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Discussion Starter #1
So, I'm no chemist by any means. Since there are a wealth of knowledge on this site related to physics and what not, I decided to start this thread.

So for starters, I know most engines are able to run on the 3 common available octane ratings from the factory. However, some things I've been reading leads me to some questions.

So, correct me if I'm wrong. 87 octane fuel is MORE combustable than 93 octane correct? So, a higher compression engine would have predetonation on a lower octane rating and need to run the higher rating? Which means e-85 and diesel fuel are some of the least combustable and have really high compression pistons to ignite fuel properly.

So, a higher compression engine that is boosted could make the same power on less boost than a similar engine with lower compression, only with less room to work with on timing correct? If this is the case, wouldn't you be able to tune a lower compression car with boost to run on any of the 3 octanes?

Lastly, wouldn't a lower compression (or worn out high mileage engine) running on 87 octane do better with a colder spark plug?


None of these questions are relevant to anything other than just expanding my knowledge base. Just, like to learn about things like this.



p.s. For the record I'm one of the looneys that doesn't always follow what "main stream" science says. So, I believe oil is an abiotic source, if anyone is interested in furthering the conversation in that direction. (even though it usually leads to climate change discussion, and that ALWAYS gets political for some reason)
 

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There is tons of info out there.


Starting from the easier to read:

http://www.eejitsguides.com/environment/fuel-octane-summary.html

http://en.wikipedia.org/wiki/Octane_rating

http://auto.howstuffworks.com/fuel-efficiency/fuel-consumption/question90.htm



http://www.faqs.org/faqs/autos/gasoline-faq/part3/index.html

Here is an interesting quote from the last one:
The efficiency gains are best when the engine is at incipient knock, that's
why knock sensors ( actually vibration sensors ) are used. Low compression
ratio engines are less efficient because they can not deliver as much of the
ideal combustion power to the flywheel. For a typical carburetted engine,
without engine management [27,38]:-

Compression Octane Number Brake Thermal Efficiency
Ratio Requirement ( Full Throttle )
5:1 72 -
6:1 81 25 %
7:1 87 28 %
8:1 92 30 %
9:1 96 32 %
10:1 100 33 %
11:1 104 34 %
12:1 108 35 %

Modern engines have improved significantly on this, and the changing fuel
specifications and engine design should see more improvements, but
significant gains may have to await improved engine materials and fuels.​

There's a start...
 

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Discussion Starter #3
Thanks. Lots of good info. I've read 2 of those before. It's actually quite a daunting task to fully comprehend it all. It seems to be the common thing to build a boosted engine with lower compression. But, the other info says (basically) that a higer compression boosted engine is actually more efficient. Just with less room for error. At least that's how I'm comprehending it at the moment. So, where does quench come into the equation? I understand that it helps fuel atomization with the "swirl" and more even displacement, but does the fuel chemistry have anything to do with that? Or just the piston/head chamber design? From what I've gathered it's mostly in the head design and allows for higher compression ratios. So, what are the quench values in the stock 4.6 engines?

Quench info. http://www.theoldone.com/archive/quench-area.htm

Nevermind. Found some good info here as well. http://www.mre-books.com/sa82/sa82_5.html
 

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Every car sold in the US is required to be able to run on 87 octane fuel. It doesn't have to run very well, but you could put regular gas into any brand new bone stock factory car and it will run and drive, it just may not make as much power or get as good gas mileage. Yes, 87 octane is more combustible than 93, which is more combustible than E85, which has an octane rating of over 100. Diesel is actually much more combustible than regular gasoline though. It has an octane rating in the 40s. The reason diesel engines can get away with such high compression on such low octane is because the fuel is not injected until the air has already been compressed. In a gasoline engine, if you have too much compression or not enough octane, as the air/fuel mixture is compressed it is heated up, and if it gets too hot, it can spontaneously combust before it is ignited by the spark plug. That uncontrolled burn is what you hear as a knock. In a diesel engine, the air is compressed first without any fuel, so there is no potential for it to pre-ignite. Instead of the ignition being triggered by a spark plug, it is triggered by the fuel being injected into the already compressed cylinder, which due to the fuel's low octane causes it to ignite as soon as it hits that hot air.

