Knocking?

[M3n2c3]

As far as I can remember, this has been happening for a few summers (possibly since I got the car, but that was a while ago now), but seems a little more pronounced since I switched to Royal Purple for my engine oil. . .

This really only happens when my car has been sitting out in the sun all day. When I'n going up the hill in to Moundhouse, the engine will make a slightly tinny, rapid tapping sound. It only happens at about 25%-40% throttle in 5th gear - enough to hold 60mph going up the hill. If I slow down or give it more gas, the noise stops. If I'm level, it stops.

There's no unusual noise when starting, no noise when driving around town, and when the car is cooler (in the morning,) it doesn't make the noise at all, even when going uphill. I always let the engine warm up before driving, and I've checked the oil - the level is fine, and there's no unusual consumption. The car does not seem to be low on power or doing anything unusual. Gas mileage seems fine. I don't remember this happening in the winter.

I don't really want to think knock. . . I'm wondering if this could be something more simple, like just valve tapping, some weird exhaust leak, or maybe Royal Purple 5w30 just thins out way too much in Nevada summer heat. . .

[Dean]

87 or 91 octane? AC on/off? Try higher octane and/or turning off AC.

What you are describing sounds like high load low-mid RPM closed loop knock.

Oil is not even involved.

[M3n2c3]

87 octane, and the AC is off. I'll try some 89 next time and see what happens.

If it is what you named, will I need to have it looked at, or is it relatively common?

Now that I think of it, I was running 91 in the car for the first year or so. . . maybe that was why it happened less?

[Dean]

Run it almost dry and try 5 gallons of 91...

The fix is downshift and get out of closed loop.

[M3n2c3]

I'll try that. I really don't want to have to switch to 91 for the summer though. . .

So it is common then? I googled and found this:

In the timing maps the closed loop region, also called the ‘knock zone’, is only in a narrow stretch of RPM range in the high load area. This is because the knock sensor cannot distinguish between detonation and normal engine vibration. But in the event poor gas is put into the car or you are simply pushing the limits too far, the ECU will attempt to save the engine by retarding the timing when it detects detonation.

It wasn't specific to the Impreza, but seems to apply.

[MPREZIV]

You're on the right track I believe. Mine does the same thing, under the same conditions, with el-cheap-o regular grade gasoline.

[cody]

WRT the oil, most run 10-30 when running RP. Supposedly RP thins more than it should as it approaches the milage/time to change it. Dunno for sure though.

[M3n2c3]

Originally Posted by MPREZIV

You're on the right track I believe. Mine does the same thing, under the same conditions, with el-cheap-o regular grade gasoline.

I've put nothing but Chevron in the car since day one. . . but yeah, I tried 91 for a while and switched back to 87. It didn't seem to make much of a difference for daily driving.

This kinda bugs me. . . the manual just recommends 87 octane. . . if this isn't unusual, you'd think there would be something in the manual saying, "but if you don't want to knock your way up hills, use 91."

[cody]

If I was going up more than a slight uphill grade @ 60mph, I'd try to be in 4th.. It's more important to do so in a turbo car, but I think high load isn't great for N/A cars either. Is your 5th lower than mine?

[sperry]

Originally Posted by M3n2c3

I've put nothing but Chevron in the car since day one. . . but yeah, I tried 91 for a while and switched back to 87. It didn't seem to make much of a difference for daily driving.

This kinda bugs me. . . the manual just recommends 87 octane. . . if this isn't unusual, you'd think there would be something in the manual saying, "but if you don't want to knock your way up hills, use 91."

At our altitude, 87 drives more like 89 anyway. Unless you've got a really bad gas station that's always way under the rating, something else is wrong.

Any chance you're getting a lot of blow-by oil running through the intake? That'll drop the effective octane of the intake charge a bunch.

[bigrobwoot]

i think the 5th in the RS might be lower than the wrx, cuz at 125 my rpms were a lil over 5200, if i remember right... and wrx's have no problem getting to 150 and higher, right?

[sperry]

Originally Posted by bigrobwoot

i think the 5th in the RS might be lower than the wrx, cuz at 125 my rpms were a lil over 5200, if i remember right... and wrx's have no problem getting to 150 and higher, right?

A bone stock WRX will have a hard time getting over 140. It's aero limited, not gearing limited.

[M3n2c3]

Originally Posted by cody

Is your 5th lower than mine?

I can't find documentation on my gearing, so I'm not sure. It's never been something I've felt the need to look up before. 2500rpm in 5th gets me somewhere between 55-60mph.

Originally Posted by sperry

Any chance you're getting a lot of blow-by oil running through the intake?

Now that. . . I don't even know what that means!

[cody]

Are you seeing oil consumption? Blow by is oil that finds it's way into the intake via the PCV valve.

[M3n2c3]

No consumption, no. I just checked it yesterday - the level's right where it should be, and I'm just about due for my next oil change. I don't know if there's any oil in the intake piping. It's still bone stock, so I've never disassembled it.

Chevron's gas was oxygenated during the winter. Doesn't that effectively increase the octane properties? Perhaps that's why there's a difference now.

[M3n2c3]

91 octane seems to cure it. 89 is a little better than 87.

In other news, while I was waiting for the engine oil to finish dripping, I decided to finally pull the intake snorkel out of the fender. It doesn't seem to have changed much except the noise. Perhaps it will result in a little better pick-up in the once summer's over, when there's actual cool air to be had.

[DUB]

Originally Posted by M3n2c3

It doesn't seem to have changed much except the noise. Perhaps it will result in a little better pick-up in the once summer's over, when there's actual cool air to be had.

The only thing that is supposed to change is the noise. You aren't going to feel any difference in the butt dyno just from removing the intake silencer.