Indycam

Can you say more on this subject ? Got links ?

Steve Weiner-Rennsport Systems

Just for clarification here since MANY people read these forums and this is a critical subject with very expensive ramifications:

1) Octane tells you a fuel's Anti-knock index and thats critical for high-compression engines (of which these are squarely in that category). If your engine needs an octane of a specific value, one should follow those instructions to the letter (number!). 964 & 993 engines need 91 or better to provide a margin in hot weather since detonation thresholds vary with cylinder head and air temperature. The Motronic system can only pull out 6 degrees of timing when the knock-sensors are active and thats not sufficient for safe operation on 89 octane gas.

2) Fuel density, aka specific gravity, affects fuel mixture and doesn't directly affect octane. Remember, MANY compounds go into gasolines and the average octane is a blend of RON & MON chemicals. Today's manufacturers are calibrated for a range of fuels that vary between seasons, but not octane. Shop for octane, not specific gravity.

If one is using race gas and aftermarket Engine Management such as Motec, stick with a particular manufacturer and a product if you can to eliminate variations in AFR's as thats usually what the engine has been mapped with.

This is a very complex subject and one can talk with the makers of race gas for a real education on this.

Indycam

You can only use 89 if you don't use the throttle or try to climb any hills and don't use any revs .
Bigstuff3 has a two fuel / two octane system , it has a system where low octane fuel is used until high octane is needed . When the driver steps on it and the low octane is no longer going to do the job , the computer switches over to the high octane .
http://www.bigstuff3.com/

Ron / Mon are not chemicals are they ?
Ron / Mon are numbers ?

ON Octane number
RON Research octane number
MON Motor octane number
BON Blending Octane number (MON+RON)/2
Rdon Observed Road Octane Number
AKI Anti-Knock Index


Bugger , I just got out my hydrometer set out of storage ...

Steve Weiner-Rennsport Systems

My bad: I left out a word and meant to write "octane-improving chemicals".

Makmov

Exactly, octane is not a good indicator of good or bad fuel.

Indycam

But he is saying the opposite of what you are saying ?
What is a good indicator of good or bad fuel ? Specific gravity ?

ChaseN

So we should all run 89? Serious - I'd like to hear your opinion.

sschultze09

Guys, I just got back from a 4 day road trip to Detroit in my 964!! No problems what so ever. Anyway after reading this thread about 5 times I am even more confused. The fuel locally marketed as non-oxygenated does not contain ethanol which is good right? But I should be running 93 octane? I have been looking but I haven’t seen 93 octane as a choice at your typical filling station. What’s up with that? As some of you have probably noticed from my other posts I am more of an electronics guy than a fuel guy. Steve thanks for the tip on Marine staybil. How much do you add per fill-up?

Indycam

Do you have a code reader ?
If so watch your knock count .
If it goes way high buy fuel with a higher octane , or make the fuel you buy into higher octane .

sschultze09

Indycam, now that makes sence to me. Thanks, I do have a code reader and will check that out.

ChaseN

What's the highest out there? I've noticed in my travels out west that 91 or 92 is often the highest available. East of the Mississippi 93, and in my area even 94, is widely available.

AFAIK the stock tune is safe on 91. But as Steve points out, it may not run as efficiently or powerfully as the higher octane blends. And yes, non-ethanol fuel is good. Availability around here is spotty in the summer, as the northern VA counties require oxygenated fuel to be sold during the hot months. Luckily I'm a short drive from the border of a non-emissions county that sells good gas year round (which the '64 is registered in...)

sschultze09

91 is all I have seen around here.

Makmov

Specific gravity. IE the density of the fuel is king.

Makmov

No not necessarily what I am saying. All I am saying is the quality of the fuel is more important than the octane rating. If you sampled most name brand fuels they are all probably pretty close in density. Where as some other off color fuel might not be as good.

Other things like oxygentated fuels reduce density.

And true to a degree what someone else said run the lowest octane you can get away with where the engine doesn't knock. However, on more sophiticated vehciles that have the ability to retard timing to eliminate knock you will loose performance. You wont here it knock but the vehicle will retard timing so you don't hear knocking, and for performance reasons you want as much advanced time as you can get without problems - generally speaking.

Makmov

Non oxygenated fuel is good.

Not all areas have 93 octane. We have 91 here because it's high altitude over 5200 ft. Many parts of the rest of the country have 98.

Used to be we could unleaded 112 at Sonoco, which they would let you pump into your street car. Pricey but nevertheless. Not sure that is the case any longer.

FWIW octane rating by definition is the standard measurement of the performace of fuel. However, what it basically measures is at what compression the fuel will detonate at. The higher the number the higher the compressing required to detonate the fuel.

When it comes to fuel density: higher densities having a greater volume of aromatics. This equates into more energy being release from the same volume.

When testing CNG fuels density of the fuel significanlly effected performance. Density has a direct corralation to volumetric effeciency and flame speed.

Sorry for and overly detailed fuel ramblings. I never really meant to be a fuel guy either but once upon a time I worked in a performace speed shop and we had a University of Wisconsin PhD come in for fuel samples which led to many interesting discussions and corrected misconseptions about fuels, which was demonstrated with a batch of Ethanol for a TAFC that didn't seem quite right which he test. The finders were a bad batch of fuel that was high in octane but very low in density. So the fuel was bad and wasn't preforming to out expectations which was putting are car off several tenths from where we knew it was previously running.

Just on a side note. This PhD was doing research on how much Ethanol you can mix with gasoline before they seperated out, which is about 15% effectively. 17% in a lab perfect world. He got a grant to by a nameless company who was trying to make a 20 or 25% fuel. Not possible according to this guy. That also led to many discussions about gasoline and ethanol but that is another discussion. What I can say there isn't any thing good about putting ethanol in gasoline when it comes to performance. It reduced density, reduces volumetric effciency, and perfromance. It does help economy very slightly, mainly it reduces emissions, and that is its main purpose for mixing it with gasloine.

In any event, octane is not a true indicator of fuel performance. It is at the point at under X compression the fuel will detonate and has little to nothing to do with how much energy is expelled by volume.

It is also why, in lower compression engines high octane fuels net zero performance gains.

Or in other words, Octane = the point of detonation under compression, but tells you nothing about the energy released from the detonation.

Example

33 kW and 29 kW for gasoline and CNG fuelled engine at 3500 rpm at full load, respectively. The octane rating of the CNG fuel is 120-130, but has a much lower density vs. the 93 octane gasoline that it was tested against.

A 4 kW difference from a fuel with a significantly higher octane number, which is 5 and half horse power.