RallyJon

More advance at lower rpms = more power at those rpms but you need higher octane to prevent detonation with the extra advance. If you run 87 octane in your 993 and experience detonation, the ECU retards the timing, costing you power.

So you're contending that ignition advance, octane and power output have nothing to do with each other?

Red rooster

Rallyjohn,

Good point but maybe slightly the wrong way round !
Higher octane fuel burns slower and so needs more ignition advance for the peak cylinder pressure to arrive at the correct few degrees after TDC .
That same advance with lower octane fuel will result in the pressure peak arriving to soon and consequent power loss.
The trick is to arrange the advance to be correct for the octane of the fuel being used .This is called mapping !
Loren is quite correct in that 91 used in a motor designed/calibrated for 87 will bring no power increase without advance adjustment, indeed there could be a slight power loss !
The pupose for using higher octane fuel is that it has a higher energy content and so with everything adjusted correctly will give more power.
Hope that helps

All the best

Geoff

cowtown

Actually, my understanding is that within a family of fuels, the lower octane fuel generally has the higher energy content (though there is no fixed relationship).

The higher octane just means lower volatility, allowing an engine designed for such fuel to use timing and higher compression to its advantage.

Red rooster

Cowtown,
Interesting. If thats correct then why bother with high octane fuel ?
You could have just started a rush to drop the compression + DME remap to retard the timing so 87 fuel can be used and 300 bhp here I come !!

Geoff

Lorenfb

I agree with Colin. He's stated it correctly.

1. Higher octane in itself, which was the implication of the previous post where the initial
statement was made, will not produce more performance.
2. Higher octane burns more slowly and is less likely to result in detonation as the flame
front procedes from the ignition point for a given CR & ignition timing.
3. To use a higher octane fuel for greater timing advances doesn't guaranty that no pinging
will occur or that the timing won't be retarded (power loss) on a knock controlled engine.
Remember, it's engine temp, engine load, true CR - carbon buildup, and not just the octane
which affects knock control.
4. When a chip supplier requires a higher octane fuel for use the his chip, you immediately
know that the timing has been "pushed" very near pinging/detonation. This is not a good mode
to have any engine operate near.
5. Having to increase the octane level, i.e. If you're lucky to find the ideal octane as one drives
from the local known area., for use with a performance chip is hardly indicative of any real
significant technical/rigorous tuning effort/effect on the part of the chip supplier. It's basically
what was done on the 911SC by just turning the distributor a few degrees, a 3 minute effort
which gave a feeling of new power.

geolab

Its getting confusing a bit here.

Fuel with high octane or low octane has the same burning characteristics.
It develops the same amount of energy, fullstop.
But the same amount of fuel compressed slightly and ignited, does not develop as much pressure
as if it was highly compressed and ignited. (as in steam and practically everything)

Diesel when compressed to a certain extent due to high temperature from compressed air, burns by itself. This is why diesel engines do not have
spark plugs but a high pressure pump(direct pressurized diesel injection in the cylinder), and warming the diesel enhances this, though only air is compressed.

gasoline also self ignites under pressure. and the octane rate is the rate at which the gasoline can be compressed the least, before self igniting.

for higher performance, engine builders highered the compression ratio.(by elongating the travel the piston does in a cylinder)
An 8 to 1 compression ratio (classic) compresses the gas to 1/8 th of the cylinder size, thus the gasoline + air size.
9 to 1 compresses the chamber to 1 out of 9 times of the original size.
But the low octane fuel will self detonate at this pressure, so higher octane is used.

So a higher compression ratio = high performance
higher octane does not burn better than low octane, but can be better compressed.
So a higher octane fuel is not equal to higher performance

Lorenfb

"Fuel with high octane or low octane has the same burning characteristics."

Fuels do have different burning characteristics! Please get an engineering
thermodynamics book. Here's a good source:

"Internal Combustion Engine Fundamentals", John B. Haywood
Chapter 9 - "Combustion in Spark-Ignition Engines", pgs 371-490

"But the low octane fuel will self detonate at this pressure, so higher octane is used"

That's correct.

"higher octane does not burn better than low octane"

But it does burn better, as you just stated above. Don't contradict yourself.

rcwelch

WOW!!! I'm off to pop some more popcorn. I'm glad I got my chip when life was simple.

geolab

Fuel with high octane or low octane has the same burning characteristics.
It develops the same amount of energy, fullstop

87 or 91 or ANY octane gasoline will produce 34 MegaJoules for every liter, if anybody likes it or not.

High Octane or low octane fuel burn at the same rate (energy wise, not timing), but self-ignite at different pressure ratings.
we are discussing extra horsepower here, and negating that higher octane gives more horsepower
But more horsepower is gained with a higher compression ratio? the answer is yes.

to obtain different octane ratings (or auto-ignition ratings) under pressure , additives are added.

I've had my share of books in the 80's thanks. And that was before the internet.

I have never stated that higher octane fuel burns better, sorry about that, no hard feelings.

just quoting I am sure if we were to discuss this over a it would be clearer, but ink on paper is sometime mal-interpreted, thats the negative part of it

cowtown

geolab, after poking around the Chevron site, I see you are right about energy content. I'd like to modify my statement to read:

InTheAir

Good reading.

Lorenfb

Initially posted: "Fuel with high octane or low octane has the same burning characteristics."

Lasted post: "High Octane or low octane fuel burn at the same rate (energy wise, not timing)"

So they do have different characteristics. Glad you clarified it.

Red rooster

cowtown/geolab,

Where on the Chevron site did you find this stuff ?
I went back to my data and saw that Premium has a higher Mj/kg than Regular but then did the density correction and found that Mj/litre is pretty much the same for both fuels !
As it is Mj/litre that the motor runs on you are correct . Well done and thanks.
Reminds me of the old saying - the day you stop learning is the day you die !!

Looked back at some other stuff and saw that the relationship between compression/octane/maximum advance has all the possibilities to get messy !
Looks like the trick is to find the best octane/advance combination that your motor is happy with ! Lots of general rules of thumb but the devil is in the detail .

All the best

Geoff

95FL993CAB

Ok
I will make some calls late tomorrow, schedule permitting. If you are still interested, please PM me with the particulars. Please avoid the short hand Steve W as there are 2 of them Please say the full name.
I will then split the groups based on preference and find the appropriate #.
I will be gone at the end of the week. I will be in Canada for a few weeks, so I am not certain this will be done prior to the Holidays.
Take care
Ron

Brad '95 C4

FYI, I believe Steve Weiner at Rennsport Systems in Portland, OR will be out of the office for 2 weeks.