Octane boosting

[Franklin2]

Here's an interesting article I came across. You can go to the paint store and add Toluene to boost the octane. http://www.elektro.com/~audi/audi/toluene.html

[pcmenten]

Lots of things will raise the octane of fuel. Alcohol, MTBE, Acetone, Toulene, Xyelene. But raising the octane above what the engine needs will actually cause a loss of power and economy. The lowest octane fuel that the engine can run on without knocking will yield the most power and economy.

[jcp123]

Interesting stuff, my Dad would love this if he still owned his Mach 1. That thing had spark knock no matter what - 9.7:1 compression, and didn't quit knocking above 3800 until the timing was retarded to 2 deg (!). Part of it was the crappy 91 octane. Toss in a gallon of that toluene though, and that would have been a whole other ballgame.

[pcmenten]

I wonder if your dad disabled the EGR on his Mach I. If he did, the problem was the advance curve of the distributor, not the octane of the gas.

When EGR is used, the speed of the flame in the combustion chamber is slower due to the lowered dilution of Oxygen/fuel (because of the added exhaust gas). To compensate for the lower speed of the flame front, auto manufacturers used a lot more spark advance. If you disable EGR, you've got too much spark advance at part throttle.

[jcp123]

I can almost guarantee it was disconnected. It was a relatively well modifed engine when he rebuilt it. Stock block, .060 over, stock 4v heads with an Edelbrock Performer 351C intake and Holley Street Avenger 670 carb. Comp cams "RV" cam...not sure exactly what grind it was, but suffice it to say it was a mild one. Keith Black pistons yielded 9.7:1 compression. Had Super Comp headers and an MSD billet dizzy, but no box. All hooked up to a 5-speed.

That said, since it was modified, would the lack of the EGR have any effect on it still? I would presume that it was tuned to run as it was set up from the rebuild.

[tdister]

Originally Posted by pcmenten

Lots of things will raise the octane of fuel. Alcohol, MTBE, Acetone, Toulene, Xyelene. But raising the octane above what the engine needs will actually cause a loss of power and economy. The lowest octane fuel that the engine can run on without knocking will yield the most power and economy.

Not always true. Some additives (aromatics, toluene in this case) have more BTU's than the gas they go in, therefore: potential for more power. Your engine doesn't care or know about octane unless it's too low, it's the other properties that matter for economy/power. And remember: higher octane doesn't in itself mean slower burning, in many cases it's the opposite. The two properties are not mutually inclusive.

Some vehicles/engines even respond better to octane boosters that have lower btu's(oxygenates), because they provide more O2 in the combustion chamber(among other things). But: engine design, rpm, fuel system type/condition, temp., and about a thousand other things (even the load on the engine at the time) change things. Some cars see an improvement, some don't. Not enough constants to give even a good "rule of thumb" on the subject. Have to do your own experiments/research on the individual vehicle as, even with similar vehicles, overall condition and things such as O2 sensor accuracy variances can change things quite a bit (relatively). Then you need to know if...

As for the write-up, it's pretty accurate. Slightly misleading on some moot points(in regard to the main topic), but it's pretty well right-on overall.

[pcmenten]

Originally Posted by jcp123

... and an MSD billet dizzy, but no box.

That said, since it was modified, would the lack of the EGR have any effect on it still? I would presume that it was tuned to run as it was set up from the rebuild.

The MSD distributor is the wild card here. It probably had a 'non-EGR' curve so that would rule out the distributor as the source of the knocking.

[jcp123]

But, since the rest of the engine was rebuilt along with it, killing the EGR would still have this effect? I'm not sure I understand...then again, I never did get all this smog stuff.

[pcmenten]

No, that engine is hopped-up. EGR won't be a factor in that engine, it's completely custom tuned.