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-   -   Octane questions with the LGT (http://legacygt.com/forums/showthread.php?t=159980)

csbrown28 04-14-2011 03:04 PM

Octane questions with the LGT
 
Before anyone is tempted to school me on octane, I know what it is and how and why it works the way it does, I was just curious if anyone here uses 87-89 octane in there LGT.

I will use 91 or higher, but again I was just curious if anyone uses 87 and what the results were.

I believe the compression ratio of the LGT is 8.2:1 which don't seem all that high, but the one place someone could school me is how compression ratio's effect turbo charged cars. I know that turbo's increase pressure but 8.2:1 seems kinda low. My '93 Probe GT had a compression ratio of 8.6:1.

Anyway, just curious....

Thanks.

cypher0117 04-14-2011 08:42 PM

the compression ratio is lower to allow the extra volume of air/fuel during full boost. The high octane is really only effective when your turbo is spooled. The added pressure from extra fuel/air brings the cylinder pressure higher than a normal combustion engine which is why you need the higher octane.

csbrown28 04-14-2011 09:19 PM

Quote:

Originally Posted by cypher0117 (Post 3349515)
the compression ratio is lower to allow the extra volume of air/fuel during full boost. The high octane is really only effective when your turbo is spooled. The added pressure from extra fuel/air brings the cylinder pressure higher than a normal combustion engine which is why you need the higher octane.

So your saying the CR is variable , but starts at 8.2:1? That would make sense.

BAC5.2 04-15-2011 03:39 AM

Quote:

Originally Posted by cypher0117 (Post 3349515)
the compression ratio is lower to allow the extra volume of air/fuel during full boost. The high octane is really only effective when your turbo is spooled. The added pressure from extra fuel/air brings the cylinder pressure higher than a normal combustion engine which is why you need the higher octane.

None of this is entirely accurate.

BAC5.2 04-15-2011 03:51 AM

Quote:

Originally Posted by csbrown28 (Post 3349550)
So your saying the CR is variable , but starts at 8.2:1? That would make sense.

Not really variable.

You have a static compression ratio, 8.2:1. Air is compressed by the turbocharger, and then stuffed into the engine where it is compressed again. This is often called the "dynamic compression ratio", because it changes dynamically. The static compression ratio never changes. Whatever makes it past the valves and into the cylinder is compressed 8.2:1.

8.2:1 is pretty low, but the 8.6:1 in your old Probe is not high. S2000's run up around 11:1. The E30 BMW's ran 10.ish:1.

Octane does little more than require greater activitaion energy to begin a combustion reaction in fuel. This greater energy requirement makes it more difficult to ignite, which means you get greater resistance to auto-ignition. Auto-ignition in the modern world is known as detonation or det. In the old days, you'd call it "pinging" or "pinking" or something silly like that. Det occurs for a number of reasons, but the end result is an ignition of the Air/Fuel mixture at a time not desired, typically before the ignition event was supposed to occur. The damages from det can be catestrophic.

So, thta's kind of it in a nutshell. The description of everything you asked would take quite some time to describe, but there are a number of posts on this board that can help you get a better idea of the answers to your questions.

csbrown28 04-15-2011 04:58 AM

Thank you for the explanation. I'm aware of pre-detonation and how it works though, I'm old enough to have experienced dieseling in my old carborated engine.

Other then questions about compression I was just curious if anyone had tried low octane in their LGT, but I supposed no one wants to be ridiculed for doing something that others would call dumb....lol

johnAWD 04-15-2011 05:12 AM

I've not used lower octane in my LGT, as I know what the result would be.

BAC5.2 04-15-2011 05:17 AM

Dieseling is something different, at least as I know the term. But you get the idea.

87 isn't "dumb", it's dangerous. Det kills. Some people have tuned for 87, but it's not worth it IMO. Its a risk I don't think is worth taking.

RabidWombat 04-15-2011 08:28 AM

The ECU has a knock sensor and can pull timing to compensate for bad gas (or 87 octane). However, the pistons are sensitive to damage. General consensus is that knock events are likely to contribute to premature piston failure and its best to avoid them.

When I was buying my 2009 my dealer specifically warned me about using 91 octane. He told me about a 2008 FXT that needed a new engine after 50k because the owner used 87.

BAC5.2 04-15-2011 08:38 AM

If the ECU is pulling timing because of knock feedback, then the motor is already knocking. That's bad.

Just run 91 or better and feel good that you aren't breaking things.

BigTDogg (MA) 04-15-2011 09:24 AM

Quote:

Originally Posted by csbrown28 (Post 3349766)
Other then questions about compression I was just curious if anyone had tried low octane in their LGT, but I supposed no one wants to be ridiculed for doing something that others would call dumb....lol

That's because it is dumb. A full tank on a LGT is what, 15ish gallons? With a $0.20 difference between 87 and 93 (what it is around me anyway) saving you $3, I can't say that would be worth my engine.

It's slightly less stupid to put premium in a regular gas car, because you really can't hurt anything, except your wallet.

Subyduby-doo 04-15-2011 09:33 AM

Quote:

Originally Posted by BigTDogg (MA) (Post 3350037)
That's because it is dumb. A full tank on a LGT is what, 15ish gallons? With a $0.20 difference between 87 and 93 (what it is around me anyway) saving you $3, I can't say that would be worth my engine.

It's slightly less stupid to put premium in a regular gas car, because you really can't hurt anything, except your wallet.

:yeahthat:I agree. Can't remember how many times I had to explain why you shouldn't put premium in your car if it only asks for regular. Not that it's going to damage it in anyway, just damage your pockets and not give you any return, or any significant return.

Montana 04-15-2011 10:57 AM

This always depends on your altitude, too. Some pleaces, we don't even get over 89.9 at the pumps.

underground000 04-15-2011 11:11 AM

The people who have ran 'regular' octane have reported worse gas mileage

cypher0117 04-15-2011 11:16 AM

Quote:

Originally Posted by BAC5.2 (Post 3349733)
Not really variable.

You have a static compression ratio, 8.2:1. Air is compressed by the turbocharger, and then stuffed into the engine where it is compressed again. This is often called the "dynamic compression ratio", because it changes dynamically. The static compression ratio never changes. Whatever makes it past the valves and into the cylinder is compressed 8.2:1.

8.2:1 is pretty low, but the 8.6:1 in your old Probe is not high. S2000's run up around 11:1. The E30 BMW's ran 10.ish:1.

Octane does little more than require greater activitaion energy to begin a combustion reaction in fuel. This greater energy requirement makes it more difficult to ignite, which means you get greater resistance to auto-ignition. Auto-ignition in the modern world is known as detonation or det. In the old days, you'd call it "pinging" or "pinking" or something silly like that. Det occurs for a number of reasons, but the end result is an ignition of the Air/Fuel mixture at a time not desired, typically before the ignition event was supposed to occur. The damages from det can be catestrophic.

So, thta's kind of it in a nutshell. The description of everything you asked would take quite some time to describe, but there are a number of posts on this board that can help you get a better idea of the answers to your questions.


i was trying to describe the 'dynamic compression ratio'

your description makes more sense though


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