Has anyone ever used colder plugs with 87 octane gas
 

I know that Mercedes is adamant that years '86+ need premium gas. But, of all the possible mods you could do to an engine, including various injections, water inductions, boring, port matching, ceramic glazing, etc..., and thereby possibly needing a different octane rating and spark plug heat range than originally specified by Mercedes engineers, do you suppose that this simple mod would be feasible:

Use 87 octane gas, with colder spark plugs? If that's not enough to eliminate pre-ignition, use a 70* thermostat?

Since lower octane is more prone to pre-ignition, my thought would be that using colder plugs would reduce than risk?

Also, my assumption is that my M119.982 is a 10:1 ratio (is this right?), which is not that high anyway.

10:1 is not that low for a gasoline engine. Using a lower range plug will slightly lower the temperature of the explosion, but it won't change the timing of the spark. If you want to use low octane gas, you will have to retard the spark to where you no longer have pre-ignition, but you will be sacrificing power. I would not recommend doing this.

10:1 is about the high end of the typical. But motorcycle engines sometimes go 12:1 to 14:1 with 87 octane, mostly because their heads run cooler.

The spark timing is definitely an issue, but I was pondering about the compression-induced pre-ignition, which is triggered by sufficiently hot hot-spots of the cylinder where the fuel/air mixture is in contact.

Reducing the spark plug heat range does not change the heat of the spark. It only changes how fast heat is dissipated into the engine head, which, in turn is cooled by the coolant fluid. If the pre-ignition triggering hot spot can be removed by (1) a colder heat range spark plug and (2) lower head temperature via a lower temp thermostat, then I suspect that the engine could successfully run 87 octane without ill effects.

Another thought would be to use water injection to cool the intake air, thereby possibly reducing pre-ignition.

Any informed opinions (either by direct experience or auto engineering background) would be appreciated....

I use 87 in a 10.7:1 CR volvo that calls for 91. No problems with stock plugs. Just make sure your engine management has a knock sensor.

we've always used mid grade.

Open hemispherical chambers tend to have good detonation resistance, as a rule of thumb. While not a true hemi, the 119 is pretty close with what I would call hemi-with-quench chamber. If you are experiencing knock then I would say it's due to the low octane or a lean condition. There are a few other circumstances that may cause knock but if you know enough to ask a question like this I'd think you'd have already addressed those issues first.

It's certainly not going to hurt anything to go a couple of heat ranges colder on the plugs, considering your geographic location. That may very well help with a knock condition but I certainly wouldn't bet the farm on it. Enriching the mixture may also help quite a bit but its no easy feat to fool an ME car and not have it adapt out your changes. A 70 degree thermostat may help on both ends by keeping the engine cool it may reduce knock tendencies, and keep the engine richer by staying in the warm-up map. That has the potential of being a double edged sword though. Check engine lights are almost a certainty. I'd also be a bit wary of constantly having a rich mixture. The potential to wipe out the rings and bore in an aluminum engine from fuel wash would keep me looking for another solution if it were my engine.

Lots of food for thought there, but I have one other suggestion. Check your cam timing. I have seen a high mileage 119 that had the trailing exhaust cam 23 degrees:eek: late from chain stretch and rail breakage. Under those conditions its easy to see that any amount of timing is a huge compromise as the dynamics of half the engine are considerably different than the other half. There wasn't any real running or knocking complaints from that engine but here at 5000+ft knocking is not a typical complaint. Nevertheless having the basic mechanicals in order goes a long way to having the engine run as well as it should.

Thanks guys for the input. I'm glad to see some of you like to break the rules. ;)

Btw, duxthe1, I'm not actually diagnosing any problems. I run my M119 on 91. My question was hypothetical. The idea just occurred to me, so I had to share.

An exhaust cam that's 23* behind the crank angle, or cam angle? Either way, that's a lot. I didn't think the tensioner could take up that much slack. I measured mine recently, and it was only about 3.5* crank angle.

I hear you on the warm-up map issue on the 70* thermostat. That's doesn't sound like a good idea.

I think the best way might be to check the variable cam retard angles during controlled conditions using a real-time scanner while being on:

(a) 91 octane with specified heat range plugs
(b) 87 octane with specified heat range plugs
(c) 87 octane with various lower heat range plugs.

Another idea might be to try the above with option (d) water injection contra-proportionate to fuel injection (i.e., max water at lowest intake vacuum).

If you all have more comments, ideas, or experience, please do post here. If promising, I might take this up as a future project.

Cheers

I rather think "heat range" describes more the plug's ability to transfer heat to the cylinder head than it does the heat of the combustion created.

Of course if it doesn't transfer heat rapidly it can become in effect a "glow plug" and contribute to detonation/pinging and of course the colder plug is the solution. To the extent detonation/pinging is eliminated I agree it lowers combustion temperatures, but this still isn't the proper description.

The engines are design optimized of course around original specifications, and changes as suggested simply reduce efficiency in the name of burning low-test fuel, and--apparently--"being a rebel who'll never be any good." It's NOT a brilliant solution.

lkchris, are you going to go around the internet hunting down my posts to harass? Who's the rebel without a cause now?

If you "think" this, then let me know when you "know" this.

Wrong. Pre-ignition.

No design is ever truly optimized, but, at best, only intended to be optimized. You'd know this if you were ever in a room with people making group decisions. And once the product leaves the factory and lands in the hands of consumers, we have the authority to do anything we want with it.

Are you saying water injection with 87 gas would lower efficiency? That's a new one.

Here at 5000ft, I run 86 octane all year long. My 103 has a true open hemispherical chamber and it tolerates it well, even on the most aggressive spark map. I always try to run the lowest octane fuel that any given engine will tolerate. There is more chemical energy in lower octane fuels, and they are more efficient. I am also a fan of water injection but the placement of the mass air meter in the 119 doesn't leave many placement options that will encourage even distribution and not kill the meter.

If your chain and rails are in good shape then playing with the cam timing isn't likely to produce positive results. I don't know this for fact but unless you own a dyno it would get real spendy to find out. I'd play with the heat ranges first and then see what you can get away with there.

That's really interesting. Running 86 at at altitude should be like running 88, so you're regularly using lower than mid-grade. I'd be very interested in seeing a picture of your sparkies, if you have them handy.

I agree that starting with the easiest thing first is best: colder plug. It would be easy to see the conditions after 100, 200 and 300 miles.

My NGK BCP5ES is running a bit hotter (pitted electrodes) than Bosch F8DC4. I'm going to get BCP6ES for running with 91. So, with BCP7ES, I might be able to run 89 at least without the timing retarding.

I was somewhat confused why my NGK BCP5ES is running hotter than the old Bosch F8DC4. So I got BCP6ES.

Today, while looking for why I'm getting bad mileage, I came across something I hadn't heard before: E85 gas likes colder plugs, by about 3 steps.

So, with the E10, it might be that we need to go a bit colder. Colder being better then hotter, maybe my choice in going one step colder might be a good thing....