99 Cobra with supercharger - Gurus Come In

[phillysrt4]

I'm asking these questions on behalf of a coworker with a 99 cobra. Usual boltons + a centrifugal blower at 6 psi. Reported A/F is in the 10's.

He's using a diablo predator to examine some engine parameters and he has a question about the knock sensor. When he's cruising the knock is usually showing around 60 (volts?) but when he gets on it in 3rd gear pulls that number goes up to about 500 (volts?). He also notices that under WOT the timing drops to around 16 degrees in lower RPMS and makes it way to around 26 degrees (I believe thats the number he told me) in higher RPMS. The timing seems to make a steady progression which gives me the impression that timing may not be pulled due to knock.

He said that he was reading some tuning threads and others have reported a knock sensor reading around 4000 (volts?) and timing gets pulled.

That being said, the questions:

1a) Is the knock sensor reading in volts? (EDIT - I ask because I always thought piezo elements worked in the 0-5V range, so is this a "scaled" number that corresponds to this 0-5 range? his whole "60 volts" thing is throwing me off)
1b) Where is the knock sensor threshold that will cause the ECU to pull timing?
2) What is the typical timing under WOT for a lightly boosted DOHC mod motor like this?

Thanks in advance for any input anyone has. I'm just not familiar enough with 99-04 ECU's to even start searching

[John4cam]

Not trying to be a smart ass, but why doesn't he just take it to a reputable tuner then just be done with it instead of running the risk of damage and even more money invested or lost?

I know there's some satisfaction in diy tuning but man that would scare me.

[phillysrt4]

I dig where you're coming from John, and I told him that for what he's doing an SCT tuner would probably be a better choice and there is a dyno about 45 mins from me where he could tune (EDIT - an authorized SCT tuner is 45 mins from us... double that for a predator tuner).

This is not his daily driver. He put the charger on, made some adjustments, did a pull, wondered WTF was going on, and backed out of it. I've done some forcefed tweaking in the past but when I did it the knock sensor output was explicitly in volts and moved from 0 to 5, so when he told me that the output of the knock sensor was "60 volts" with almost no throttle and "500 volts" on the WOT pull I got confused, and I don't think the units the predator is giving are volts.

I too strongly advised him to go with an SCT tuner with hopefully a safe email tune to get to a dyno and tweak from there. His tuner just doesn't sound like it has the RPM resolution and parameter "granularity" to do a good tuning job for his needs (EDIT2 - his predator must be an older one because he says he only has two RPM ranges to adjust in whereas the one on diablo's website seems to have 3).

EDIT - i'm just trying to make sense of the numbers he's getting both in terms of the knock sensor and whether his timing is being reported correctly.

[John4cam]

I think the knock sensor is read in ohms...

[phillysrt4]

Well for testing if its bad you can use an ohmmeter because you should get a very high resistance value (in the 100's of kOhms) but if you hook a sensor up to a scope and tap it you can see a waveform which means its generating a voltage (which makes sense because a piezo sends a voltage signal when tapped/squeezed).

EDIT - but based on what you say and what he's telling me, I could see a reading of 60 ohms to 4000 ohms. hmm...

What he's telling me is just too conflicting and isn't making sense to me. I think I might just tell him to buy an SCT from our "local" distributor who also has a dynojet in house and be done with it.

EDIT - thanks for checkin in though John, I appreciate it. :)

edit to edit - i've been up all night and not thinking clearly... lets try this again after some sleep.

[Cam99]

Can't be voltage like that on a 12 v system. Mili-volts maybe? ohms?

[phillysrt4]

yeah I'm thinking either ohms or mV

[Martin0660]

No idea on the newer stuff, but older Ford Knock Sensors report FREQUENCY.

[SAC81]

1a) Is the knock sensor reading in volts? (EDIT - I ask because I always thought piezo elements worked in the 0-5V range, so is this a "scaled" number that corresponds to this 0-5 range? his whole "60 volts" thing is throwing me off)
1b) Where is the knock sensor threshold that will cause the ECU to pull timing?

2) What is the typical timing under WOT for a lightly boosted DOHC mod motor like this?

Does his PCM have a PID called 'KS_SPARK'? Not sure about the older Ford PCMs, but the newer stuff (at least Spanish Oak and newer) use this PID to show how much the PCM is retarding timing due to knock. To me, it's easier to figure out then trying to read the actual HZ the knock sensors are reporting.

I will say that my car reports ~2500-3500 HZ on the two knock sensors at the top of each car at WOT, and the PCM isn't pulling any timing. I've read elsewhere on the web that Ford sensors generally read real detonation at or above 5000 HZ.

26* of WOT timing on a boosted car is really, really aggressive. Is he sure that's what it's recording? That's a boatload of timing. Then again, with AFR in the 10s and low boost he may be able to get away with it. I've never tuned an older 4V like that, but in general if I were tuning that car, I'd probably shoot for AFR in the 11.5-11.7 range and 15-17* of WOT timing.

[phillysrt4]

No idea on the newer stuff, but older Ford Knock Sensors report FREQUENCY.

Ah... that makes sense too!

Does his PCM have a PID called 'KS_SPARK'? Not sure about the older Ford PCMs, but the newer stuff (at least Spanish Oak and newer) use this PID to show how much the PCM is retarding timing due to knock. To me, it's easier to figure out then trying to read the actual HZ the knock sensors are reporting.

I'll have to ask him about that PID and I'll let you know. I see why having that would be easier to read and makes tuning less... painful...


I will say that my car reports ~2500-3500 HZ on the two knock sensors at the top of each car at WOT, and the PCM isn't pulling any timing. I've read elsewhere on the web that Ford sensors generally read real detonation at or above 5000 HZ.

26* of WOT timing on a boosted car is really, really aggressive. Is he sure that's what it's recording? That's a boatload of timing. Then again, with AFR in the 10s and low boost he may be able to get away with it. I've never tuned an older 4V like that, but in general if I were tuning that car, I'd probably shoot for AFR in the 11.5-11.7 range and 15-17* of WOT timing.

Thanks for the feedback. I'll let him know the general findings of what everyone here has had to say.

Thanks everyone!

[phillysrt4]

Oh btw- for the knock sensor he claims it has a "V" after the number (e.g. 60V) but Hz or mV makes more sense to me

[Cam99]

Tell him to get a LSX. Problem solved

[Mitternacht]

Tell him to get a LSX. Problem solved

No, that's the easy way out. LSX is for quitters.

[phillysrt4]

^^^ x2 :)