Should 100 octave affect mileage?
 

The local Sinclair fuel station in Los Angeles sells 100 octane racing fuel. Recently, I have been filling up the tank exclusively with 100 octane, for about 8 or 9 tanks, but I noticed that my fuel mileage seems to be 3 or 4 mpg lower than it was before. The first tank was a long trip on I-5 through The Grapevine, and I got 27 mpg. Since then, though, it's been 19 or below. I usually get 21 to 23 mpg.

Does 100 octane burn so much faster that the F20C ends up consuming more fuel per mile?

Sorry if this is a naïve question - I've never had access to 100 octane before. Maybe my right foot got heavier as an unconscious result of running race fuel.

No that fuel actually burns slower / cooler, so if your car doesn't use the extra octane to advance timing and you don't have knock that is retarding timing (in other words it doesn't need the extra octane) then you will just make less power. So you will use more throttle to maintain the same speed as when you were running 91 Octane, hence you have more fuel consumption. I don't know what your 100 octane fuel uses to boost octane levels, if it uses more ethanol then you will get less fuel economy naturally as ethanol has less BTU's than straight gasoline. Ethanol gives you less fuel economy but burns cooler and can be used to provide more octane. I would say 92-93 Octane would be the maximum needed, 91 Octane minimum works well in most situations where you don't have knock.

Just my thoughts on this, others may think differently. If the 100 octane fuel uses MMT to boost octane also stop using it, as it can kill your cat. converter and foul your plugs and 02 sensors etc.

As is explained everywhere octane is not a measurement of power potential and as just noted is a resistance to ignition by burning slower. My US-spec Owners Manual (and everyone else's) states 91 octane or higher is needed to prevent engine damage by preventing detonation. Maximum 10% ethanol blend. No mention of "mo powa babeh" going any higher. Note North American "pump octane" is seemingly lower than the UK and EU ratings for the same fuel due to how it's measured. US 91 octane corresponds to Euro 95.3 Ron so don't think the Brits are really getting higher octane 95 (US91) and 98 (US93).

Good octane converstion article.

And don't forget there also 100LL AVGAS.

-- Chuck

It only has a lower tendency to pre-ignite which is why higher compression engines use high octane fuel. Should not affect your fuel economy but also no reason to run it unless you have a more aggressive tune that can take advantage of it. For example a lot of tunes (the one on my AP1 for example) are built around 93 octane and can take a little advantage of the higher octane.

Having returned from Portugal I was initially impressed by the octane ratings they have, but I didn't realize they aren't too far off our octane , I think the lowest they had was 95 and up to 98, with E5 and E10 ethanol levels, I don't think they have anything that compares as low as our 87 though , but I could be wrong.

I'm hoping one of our EU or UK members can still find his owner's manual and tell us what RON octane is specified. I suspect it's a minimum of 95 RON which is the same as US91. There's a sticker inside the fuel door on North American cars too.

-- Chuck

German Owner manual and Sticker inside the fuel door state:
The prefered and recommended RON / ROZ Octane here in Germany / Europe for the F20 C2 engine is 98 Octane. If this is not available, 95 Octan is allowed, but this could maybe cause a loss of power.



https://cimg4.ibsrv.net/gimg/www.s2k...d7d090df67.jpg

Matches nicely.

Colorado which has an average altitude of 6800' (2100 meters), for example, markets 85 octane as "regular" but still has 91 as "premium." Premium in Alaska can be as low 90 which is below the recommended fuel for our cars. (Never bought gas in Alaska so that's just what Google says.)

-- Chuck

Didn't the Japanese version of the S2000 always make a couple more hp in Japan, due to higher octane availability there and different tuning as a result ?

Are there any good threads on "tuning" here at S2KI ? (specifically: tuning of just the stock engine, without considering suspension or other non-engine components)

I don't want to derail this thread into a tuning discussion, but the topic has been mentioned a couple of times already.

My experience is limited to twisting a distributor to adjust the timing on a carbureted engine, but I imagine that a computer-controlled engine like the F20C might have more than one variable to adjust.