6500rpm Flat spot
 

could this be the VDI? or maybe this is normal because i have the stock exhaust manifold and cats? the 6pi work because i can feel the escalating torque from about 3500 upwards but when i hit 6.5K it feels like something's wrong.

help please.
Thanks

Assuming you have a series 5 car (89-91), the VDI should activate around 5800 RPM.

Find the actuator under the upper intake manifold on the driver's side (with the engine off), and pull down on the rod coming out of it to see if is stuck or not.

You could have someone watch the actuator while you rev. the engine up in neutral to see if it opens too.

My VDI did not work when I got my car, and the problem turned out to be the contacts on the solenoid valve were corroded. There are two solenoids with brown and white dots & connectors (driver's side of engine near the alternator). These control the VDI and 6PI on the series 5 cars by allowing air from the air pump to pass to the actuators. I suppose you could have a vacuum leak too.

All I had to do was to clean the contacts to get the VDI to work. You should be able to feel the acceleration increase in first and second gear as you pass 6k RPMs if the VDI is working. The exhaust manifold & cats should make no difference, because they are not part of the VDI actuation system.