stock BR-Z, much detonation during shifts: tune?

[elefunkd]

Ah, gotcha. Dealer visit it is, afterall. I so wanted to avoid that hassle, but, moving forward and such, it seems inevitable.

Thanks again.

-T

[Lonewolf]

Originally Posted by ft_sjo

They all do it. The 'di failure' paranoia threads is dealt with by an updated calibration, however that won't resolve the 4k RPM tip-in det you're talking about. Most of the doom mongers won't know the difference though and will mislead you.

Good to know that you know everything....

[jamesm]

Originally Posted by arghx7

How do you know it was knocking? Are you sure you're not just hearing driveline noise? Do you have ECUtek logs showing the knock?



I wouldn't say it's "desireable" condition, but it's certainly a normal one. Just about every modern engine runs on the edge of knock under certain conditions because that gives the best fuel economy. And knock isn't an all-or-nothing thing. There are degrees of severity.

'on the edge of knock' and 'knocking' are not the same thing. if you're engine is knocking, something is wrong.

[fstlane]

My BRZ with 2500 miles on it has behaved this way since day one. Any quick shift regardless of the rpm will result in a brief period of detonation. I have also used the knock detector plug in for the Torque app which confirms what my ears hear. I have tried 91 octane gas from a number of different top tier stations as well as blended 100 octane pump fuel at a ratio in the 93-94 range. Unfortunately I feel a trip to the dealership will result in nothing but a wasted trip with a "this is normal" outcome. For now I'm just trying to slow my shifting down a bit and keeping my engine completely stock until a solution is found or a mechanical failure happens. Please share your findings. Thank you!

[elefunkd]

Originally Posted by fstlane

My BRZ with 2500 miles on it has behaved this way since day one. Any quick shift regardless of the rpm will result in a brief period of detonation. I have also used the knock detector plug in for the Torque app which confirms what my ears hear. I have tried 91 octane gas from a number of different top tier stations as well as blended 100 octane pump fuel at a ratio in the 93-94 range. Unfortunately I feel a trip to the dealership will result in nothing but a wasted trip with a "this is normal" outcome. For now I'm just trying to slow my shifting down a bit and keeping my engine completely stock until a solution is found or a mechanical failure happens. Please share your findings. Thank you!

I will definitely update....
But, what a disappointment in terms of lack of r&d on the manufacturers part.

-T

[StormTrooper]

I'm very curious how you're managing it and how any dealer could call this normal.

I have 6500 miles on mine live at 4500 ft and have to use crappy 91 oct gas.

I drive aggressively including shifting in around and after the torque dip and have never noticed this.


If Toyota or Subaru calls this "normal" I'd definitely get it in writing so that when something happens they have to fix it. Reminds me of the oil consumption threads where people were told a quart per change was acceptable.

[NotSoJDM]

I have the same issue. For as long as I can remember. My car has 29k on the odo and around 15 autocrosses. Hasn't gotten any worse since when I first noticed it. But my catback does make it much more audible.

Keep us posted on how the dealer trip pans out. Im sure youre already aware, but take everything you read in here with a grain of salt.

[StormTrooper]

After driving my car again I have to ask why you would ever notice this.... If you are shifting at 4k you're not in a hurry....

[Cameronghanson]

If it makes a weird harmonic sound when engaging the clutch there are some threads about that. I had that too and had injectors and fuel pump replaced...they went away and within a month or two they came back

[arghx7]

Originally Posted by fstlane

My BRZ with 2500 miles on it has behaved this way since day one. Any quick shift regardless of the rpm will result in a brief period of detonation. I have also used the knock detector plug in for the Torque app which confirms what my ears hear.

I don't know the exact formula your Torque app uses, but the only way it could work is by taking some kind of "before and after" comparison or some kind of statistical analysis of spark timing. It can't read the knock sensor over the universal CAN bus. So you're still at the mercy what the knock sensor is responding to. The knock sensor can be responding to driveline noise, or it could in some cases be a little extra sensitive to combustion noise depending on how the engineer set the background learning signal processor and the knock judgment threshold.

Originally Posted by jamesm

'on the edge of knock' and 'knocking' are not the same thing. if you're engine is knocking, something is wrong.

I know what you're thinking. "It's either knocking or it's not. If it's knocking, that's bad, and no engine should be knocking on a regular basis because it will cause damage." Wellll...

Ask trained engineers or engine technicians observing the same engine whether the engine is "knocking too much" and you'll get a different answer. There are two different ways of judging knock. There's a cylinder pressure trace method, and there's an audible microphone method. Notice that I didn't include a knock sensor in that list. The knock sensor reading, as processed by the ECU, is a reflection of some judgment call made during development in terms of sensor placement, calibration of the noise learning, and judgment of the "real knock" threshold.

Cylinder pressure, actual microphone, or knock sensor?

First, somebody decided where to put the knock sensor, which was a series of compromises in and of itself. Then during calibration of the knock system, somebody was physically listening to the engine or looking at the cylinder pressure trace. There are two schools of thought on knock judgment. The "microphone guys" swear that it's the best way to judge knock, and that with proper training it can detect real knock before a cylinder pressure trace ever could. The cylinder pressure guys say the same damn thing, claiming it's more representative than a microphone could ever be.

So, you get two guys listening to an engine and they will have their own opinions on how much noise is too much, unless they were trained by the same guy. And their judgments would then be reflected in the knock sensor feedback system on the ECU, which ultimately affects you the driver. But what about the cylinder pressure trace? Combustion pressure couldn't lie, could it?

Let's look at my cartoonish representation of a typical pressure trace at high load and MBT timing (minimum spark advance for best torque). X axis is time/crank angle, Y axis is combustion chamber pressure measured by pressure transducer.



so that's MBT. Usually you have 50% burn by 6-10 degrees ATDC firing, and peak pressure (shown above) maybe 15-20 degrees ATDC. Now here's a retarded combustion phasing under full load. It's retarded because at a given rpm, we retard ignition timing with increased load to mitigate knock. In this example, peak pressure is around say 30 degrees ATDC.











All of these made different levels of power and fuel economy, and often the uglier pressure traces perform better (except for when they don't). So which one is knocking "too much" ? But why not pull timing more so the pressure trace is less noisy? Well if you decide to do that you'd lose output and heat up the exhaust, requiring you to richen the mixture to keep temps down.

Remember, the knock sensor on your car can't read cylinder pressure and it doesn't have a human being sitting there with a microphone. It's just an approximation of that process. Setting an OEM knock control system is very difficult and except for extreme cases, there is no universal consensus on how much knock is too much. It's all somebody's judgment call, magnified by production tolerances and other factors.

[mad_sb]

Nice post @arghx7 :-)

[Suberman]

Jeepers guys, transient detonation will impact your engine in exactly zero way.

Supercharged engines detonate all the time. It's how the knock sensor works to control boost pressure. Basically a knock sensor controlled engine MUST detonate in order for the knock sensor to sense knock. Only AFTER the engine detonates will boost or timing be knocked back (pun intended).

Serious piston holing detonation just cannot occur in these engines.

[arghx7]

Originally Posted by Suberman

Jeepers guys, transient detonation will impact your engine in exactly zero way.

I'm not sure I'd go that far... remember the injector issue on some of these engines and the new knock control settings from the dealer reflash.

I was just trying to point out that this is a gray area, not black and white. It's a murkier issue than "my car is definitely knocking and any knock is bad"

[elefunkd]

Originally Posted by StormTrooper

After driving my car again I have to ask why you would ever notice this.... If you are shifting at 4k you're not in a hurry....

This makes me realize that there is such a thing as a "stupid question"