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post #1 of (permalink) Old 01-15-2000, 11:01 PM
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93 octane in stock engine

I have always heard that if you don't have a high compression engine, that there is no point in running anything more than regular 87 octane unleaded fuel. I know some people that run a tank of 93 octane occationally to "clean out the engine", but it seems to me this is not required (other than to get some of the additives like fuel injector cleaner, etc that some companies put in their premium fuel). Anyone have thoughts on this?

Chuck
1995 YJ 4.0 5spd, 3" body lift, 32" Bridgestones (w/damn 3.07 gears), Durabak
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post #2 of (permalink) Old 01-15-2000, 11:15 PM
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Re: 93 octane in stock engine

welp i know i'm only 19, but heres what i think (weather its right or not), u put 86 octane in your car, in my 88 wrangler with the carter carb, it ran like crap, u put 87 its ok, now assuming the 87 is actually 87 octane, and not a watered down 87 to make it 86 which runs like crap, i used 89, once in a while i would put in 92, it ran great with 92, put in 93, it didn't like it, now for my 95 wrangler with fuel injection, i use 87 it runs good, i put in 93, i think it runs alot better on the highway, now i only use shell gas, and this is just what i think, its probably not doing anything differnt at all but for some reason just because its a higher octane i "think" it does, but who knows? anyone got any more info, sorry for the long, speech hope it makes sense

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post #3 of (permalink) Old 01-15-2000, 11:28 PM
 
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Re: 93 octane in stock engine

I've read somewhere(don't know in which magazine) but by adding the octane, the point at which the fuel combusts is higher. So in theory 87 octane burns at a lower temp while 93 would burn at a much higher temp. So in cold weather your supposed to run the lowest octane.

87 wrangler

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post #4 of (permalink) Old 01-15-2000, 11:30 PM
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Re: 93 octane in stock engine

actually i believe that, my temperatue gauge has gone up a little bit since i currently am running 93 instead of 87....good point!

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post #5 of (permalink) Old 01-15-2000, 11:45 PM
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Re: 93 octane in stock engine

Yeah, the 93 octane is "more difficult" to burn. The only reason I see to use it is to avoid detonation, which you would get in higher compression engines by using low octane fuel (since the lower octane fuel, which is more volitile, ignites too soon in the higher temperatures of a high compression engine). In the 2 stroke engine world, you can run up to about 180-185 psi on 93 octane - after that you have to go race fuel to avoid detonation. Not sure if that translates to the 4 stroke world or not, but I would assume it does.

Chuck
1995 YJ 4.0 5spd, 3" body lift, 32" Bridgestones (w/damn 3.07 gears), Durabak
post #6 of (permalink) Old 01-16-2000, 07:56 AM
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Re: 93 octane in stock engine

I run 93 in my 305 TPI all the time and I still get a pinging cruising at 75 MPH. I can accelerate or
decelerate and it stops but if I continue to maintain the same speed it pings.

My understanding was that the higher octane retarded combustion and caused it to burn longer, rather
than in a quick flash, not hotter. Kind of like one of those time released 12 hour cold medicines.


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post #7 of (permalink) Old 01-16-2000, 04:11 PM
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Re: 93 octane in stock engine

On the 305, check the operation of the EGR valve, it controls cruise detonation along with NOX reduction (emissions). If not hooked up or if the passages are plugged, this could be the cause, along with too much vacuum advance on earlier motors. Most low compression engines will run better on regular. Chrysler issued a TSB relating to high octane fuel and volutility issues. Modern gasoline by law has to contain detergents to keep fuel systems clean.

post #8 of (permalink) Old 01-16-2000, 04:17 PM
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Re: 93 octane in stock engine

That's right, higher octane burns slower (or rather, later) than lower octane fuel, which prevents detonation in higher compression engines. If you're not getting detonation (knocking, pinging) on the lower octane fuel, higher octane fuel will do absolutely nothing for you except empty your wallet faster (no small matter considering a Jeep's crappy mileage). In general, the higher the compression ration, the higher octane required. However, some modern computer controlled engines (dunno if any newer Jeeps fit in this category) have a "knock sensor" that will retard the timing if it senses detonation. In this case (only), you won't hear any pinging, but there will be some power loss as the engine adjusts itself to the cheaper gas.

OTOH, some cars ('70's Fords in particular) would be pinging all the time no matter what fuel you fed them. It was annoying, but normal and expected.

-Dana

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post #9 of (permalink) Old 01-16-2000, 10:10 PM
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Re: 93 octane in stock engine

I had a great debate resolved on another board about the statement: "Use only regular fuel because higher octane fuel will not benefit your motor". The wording of this bothered me and I asked for an authoritative explanation as to why ....in the hot weather my Corsica ran like crap on regular and ran better on premium. I'm gonna keep this short (well...short for me[img]/wwwthreads_images/icons/smile.gif[/img]) ...I asked if the statement meant that you'ld see no increase in engine reliability and longevity or did it mean that it wouldn't do a darn thing for you (which I had an argument with). As I suspected the "generic" wording of the statement is intended to advise otherwise uniformed consumers that premium fuel is not a wear saving and service saving component of vehicle ownership. It DOES NOT mean that an engine requiring low octane fuel will see NO DIFFERENCE in performance. If you tow ........hill climb....dune..or operate a POS 88 Corsica 4cyl auto with the air on in hilly Penna rural/suburban traffic on 90 degree days ......you'll reap benefits in "drivability".......albeit at a price.

I agree with the EGR on curing detonation at higher speeds .....the introduction of the inert combustion gas cools the combustion process (to reduce the Oxides of Nitrogen)......if defective, however, it is usually accompanied with some low speed or idling side effects.

70's Fords......before we got married my wife had a 75 or 7? Pinto .....you know the one Hugh Downs advertised with the "...over 550lbs of road hugging weight"?......it knocked no matter what fuel you put in it ......it couldn't get out of it's own way.

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post #10 of (permalink) Old 01-16-2000, 10:52 PM
 
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Re: 93 octane in stock engine

I've also heard it said that higher octane gas will help older, high mileage engines perform better, although I've never seen evidence of this. Anybody out there ever hear of anything like this?[img]/wwwthreads_images/icons/smile.gif[/img]

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