Do you use 87 gas or 89?

[ktsubaru]

Hi.

 

I drive 09 Outback 2.5i Automatic.

It has 4000 miles on it.

 

I notice that (specially when A/C is on) the motor knocks on 87Gas when driving less than 20mph, specially from 0mph to 10mph.

 

The similar phenomenon occurred on my 08 Outbacksport 5spd.

 

This doesn't happen when I use 89. Does anyone have similar issues?

[Fairtax4me]

Usually 87 should be fine. But that being a brand new engine you're probably burning a bit of oil while the rings are still breaking in. Even a small amount of oil can cause pinging by destabilizing the fuel. I'd run 89 for another couple thousand miles and then try switching back to 87.

[zyewdall]

My understanding is that most modern engines should automatically adjust the timing to avoid pinging, even if you use low grade gas -- loosing power due to doing this though. Older cars are more picky and it depends more on the compression ratio... but... not sure if that's true.

 

Most of my older subarus I've noticed very slight pinging in the summertime on steep hills with the lowest grade (I'm assuming that you are referring to 87 as the lowest grade... here in colorado, 87 is mid grade, and 85 is the low grade). So I generally run the mid grade stuff. Not enough experience with the newer ones to decide on them yet. I have never run the high grade (89 or 91) in any of my cars... only in the chainsaw, which is very picky.

[Fairtax4me]

The ECU can and does adjust timing to a degree. But it's not always ignition timing that causes pinging. Fuel quality, air quality, air density, air temp., combustion chamber temp., and many other things can affect the way the mixture ignites, and how stable it is before and after the spark plug fires.

 

Pinging/detonation occurs after the spark plug has fired. So there is heat and pressure building in the chamber which causes the mixture to burn unevenly and/or cause spontaneous combustion in another area of the chamber. These two separate events cause a rapid pressure spike in the chamber which creates a sound wave that resonates through the block and internals of the engine, similar to that of a hammer strike.

Funny thing is that it's an almost mechanical function of internal combustion. No mater what size the engine is, or who made it, the sound produced by detonation, though not always audible to human ears, is always the same frequency.

 

The basic underlying cause is an unstable/incorrect fuel:air mixture for the conditions that the engine is operating under.

[zyewdall]

Thanks for the explanation. I don't really understand gas engines that well compared to diesels....

 

Like... why is running lean bad for gas engines, whereas diesel engines running lean is what you want, to avoid black smoke and melted pistons?

[Fairtax4me]

Yeah they're two entirely different beasts. I've been picking up some info about diesels here and there from the techs at work. I always wondered why diesels don't make vacuum, why you have to have a separate pump for vacuum driven parts such as the brake booster and engine shut off valve. I was watching as one of the techs disassembled the top end of one for a head job and finally noticed it had no "throttle body". Technically it does, but not in the same sense as that of a gas engine.

 

Running lean is bad for gas engines due to the nature of the combustion of gasoline. A leaner mixture is a hotter flame. While it does make more power, (to a degree) it will eventually lead to hot spots in the combustion chamber that will act similar to a glow plug on a diesel. The thing about detonation is, most of the time it is due to high temperature in the CC, an overheating engine will start pinging like there's no tomorrow. But it's a snowball effect. The higher pressures created in the cylinder by the detonation make the CC temperatures even higher. At some point some some part part of the internals (exhaust valves most of the time, or carbon buildup) will start to ember (glow) and a hot spot of that nature can ignite the fuel as it enters the cylinder during the intake stroke. That means near instant death for a gasoline engine. You might get away with that once. What happens is the ignited mixture is already extremely hot, add to that the pressure created by the compression stroke. That can drive the CC temperature well upwards of 3,000°F which is enough to instantly melt a hole in the top of the piston.

 

http://www.aa1car.com/library/piston_preignition_damage.jpg

[zyewdall]

Ah... so that's why they can get better gas mileage from the direct injection gasoline engines, because they can run it leaner, but it doesn't have the danger of compressing a fuel charge that's already ignited, because the fuel doesn't get introduced intil later, just like a diesel engine. Hmmmm.

