My Microtech takes its boost reading from a nipple on the rear part of the UIM just before the LIM. At the same vertical level, my Profec-B Spec II takes its reading from another nipple above the front rotor. For some reason, the vacuum reading on the Profec-B has been different from the Microtech. The Microtech reading is a solid 3 inches higher. They used to read the same. Is it possible that one rotor is pulling more vacuum than the other causing this problem or is it likely that I have a leak in the vacuum line going to the Profec B? The car idles smoothly, has no problems with the AC on, and pulls hard.

Thanks.

 

very possible one rotor is stronger...especially on rebuilt engines.

maybe one OMP nozzle doesn't lube as well as the other causing one rotor to wear a little faster...this is just one example.

 

Well, this condition just started about 40 miles ago. The motor has about 15k miles on it. I premix 1 oz to 1 gallon, and my OMP is still functioning.

 

I seriously doubt that there can be that much variation from one part of the intake manifold to another, no matter what the condition of the engine. I'd suspect one of the vacuum lines is blocked, leaking, etc., or one of the calibrations has changed.

 

I'm not sure what nipple you're using on the front rotor, but two things spring to mind if the nipple is actually reading direct from within the housing:
- the combustion volume has changed since sweeping past the intake, thus compression has begun
- the velocity of the air over the nipple is different, and thus affects the vacuum reading.

The vertical level is mostly irrelevant when working air (important with liquids though).

Another question: is one of the nipples before where the throttle passages rejoin? Do both gauges read the same under boost?

Dave

 

well, put the same gauge on the rear UIM runner...see what it reads, and then put it on the front UIM runner. Should solve that mystery.

I'm talking about the nipples closest to the LIM.

 

Ahh, ignore me. "above the front rotor" I didn't realize he meant still on the UIM.

I agree with DaveW, they should be identical, all of those nipples at the bend in the UIM.

Dave

 

Try switching them to determine what's going on.