Need help understanding scan data

[Aurora40]

Yes, but MAP is the pressure in the manifold. Why would it increase with VE? I'd think if anything, it would be the worst where the VE is highest, since that's where the intake system would be most taxed.


I do have sort of a side question though... Is total airflow highest at the torque peak? I realize each cylinder is filling the most completely, but that event is happening less frequently than at higher RPMs. If I were to guess, I'd think the total airflow would be highest at the power peak, but I'm just guessing.


Edit: Also, Dominic, that's a good idea about looking at the MAP with the engine off. It's funny, but that never occurred to me. I always tried to look at it at WOT at a low RPM, figuring that the intake would be pretty close to atmospheric there. But now that you say that, it seems so obvious.

[XfireZ51]

Originally Posted by Aurora40

Yes, but MAP is the pressure in the manifold. Why would it increase with VE? I'd think if anything, it would be the worst where the VE is highest, since that's where the intake system would be most taxed.


I do have sort of a side question though... Is total airflow highest at the torque peak? I realize each cylinder is filling the most completely, but that event is happening less frequently than at higher RPMs. If I were to guess, I'd think the total airflow would be highest at the power peak, but I'm just guessing.


Edit: Also, Dominic, that's a good idea about looking at the MAP with the engine off. It's funny, but that never occurred to me. I always tried to look at it at WOT at a low RPM, figuring that the intake would be pretty close to atmospheric there. But now that you say that, it seems so obvious.

Bob,

Remember what's being described by MAP. Its the differential pressure in the manifold v atmosphere. So higher MAP means you are getting closer to being equal to no difference between the manifold pressure and atmosphere. Anything less than that means there is something inhibiting that airflow. VE dropping after torque peak is easier to see on OHV motors. In tuning my 84, the VE number would drop fairly rapidly from torque peak(3800rpm) to power peak (4800rpm). I just did a datalog session yesterday at the track and MAP was 99.3kPa from 4900 to 5800rpm then dropped to 97.9 kPa up to 7200rpm where 100kPa was 14.6psi at Key ON. As for VE, don't forget that at higher rpms, things are happening very rapidly. Dropping VE says you're not filling those cylinders simply because air has inertia and only so much gets in within a rapidly shrinking window of time. Gotta love those DOHC motors.

[Aurora40]

Right, doesn't the MAP just indicate how well the air filter and TB are working to keep air going into the motor? Maybe we're saying the same thing and I'm misunderstanding you, or maybe I've got it wrong (again)?

If you are seeing the MAP pressure drop above 5800 rpm, doesn't that mean your intake system (TB, air filter, duct, etc) is not able to keep up with the consumption rate of the motor? Ideally wouldn't it be the case that the MAP would not drop as RPMs increase?

It sounds like what you describe is what I see. As RPM increases, the MAP reading drops fairly linearly. But VE doesn't drop in the same way. At 1,500 rpm, the engine has worse VE than at 4,500, correct? At 1,500 rpm at WOT, the MAP reading will definitely be higher or as high as at 4,500.

Also, the MAP is not differential, it is an absolute pressure measurement, isn't it? For example, the sensor can differentiate between a day with high ambient pressure vs a day with low ambient pressure, can't it?

Sorry for all the questions, I'm just trying to understand.

[tpepmeie]

Bob,
you understand this better than you give yourself credit for. Ideally, there would be no pressure loss through the throttle and intake at WOT. That is rarely the case, but usually the pressure drop is only slight (say, 1 kpa). If the Key on Engine off MAP reading is 99.6, then you might see 98.6 or so at WOT, and it will get worse with increasing RPM. Whoever suggested the OP to check for collapsing air duct may just be on to something.