The last gas question

[severous01]

wow, there's a LOT of seriously wrong info in this thread.

if you done your research, you know that higher octane DOES BURN SLOWER and is used in higher compression, more advanced timing applications for this reason....the spark is well before TDC for this reason, the fuel will start to burn well before TDC and then explode ATDC. higher performance engines REQUIRE this fuel because of advanced timing...and that's all you'll get with any of the aftermarket programmers....maybe a little fuel trim to enhance new timing. any way, the fuel is the same, and is actually has more addatives to make it lower octane vs higher...and lower octane fuel is cheaper than higher octane.

facts and myths: http://www.state.mn.us/mn/externalDo...ctaneFacts.pdf

octane rating is the percentage of resistance to knock....yes, even tho they have 102 and 104+ or whatever. the higher the octane the more energy required to cause a burn or release of energy.

when an engine senses knock it fixes the problem by retarding timing. if you know anything about old school (distributor ignition) you know you can do A LOT with a simple twist of the Dist...2 or 3* can net you up to 20 hp in some cases (5.0 mustangs for instance) but at WOT you'll need high octane fuel...hence the retarding issue from the factory. efficiency.

the higher the compression, the higher the temps. higher temps inside means a higher chance of pre-ignition or knock...or ping...or whatever you want to call it. and since all engines now are computer controlled and most have knock sensors....the pcm would much rather save the engine by retarding and fuel efficiency than popping a hole in a piston cuz your dumbass decided to ignore it or keep goin cuz you 'had to'. any way, do some more research on wiki or the site i provided.

[Yaris Hilton]

Originally Posted by severous01

wow, there's a LOT of seriously wrong info in this thread.

if you done your research, you know that higher octane DOES BURN SLOWER and is used in higher compression, more advanced timing applications for this reason....the spark is well before TDC for this reason, the fuel will start to burn well before TDC and then explode ATDC. higher performance engines REQUIRE this fuel because of advanced timing...and that's all you'll get with any of the aftermarket programmers....maybe a little fuel trim to enhance new timing. any way, the fuel is the same, and is actually has more addatives to make it lower octane vs higher...and lower octane fuel is cheaper than higher octane.

There is some wrong info (slower burning with higher octane) and some basic misunderstandings in your post. If you'll do your research, you'll find that what's slower about high octane fuel is the pre-flame phase of combustion during which the initial reactions leading to autoignition occur. The flame velocity and duration of the flame are not affected. The pre-flame reactions occur when the fuel is heated by compression, and by compression of the unignited part of the fuel-air mixture by the expanding ball of flame behind the spreading flame front. If you increased the compression ratio, at some point you'd have the fuel autoignite or detonate without any spark being fired at all, which is an upper limit on the compression ratio that can be used with a given fuel. Detonation that we're discussing is what occurs when the spark fires before any autoigniton occurs, but part of the charge autoignites before the flame front has spread through the whole combustion chamber.

In all engines, advanced timing is necessary because the flame front travels across the combustion chamber at a finite speed from the spark plug after it starts the fuel burning. The spark fires before top dead center, but full ignition of the charge occurs sometime after TDC. Most efficient conversion of the energy released by the burning fuel occurs when the fuel burns near the top of the power stroke, expanding more and therefore doing more work as the piston goes down. If the octane level is too low, detonation can be suppressed by delaying the spark so the piston is going down and relieving the pressure on the yet unignited part of the mixture so it doesn't autoignite before the flame front reaches it. As more of the fuel then burns at a larger combustion volume, the peak flame temperature may go down a bit but the engine actually runs hotter because more of the cylinder is exposed to the near-peak flame temperatures. The exhaust temperature will also rise, because the flaming gas has not cooled as much by expansion on the power stroke. A higher compression engine will need higher octane fuel to be able to use its optimum spark advance level and operate at maximum efficiency. Other factors like air intake temperature, combustion chamber surface temperatures and combustion chamber turbulence also affect octane requirements and optimum spark timing.

Preignition due to hot spots in the combustion chamber from carbon deposits and such is different from detonation, but detonation often causes hot spots to light up and cause preignition, and preignition often causes detonation.

Wikipedia has a good explanation of knocking here: http://en.wikipedia.org/wiki/Engine_knocking

That link to the state of Minnesota's "Octane Facts" is a good consumer-level explanation of fuel octane, that doesn't contradict anything I've said.

A very helpful, more technical reference is the Gasoline FAQ: http://www.faqs.org/faqs/autos/gasoline-faq/

There are many technical papers available online on fuel combustion that a bit of creative Googling will turn up. I've spent many hours reading quite a few of them. If you're really interested in the chemistry and dynamics of it, I'll recommend the seminal text Explosion and Combustion Processes in Gases, by Jost Croft, McGraw-Hill, 1946. (A translation of a German text that the U.S. government promulgated to advance aircraft engine and fuel technology at the end of WWII. I found a copy on eBay a few years ago.) Pretty much everything known about combustion in engines now is drawn from the information first assembled there, with refinements developed over six decades of further experimentation and improved sensing technology.

I'm not arguing to be argumentative. I'm just sharing information I've spent a lot of time and effort learning, and correcting a common misconception about burning speed being different as the octane level changes.