87 Octane?
 

I have a 1989 Formula with a 7.4 Merc. I bought it from a guy in Florida who told me I had to use the high Octane gasoline (93). It says in the manual you can use 87 or higher. Most of you are smarter and wiser than I about this type of stuff. I just wanted to hear whether any of you guys thought it mattered or if I can put the cheaper stuff in.

If it is a stock compression motor run 87,,, 93 will actually hurt performance,,, 93 is for high compression motors as it has a slower burn rate. You would notice it even more in a computer MPI motor.

What about when your ECU is reprogrammed (i.e. by Arizona Speed & Marine) and they recommend 91 octane? They've advanced the timing some, but obviously compression does not change. Is it better to run the timing stock and use 87 or 89 octane instead? Just something I've been wondering since I've been putting 93 in my boat since ASM did the ECU.

zog.

You bought your boat from an idiot. The expensive marketing campaigns the gas companies put out really does work, as is proven by this herman.

When you KNOW you are gonna leave the boat in storage for a while, it won't hurt to run midgrade cause it has a better additive pack in it, and will tolerate storage for a longer time without breaking down from evaporation and degradation BUT you'd be better off to pour in Sta-Bil in 87 gas and run it thru the motor for tha last 10 minutes before you store anyhow.

Ric,

If ASM didn't change the engine mechanically, then mix and timing are the only things they could have changed. If they now recommend 91, then it will be for "best power" taking into account the changes they made. If you run 87/89, then the knock sensor will make sure you don't hurt anything but you might lose a few degrees of timing and a smidge of power. Bottom line is, sounds like you can run whatever you want and you'll get whatever timing curve your gas can tolerate. If you run 87, you'll still be no WORSE off than you were before the ECU change.

(I'm not sure how "gradual" the ECU is on backing out timing from the knock... If it is gradual, you're in high cotton - if it is overreactive, you may notice the motor go flat on cheap gas - you'll have to test it yourself)

The problem is, I don't trust the knock sensor. They sure as hell don't work on American cars. My Merc Mountaineer detonates all the time going down the highway w/ 89 octane, especially in colder weather.

RIC

Check for a vacuum leak or for proper EGR operation on your 'neer. It shouldn't be doing that. I don't think it's a knock sensor issue. If it's got miles on it, do a decarbon job on it.

The Merc knock sensors usually work "too well", pulling out timing at the hint of knock or electrical noise nearby.

M

I wish I had you with me every time I took my Mountaineer back to the dealer (since 10,000 miles) under warranty complaining about detonation. Their answer was always, "we cannot duplicate the problem" and "if you use anything but 87 octane, it will cause a problem" and "if you are using 87 octane, try another gas station". They always had an answer. I just got fed up with their crap and decided to live with it. My 94 Mustang GT did the same damned thing for the 67,000 miles I had it. They finally told me I had to put 93 octane in that one (which isn't true).

I'm guessing the injectors get dirty over time. I only have 58,000 on the Mountaineer 5.0L.

And what's a decarbon job? Sounds like tearing things down and cleaning up the combustion chambers. I'm not up for that.

Ric232, I have heard similar complaints to yours many times. Most of my work is GM or performance only, but I would try a different dealer. Also, a lot of performance shops have the equiptment to check your system and might be a better source for the answer than the dealer. There are simple checks for the knock sensor that most hot rodders know and only require a hammer and maybe a long punch, maybe a timing light. GM used to teach this method and it has never let me down, no matter whose system it is.

In other words, tap on the block right next to the knock sensor and see if the engine rpm changes. If it does, you knock sensor is fine.

BT :cool:

Would a knock detector think lifter clatter in a solid lifter motor is detonation? Just curious.:) Doesn't some tranny fluid run through the motor just like the winter fogger oil help to get rid of carbon build up? Don't do this unless someone verifys this is true. I heard this at the race track or something so you never know.:p