brillo

I decided to experiment with different grades of gasoline to see what effects it would have on mileage idle power etc….given the varied responses and posts about the subject. I have about 4,100mi on my car.

In Texas, we only get 93, not 91, so our choices are limited to 87, 89, and 93. I seem to get better mileage on the lower grades, but my idle will jump around occasionally for no apparent reason on low and mid grade gas. Any idea what could cause that? It isn’t consistent, which is what I find weird, it will jump from 800 to 1500rpm for no apparent reason a few times and then go away, only to come back later.

I searched the various octane posts with no answer to this weird idle. Any thoughts on what could be causing this?

bigblockbeater

what is happening with the lower grade fuel is simple. the gas doesn't burn as well as the 93 so some of it gets expelled into the exaust unburned. it catches from the pipes heat and causes a backfire. all be it very suttle but it is there. try listening to your tailpipe with a warm idling engine. put a hand on the car and feel for the vibrations. every time you here a little puff the idle will drop briefly. i think you should be careful of fouling the plugs with the cheaper gas.

brillo

I didn't mean to imply the quality of the gas was lower grade, Its all shell gas, be it 87 89 or v-power, so i don't think its a quality issue, unless I'm misunderstanding you....

brillo

bump

bigblockbeater

i didn't mean like bottom of the barrel gas, i was just refering to the octane level. want to know a secret about gas. its all the same. i work on an oil tanker and we haul fuels from all kinds of plants to many different suppliers. no matter who makes it it still has to have the proper octane and detergents. i use 93 also because 91 is not as available and at idle my car will backfire too. its just the nature of the wankle. sometimes i make it backfire on purpose, you can't hurt any valves lol.

robertdot

The basic premise is this: the lower the octane, the less pressure+heat will cause it to combust. Hence, the 10:1 compression ratio suggests 91+ for the RX-8. If you are using less than 91 and things are weird it might be because it is combusting too soon, which might explain the odd idles (even if the ECU's knock sensor makes up for it). You might get higher octane ratings in TX because it gets alot hotter there than in other places and that keeps engines from detonating/knocking.

Octane describes how much pressure/heat gasoline can take before it combusts. The lower the octane the less pressure/heat it takes to combust. 10:1+rotary = high pressure+high heat. There is a reason Mazda suggests 91 octane: To protect those of us in HOT environs from detonating our engine!