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Old 12-25-2005, 06:38 PM   #1
rek888v2
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Default Engine knock at low RPM's

I recantally had my STi tuned at Vivid Racing. No issues, until a few days later. Now i expected some type of detination and saw it. Today while drivinh to Wally World (Walmart for those who aint down!) and i saw the check engine light flashing. That was in 2nd at roughly 3000 RPM's. Next I saw the same in 4th at roughly 3500 RPM's. Now, I searched the forums and found no real answers to what knock is, what causes it and how to stop it. Any info about it would be helpful.

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Old 12-25-2005, 06:58 PM   #2
cdvma
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If you could grab a UTEC log during the knocking that will help us diagnose otherwise its no more accurate than a guess. I'm leaning that its just too much timing but can't tell for sure.
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Old 12-25-2005, 07:50 PM   #3
rek888v2
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I will see what I can do.

Last edited by rek888v2; 12-25-2005 at 08:19 PM.
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Old 12-25-2005, 09:11 PM   #4
supermarkus
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Check your up-pipe heat shields just in case, I've heard of heat shield rattling setting off the knock sensors.
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Old 12-25-2005, 11:13 PM   #5
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Possible, but unlikely. I'd look for other explanations.
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Old 12-26-2005, 01:09 AM   #6
rek888v2
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I dont think its the heat shields. I noticed today that it only does it when im already in the gear, say 4th while doing 40 MPH and I hit the gas. The car boosts at a low RPM, then knock occurs. Im thinking its cause the car is leaning itself out (my AF gauge was in the red), while trying to create higher boost at an RPM that isnt pumping the right ammount of gas to sustain it.
How bad is knock for the engine and what is it affecting? Any info would be really helpful.

Last edited by rek888v2; 12-26-2005 at 01:14 AM.
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Old 12-26-2005, 01:50 AM   #7
keaniegenie
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i've had the same problem with similar upgrades (utec, txs shorty intake, helix flex uppipe, invidia catless dp, perrn pulley, and a txs tmic). if I shift, i'll get knock if I hit the gas too hard. similar rpm patterns. pulled back the boost and timing and I'm still having the problem.
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Old 12-26-2005, 03:34 AM   #8
supermarkus
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You should post your datalogs. What type of AF guage are you relying on for AF data? I'm guessing a narrow band (virtually useless for tuning). Knock will eventually destroy your engine, until then your engine management will pull timing out and dump fuel to avoid knock and leave you low on power.
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Old 12-26-2005, 03:36 AM   #9
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Yo Supermarkus. Up late too? PM sent, Cheers!
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Old 12-26-2005, 10:15 AM   #10
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Quote:
Now, I searched the forums and found no real answers to what knock is, what causes it and how to stop it. Any info about it would be helpful.
There is actually quite a bit of information out there but you would be more likely to find it searching for information on "detonation"


Quote:
How bad is knock for the engine and what is it affecting? Any info would be really helpful.
Knock (detonation) ranges from imperceptible low grade detonation to a brutal knocking sound depending on how severe it is and at what rpm it occurs at.


Some engines can tolerate low levels of knock for very long periods of time.
These are typically low performance "over built" heavy duty engines.

Low rpm Knock tends to sound more like a solid "clunking" knock (where it got its name) and sounds about like someone beating on your engine with a big hammer ---- which is effectively what is happening if it is severe.

At Higher rpms knock progressively gets sharper and higher pitched in sound, and is often discribed as sounding like a handful of pebbles shaken in an empty tin can. The is where the other common name for knock ( pinging --- or pinking in the UK) comes from.

It can also do damage to the engine if severe.

On our engine it usually damages the piston rings or breaks the ring lands on the pistons. This destroys the piston ring seal which leads to a drop in performance and lots of oil getting into the combustion chamber (which makes the detonation worse).

Our engine is prone to detonation (when compared to engines like the 4G63) due to its large diameter bore compared to its stroke.

If you are beating on the car you MUST put adequate octane fuel in it for your level or tune or you risk killing the engine.

Engines are most sensitive to detonation near torque peak rpms, so trying to get huge torque numbers at 3000 - 4000 rpm is just asking for trouble.
As engine rpm goes up, detonation becomes harder and harder to initiate. At F1 engine rpms they can run fuel which would be unusable for your engine.

The most effective things you can do to reduce detonation are, (ranked roughly in order of effectiveness):

1.) adequate fuel octane
2.) reduce the temperature of the intake air charge
2a.) use water or alcohol injection ( accomplishes both of above)
3.) reduce the engine coolant temperature.
4.) lower cylinder pressures by lowering boost or retarding ignition timing.
5.) reducing load on the engine (down shift instead of lugging the engine at low rpms)


It would be very easy to write several books on what detonation is and it is a much debated issue in performance circles. It has been studied for over 100 years and is still not well understood even by automotive engineers.

The short version is it is a form of abnormal combusiton that occurs when cylinder temperatures and pressures exceed a critical value for the fuel being used. Lower either the temperature or pressure and it will go away.

Ignition advance is often used as a quick and easy way to reduce cylinder pressures, the fuel air mix is usually cooled by injecting excess fuel. (not a particularly effecient technique but very simple to do).

Detonation is caused when normal fuel air combustion suddenly converts from a very rapid but progressive burn to an explosive detonation. It causes supersonic shock waves to develop in the combustion chamber and can elevate cylinder pressures several times above their normal level. Serious detonation can split cylinder walls, knock big holes in pistons, lift cylinder heads and blow head gaskets.

In short -- AVOID IT in any engine but especially in the Subaru OEM engine.

Larry

Last edited by hotrod; 12-26-2005 at 10:22 AM.
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Old 12-27-2005, 12:33 PM   #11
rek888v2
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Thank you everyone. I went for a logging run today and realized this. I was in AZ with 90 octane. CRAPPY!!! I am now in TX and filled up on 93 this morning when I rolled in here. I am almost positive by what the UTEC was showing that it was the crappy gas. I also saw from what was posted that the knocking was low, unnoticable by the ear knock. I was in 6th and hit it. Knock occured, but no actual knocking sound. When I get home to NC, I will have more time to log some runs, but if need be, I guess i'll have to push my ignition timing back a few degrees. Again, thanks for all the great info!!! It really did help alot!
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Old 01-02-2006, 06:41 AM   #12
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I would def consider retuning for 93. Definite advantages and gains.
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Old 01-02-2006, 10:30 AM   #13
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It is cuz of the gas. I got bad gas the other day and had knock at low rpms like you and I also have UTEC. It took two full tanks of 93 to flush it all out and stop the knock. You can try some of that octane booster stuff too but it is a pretty big rip off. I would just take it easy for a couple tanks. Or fill up with 100+ if you have that available around you.
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