PCMtec

[engineermike]

There's this Whipple phenomenon where the supercharger sets off the knock sensors in the 1000-2000 rpm light-throttle acceleration from a dead stop. The Whipple remedy is to limit knock retard to .5 deg at that load and speed range. I decided I didn't like that solution so I started using PCMTec to try to find a better solution. What I learned is just how little I knew about how the knock sensing works on these.

PCMTec allows you to log dozens of parameters that lead up to knock retard, not just raw per cylinder retard.

Apparently, the PCM is always calculating a "reference level", or a rolling average vibration from the engine. There is a running "knock integration" that represents the current level of vibration. It then divides the knock integration by the reference level to determine a ratio. When this ratio exceeds the "knock threshold" in the tune, it is interpreted as knock. The "knock intensity" is the ratio minus the knock threshold. The knock intensity is used to determine the amount of timing removed, which comes from another table in the tune.

I was able to catch the whipple SC rattle in a log and you can see the knock reference level, knock ratio, and intensity. This is false knock, but still a good graphic showing how the algorithms work.

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[Grimreaper]

thats outstanding to be able to see. always guessed the intensity from kr.

you going to adjust the threshold? assuming you are on e still. whats map during this event? wonder if the bypass closing is adding to this.

neg adding or removing spark in per cyl in the software?

 

[engineermike]

you going to adjust the threshold?

I’ve been contemplating this. With the peak knock ratio being under 3, threshold at around 2, and the intensity, being ~2.x - 2, ranges from 0 to 0.8, I understand my options.

The problem with limiting knock retard is fairly obvious. The problem with simply raising the threshold is that it not only makes the pcm blind to light knock, but also reduces the reaction to it since the calculated intensity will be less as well.

I see two options now: 1) increase the threshold but also increase the kr amount once the threshold is met. Eg raise threshold from 2 to 3 and then increase the amount from 1.25 to 2.5 at 0.0 intensity. 2) reduce the amount to 0 at 0-1 intensity. This would have the same effect but leave the ratio and intensity data and calcs the same as they are now.

I initially attempted to raise the reference level (through constants) because that would be technically correct but then learned that the rattle starts suddenly so even at higher reference levels it still shows as a spike in ratio.

assuming you are on e still. whats map during this event? wonder if the bypass closing is adding to this.

Speed is 1000-2000 rpm and load is typically .3-.4. It’s the same speed and load as cruise so I don’t want to neuter my knock system. Bypass valve doesn’t close until much higher loads.

neg adding or removing spark in per cyl in the software?

Negative is retard. You can see all cylinders actually go to 10+ deg of retard when this happens. Yes it’s on e, so I know it’s false. Also, even on gas it will run steady state at the same speed and load with no trace of knock.

 

[K4fxd]

I'm N/A and get the same false knock in the same range you do. Usually 1200 to 1800 RPM and .3 to .4 load when adding throttle. It won't do it every time.

I'll have to look at logging these. If you do find a safe way to get rid of it can you share it? Thanks.

 

[shogun32]

Usually 1200 to 1800 RPM and .3 to .4 load when adding throttle

What are you doing running around with so little rpm? Exhausts get all boomy right around 1500-1800 too.

Maybe the resonance is a function of the engine mount, headers, and trans?

 

[K4fxd]

What are you doing running around with so little rpm? Exhausts get all boomy right around 1500-1800 too.

Maybe the resonance is a function of the engine mount, headers, and trans?

Got to pull away from stop lights.....

 

[engineermike]

What are you doing running around with so little rpm? Exhausts get all boomy right around 1500-1800 too.

Driving normally(?).

The corsa touring is probably quieter than stock under 2000 rpm, yet full 3” straight through design.

 

[shogun32]

Got to pull away from stop lights.....

Well sure but you pull right thru that range in half a second and never see that range again unless stopped...

My point is the resonate frequency of the motor and trans and the huge oscillating/vibrating assembly known as the headers. They are only supported on one end: the ports. The other end is a flex joint and the cats are at the unsupported end, and not exactly light.

 

[K4fxd]

I know it is false because it does it on e85 and does it if I pull timing way back in that range. It's aggravating as hell.

 

[K4fxd]

Well sure but you pull right thru that range in half a second and never see that range again unless stopped...

My point is the resonate frequency of the motor and trans and the huge oscillating/vibrating assembly known as the headers. They are only supported on one end: the ports. The other end is a flex joint.

I've got an auto so the car cruises in that range most of the time.

It does go away after the engine is fully warm. Something might be loose that tightens up after it gets hot.

 

[shogun32]

I've got an auto so the car cruises in that range most of the time

Another thing to fix. Rpm floor should be 2100 unless under 18 mph and no throttle.

 

[K4fxd]

Another thing to fix. Rpm floor should be 2100 unless under 18 mph and no throttle.

I like to get 18mpg in the city.
In sport mode where I have it shifting at 2500 rpm light throttle it drops to 13

 

[engineermike]

@K4fxd are you running flex fuel?

 

[K4fxd]

are you running flex fuel?

Yes

I don't get 18 on e85.
e85 is 13 in D and 9 in sport. I really wish Ford had given us a 20 or 24 gallon tank.

 

[engineermike]

Ok at least on gen3 there’s a table to change knock retard “amount” as a function of ethanol. The blend table is zero’d from ford. So basically, you could zero the amount at 1000-2000 rpm and .3- .5 load on e85 only. I certainly feel this is safe but doesn’t fix the issue on pump gas.

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