Help me understand my MAP readings

[loren]

I've made some observations recently that I don't quite understand.

First, a question... I'm pulling my MAP input from the vacuum source on my throttle body, which comes from just past the throttle plate in the intake tract. Is this an accurate and true vacuum source, or should I drill a MAP source directly into the intake manifold, closer to the head? The reason I ask is that I can't get the engine to idle with less than 60-65 kPa, and it doesn't even dip below about 40 kPa in a lift-throttle situation. I do have a somewhat lumpy cam, but it just doesn't seem quite right.

Now, what's happening that I can't make sense of:

Acceleration at full-throttle gives ~100 kPa, which should read the appropriate 100 kPa for whatever the RPM is. Fair enough. The car happens to run like crap in this situation, as if it's starved for fuel.

The weird part is that if I give it about 3/4 throttle (which is actually 60% by my datalog), I see ~100kPa... the difference is that in this situation, the car pulls nicely.

My seat-of-the-pants (sorry, the WBO2 is still inop) concludes from these two observations that the car is getting more air at full throttle than at 3/4 throttle, and thus needs more fuel.

The part that I don't understand is that if the kPa is the same at 3/4 throttle and full throttle... how can the MS know to give more fuel at full throttle?

[Mike_Robert]

If you are obtaining your signal from a "ported" vac source you will see very odd behavior similar to what you are observing. These were typically used for vac advance on distributors and emissions control purposes and are not suitable for use as a MAP signal to an MS. They were typically situated where they would be on the atmospheric side of the throttle plate when the plate is closed but on the engine side when more than very slightly opened. There's not enough info in your description to tell exactly where the signal source is but it would likely be better to obtain signal further away from the plate.

-Mike

[loren]

If you are obtaining your signal from a "ported" vac source you will see very odd behavior similar to what you are observing. These were typically used for vac advance on distributors and emissions control purposes and are not suitable for use as a MAP signal to an MS. They were typically situated where they would be on the atmospheric side of the throttle plate when the plate is closed but on the engine side when more than very slightly opened. There's not enough info in your description to tell exactly where the signal source is but it would likely be better to obtain signal further away from the plate.

It's definitely on the engine side of the throttle plate, but "just". Sounds like I need to stick a pipe in my manifold and be happy.

I needed something to do tomorrow.

[loren]

Well, fuuuu...

I just found a photo I took of the bottom side of my TBI. The vacuum port is on the correct side of the throttle plate, but the throttle plate opens INTO it, so that once the throttle is more than a little open, it is then effectively a "port" vacuum source.

I'll fix it tomorrow. Will be interesting to see what effect this has. As mind boggling as some of this stuff is, I do enjoy learning from my mistakes.

Thanks for the tip, Mike!

[Mike_Robert]

Good stuff. Learning is cool!

-Mike, the 49 year old freshman....

[loren]

Update: Pulling vacuum from the manifold instead of the TB did help some, but didn't make as much of a change in vacuum as I'd hoped. MAP at idle is still in the 55-60 range, which is better than it was. But, the resolution of the MAP signal is much better. It's not as "on/off" as it was, and I've been able to tune the idle and drivability characteristics MUCH better. Even without the wideband, I've found a tune that's very drivable and seems to be making at least most of its potential power.

So, life is good. Lesson learned: use a vacuum port from the manifold if at all possible.

Gonna try to fix the wideband this weekend.