S4 N/A with LT10s - Odd High RPM Vacuum readings
 

Hey everyone,

I have an '86 GXL that I put an LT10s into, and finally was able to take it to be tuned on the dyno today. All went well, save for some really odd vacuum readings high in the RPM range. As you can see on the Dyno graph, the curve was nice and steady then suddenly went all over the place and the pressure readings from the microtech went up and down wildly:

Now, I've taken the BAC out and I'm drawing the microtech vacuum line straight from that location. Here is what I was thinking: As the pressure readings are out of whack, is it that this location is inefficient to draw vacuum from at high RPM? Where is a good place to attach the line to on an S4 N/A intake? Or is the S4 intake simply not efficient past "X" RPM? Or am I way off base with all of this?

Kind of an odd thing....
Any thoughts?

It looks like you may be experiencing noise. How is the system grounded? Is the CAS connected with the shielded cable that came with the Microtech harness and what condition is the connector?

Pulling vacuum from the dynamic chamber may not be the best place. Try taking it from the same place as the stock pressure sensor.

your having the same problem i had .... is tricky to tune it but no big deal

The system is grounded quite well - straight to the battery. I did use the shielded cable that came with the microtech and used a new connector. Would Noise make the pressure readings go wild?

Where is the stock Vacuun pickup point?


How did you fix it - just by tuning it or did you move the Vacuum pickup point?

Thanks for the help guys!

If you want to settle the signal down coming into the microtech.

Use a 1/4inch fuel filter, it will settle all spiking on the map sensor.

But Id say the microtech is picking up that at high rpm your inlet system has become a restriction.

I understand the FD has a small filter in front of their MAP sensor, so I was going to look into picking up one of those.

When you say that the inlet system has become a restriction - what do you mean by the "inlet system"? The nipple that the tube is plugged into, the actual filter that's sucking in air, or the whole intake?

Thanks!

the whole intake.

Basically the engine is pulling more air then your inlet manifold can supply and starts pulling vaccum again.

The 2nd gen also has a small pill in the boost sensor line, probably for this purpose as well.

The dynamic chamber is designed to exploit pressure waves from the opposing ports and intake reversion. Personally I would not pull vacuum from that point as it will be very turbulent (and run into about 2 PSI of "boost" at high RPMs).

As mentioned, a pill in the line will help greatly.

Sounds good - I'll add in the filter from an FD and move the Vac picckup to the stock location - that should clean things up.

Thanks!