High Octane II

[V1]

I filled my car with Shell V-Power by accident one day. Then I decided, what the heck? So I tried it for a few more tankfulls - to test mileage theories, but at least to reap the cleaning benefits.

To my surprise, my mileage did not drop as I expected it to. My car even felt like it ran better, but I know that might have just be me. Here is my question.

Mazda says this car requires "87 or better." Most people believe that 93 is a waste for this car (10:1 compression). It probably is, but is there really any harm to the car if I keep using it? I also add FP60 every fillup.

Another question: Is it likely that using higher octane will cause fuel dilution? When I changed my oil yesterday, it just smelled different than I'm used to.

 

[kang]

V1,

The octane rating of gas is just its ability to resist pre-ignition, or ping, nothing more. There is no more or less energy in high octane gas as compared to regular gas. Engines with higher compression need higher octane, as do engines with a lot of miles on them.

Most cars these days can accommodate pretty much any octane gas you give them. There are knock sensors on the engine. They report pinging to the ECU. If pinging is detected, say due to low octane gas, the ECI will retard timing (sometimes on individual cylinders) until the pinging stops. Reducing the timing can also reduce power and efficiency. If you use higher octane gas the ECU won’t have to retard timing. But if your ECU is not retarding the timing to prevent ping when you use low octane gas, putting high octane gas won’t make a bit of difference (except to your wallet).

As to your other comments, no, I wouldn’t expect mileage to drop with high octane gas. In fact, if you are getting more timing with the high octane gas, your mileage might even go up a bit. But usually the increase in mileage is not cost effective, e.g. it’s still cheaper per mile to use low octane.

I’m not surprised that your car felt better. You were probably getting some inaudible pinging with low octane that the ECU corrected for by retarding timing. There is nothing wrong with this, and no harm will come to your engine because of it. The system is doing what it is supposed to do. With the high octane, you got a bit more advance and the car feels peppier. If this increase in pep is worth the extra cost of premium to you, go for it. There will be no harm caused by it.

Octane has nothing to do with fuel dilution. There is no relation there at all.

 

[V1]

Kang, thanks. One of my other concerns was if premium causes more soot or carbon to build up both in the combustion chamber and/or on the exhaust valves

I understand the concept and significance of timing advancement, but is that enough for my car to burn up the extra octane in premium?

 

[kang]

I think premium can cause more carbon build-up. That's why they typically put more additives in it.

I don't know what you mean by "is that enough for my car to burn up the extra octane." If the performance increase with premium is worth the price increase to you, go for it.

 

[V1]

I meant, does timing advancement allow the higher octane fuel to burn completely? It would really be interesting to know for sure how carbon buildup with premium compares to just regular gas. Maybe I just have to wait to see what my spark plugs look like when I pull 'em. Thanks for the info, Kang.

 

[kang]

Yes, I'm sure you get a complete burn.

 

[Shaman]

Did your mileage increase?
Did the “horsepower” increase

For example, my father had a 90 Honda Civic, bottom of the line, with a 1.5L and a 4 speed. With testing, he got better gas mileage with premium than he did with regular, and the premium worked out to be cheaper to run than the low octane. You can try to find out why your car uses premium better( probably carbon buildup), and solve it (piston soak, fuel power, injector cleaner, pull the head and clean the valves and pistons manually), or you can just run the premium and pay the difference. It all boils down, as always, to cost.

 

[V1]

Carbon buildup should not be a issue on my car - it's an '04 with about 43,000 miles now. In fact, the exhaust pipes are still completely soot-free. Even after about a dozen tanks of premium.

Mileage may have gone up by 2 mpg's or so with 93, but with the cooler weather and all, it's hard to tell if that's really better compartively speaking.

As far as I can tell, the engine runs smoother with 93. I can't speak much for the power difference though - no clue. I don't trust the seat.

Maybe I will revert to regular gas to see what happens. Right now my car runs better than ever.

 

[kang]

quote:

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Originally posted by V1:

Right now my car runs better than ever.

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Can't complain about that. If you are worried about extra carbon build-up with premium, a quick Italian tune-up will take care of it!