[subaruplatt]

I find that the 2.5i in our impreza runs fine on regular.

But it runs smoother and has more power with mid-grade.

For the last couple of months I have been biting the bullet and buying mid-grade as I feel that the car actually runs better.

[Fairtax4me]

Ah... so that's why they can get better gas mileage from the direct injection gasoline engines, because they can run it leaner, but it doesn't have the danger of compressing a fuel charge that's already ignited, because the fuel doesn't get introduced intil later, just like a diesel engine. Hmmmm.

 

That's part of it. The main thing is atomization of the fuel, which makes a much cleaner, more efficient burn. Direct injection uses a MUCH much higher line pressure, in the 1,200 - 1,800 psi range (some even higher maybe), whereas port injection is around 35 - 70 psi in most applications. The Mercedes Benz new Bluetec common rail diesels with direct injection run over 20,000 psi.

[torxxx]

run 89 octane. Most new cars recommend running 90+ octane if you read teh owners manuals. Knock sensors do retard timing when the engine knocks but it will only work to a certain degree.

 

One other thing to remember is these engines are very prone to piston slap due to the fact that they are flat engines..

[doc526]

Yeah they're two entirely different beasts. I've been picking up some info about diesels here and there from the techs at work. I always wondered why diesels don't make vacuum, why you have to have a separate pump for vacuum driven parts such as the brake booster and engine shut off valve. I was watching as one of the techs disassembled the top end of one for a head job and finally noticed it had no "throttle body". Technically it does, but not in the same sense as that of a gas engine.

 

 

The cool thing is that engine speed is governed by the amount of fuel injected into the cylinder not air.

[valvestem]

My wife always pays for the mid-graded in her Subees since 2000, and she says the cars run better. Who knows? I have a 2010 Fozzie, that "seems" to run a dite better with mid-grade than 85. I really haven't had the car long enough to evaluate the gas, and all my observations are strictly seat-of-the-pants, not entirely scientific.

[zyewdall]

That's part of it. The main thing is atomization of the fuel, which makes a much cleaner, more efficient burn. Direct injection uses a MUCH much higher line pressure, in the 1,200 - 1,800 psi range (some even higher maybe), whereas port injection is around 35 - 70 psi in most applications. The Mercedes Benz new Bluetec common rail diesels with direct injection run over 20,000 psi.

 

I've never driven the modern mercedes, but the modern cummins in the brand new dodge trucks are rediculous -- they don't even sound like a diesel any more at idle. Plus, quite a bit of power.... we blew a WRX off the line in a 3500 dodge dumptruck full of scrap steel (okay, the WRX wasn't exactly paying attention when the light turned... but he was still a little suprised)

[Fairtax4me]

I've never driven the modern mercedes, but the modern cummins in the brand new dodge trucks are rediculous -- they don't even sound like a diesel any more at idle. Plus, quite a bit of power.... we blew a WRX off the line in a 3500 dodge dumptruck full of scrap steel (okay, the WRX wasn't exactly paying attention when the light turned... but he was still a little suprised)

 

There is a tone that is very diesel about the engine when it's running. The familiar clatter is still there, but it's so muted in the new design that just a regular conversation will drown out the sound of the engine. And the biggest difference is just in the way the fuel is injected.

[mike97]

Hi.

 

I drive 09 Outback 2.5i Automatic.

It has 4000 miles on it.

 

I notice that (specially when A/C is on) the motor knocks on 87Gas when driving less than 20mph, specially from 0mph to 10mph.

 

The similar phenomenon occurred on my 08 Outbacksport 5spd.

 

This doesn't happen when I use 89. Does anyone have similar issues?

 

i have had that same problem .I own a 97 sub leg 2.5.now i use 93 only and runs better and more power.89 will work just not the performence i like