Octane requirement question

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pjc360

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I've got a 300 hp crate 360 magnum engine in my 91 power ram 150, it was purchased brand new by my father in 2003 and I acquired it in 2010, it's a low mileage engine, it had 18,500 miles on it when I got it, and I have barely put 5000 miles on it.
My father always ran 91 octane in it and said it had to have 91 ran in it, we live in Idaho and we are 4000 feet above sea level.
This engine is advertised with a compression ratio of 9:0.1 and I checked the cranking compression with the engine fully warmed up and the carburetor opened up all the way and every cylinder read 150 psi on the dot.
I've been running ethanol free 91 octane fuel in it ever since I got it, but I noticed the other day the only gas station that sells eathanol free premium jacked the price of it up to where it is 22 cents a gallon more then the standard 91 octane with 10 percent eathanol.
Fuel prices where I live and their octane rating is 85 octane with 10 percent ethanol at 3.04 a gallon, 87 octane with 10 percent ethanol at 3.14 a gallon, and 91 octane with 10 percent ethanol at 3.24 a gallon, and of course ethanol free 91 octane at 3.47 a gallon.
Oh and there is a station that sells 87 octane with only 5 percent ethanol for 3.14 a gallon as well.
I'm just wondering can I run 87 octane with 10 percent eathanol in this engine and be ok? I've been told when you live higher in elevation the octane requirement is lower because the air is thinner, so I was thinking since I'm at 4000 feet above sea level, and the compression ratio is 9:0.1 that I should be safe running 87 octane, I don't know much about cranking compression and how that helps determine the octane you should run, but all 8 cylinders read 150 psi on cranking compression.
I'd just like to save 33 cents per gallon at the pump if I can, because that's quite a bit. Specially when your filling up a 30 gallon fuel tank. That's darn near saving 10 bucks right there.
I know timing plays a part in this and my initial timing is 16 degrees and my total timing is 34 degrees and I have another 14 degrees of timing added at idle from the vacuum advance.
And my timing is all in at 3000 rpm.
What are your thoughts? Should I be safe running 87 octane fuel?
 
With a 15 gallon tank, you're talking about 3.3 dollars for the .22 cents difference. Are you serious?
 
I have a 30 gallon tank, and I save 10 dollars at fill up when I spend 3.04 for 87 octane and 10 percent ethanol vs 3.47 a gallon for ethanol free 91 octane fuel.
I fill up about twice a month, more during hunting season.
So that's saving at least 250 a year running 87 octane. And that's assuming I only fill up twice a month, like I said during hunting season I bet I fill up once a week for the whole month of October.
 
Sometimes you get just as many miles to the dollar out 91 that you could get out of 87, so be sure you are not shooting yourself in the foot.
 
Buy some and try it out If you go with the 87 E10, you may as well get the 87 E5 for the same price. It's the same exact octane, but the lower ethanol content will give you ever so slightly better mileage.
 
And remember this the octane requirement varies with load and rpm and altitude. If you drive like I often do now that I'm semi-retired, you might get away with 70 octane. If you're pulling a trailer, and need to really hoof it, maybe you do need 91.

I would try the lowest scum gas I could find and,on an empty tank, try 5 or 10 gallons of it to see what I could get away with. If it gets into detonation, I would add an equal amount of 91 to arrive at an octane somewhere close to half way in-between . If it doesn't detonate on the scum, I'd run it. If it only occasionally detonates on it, I would keep track of the circumstances, and if it works out, perhaps a slight timing change to either the amount or the rate will fix it right up.
Typically, on a non-computerized vehicle, an octane change will not significantly change power output, so long as detonation is not encountered.
If you're gonna drag a moose outta the bush, at lower rpms, and large throttle openings,Use low range and listen hard for detonation, or just run the good stuff. But honestly 4 octane points at 4000 ft, at 150 psi,I can't see it being a problem.At all.
 
Problem is I don't know what to listen for, I've never heard pinging or detonation.
And the exhaust on this truck is pretty loud, specially in the higher rpms, so I don't think I'd be able to hear it if it was going on.
I just figured I could ask if theoretically this engine could be ran on 87 octane given the higher altitude and the compression ratio of 9:0.1 and the cranking compression of 150 psi
 
Get it up to about 2,500 RPM's accelerate hard in high gear.
You'll hear it if it's going to do it.

It sounds like a small hammer pounding on your engine block.
 
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