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View Full Version : Safe octane for 10.2 to 1?



lutzy
02-06-2011, 12:44 PM
Just bought my car and the engine specs are 10.2 to 1 compression..what's the recommended octane I should use or is it just a trail and error? The last owner had 106 octane in it for the dyno and it's still on it's first tank..

Vegas69
02-06-2011, 12:59 PM
Depends on the camshaft. I have 10.3:1 and run 91 octane no problem. 106 is way to much.

ChevelleNV
02-06-2011, 01:21 PM
Ya, 106 is way to much for that compression level, you should be able to run 91 without any problems, if it pings on ya throw a little octane boost in it. but i doubt it will. my engine is 12.2:1 and I can get away with 91 with a little octane boost in it.

wiedemab
02-06-2011, 02:09 PM
Timing will play into it as well. I ran 93 octane in my 5.0 Ford that was right at 10.5:1 without any problems. I didn't have the timing tuned super aggressive though. I would think you would be OK with 91 to 93.

68Formula
02-06-2011, 02:28 PM
Whats the camshaft duration and lobe separation? Aluminum or Cast Iron heads? What is the shape of the combustion chambers and how is the spark plug placed? Have the valves and/or chamber and/runners been thermal coated? Are the heads reverse cooled? Is the valve arrangement of the engine such that two exhaust valves of cylinders are directly adjacent? What is the heat range of the plugs being used? What is the high ambient temperature that the engine may be run in? What is the quench distance?

As you can see, there are a number of factors that can affect the required octane. Literally one engine with that SCR might need race fuel octane, another might even get away with 87. And don't forget, the tuning of the engine can affect this as well.

68sixspeed
02-06-2011, 05:46 PM
florida, low altitude, can you get 93? hopefully so, if you have aluminum heads you should be ok, I use to have a 358 with 10.4:1 and aluminum heads, a good size roller cam which did probably bleed some compression but it ran good on 93 unless I got a bad tank. My current motor is 11:1 and just makes it on 93, but the aluminum block acts as a bigger heat sink plus the cam is pretty big for a small block. (.700 intake lift)

6'9"Witha69
02-07-2011, 04:56 PM
I run 10.8:1 with a 383, Dart Pro 1 215 Al heads, custom Isky Hyd Roller with 113-114 Lobe Seperation. I run 91 all day long.

wmhjr
02-07-2011, 10:33 PM
68formula is exactly right. Compression is only one factor. I'm running heavily ported aluminum heads, a good size roller cam and just over 11:1 compression. I need less than 30 deg timing. Engine runs cool, and I can run very hard in hot weather all day long on 91.

wiedemab
02-08-2011, 04:57 AM
All very good points - - - I was being pretty simplistic with my response. My 10.5:1 motor had aluminum heads and a FMS "B" cam, which is pretty mild.

MonzaRacer
02-14-2011, 08:35 PM
OK as for most builds , I go by the rule of thumb, 160to 180 cranking compression will live on just about anything especially with aluminum heads, 180 to 200 plus then you will need premium.
Aluminum heads will give you considerable leeway on octane.
Now if you want to alleviate all cooling issues , go to www.evanscooling.com and look into a change in coolant. NPG+ and some flush to get ride of old coolant or water and your good to go.
The actual possibilities with Evans NPG+ are pretty astounding. 375 deg F boiling point with no pressure, -40 deg F below,it never fully freezes just gets little slushy. No corrosion because it isnt 2 different liquids and 2 different metals, like a battery.
Oh and no nucleate boiling or hotspots and on most high performance engine even allows for more timing for more power.
But unless your cam is super short on duration your good with premium.
also if your looking for best torque out of your combo just keep advancing (within reason) the cam timing till you get highest peak torque if it keeps rising but never levels off then the cam is too small for heads or heads are overkill.
Cranking compression will be more of an indicator of octane requirements over static compression.