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Pavlo
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Joined: Nov 15, 2006
Posts: 18

PostPosted: Sat Nov 18, 2006 12:42 am Reply with quote Back to top

I know you are the expert at this, but now why do you advance the timing until knock? And timing that is advanced higher then the factory have it set, will make you run higher octane then the minimum.
This is something I can never understand, I've heard too many different variations on what exactly does octane show. Does it require higher temperature to ignite it or what (premium).
If you can please explain this to me.
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Misha
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Joined: Aug 02, 2006
Posts: 705
Location: McLean, VA, USA

PostPosted: Sat Nov 18, 2006 1:44 am Reply with quote Back to top

This one is a whole lot of a story. I don't promise I put it here tomorrow, but I'll try to do my best to complete this in the nearest few days.
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Misha
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Joined: Aug 02, 2006
Posts: 705
Location: McLean, VA, USA

PostPosted: Fri Nov 24, 2006 3:18 pm Reply with quote Back to top

Ok, pavlo, I did post a new encyclopedia article on octane rating, and updated article on detonation. Please, read them first, this should pretty much answer your second question. Summarizing it here, octane rating basically shows how high or low air/fuel mixture auto-ignition temperature is under high pressure, so you were pretty close with your thoughts.

Coming back to your first question. There is an optimal ignition timing advance for every combination of engine running parameters (like rpm, throttle percent, etc.). If you keep all parameters constant and just change the timing, you can draw a diagram like this:

Image

This is how every gasoline engine is initially designed – to be able to work optimally on a particular fuel grade. Engine in this example was designed to work on 87 grade gasoline. The next picture shows the behavior of engine designed to work on 92 grade gasoline.

Image

It will detonate on anything lower than that, but if you retard the timing to 10-12, it will work fine on 89 grade, and if you retard to 4-6, 87 would not be a problem. BUT, being non-optimal, you are loosing power and fuel efficiency, too…

I read on many forums that people do advance timing on later Honda models, and get some HP gains out of it (using higher grade gas of course). I have to see the dyno yet, but if this is for real, the only viable explanation I can come up with would be that those engines were designed to use higher octane gas, and Honda just retarded the timing to adapt them to regular US gas, instead of properly re-designing them. Why? IDK.


Last edited by Misha on Sun Dec 17, 2006 1:29 pm; edited 1 time in total
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Stimpy
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Joined: Sep 25, 2006
Posts: 107

PostPosted: Sun Nov 26, 2006 3:10 pm Reply with quote Back to top

one question..

The higher octane must be better in some way right? I mean, why aren't 'performance' engines just set for 87 instead of 93?

what exactly DOES the higher octane gas allow the engine designers to improve?


..well, it was one question in my mind at least.
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Misha
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Joined: Aug 02, 2006
Posts: 705
Location: McLean, VA, USA

PostPosted: Sun Nov 26, 2006 5:53 pm Reply with quote Back to top

Yeah, sure, there is one key parameter that affects fuel efficiency and power simultaneously - and I'm pretty much sure you heard of it many times. It is called compression ratio. It defines engines thermodynamic efficiency, in other words how good engine is using the heat received from gas burning. And how high you can go with CR is limited by detonation, and detonation is controlled by octane rating…
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arun
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Joined: Dec 25, 2009
Posts: 100

PostPosted: Fri Jan 15, 2010 11:55 am Reply with quote Back to top

Eventhough octane is adjusted to optimum level and the engine runs smoothly, Is it safe for the engine for a longer run?
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