Understanding Intake Manifold Pressure Waveforms

OK, so I've started playing with my diy pressure transducer and ac coupling adapter. I'm posting this in the techniques forum since the purpose here isn't necessarily to fix this car, but to learn how to read intake manifold pressure waveforms.

The car is my very own trusty 1999 Nissan Almera ( Euro Sentra ) with the DOHC GA16DE engine and no VVT. As a reference I'm sticking in this running compression waveform image I pulled from a Pico tutorial on youtube:



Red trace is the injector, green trace is the ignition. One can see that the injector usually fires around the time the intake valve starts to open, which is always before TDC.

The engine pulls about 20.5 inHg ( -10.5 psi ) at idle. The purple trace is the #1 injector and the firing order is 1-3-4-2. Images taken with a -14.5 to 30 psi transducer which translates to 0.011125 psi per millivolt. The signal is AC coupled so I can get more detail. Captures are 20ms and 20mv per div.



Here the engine is idling at 714rpm. Peak to peak voltage is about 40mv, which translates to about 0.445 psi ( 0.9 inHg ) of variation in pressure.

The first thing that I noticed is that the injector fires about 34* before peak manifold pressure is reached. I happen to know that the injector fires around maybe 10*-12* BTDC on this engine, which should also be about the time the intake valve would start opening, and this corresponds to the PICO example above.

Yet manifold pressure is still increasing for 34* after the injector fires, and that would seem to indicate to me that IVO isn't occurring till about 24* ATDC, which of course makes no sense.

So I'm thoroughly confused regarding the relationship between the injector firing and valve opening on the one hand, and their relationship to peak and trough manifold pressure. I assume the injector firing and IVO happen close together, and I would think that manifold pressure should be decreasing at that point, not increasing.

The next thing that stood out to me is the seeming bounce in manifold pressure I see at each peak, circled in red. I'll come back to this later.

This next capture is during a throttle snap, at about 3150 rpm.



The main thing that stands out here is the consistently lower peak pressure on what I assume to be the #2 cylinder.

This last picture is holding the rpms steady at about 1500.



You can see here more pronounced that "bounce" that I circled in red in the first capture. It's just lower down the declining slope here. You can also see what looks like a pressure bounce at the troughs. And of course one can clearly see again what looks like lower peak pressure on cylinder #2. I would think they should all be uniform, on the other hand we're only talking about a difference of my 0.13 psi, so I don't know how significant that really is.

Anyways, I'd love to have peoples comments on this.

Right after I posted this I ran across this super long thread on IATN about vaccum waveform analysis:

members.iatn.net/forums/search/search.as...rum12&ft=1&fv=4&qv=1

I haven't read it yet, but thought others might find it interesting.

When you take intake manifold pressure readings you are looking for vacuum pulls. Focus on the valleys of the waveform and see that they are uniform. As for the Pico
waveform the injector is firing during the exhaust stroke as the in cylinder pressure waveform indicates. The intake valve will open and pull in the mixture as the piston travels to BDC.
When you sync injector to the intake vacuum waveform you have to offset it by two pulses to the right for four cylinder engines. That will be your number one cylinder.

They always say a picture is worth a thousands words.

CH1 300 PSI Transducer in cylinder #1
CH3 Primary voltage at the dizzy coil
CH6 -14.5-30 PSI Transducer AC coupled at the intake manifold
CH7 Injector voltage cylinder #1



I'm assuming the shape of the intake manifold waveform is altered from normal due to #1 cylinder being disabled. The primary ignition events are about 6-9* BTDC which means their trailing edges are only maybe 2-3* prior to being 180 markers.

Still have allot of reading and thinking to do here.

love to see that,is it subscription only

Removed Link.

3000+ People should have it by now, we can always share again!

Unbelievable but true: It´s almost 20 years that Tom Clozza started throwing this out on IATN. Hats off to him!

And no, back then it was not "by subscription". Thank you Solera

superb many thanks

This really is a great article,using it ive realised,i have valve train issues

You do have to remember latency on the cheaper pressure transducers. I check using a timing light.

So you may be closer for injector firing to tdc than you thought.