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I was wondering about the different grades of gas and was wondering if it was okay that I have always but unleaded 87 octane in my tank? Any of this matter? I think this is a 90's engine block, if that matters.
 

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I was wondering about the different grades of gas and was wondering if it was okay that I have always but unleaded 87 octane in my tank? Any of this matter? I think this is a 90's engine block, if that matters.
What matters most is compression ratio and timing. At a higher CR, there is a higher risk of the gas pre-igniting while the piston is still moving upward. Not good! Higher compression engines need higher octane gas, simply because it takes longer to ignite.
 

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Discussion Starter #3
What matters most is compression ratio and timing. At a higher CR, there is a higher risk of the gas pre-igniting while the piston is still moving upward. Not good! Higher compression engines need higher octane gas, simply because it takes longer to ignite.
answered perfectly.
 

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Keep note that a lot of newer vehicles have knock sensors and electronically controlled distributors, so when a knock is detected timing is retarded. It can reduce pre-ignition/detonation...
 

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lost me with that last one. sorry. :eek::eek:
When the gas ignites before it is supposed to, called preignition or detonation, it makes a certain knocking sound.

Basically, say you have an LT1. On these motors there is a special sensor that "hears" this exact knocking sound. Also, the distributor's timing advance is controlled by computer. That is how you can re-program timing curves by flash tuning the computer. If I remember, LT1s have a compression ratio of 10.5 or 11:1. Pretty high so you need high octane fuel. Now, say you accidentally put plain old 87 octane in there. In a 70's (for example) motor with that CR, you bet there would be detonation problems! On an LT1, however, the computer hears the knock detonation makes and retards the timing. In turn this can eliminate the detonation.


Why you think this can help detonation? Well, for simplicity lets use this example, even though it is NOT the actual distances! Say your spark plugs fire 1/4" before the piston is at its highest point when you use premium fuel. By the time it is at the highest point the burning gasses are forcing the piston downward. When you accidentally put the regular gas in it, the burning gas starts pushing the piston down while it is still going up!:eek: So now you need the spark plugs to fire 1/8" before the piston's highest point.

An LT1 will change timing so the spark plugs fire at that 1/8", because it has the sensors, computer, and control to do so. MUCH better than any plain old 70s design can do.:yes: Basically the non-computer motor will keep firing at 1/4" and destroy your motor.

Hope this makes it easier instead of harder!:D Now is probably a bad time to say there are exceptions to the rule though.
 

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Prignition

My 350 small block runs fine while I drive it but when I turn it off I'm getting preignition ! Any idea's ? Besides the timing !
 

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Idle might be too high, choke may be still on, the idle circuits are too rich and need to be dialed back?

What do the plugs look like? If they are awash with gas, then your carb is too rich.

ray

Probably should have posted this in the drivetrain section.
 

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I always run 93 octane in my 406 w/ 10:1 compression. However, I always wonder how fresh the 93 octane gas is at the gas stations? It's just not used like the 87 is.
 

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Luke, what you are describing is called run on or diesel img. It's from unburned hot carbon causing fuel to ignite without a spark. Not real good for the engine. One near term solution is to use a higher grade fuel. At least a mid grade. So called regular was 89 octane years ago, now at 87 the old engines don't like it. You may be able to get some help from different spark plugs, but for myself, I run 92 or higher in mine.
 

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I like to run 92 or higher in all my TOYS ,With that said when I run 92 or better in my 2002 chevy truck (5.3) I get better MPG





. Later Larry Mc
 

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run on

Luke, what you are describing is called run on or diesel img. It's from unburned hot carbon causing fuel to ignite without a spark. Not real good for the engine. One near term solution is to use a higher grade fuel. At least a mid grade. So called regular was 89 octane years ago, now at 87 the old engines don't like it. You may be able to get some help from different spark plugs, but for myself, I run 92 or higher in mine.
Hey Pete ,
I'm using 93 octane , pulled the plugs and they looked fine . Would a leak in the intake have anything to do with it ? The car had been sitting for a few years , I just got it in sept. 2014 and am putting it together. It starts fine , drives fine but once its warmed up and I try shutting it down that's when I'm getting it . Carb maybe bad ??? I'm just looking for any answers . Thanks
 

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Luke, not to hijack the thread, but I'd start with what's easy. The carb, especially if it's been sitting for a few years. Intake gaskets would be unlikely to cause run on. You may want to go to a shop that specializes in dialing in cars. I use Carb Connection in Kirkland, Wa, but since it's about 3000 miles +\- from you that probably isn't an option. I've slowly learned that as much as I enjoy fixing things myself, sometimes I'm just better off taking it to the right shop. Often times the shop will tell me what I can/need to do myself then have them put the final touch on it. PM me with your progress.
 

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Run on

Luke, not to hijack the thread, but I'd start with what's easy. The carb, especially if it's been sitting for a few years. Intake gaskets would be unlikely to cause run on. You may want to go to a shop that specializes in dialing in cars. I use Carb Connection in Kirkland, Wa, but since it's about 3000 miles +\- from you that probably isn't an option. I've slowly learned that as much as I enjoy fixing things myself, sometimes I'm just better off taking it to the right shop. Often times the shop will tell me what I can/need to do myself then have them put the final touch on it. PM me with your progress.
Thanks Pete I'll keep in touch !
 
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