According to numerous FORD technicians...
 

and the owners manual states 87 octane is the only gas u should burn, unless you have a supercharger. The Ford technicians said that our engine(4.6 or 3.8) is "desingned" to burn 87 octane and burning "premium" could make knocking/pinging worse, as well as harm the engine down the road! This is just really strange to me, wouldnt you want to burn a higher octance gas to keep to keep the injectors, valves, etc.. cleaner?? Whats your guys take on this, just curious??

The technicians are largely correct. You shouldn't burn fuel with higher octane than 87. The stock engine is designed to burn 87 octane.

Now if you advance you timing, add a chip or supercharger, whatever, you may have to run higher octane fuel.

buying a higher octane fuel than your engine needs
 

is not only a waste of money, but will lose you power.
i used to laugh at the guys running N/A, stock compression motors buying the race gas at the track and wondering why their ET got worse...

LOL!

my cobra says right on the cluster "use only premium unleaded fuel", of course with the blower, it is all that much more important to use a more stable (higher octane) fuel.

Today's higher octane fuels have the same additives as the 87 octane. The only thing you're doing is wasting money if you have a stock engine.

I use sunoco 94 is that a waste? Ihave my timing set to 14 deg.

Will

pinging
 

i've never heard of running 92 octane in an 87 and causing pinging. that's going against the grain. but i'm no expert.

for anyone who doesn't understand:
the octane rating of gasoline is a scale on which the burning characterstics of gasoline are COMPARED

pinging
 

i've never heard of running 92 octane in an 87 and causing pinging. that's going against the grain. but i'm no expert.

for anyone who doesn't understand:
the octane rating of gasoline is a scale on which the burning characterstics of gasoline are COMPARED to gasoline. not how much octane it contains. it contains zero.

i meant to say ' COMPARED to octane' not gasoline.

Higher octane gas will take higher temperature/pressure before it combusts than lower octane. That's why you need it for forced induction, advanced timing, or high compression; it prevents pre-ignition, which is when the explosion happens before the spark plug fires because of high pressures and temperatures. I can't really see how it would hurt anything to use higher octane gas, and it doesn't make much sense that it would make knocking/pinging worse. It's just a waste of money if you don't need it.

If you use higher octane gas, it will not burn as quickly as lower octane fuel. Therefore, you can be left with partially unburned fuel and carbon buildup. Another note, super and premium fuel has a greater chance of containing ethanol, which will decrease your fuel economy.

Interesting and helpful feedback guys. Thanks for the responses! I've been burning 87 octane ever since I got my stang a few months ago, but my 2 prior cars said to use "premium" gas only.... thats why I asked. ;)

I guess FORD must of made a mistake when they stamped "premium unleaded fuel only" on the fuel gauge on my 98 COBRA...........

Well?
 

So should I continue to use 94 octane or am I waisting my money? Thanks.


Will:D

I use 91 cause of my chip. Is that ok??

If your chip instructions said to use 91+ octane you better. I have to run 92 octane + or I get detonation with my chip.

Listen to that rating. I get waaaaaayyy better gas mileage when I use 87 (Not that I care about gas mileage). Premium is a waste of money. It confuses you EEC if you switch a lot too.

The GT uses 87 octane. Your Cobra uses the good stuff.

cobrazex ...

I do believe this string started talking about a car whose owner's manual stated it should use 87 octane, and most answers were geared to that question.

Obiviously, if you have a different engine, and the owner's manual states another octane, I would think Ford stated that for a purpose.

if i run anything in mine less than 91, the valves chatter like crazy. same thing with my old contour svt. since reading these threads i started using 87. no dif (well maybe 1 mpg (WAHOO!!) better). i am just running a pretty much bone stock 98 gt. any insight, people?

Right off the bat I'll say it's not the valves, which aren't really directly affected by octane ratios.

The preignition generally happens when carbon built up on the piston (or cylinder head) creates a "hot spot." The hot spot is caused by the carbon holding a higher temperature than the metal, and it gets so hot that it inititates combustion before the spark. This is how a diesel engine works without spark plus by the way. Unfortunately, it's a lot more damaging in a gas powered vehicle because there is still a spark. The spark happens a fraction of a moment after the hot spot starts combustion from it's point. The two burning fronts eventually collide with one another which causes a powerful blast noted as an audible "ping" or "knock" from outside of the engine.

If you have been running higher octane fuel in your car for an extended length of time and you switch back to 87 octane, not only is there likely to be some carbon buildup due to incomplete burning (see one of my posts above), but the computer is also used to the 91 and it's adjusted for it.

Advancing timing causes an issue with lower octane fuel because of the burning properties. Lower octane fuel burns faster and is more explosive than high octane fuel. If you bump your timing too much, you wind up having the piston colliding with the flame front created by the spark. The piston is on the compression stroke, the spark goes off say 15* before the piston reaches the top "TDC" and the flame wave travels so fast, it runs into the piston. If your fuel burns slightly slower, like high octane fuel, your piston can reach the top before the flame front is at maximum force, thereby avoiding a powerful collision inside the engine.