As for the amount of power based on boost and compression, it isn't that simple. Boost is just a measure of restriction due to the turbo or supercharger pumping more air than the engine can move. If you improve the air flow of the engine with better flowing heads, intake, cams, exhaust, etc., then boost will go down while power will go up. As far as what kind of timing you can run, that depends more on dynamic compression than static compression ratio, which means the actual pressure created in the cylinder with the engine running under full load. If you increase the static compression ratio, but also increase the overlap in the cam, you could theoretically keep the dynamic comression ratio the same, and make more power, but at the potential cost of worse gas mileage and higher emissions. Adding timing will add pressure and heat into the cylinder, which will improve power up to a point, and if you go too far with it, it will cause detonation. As for what creates the most power, that depends on the individual combination. There is no universal answer for all engines that you will be better off with more compression and less timing, or more compression and less boost, or less compression and more boost, or anything like that. It all depends what the combination responds best to, and that is going to be different depending on so many factors.

As for running a colder plug, you want to run the hottest plug that you can that will not cause pre-ignition. Keeping the plug hotter will help it to burn off contaminants and prevent the plug from fouling out, as well as allow the ignition system to work better. The heat range has to do with how much heat the plug can dissipate, so as you create more pressure in the cylinder, you create more heat, and so you need to go to a colder plug to keep its operating temperature the same. Going to a colder plug on a car when you have not increased the dynamic compression is going to result in fouling plugs, more strain on your ignition system, reduced power, worse gas mileage, and higher emissions. If you have a car that reccomends premium fuel, and you want to run regular in it, the colder plug might allow you to do that, but the cost in terms of the plugs and the increased fuel consumption would be more than the cost difference of just running premium fuel in it like it was designed for.
 

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Discussion Starter #5
Thanks Mikey. I'm a little behind on how diesel engines work. There is a lot of variable that come into play. I mostly got on to this because I was pondering compression ratio for the pistons I wanted to order but ended up getting more in depth with my reading.
 

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There is tons of info out there.


Starting from the easier to read:

http://www.eejitsguides.com/environment/fuel-octane-summary.html

http://en.wikipedia.org/wiki/Octane_rating

http://auto.howstuffworks.com/fuel-efficiency/fuel-consumption/question90.htm



http://www.faqs.org/faqs/autos/gasoline-faq/part3/index.html

Here is an interesting quote from the last one:
The efficiency gains are best when the engine is at incipient knock, that's
why knock sensors ( actually vibration sensors ) are used. Low compression
ratio engines are less efficient because they can not deliver as much of the
ideal combustion power to the flywheel. For a typical carburetted engine,
without engine management [27,38]:-

Compression Octane Number Brake Thermal Efficiency
Ratio Requirement ( Full Throttle )
5:1 72 -
6:1 81 25 %
7:1 87 28 %
8:1 92 30 %
9:1 96 32 %
10:1 100 33 %
11:1 104 34 %
12:1 108 35 %

Modern engines have improved significantly on this, and the changing fuel
specifications and engine design should see more improvements, but
significant gains may have to await improved engine materials and fuels.​

There's a start...
Either that guy sucks at math or just made those numbers up. I wonder what he considers "modern engines", because as far as I know, modern engines came about long ago, lol. Either way, I'm not even on that chart :rofl:
 

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Either that guy sucks at math or just made those numbers up. I wonder what he considers "modern engines", because as far as I know, modern engines came about long ago, lol. Either way, I'm not even on that chart :rofl:
Those figures are actually in a book I have; that's why I found and posted them. :)

The book was written in the 60's, and the covers and a bunch of pages are lost to whatever garage it came out of, lol.

They're "Carberated" numbers, on old school valvetrains; 'modern' FTP is non-flathead, I suppose, from the book I have. :)

He11, they probably still work for Chevys! :rofl:

I thought you'd wait awhile before chiming in, truthfully. :) See who's awake and all...

I said "There's a start." That really was the start, flat heads and delicate valves.
 

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that explains a lot. I anticipate 64% thermal efficiency at my compression level. Roughly ~5% more efficient than a 9.4:1 PI 2V ;)

I remember looking at thermal efficiency via that formula and knew that something was off. You know me, can't resist numbers!
 
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