchase, and the trip mileage.

7) Divide miles/gallons and you have a reasonably repeatable method (I didn't say accurate).

-Alt
 

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Replying to Topic 'Using premium fuel (91oct) in a car designed for regular (87oct)'

Alt, I would recommend your method except the "almost run dry" part. You don't have to do that, and you really shouldn't do that since you then pull along all the crap that gathers in the bottom of the tank right into the fuel filter.

The method is fine, but you don't have to go so far as almost running dry. Instead repeat the method two or three times and add the total consumption/mileage. Then you get a more accurate average also.:)
 

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Replying to Topic 'Using premium fuel (91oct) in a car designed for regular (87oct)'

You may also live in an area that oxygenates the fuel in winter to help control smog - that kills the gas mileage. it is possible that the 87 and 91/2 octane are oxygenated slightly differently.

MM
 

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Replying to Topic 'Using premium fuel (91oct) in a car designed for regular (87oct)'

LOL i didn't mean literally dry... that's not at all necessary for this method to work. I merely meant try to use as much gas as possible if only to reduce the percentage impact of the variation between pump trip points when you're filling up. 1/4 tank should be fine.

-Alt
 

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Replying to Topic 'Using premium fuel (91oct) in a car designed for regular (87oct)'

In fact, don't even measure the pump trip point. Just the start and stop ones.:)
 

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Replying to Topic 'Using premium fuel (91oct) in a car designed for regular (87oct)'

The individual measurements are not interesting. Just note the amount of gas you fill each time, and how far you've driven during these miles...

1. Fill her up. Take a reading of the ODO.
2. Drive as you wish. Fill it up whenever you wish. But make sure you note HOW MUCH you fill it. You don't need to note what the ODO says every time.
3. Whenever you wish to get a reading on the mileage: fill her up, note the ODO reading and do the math.

The only gain from noting the ODO every time you fill her up is that you then can calculate changes in the mileage over a period of time. If you have a driver's journal in your car and note how much you fill it up and the ODO meter reading every time, you can do various calculations based on this simply by calculating the average between any two.

- Latest tank mileage
- Average mileage
- Average on winter/summer
- Before/after changing tyres
- etc etc
 

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Replying to Topic 'Using premium fuel (91oct) in a car designed for regular (87oct)'

Ahhh you misunderstood me. I meant "the point at which the pump trips"... that is, you are making the assumption that the vapor lock mechanisn is always tripped at 12 (or whatever) gallons, and not 11, then 12, then 14, then 10. Capiche?

-Alt
 
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