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Old 05-05-2004, 09:23 AM   #1
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Against the suggestions of many people I went ahead and ran 89 grade gas (midgrade) in my car since it'* about $11 cents cheaper a gallon than premium. What I have found, is that my car doesn't run like it used to. In fact, it runs better! It seems to have a bit more power and my gas mileage has actually gone up, instead of down. I usually can drive around 50 miles before I leave full, this time I'm at 75 miles and I'm still on full. The only mods I have that might allow this is a gutted air box, K&N filter, removed MAF screen, and exhaust upgrades. Has anybody else ran midgrade without problems on their L67 like I have been? The engine runs fine and has more power it feels like so I dont' really think I'm having any problems with engine knock. comments?
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Old 05-05-2004, 09:58 AM   #2
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Its not something I would continually do there Custom, however, that toulene mix that Bill posted is the best way to get low to mid grade gas to work for you.

Over the long term...the lower grade will have adverse effects.
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Old 05-05-2004, 10:12 AM   #3
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Scan the car for KR, I'm sure you'll have some.
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Old 05-05-2004, 10:13 AM   #4
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As long as your not getting KR you can run as low a grade as you want. In fact you will get more power from less octane. However if you ARE getting KR then running any octane that is giving you KR is bad for the engine.

Lower octane gas burns easier hence the more power, however higher octane can in some cases give your more advanced timing if the computer sees the right conditions and commands it.
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Old 05-05-2004, 10:29 AM   #5
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Im sorry its off topic, but what is the MAF screen?
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Old 05-05-2004, 10:45 AM   #6
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Quote:
Originally Posted by Sigma
Im sorry its off topic, but what is the MAF screen?
Its a screen in the throttle body that straightens out the air flow into the intake.
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Old 05-05-2004, 10:46 AM   #7
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Quote:
Originally Posted by Sigma
Im sorry its off topic, but what is the MAF screen?
MAF= Mass Air Flow
The screen sits in front of the MAF sensor and aids laminar flow which the sensor uses to determine how much air your car is getting.
The screen can impead air flow and some of us have removed it.
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Old 05-05-2004, 10:47 AM   #8
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Just noticed the MAF thing. If you have no way to scan your car I would put the screen back in. Sometimes the MAF reading can be affected screwing up your A/F ratio.
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Old 05-05-2004, 10:59 AM   #9
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what is KR? Id love to put 87 or 89 in mine too, it costed me 55 bucks to fill it on Saturday with 91...
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Old 05-05-2004, 11:06 AM   #10
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Quote:
Originally Posted by SSEimatt93
what is KR? Id love to put 87 or 89 in mine too, it costed me 55 bucks to fill it on Saturday with 91...
KR is knock retard. When the computer detects predetonation in the cylinders (by using knock sensors) it will pull timing from the car in order to get it under control. predetonation happens when a hot spot in the cylinder ignites the fuel charge before the spark from the spark plug does. When the two parts of the fuel charge ignited from 2 different sides (1 from the hotspot, 1 from the spark) collide this causes knock. Too much of this can destroy a piston. Higher octane is harder to burn so the its harder for a hotspot to set off the charge the reducing knock. Being harder to burn however means less explosion from the charge and thus less hp. However if your not getting knock due to higher octane then the computer can advance the timing of the car resulting in more hp so its kind of a trade-off. Make sense?

The advantage of high octane is that the computer can always advance timing when conditions are right, so your car can only perform better. Using low octane fuel with no knock is ok until something happens to offset the balance (you develop a hotspot, either through booststacking, or heatsoak) and the only thing the computer can do is pull timing to try to cool it down, this significatnly reduces your hp and can damage the engine. I for one will always use 94 or better. I could possibly get away with 91 but with my mods if something were to happen and the computer couldnt compensate then by-by engine.

This is why running lean in a supercharged car is so dangerous. More air and less fuel means its easier to combust so a hotspot can set it off a lot more easily. A custom PCM can add fuel to help make the fuel charge more resistant to predetonation but this also reduces hp. Getting the air/fuel mix just right so your not getting knock but not making the charge too fuel heavy is a fine line and a great place to try and tune the car.

Leaner = more horsepower but also closer to the edge where if something goes wrong it could be catastrophic for the engine.
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