Fuel Octane

Here's an interesting tidbit. Regards to ethanol, it has a different specific gravity to gasoline. Why does that matter? In modern EFI it really doesn't and esp doesn't with those engines fitted with sensors so that fuel density ratio can be adjusted.
What about CIS, ah well that's the problem. CIS was engineered around gasoline fuel density. It's an hydrolic system using air mass. There's no compensation for any added ethanol. The AFR will be altered and the mpg will suffer. Esp as ethanol has less BTU then gasoline.

 

Ethanol is slightly denser than gasoline, but do the math, it works out to be a negligible difference in density for an E10 blend. In fact, different grades/blends of pure gasoline have as much or more variability in their density depending on their blend and additives, oxygenation, seasonal changes, etc. I don't know what level of variability is down in the noise, but for decades engines have had to deal with this even with pure gasoline.

 

ethanol boils at a lower temp. bad for vintage cars.
ethanol gives worse gas mileage.

 

OK - I am not saying that ethanol should be added to gasoline - I don't think it should but that's a whole different, and sadly, political, discussion... I totally agree/am familiar with the problems it can cause in systems, especially in storage if untreated.

Ethanol has around 2/3 of the energy of an equivalent amount of gasoline so we are removing 1/3 of the energy from 10% of the fuel. I'm no math expert but with the help of the calculator built into my Mac's operating system it seems to indicate that a gallon of 10% ethanol fuel has 96% of the energy of a gallon of pure gasoline.

Assuming the calculations are correct, from an MPG standpoint that means my 328, which averaged 23 MPG on three different trips across the US at 70-80 MPH with 91 (US) octane 10% ethanol gas would average 24 with straight gasoline. The difference doesn't strike me as worth worrying about. BUT, I will admit I spent a lot of time in big-block US muscle car where 1/2 of that mileage was quite good so my expectations are somewhat low!

 

My euro 308 GTB runs happiest on 87 and I drive it all the time. Starts better, accelerates more quickly, hot starts better. 87 has been a blessing. My first year I did the 91 all year because I was stupid.

 

@Hannibal308 Interesting, my Euro carb GTB dry sump is really happy with a 50/50 mix of 91 octane non-ox and 100LL AVGAS

 

That’s nice.

 

Thanks @Hannibal308 , given your insight/experience and the Engine Masters YouTube video, I'll go to straight non-ox (& tempted to try your 87 too...). I had used the 100LL to keep some lead in the engine originally designed for leaded fuels, but thinking that's bunk.

 

All good. I live in the middle of nowhere and put about 200mi per month on my car. Sea level to 7k feet between 55 and 105F and I’ve been happy with 87 since one of our bros in Germany suggested I try switching because my car hated starting hot. Since then she starts better cold and hot, runs better at altitude, and never seems to complain or fuss about anything I do to her throttle-wise. I’ll never switch again.

 

Whats your compression ratio?

9:1 or lower is fine for 87, maybe even 9.3:1 since the heads are aluminum Octane numbers i use are (R+M)/2.

if you are at 10:1 or higher, you should use 91.
Doug

 

+1

Stock 308’s run just fine on 87. Hot starting can be an PITA for carbs in the summer as ethanol has a lower boiling point.

Helpful hint for GT4 owners - if you don’t plan to drive in the rain you can easily remove the metal rain-drain that attaches to the underside of the engine cover cooling vent - it almost totally blocks the vent! This will significantly lower engine bay temps and help with hot starting. Takes about 5 min to remove/replace.

 

I'm no expert on this stuff at all, but I recently switched from 93 E10 to 90 non-ethanol. My 328 is running better, but I have no idea if it's the change in octane or the removal of the ethanol. Appreciate everyone's insight on this thread.

 

"My 328 is running better, but I have no idea if it's the change in octane or the removal of the ethanol.'

It may be running better because you BELIEVE it's running better.

A dyno test some years back - I seem to recall they used a Miata but I'm not sure about that - anyway, 10% 87 and 91 octane ethanol gas made more power than 93 non-ethanol gas when the engine was tuned to it's best with each type of gas. They also tested 105 octane racing gas which produced virtually no gain - I think it was something like 1 HP over the E10. FWIW, E85 produced considerably more HP than any non-ethanol gas.

 

Ethanol will always make more power if you can tune for it. This is especially true if you run a turbo.
If you are running stock system I would avoid ethanol altogether if possible.

 

My experience in the BBI, e46 M3 and harley sportster is that pump gas is crap. All 3 of those vehicles run just fine on pump gas or so one thinks, until one tires something better.

Some background, the e46 always had a slight low rev hesitation, and I always thought the throttle until 5k rpm was lazy. Switching to a mix of 50% race 100(still has entanol) and 93 gave a crispness to the throttle everywhere and I suppose that cars computer could take advantage of the extra octane.

The sportster, same thing, crisp immediate throttle response everywhere and a happiness with revs.

The BBI, well it now runs like I remember it did in the 90s. Same thing but even more exaggerated, ie a sharp throttle, it never feels now like the motor is catching up to the throttle, its just there.

I think pump ethanol fuel just burns like crap and you feel that in lazy throttle response and poor low down tractability.

In flroida we can also get 90 octane ethanol free fuel. in my old chevy equinox it simply runs better.