87oct fuel doesn't normall have ethanol, which is generally formulated at 10% by volume in 89+oct fuels. Ethanol has a higher octane rating than gasoline, I think about 104, so adding it is cost effective to increase octane ratings. Ethanol may also burn 50% cleaner than gasoline, but it requires 2x as much ethanol to get the same power as gasoline, so your car burns 2x as much of it. 10% (minimum) formulated ethanol gas will therefore reduce your fuel economy by .10 x .50 = .05 or 5%. That's about 1-2mpg.

Hey UNIT....as a side job maybe u should become a Ford technician. You seem to really know your Sh*t!:D

Your Cobra has a much higher compression ratio which creates more heat and will make lower octane fuel flash earlier or pre-ignite. I'm sure if it was 9:1 instead of 9.85:1 it would be a different story. Hell, I would have my car tuned on C-16 if I thought I wouldn't go through a 55 gal. drum every 2 weeks @ over $300 a barrel. I have seen cars tuned on pump gas make 490 rwhp and then switched to C-16 on the dynoand a new tune and make over 150 rwhp more. I guess 22-23 points makes a huge difference.

Unit ... question for you.

My 2000GT has seen nothing but regular gas. It was a solid car to drive always consistent.

My 2002GT seems to go on Dog days when I put lower octane. So I have started putting 90+ octane gas in the car and even 94 + octane booster when I go to the track. It seems to behave consistently well when I put the higher octane gas. Could my engine ask for this type of gas??? Is that possible? or is it more likely that it is still adjusting being relatively new ~5000miles.

Well, how come when I burn 87 octane my car knocks. When you give it half or more throttle it knocks so loud that is all you can hear?????

Thanks

Lizard King I have a 2002 GT as well, build date 3/2002 :D and it's got about 1700mi on it now. I have run 89oct in it before, and I run 87 in it right now. I haven't noticed any pinging/knocking or performance degredation in the seat of the pants. Do you always get gas at the same place? I never get gas at Holiday here because of their "Blue Earth" gas campaign. It's possible the place you get your gas from is subpar? Also, I know you have a couple mods now. Anything that plays with the timing could be effecting your car, or possibly a defective anti knock sensor is pulling timing?

Did you have your car at the track when you were running 87? I'd be curious if it traps any higher on higher octane fuel.

Also, different cars do run differently. Take the old 5.0's as an example. In proper tune, people are able to bump the timing from 13-17* on premium gas. That's a 4* difference.

The difference on the 99+ cars is going to be obscene. Tolerances in the head are TERRIBLE on the new GTs. I've never seen anything so ridiculous in my life. I don't know who the moron working on the PI head design team was, but the tolerance is +/- 1.5cc in combustion chamber size. Same as the older heads, but the problem is the overall combustion chamber size on the PI heads is a meager 44.75cc. The pistons have a 17cc dish to them, but still. Your head's could have combustion chambers anywhere from 43.25cc to 46.25cc under Ford tolerences. Now, for the sake of argument we'll say your car is running really strong and you have at least one cylinder with a 43.25cc combustion chamber. Your compression ratio on that cylinder is going to be about 9.25:1, whereas if you have at least one combustion chamber with a max tolerance of 46.25cc, the compresion ratio will be 8.75:1. That's 1/2 point of difference between minimum and maximum compression from cylinder to cylinder. It's certainly concievable to me that a few real good factory "freaks" are going to have a lot of the 9.25:1 CR's and they might favor higher octane fuel a little more. There are also the "slow" 4.6's that will be making power based on a lot of the 8.75:1 CR combustion chambers. Of course the computer is designed to make sure the engine isn't knocking, and that means a little internal timing retard and more fuel on an engine that might be doing a little pinging. Of course, this is all grasping at straws because nobody knows what your head would truely cc out at. If your car truely runs better, and traps higher in the 1/4 under higher octane fuel, then you have a sensor malfunction, or your combo really does need the higher octane fuel to keep from pinging.

bstang46, How long have you been running high octane fuel in the car? If there is carbon build up, you can certainly run into the problem you are describing, although the anti-knock sensor doesn't seem to be working super good? If you've been running high octane fuel in the car for an extended period, the computer may base it's injection and timing curves on the higher octane fuel? I'm not sure how internally adjustable the newer computers are. I know on my 87GT, driving it hard lets the computer adjust itself to a more aggressive state of tune.

It knocked only when I used 87 ocatane. I tried the 93 to see if it would quit. So I am not sure what the problem is. There could me many things that have caused it. I used 87 first like the manual said.

Thanks

Thanks Unit for the help. I think this car is running some higher compression... I really seems to like the high octane stuff.

It'd certainly be nice to think you have a freak on your hands. They really are a lot of fun! :D

Seriously though, you'd have to run back to back at the track. With 87, and then a higher octane to see if it's true that your car does better on high octane. If you don't trap any higher, you're not making any more power. It's still not a guarantee that you have a freak composition of small cc combustion chambers, the only way you can find that out for sure is to take the heads off and have them cc'd. Who knows?