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

Right on blue thunder!!
WETTE VETTE, most knock sensors won't pick up lifter noise. It takes a solid knock to trigger the sensor. I have many solid lifter motors running with knock sensors and have had no problems.

2 kinds of knock detection. One (the most common) uses a knock sensor "sending unit" that is tuned to respond only to noises in a certain frequency rance (3,500 Hz it seems like I remember). Most other motor clicks and clacks are at a different frequency than detonation.

Other kind of sensor is built into the ignition computer and measures coil voltage feedback and can somehow detect a change in the ionization gap at the spark plugs. When detonation occurs, apparently the "gap" voltage abruptly changes. Anyhow, the computer can detect this and makes changes accordingly. I like the separate sensor idea.

As far as a decarbon job, I ask this: Have you ever blown a headgasket on ANY water-cooled motor? If so, then you have certainly noticed that the cylinder that had been "seeing" coolant looks like it has been scrubbed clean as a baby's butt. This is cause the combustion chamber was running hot, and all the carbon was hot and glowing - then the water hit it. The water puts the carbon chunks into thermal shock and they act just like throwing an oven-hot glass bowl into cold water - POW! - cracks it into pieces, off it goes out the exhaust. Leaves behind a shiny clean carbonless combustion chamber.

Here's how I decarbon: Get the motor completely up to operating temperature. Pull the air cleaner/flame arrestor/intake hose, etc off the top of the carb/throttle body. Start her up (if it's an injected motor with an airflow sensor (hot wire meter, flap, etc) then it will be cantankerous without the intake hose hooked up but it WILL run (it is actually helpful to blow a hair dryer on COOL into the airflow sensor on some vehicles, but I have always been able to keep them running by moving the throttle linkage slowly and gradually). Anyhow, rev her and hold her at around 2000 rpm.

Using a "mist" setting on your garden hose nozzle (if you ain't got a mist setting, youll need to buy one that does), start spraying mist into the carb/throttle body. Continue this, getting pretty aggressive with your misting, obviously avoiding shooting enough water to get dangerous... Five minutes of this on your car will leave a 2-foot diameter ring of black nast under your tailpipe and all your pistons and combustion chambers will be clean clean clean.

This usually works wonders for high mile motors that have recently begun to ping incessantly.

There are also commercial decarb liquids that you pour in and they can work well also. I prefer to use water for the decarb, but the commercial chemicals will also clean the backsides of the intake valves and other parts that get gunked up.

mcollinstn, you are right on. The voltage required to fire a plug is contingent on the medium and the pressure. Knock is caused by very high pressure spikes in the cylinder which causes the voltage spike. In 1962, Oldsmobile had water injection on it's 215 inch turbocharger motor. If you took a head off of one or these that had a properly operating water injection system, the pistons and chanbers looked brand new clean. Water injection was used by industrious horsepower freaks long ago and this is how I decarbon an engine now. You have some great info!!!

I'm just a sponge. I kind of absorb info, among other things.

(you should see how I absorb cheeseburgers!)

Two questions fellas... aren't you worried about embrittlement of your exhaust valves when being quenched by 212* H2O? I've heard of that trick before, just never had the nads to try it.

Second, I though detonation occured after the ignition spark ceased. If that is true? how does one measure anomallies in voltage requirements of the ignition? Seems pre-ignition would disrupt the cylinder pressure more than detonation and cthereby cause ignition anomallies.

Ditto on the sponge thing mcollinstn.

BT :cool:

All of this brings me back to my original question. If my automotive knock sensor won't stop the pinging in my car (and I've witnessed the same thing in fairly new GM vehicles also), can I really trust the Merc sensor?

Ric232, just do the hammer test. It works. If you need specifics, pm or call me and I'll be glad to help. If the test works, I would trust the sensors. If it fails you need to fix the problem. I can run you through the steps. I'll post them if enough want it or ask maybe mcollinstn.

You've got to be kidding??? If this is safe, why in the heck does everyone get freaked out about a little water reversion in marine engines?

Ric232, the "reversion" you hear about is water coming back through the exhaust. Sometimes it comes in when you turn the engine off and sits there in a cylinder and causes rust. Can also cause misfire, etc. I have NEVER had this problem. Almost all modern day water in cylinder problems are condensation. Watch a Winston Cup team. They do something to stop air from entering the engine every time they stop it. If it will be left even overnight, they will fog it down.
The small amount of water it takes to decarbonize is small and will go right out the exhaust as steam. Notice, you only FOG or mist into the intake!!!

