Octane requirement

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pjc360

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I'm running a 300hp crate 360 magnum, it has 20 thousand miles on it right now, it has 9.0:1 compression ratio, I'm running 16 degrees of initial timing and 34 degrees total timing.
I live 4000 feet above sea level, I ran a compression test on it while warm with the carburetor wide open and it read 150 psi on every cylinder.
Based on that information am I safe to run regular 87 octane fuel in it? Or should I run 91 octane fuel in it? My timing is all in by 3000 rpm.
I can't hear any knock h running regular 87 but the exhaust is pretty loud so I'm not sure I would hear it if it was knocking.
What do you guys think?
 
I run 91-94 in everything. Even my lawnmower. When we are talking cents, why play around. 10 gallons we are maybe saving 2$ or so.

My cranking compression is 160. timing is 17/35 all in by 3500. I run premium.
 
I think you are borderline, depending on what the cam is bleeding off. Can you get 89. I think that by using a higher than needed octane you spend more money and it is possible you will lose power.
 
We can only get Non-Oxy fuel in 91 octane here. So I run Non-Oxy Premium always
 
I thought I read when you live at higher elevations the octane requirement drops because the air is thinner.
 
Much depends on engine design technology and controls. I have a brand X, that runs great on 87, 13:1 compression, gets 40 mpg at 70 mph. It also has 77.5 Hp/L vs 50.8 Hp/L for the 300 Hp 360cid.
 
pjc360 said:
I thought I read when you live at higher elevations the octane requirement drops because the air is thinner.
Click to expand...

This is true. Less air means less bang at TDC.

My 360/300HP magnum crate was a measured 8.8:1 when I pulled the heads for porting. I milled .010 and went from the OEM .040 gasket to .028. I'm now at 9.3:1, run 27* intial 34 total on 89 Octane at sea level.

Next time you need gas put a 1/4 tank of regular in and drive it. Accelerate in 3rd gear and see if it pings. You can also pull the plugs after driving and check the porcelain with a magnifying glass for tiny silver specks.

My guess is if you are not running a carb with a super lean fuel curve the regular will work.

The horror stories of cylinders pinging in Magnum motors is mostly due to the beer barrel gasket sending oil in the cylinder. Also a dirty fuel injector will make a cylinder lean and the ECM only gets an average from the O2.
 
Ive been running 87E10 in my 360 at 10.7 Scr/8.6Dcr, at 900ft, for over 100,000 miles with Zero problems. Now, it does have aluminum heads, which they say sucks about a half to .8 of a point of compression. So that might be a 10.2/9.9Scr equivalent.
At a true 9.0Scr,and at your altitude,your Dcr is so far down, you should be able to run the cheapest skunk-piss out there.
However, if you really want to know, and you cannot hear detonation, this is what I would do: I would get one of those windshield mounted accelerometers, that spits out horsepower graphs. Follow the directions in the box. Forget the disappointing numbers that it spits out. Look at the graph it generates.Capture it, to a paper copy. Repeat the test with 2 or 3 degrees less power timing. Again capture the graph. Run the tests several times to verify repeatability. Then start adding timing, 2* atta time. Do not exceed 38*. Label all the graphs as they are generated. Compare them. The one with the highest horsepower, and the smoothest line is the sweetheart.This takes care of the power-timing.
Next you can use the same tool to dial in your WOT jetting. If you lean out the mains for best power, then you should revisit the power-timing.
 
Engine is in a truck, a 91 4x4 short bed.
I can't hear any pinging on regular, so I'm not sure. I still have the manual that came with the engine and it says must run at least 91 octane fuel.
So maybe I should just spend the extra money and run 91 octane and be safe.
 
If it isn't detonating, running extra octane is just wasting money. It doesn't do anything for you.

With a compression ratio at 9:1 I'd be surprised if it needed 91 octane. Should be just fine with 89, and at that elevation I wouldn't be surprised if it ran on 87 without detonating.
 
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