Gas options

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mattjw

mattjw

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I have a ‘73 Scamp with a 318. Owners manual says to use 93 octane. I can get 93 with ethanol and use an additive, or 90 without ethanol and use an octane booster. I’d appreciate some thoughts on which is the better option. Thanks
 
I'm not afraid of ethanol fuels. I'd use which ever fuel is cheaper of the above two you posted. Both will be fine so save $$$$$ where you can.
 
Strange that the owners manual says 93. A 318 is a low compression engine and should be fine with 90...
 
and in 73', 93 octane was low end gas. my 73' will ping pretty good in summer on anything less. of course the motor is tired and the distributor has never been re-curved, so take that into account
 
I run pump 87 in my 69 318 all the time, doesn't seem to care. It's my daily, so the cheapest gas I can get is the best gas. When I've left the car at home while I'm at school, I go find ethanol free to put in it, just to help the shelf life of the fuel and the rubber from getting eaten, but I'm not completely sure it does anything, just a little placebo peace of mind at worst.
 
68Valiant225 said:
I run pump 87 in my 69 318 all the time, doesn't seem to care. It's my daily, so the cheapest gas I can get is the best gas. When I've left the car at home while I'm at school, I go find ethanol free to put in it, just to help the shelf life of the fuel and the rubber from getting eaten, but I'm not completely sure it does anything, just a little placebo peace of mind at worst.
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You can also use some Stabil in the tank and drive if for a bit so it goes through the system before you shut it down.

Clear fuel is hard to find where I'm at. Can't find E85 either, although I'm told it's everywhere around Tri cites but I ain't driving an hour one way for fuel!!!!
 
You would think a 318 would run on low octane.
But if it does ping with low octane ethanol free, try low octane with ethanol. I read somewhere that ethanol raises the octane levels higher than the posted minimum ratings.
I run my 340 with 93 unleaded with ethanol and I have 10.25 pistons. I do try to add a lead substitute but it never pings even if I don't. But I always add stabilizer to it if it is in there more than two weeks.
 
I'm with the can't even imagine a 318 needing anything more than 87 group unless it's a built performance motor, has timing issues, or is loaded up with carbon.
But since that is all the info we got on it I would have to go with one of the latter two.
 
That would be 93 of the old octane rating system back when super premium was 100
put in 5 gal of 87 and see if it pings
then check your timing
you might be able to curve your dist to take advantage of 93 but I doubt if you would save any money over 87, more power a little
btw how's your timingchain- somethingthat matters
 
Pressure and timing, heat or hot-spots in the chamber, plus loading, are the things that affect what octane fuel to run.And altitude.
Tennessee has a wide range of elevations from less than 200ft to over 6000 ft. So there is no way to get yur tuning right for the full range.
If you want to tune for 87 gas, you are gonna have to tune for a much smaller elevation window.And if yur car has 2.2s in the back well it ain't likely to ever be happy with 87gas.
 
A factory spec ‘73 318 shouldn’t even need 87 octane. At any elevation above sea level that still has a road.

Compression ratio from the factory in ‘73 was advertised at 8.6:1, and in reality they were probably closer to 8:1, the pistons were almost always further down in the hole than spec. I ran the 318 in my Challenger exclusively on 87 octane from sea level to over 8k feet, detonation was never an issue. The rebuilt 340 in my Duster has a calculated compression ratio of 9.8:1, and it does ok on 91 octane. I could probably throw a little more timing at it with more octane, but that’s 9.8:1. None of that is ethanol free, just standard pump fuel in California which always has some percentage of ethanol. It even varies from the summer to winter blend too, never even noticed on the 318.

If a factory compression 318 won’t run on 87, there’s something seriously wrong with the tune. Unless of course the compression ratio isn’t factory. Even then though, you shouldn’t need much more than 87 until you’ve gone higher than the mid-9’s for compression.
 
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All I'm suggesting is that an elevation change of 4000 feet represents a cylinder pressure change of about 20 psi. And a jet change of two sizes at WOT.
All that change will drive the Part-Throttle tune crazy, and when it goes lean, detonation is sure to appear with 87 gas.......... especially if the engine left the factory already tuned lean;
and, IMHO, the recommended 91 gas is pointing exactly to that.

But I totally agree that any 318 should run on 87, less even, when tuned for it, and operating within a narrow elevation range. The Effective compression ratio, could be down to 4/1 at times, and that will support skunk-pee quite nicely..
 
AJ/FormS said:
All I'm suggesting is that an elevation change of 4000 feet represents a cylinder pressure change of about 20 psi. And a jet change of two sizes at WOT.
All that change will drive the Part-Throttle tune crazy, and when it goes lean, detonation is sure to appear with 87 gas.......... especially if the engine left the factory already tuned lean;
and, IMHO, the recommended 91 gas is pointing exactly to that.

But I totally agree that any 318 should run on 87, less even, when tuned for it, and operating within a narrow elevation range. The Effective compression ratio, could be down to 4/1 at times, and that will support skunk-pee quite nicely..
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Totally depends on what elevation your tune is for. I drive my sea level tuned cars over the 8,000 foot elevation passes here without issues. They run a little rich, no big deal. The difference from 0 to 4,000 feet is pretty negligible, maybe a tenth or two on my AF/R gauge. Idle speed slows down a smidge.

Now, if you tuned for 8k feet and then drove to sea level you might have an issue going into lean detonation. But not the other way around. And a low compression 318 tuned for 4k feet works just fine at sea level. You lose maybe a tenth or two on the AF/R, which shouldn't hurt anything unless you're already way leaned out.
 
The 93 octane recommend in 1973 is probably the Research Octane scale.
Today, the octane rating displayed on the pump is the average of Research Octane and Motor Octane. Generally, Motor Octane is around ten points lower than Research Octane, so lower that 93 by 5 to come up with 88.
Try the 87, make sure your timing is correct, and maybe use a combustion chamber cleaner like Gumout Regane of Chevron Techron.
 
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