blue - under no load, the combustion temps (and more importantly, the EGTs) are moderate and the exhaust valve will be far below a temp point that will affect it in a metallurgical fashion. In fact, at such low speeds, the exhaust valve has adequate "on the seat" time that it has trouble getting much hotter than the valveseat itself. After all, the metallic valve wants to transfer its heat to the seat, and up its stem, and anywhere else that it has a pathway in which to conduct heat. The carbon on the other hand is a BUILDUP, not a solid formed with any sort of structural molecular bonds. While carbon compounds can indeed be used to form things such as alternator and starter brushes which DO conduct heat and electricity, the carbon present on the face of a valve or piston crown is a "residue" or leftover byproduct of the combustion process. It is impure and contaminated with lubricants and dirt and other things that do not allow it to form in the same manner as the alternator brush or even pencil lead. This carbon is just kind of "sitting around" unable to conduct heat in any significant manner. It is this primary reason that is becomes an issue to us in the first place. Since it cannot conduct combustion heat to other components, it just sits there and gets hotter and hotter until it becomes a thousand little glowing spots in your combustion chamber that will "light off" your fuel mix before the plug ever fires (remember those COX airplanes we played with). It is this same glowing layer of carbon chunks that keeps Aunt Marge's Buick running when she turns the ignition off... Anyhow, these glowing, nonconductive chunks - when hit with a water mist, will crack and break free. The exhaust valve will simply cool its face a bit quicker - no big deal.

The preignition thing? Sure, preignition occurs before spark. Apparently the system treats THOSE discoveries differently than it treats those that occur AFTER the spark commences. Let's think about it... If the wavefront or spike from the detonation occurs DURING the spark (I'm not sure what dwell time in crank degrees would be for a spark at 5000 rpm), then it would be easy for it to "see" this. If the spike occurs AFTER the spark STOPS, then the system would need to somehow still be able to monitor the gap voltage somehow. I'm just throwing stuff out there - I don't know the answer. I only know that this is the newest method that is catching on for controlling detonation. To monitor "after spark" conditions, it would seem like the motor would need to have separate ignition coils to accomplish this, although they might can make it work in a one-coil system by utilizing "relative" boundaries for each cylinder - for example: a motor only fires every 90 degrees of crank rotation, so a "relative" boundary could be programmed whereas any voltage anomaly that occurs would have to be mapped to a "zone" with a span of 90 degrees (like maybe a time factor equivalent to 20 crankshaft degrees before spark initiation to 70 degrees after spark initiation). That way, any anomaly in coil voltage could be attributed to the correct "zone" and determined to be either a prespark anomaly or a postspark anomaly and handled accordingly.. Who knows...

Ric - I still don't think your truck problem is due to the knock sensor. Like 1500 says, smack on her and see. A timing light will show the change for sure. I like to take a metal rod (I have one about 3 feet long and 3/4 diameter) and hammer on it a bunch of times (prolly 3 times a second). If you are watching the timing mark (somebody else watching of course) it'll swing WAY out of whack if it is working. Another thing, Ford timing can still be set by moving the distributor. If somebody at some point tried to set the timing, they could have done it incorrectly. To do it properly, or to check to see where it is set - do the following: find the single-wire plug in the harness to the distributor. It will be on that section of the harness and will simply have a plug that you can plug and unplug. Unplug it. The computer advance will now be disabled and you can check your baseline timing with a timing light in the normal fashion. Check it. If it is advanced then set it back. When done, plug the plug back in.

Mist, safe? Here's the deal: Mist is compressible. Water is not.

If you're sitting there idling and getting some water reversion cause your riser is leaking and you have a wild cam, the water will slowly puddle in the bottom of the exhaust where it turns the curve back to the exhaust port. You'll soon have anough water in there that the exhaust can't get by it in either direction (the wild cam will have the exhaust port experiencing a flow that reverses as the piston begins its intake stroke and starts downward). The next exhaust pulse splashes the water out of the way as the gases leave the cylinder - then the intake opens, the piston starts down and the exhaust valve is still open. This time, aided by the piston suction the water now doesn't stop at the bottom of the curve - it goes all the way into the cylinder. Valves close. Piston goes up. Air compresses all the way till it hits water. The piston stops, crank doesn't. Rod bends. Motor quits. All of the above is bad enough in a stationary situation. Add an idling boat in a narrow inlet with lots of boat wakes rocking it from side to side and on one of those rolls, a cylinder will suck a double-dose - maybe several cylinders. It's a problem. Add the corrosion and rust element and you can see that there are other issues that are contingent on ensuring dry exhaust pipes. This situation is far different than what we are proposing here.

Don't take my word for it. Go pay somebody a couple hundred $$ to decarb your motor. No sweat off mine. Chances are, regardless of what they tell you they've done, is that they will spray their hose in your motor...

Good stuff mcollinstn. Thanks,

BT :cool:

Gents,

Thanks a lot for all your replies. As far as winterizing, I am one of the lucky few that lives far enough south that I can use the boat all year long so that is really not a problem. It will feel good saving 20 cents a gallon at the pump.

I saw a guy using a squirt bottle to put water through the carb. I was wondering what the heck he was doing. One guy had it idled up a little, while the other fella squirted away!