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Old 03-27-2012, 12:27 PM   #26
monkecmonkedo
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Quote:
Originally Posted by Hen View Post
Referring to the start of the thread and the varying AMM signal, is there any way to check that it really is intake pulses you are seeing? And not interference from ignition leads, etc?

It seems impressive that the intake pulse can still be measured after passing out past the throttle, through the intercooler and most importantly out through your turbo (especially on the WOT run where the turbo would be spinning at full speed).
If you look at the WOT data in my second post, you can see the amm signal is a gradual up and down, not a square wave like the ignition pulses. That pretty much rules out ignition nose as a potential source.

I think of the amm signal this way: the turbo is being throttled via the Watergate to provide a constant 15 psi. The intake plumbing is full of compressed air until a valve opens, pulling half of a litter of compressed air in. this results in a pressure drop at the intake valve that travels back through the plumbing until it reaches the turbo, which closes the waste gate, generating more pressurized air to make up for what was drawn into the cylinder. The air for this travels through the amm, which is what I'm measuring. Basically, the airflow through the intake it's a series of pulses, not a continuous smooth flow. It would be interesting to have a pulse dampener in line, which would make the ammsignal less spiky our to do this electronically.
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Old 03-27-2012, 03:17 PM   #27
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Originally Posted by davidjmonk View Post
If you look at the WOT data in my second post, you can see the amm signal is a gradual up and down, not a square wave like the ignition pulses. That pretty much rules out ignition nose as a potential source.
Interference is resonant, it can show up as a sinusoidal wave on a signal line. Not saying you have any interference issues, but just because a signal doesn't exactly match a wave shape doesn't rule interference from that source.

That said, I'd imagine there's some sort of buffer either in hardware or software that dampens the MAF input. Probably some sort of derivative from the same buffering method on ECU's that used vane air meters.
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Old 03-28-2012, 06:32 PM   #28
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I understand that the intake pulses will "flow" through the system but I would find it surprising if a turbo can accelerate up and down, or a wastegate can open and close noticably at 30Hz (approx 3 cycles in 0.1s from the original idling datalog). My gut feeling is that by the time the intake pulses had passed all the way out to the AMM they would be smoothed out and was just wondering if there could be another explanation for the signal variation which matches RPM.

EDIT - Sorry just re-read and I apologise if I come across negative. I think these sort of measurements are really interesting and seen all too rarely.
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Old 03-28-2012, 07:58 PM   #29
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No worries. I think it is tough to think about how things work at the speeds our engines are running. That is what makes these measurements so interesting. I agree reading the above posts that it could be electronic noise, but my gut feel is this is really how it works.

The turbo doesn't need to spool up and down at this speed, nor does the wastegate. The wastegate actuator actually dampens the pulses, so the wastegate is actuated on the average manifold pressure. Since I've got a MBC, that is another source of dampening/averaging. So the turbo is sitting at a relatively constant speed, fluctuating slightly based on the rpms and wastegate flow. All the turbo wants to do is flow air to maintain a pressure dictated by the MBC. When the valves are closed, it is at that pressure which in effect dead heads the airflow. When a valve opens, the air needed to fill the cylinder drops the pressure, and a little air flows in, which the AMM measures. Think about it like pulsing your finger on and off a garden hose end. Water only flows when your finger is off the hose.

But maybe I'm thinking about this all wrong. BTW, I'm working to remake my amm cables using all shielded wiring with the shields grounded on both ends. That should help rule out electronic noise and is a good practice anyway.
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Old 03-28-2012, 08:14 PM   #30
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amm needs to be able to detect reverse flow.....

a voltage is fine , but which direction is it ?

cool datalogging tho ... do the intakes pulses match the opening/closing events of the valves ?
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Old 03-29-2012, 11:08 AM   #31
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Originally Posted by davidjmonk View Post
But maybe I'm thinking about this all wrong. BTW, I'm working to remake my amm cables using all shielded wiring with the shields grounded on both ends. That should help rule out electronic noise and is a good practice anyway.
Thinking out loud here, could you rule out noise by datalogging a piece of wire run round the engine bay but not connected to anything, or perhaps to a steady voltage supply (12v through a resistor), and then look for the same spikes?
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Old 04-03-2012, 04:26 PM   #32
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Just turn off ignition for couple seconds, if amm signal is different, then there is noise.
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