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Hi Nick,

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THE DIP? Well, the way I see it is that when your first crack the throttle, there is a sudden inrush of air to fill a certain amount of area due to the low pressure in the intake. This area includes the intake tracts and the plenum. Once this area gets filled up very quickly and has to slow down, the dip occurs. Quite often we see a ringing after the first peak up until the second peak happens. I think of it as sound waves bouncing back and forth until the air stabilizes. The second peak is what the engine is actually inhaling. Certain engines don't even have a second peak. Of coarse this is snapping the throttle as quickly as possible without raising engine rpm and doing it at the throttle body.

Your capture is interesting. Normally the first peak is the highest. What engine is this?

Something you might be interested in when doing those snap throttle tests on analog MAF's is using the single trigger feature. Also, no need to include TPS in this test. That is just wasted hook up time. Single trigger off the MAF signal with a little pretrigger built in. I like doing it this way so I don't have to be on my toes ready to hit the space bar or stop button.

Yes, I have captures of contaminated MAF's, but none with restricted exhaust. Only scan data of it.

Hi Nick,

You bring up some very valid points and good observations.

Up to now I've always conducted the TPS/MAF signal comparison by sharply snapping the throttle, as you say, it makes sense to think then that the first peak would be the highest because you are filling/maxing the inlet so how can you beat that?  Having said this the captures i posted does seem to prove this wrong.

The trend I see is directly related to throttle response. As I said previously, some engines don’t have a second peak. I attribute this due to lack of throttle response. The RPM will barely change. Other engines have a quicker throttle response and no matter how fast we try to snap the throttle, the RPM picks up. I think you have a well oiled responsive engine there.

Looking on the flip-side, once the intake has filled and starts to pump for real, would it not make sense that at higher RPM the intake airflow would be higher quite simply because its pumping more within a given time?

Not usually. With most modern engines the fill is more predominate. Which ever happens first is really not the issue. The important thing is to document what is normal and then we can make a judgment call.

Do you think holding the throttle for longer holds any diagnositc value?

No I don’t. You captured what you needed on the first snap for reference.

I suppose if you combined RPMs with MAF would offer a better picture. Do this enough times prehaps and you'd get a feel of what to expect, and also i suppose may be easier to spot plugged exhausts.

I think the degree of restriction comes into to play here with a snap throttle test. If the restriction isn’t bad enough to increase manifold pressure at idle, I don’t believe it would show up. That response test is only happening within a half second. Now if we factor in RPM due to the quick response, you have a very good point. Can this engine exhaust properly? The key is having a known good capture or scan data under the same conditions.

Very good discussion Nick,