A weird question here: Which octane should I use for '78 AMC 360?? We actually discussed this the other night, but no one knew for sure. Can you help us?

The regular stuff (85-87 octane depending upon your area) will be just fine. Unless you have thrown in some high compression pistons you shouldn't get any detonation (pre-ignition). Octane is a measure of resistance to detonation, nothing more. Many misinformed people think there is "more power" in a higher octane fuel. The oil companies don't try to educate people about this misconception because they can sell the higher octane stuff at a higher price. The big secret is that the higher octane stuff doesn't really cost signifcantly more to produce, it's around 10 cents per thousand gallons.

My other car is a bulldozer!

It's is nice to see an educated jeeper....As utahjeepr said, octain is resistance to burn/detonate. Too high of an octain rating will actually cost you power and will cause carbon deposits in your combustion chambers. These carbon deposits will lead to pinging even with higher octain gas and may cause the engine to deisel when you shut it off.

5.0s and 4 speed = lots of broken parts!!!

It's is nice to see an educated jeeper....As utahjeepr said, octane is resistance to burn/detonate. Too high of an octane rating will actually cost you power and will cause carbon deposits in your combustion chambers. These carbon deposits will lead to pinging even with higher octane gas and may cause the engine to deisel when you shut it off. Use regular and set your timing so the engine doesn't ping under load, if the engine feels weak with this timing go to 89 octane and turn up the timing a little till the power comes back but the pinging doesn't.

5.0s and 4 speed = lots of broken parts!!!

AHMEN , use the lowest octane that your motor will not ping on . Lower octane burns faster and results in more HP .

OK, so which is worse: using high octane in vehicle requiring low octane or the opposite?


'91 YJ w/stuff, http://members.home.net/t.molnar

OK I have a question then...if I have upgraded to the TR cap and rotor and have a Jacobs Multi Spark Box with a Jacob's Ultra Coil and plugs gapped to .50 with this much more sparking power would it be and advantage to use an 89 octane fuel or stick with 87?

I tend to disagree with some of you here. Higher octane fuel CAN give you more performance because of it's high anti-knock qualities....but will do your motor no good. That was why there was a BIG campaine to discredit the use of premium fuel. The industry was taking advantage of an uniformed consumer thinking that premium fuel was of higher quality, which it was at one time when only a few exotic euro-cars had fuel injection. BUT-you try towing something up grade with regular and then do the same with premium in the same vehicle you will experience better "drivability". It's the same with most vehicles. In the summer with the air on my mothers Corsica couldn't get out of it's own way...having to frequently use passing gear to accellerate. On premium, however, it was noticably more "drivable"(I don't like the word powerful - it was a 2.0 4banger). You merely extend the envelope of acceptable performance to the extreme.

So CAN premium yield more "drivability" from your "regular" engine? Under some circumstances, YES (heavy loads, hot weather, towing, etc.). Is it doing much more than thinning your wallet faster...in most cases NO.

So, to answer your question, IMHO I think that you should run the cheapest stuff you can find that gives you acceptable drivability. If you are unhappy with the drivability of 87 (that is, it can't get out of it's own way) ...go to 89.


GeeAea

Now that I've quit smoking my doctor informs me that I'll die in much better health!!!

Higher octane only gives you more performance on engines that were designed for it. If your engine has a knock sensor that retards the timing when it detects knock then a high octane fuel will give you better performance because the ignition can be run advanced and give you more torque. An engine without a knock sensor will actually see slightly reduced fuel economy, emissions, and power because the ignition is set up for regular fuel and the extra goodness is going right out the tailpipe because the higher octane hydrocarbons are only partially burned. If you use high octane and recurve your ignition for it you will get better performance. High octane unleaded was not readily available in 1978 so your engine would be fine on regular.
Regards,
Josiah

Hillsboro, OR
1955 Willys CJ5 Buick 225 V6 160HP 270ft-lbs, T90 trans, Warn OD, PTO winch, Spicer 18 T Case, RS9000's, Dana 25F/ 44R,
5.38:1 gears, 11" brakes, Bestop Supertop, Hurculiner

Sorry..but I still beg to differ with you. Higher performance is possible from a "regular" engine on premium. I agree with the part about the knock sensor....but that's what I said. In a vehicle that was under extreme conditions (hot ambient temperature, heavily laden, towing, etc. ALL THESE THINGS CONTRIBUTE TO KNOCKING) it will extend the envelope of acceptable drivability. There is no extended envelope for an engine designed to run on premium....only a reduced one by using regular...like my old girlfriends 560 SLC with the warning "if you can't find premium fuel don't exceed half throttle and accelerate slowly".
As far as I know there were no computer controlled timed engines in existance in 1978 (that is those that actually set the timing making it "unmodifiable") ......that is it was still set by dialing in the distributor ..or in the extreme case of certain year Oldsmobiles ..dialing in the cam to set the "static" timing. An exception to this may have been the Chrysler "lean burn" setup where they used extensive timing variations during startup and such.

Auto industry rhetoric says:
Mild knocking is considered acceptable because it means that you're extracting the maximum power out of the fuel that you're using. This is true but this in NO WAY MEANS that you're getting the most performance that the engine is capable of.
Catch my point here:
You're driving up a hill with the air on ..it's 95 degrees out, and you've got the kids and the in-laws and their luggage in the car. ...you're running regular...YOU WILL START TO KNOCK perhaps beyond mildly. Beyond "mild knocking"(which is "lugging" an old econo-driver trick) you are, any way you look at it, experiencing disrupted combustion of a ruptured flame front and are producing less power than a motor that is not excessively knocking (or not knocking at all). That is, the industry rhetoric is designed with the average consumer with a mass use basis in mind. You wouldn't want the entire jerked-$ff public wasting btu's that they didn't need to .. it's more policy than truth.

Now it won't give you any longer engine life, in most cases and it won't give you any better gas mileage.

Anyone who says that there is NO DIFFERENCE in performance has never tried to make their vehicle perform...or have never tried it to see the difference. Perhaps it's different with 3rd world designed rice burners ..if that's your daily driver....but the only vehicle that I've ever owned that didn't show some improvement in (qualified) performance was my 1977 Chevette.

Heck in my TJ ...it's like night and day...especially in the summer months.

GeeAea


Now that I've quit smoking my doctor informs me that I'll die in much better health!!!