High compression
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From: Crawfordsville, IN
Car: 1991 Galant VR4
Engine: 2.0L 4G63 Turbo
Transmission: 5sp transaxle/transfer case
High compression
Is there a rule of thumb for how high your gas octane level has to be to run a certain compression ratio? Ex: 10:1 requires in and around 93 octane.
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From: clinton,tn
There are to many variables to say what cr a specific octane gas can support. Alum. heads generaly will support more compression than iron heads can, then you have cam specs to deal with as well as combustion chamber design and so on and so on and so on. Oh did I forget to mention ignition timing. Thats another biggie.
Steve
Steve
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Not really. Theres alot of variables, the main one being the cam.
I have a car that has 10.3:1 and will run on 87 as long as it isnt 95-100 outside. Kevin is running almost the same combo I am (different cam) and he has to use octane boost at the track.
I have a car that has 10.3:1 and will run on 87 as long as it isnt 95-100 outside. Kevin is running almost the same combo I am (different cam) and he has to use octane boost at the track.
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Saint,
There are a lot of variables. The static compression ratio only hints at the cylinder compression pressure. A "nominal" engine at 9:1 static with a good cam selection can create more compression pressure than a some poorly designed 10.5:1 engines. Compression pressure in the cylinder is more of a factor, along with spark timing and engine/chamber temperature.
An engine with a non-functioning EGR can get very hot chambers and tend to detonate more than an engine that has a proper EGR. Alocohol blended fuels can also provide some cylinder cooling while providing very good cylinder pressures, since the alcohol component of the fuel burns a little more slowly and cooler than raw gasoline. (Incidentally, there is no power loss with ethanol blends, but that is a topic all by itself.)
Excessive timing advance can require more octane and cost power. There is a point where timing can be advanced too much. The control system makes a huge difference as well. The LT1s had an OBDII system that would control spark and detonation better, and allowed the use of 91 octane fuel at 10.5:1 with iron heads.
There is no set number, but increasing the compression ratio will certainly not lower the requirement for octane. It just might not require you to use a higher octane.
I've got an 11.1:1 355 that can run well on 93 octane gasohol, so the picture gets even more fuzzy.
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Later,
Vader
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"What a Day..."
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There are a lot of variables. The static compression ratio only hints at the cylinder compression pressure. A "nominal" engine at 9:1 static with a good cam selection can create more compression pressure than a some poorly designed 10.5:1 engines. Compression pressure in the cylinder is more of a factor, along with spark timing and engine/chamber temperature.
An engine with a non-functioning EGR can get very hot chambers and tend to detonate more than an engine that has a proper EGR. Alocohol blended fuels can also provide some cylinder cooling while providing very good cylinder pressures, since the alcohol component of the fuel burns a little more slowly and cooler than raw gasoline. (Incidentally, there is no power loss with ethanol blends, but that is a topic all by itself.)
Excessive timing advance can require more octane and cost power. There is a point where timing can be advanced too much. The control system makes a huge difference as well. The LT1s had an OBDII system that would control spark and detonation better, and allowed the use of 91 octane fuel at 10.5:1 with iron heads.
There is no set number, but increasing the compression ratio will certainly not lower the requirement for octane. It just might not require you to use a higher octane.
I've got an 11.1:1 355 that can run well on 93 octane gasohol, so the picture gets even more fuzzy.
------------------
Later,
Vader
------------------
"What a Day..."
Adobe Acrobat Reader
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