silverx8

I heard that lower octane could increase power from a high revving engine. In theory, the high octane gas doesn't burn fast enough to get the power out before the next cycle.

Fanman

I've heard & read the exact opposite. The engine will retard the timing (to prevent detonation) with lower octane fuels costing you hp. I posted an article from Car & Driver a while ago on the effect of octane & fuel. In the M3 & Saab Turbo power/times were down substantially when it was filled with 87 vs. 91/93 octane fuel.

Aseras

my car runs WAY WAY better on 87 than 93/91. as long as it is good name brand 87. don't use shitty stuff. better gas milage too. like 280-320 a tank diffrence between the two.

Fanman

Here is the article from Car & Driver. If you are looking for more power then stay with the premium, better gas mileage then 89/87. The engine will retard timing (decreasing hp) on lower grade fuel.

http://www.caranddriver.com/article....&page_number=1

silverx8

This is what the board is about. I read the same article and got a totally different impression. I read from the article "premium fuel does not necessarily pack more energy content than does regular", so low octane does not mean low power. In regards to the rotary: "An engine's tendency to knock is influenced most by its compression ratio, although combustion-chamber design also has a large effect". I think that the rotary is a very different combustion chamber design and that should have a large impact on knock tendency.

In conclusion:" The Accord took a tiny step backward in power (minus 2.6 percent) and performance (minus 1.5 percent) on premium fuel, a phenomenon for which none of the experts we consulted could offer an explanation except to posit that the results may fall within normal test-to-test variability. This, of course, may also be the case for the gains of similar magnitude realized by the Ram and Mustang."

The low octane gas was better in a few cars with piston engines. Only the highly tweaked and turboed cars lost power (the M3 and SAAB). I say that until proven otherwise it is probable that the regular can perform fine, if not better.

I am running regular now and find that it runs pretty well and surges less, with the same power.


:D

Gomez

People way smarter than me say to use the minimum octane rating you can to avoid detonation. Richard Paul discussed this at length some time back.

Fanman

The example of the Accord is a little different in that it is built to run on 87 octane, same as the big V8's in the test. The way engines are set up are they are optimized to run at a particular octane of gasoline. For example lets use 10 degrees of advance as an example. At 87 octane the Honda Accord is running 10 degrees of advance. With 91 octane it is still running at 10 degrees of advance so you are not seeing any improvements (or in the Mustangs case very slight improvement), whereas with engines like the M3 or the RX8 they are optimized to run with 91 octane. Using the 10 degrees of advance example...If you are using 91 octane you are running optimally at 10 degrees of advance. If you sport for 100 octane, you are still running at 10 degrees of advance. But if you start using 87 octane, you are no longer running the engine with the "optimal" fuel map and the engine retards engine timing. So instead of 10 degrees of advance, you are only running with 5 degrees of advance. The engine senses you are running lower grade fuel and retards timing appropriately & cutting hp. It's not like a carbuerated engine where you adjust timing manually. The electronics in this car do so automatically. I'm not sure where you assume that the RX8 engine isn't a "highly tweaked" engine ala the M3 engine. The compression ratio on the Renesis, and other high hp, small displacement engines are very high.

Also remember that this article was printed in 2001. Since then, the Accord has upgraded from a 3.0 L 200 hp engine, to a 3.0 L 240 hp engine. A few years back they ran a similar article & in this one they had a Honda Accord coupe with the new 240 hp engine & a BMW 330Ci. The Honda with 91 octane this time gained over 10 hp. The higher you are pushing the small displacement engines, the larger need you will have for high octane fuels.

In this case I will disagree with the people that say you can get away with 87 octane. All you are doing is retarding the timing on your engine. If you are just driving around town & don't really care about hp then go for it. I prefer spending the extra $3 & optimizes my car at all times.

Celronx

iTrader: (1)

My car definately ran better in the cooler months using 87 octane(Shell). If I used 93 my milage went down by about 10-15% and I had tons of hesitation and jumpy idle. On 87 the car felt stronger and since I didn't get any detonation I used it. I also noticed that the black soot on the tailpipes went away.

When it started getting warmer (mid-upper 80's) I got some knocking at higher RPMs. Specifically, it would knock above 6500 RPM if I was pushing the car while going up hill, but only if the AC was on. I tested it just to make sure what was causing it. Since I was using a lower octane, I was paying attention to when it happened and what the ambiant temps were. I changed to 89 and haven't had any problems. Except for some of the soot returning.

I'm sure that I'll have to go to 93 once we get up to the high 90's.

I'll just use what makes my car run best.