Why has my STi II got a serious flat spot around 5500-6000rpm? This is especially obvious in forth gear, the car literally slows acceleration and then goes again.

The results of my rolling road session can be seen below.

 

Ian,

At a guess, I'd say the car started to det', and the Jecs ECU retarded the ignition.

This is more than likely due to excessive heat build up on the rollers. Could you feel the "dip", or hesitation before the dyno, or was it more apparent after. If after, try an ECU re-set.

Mark.

 

Mark,

This has been happening both before and after the rolling road session. The graph just proves it.

The rolling road session was at Powerstation, and credit to them they had a 'blower' straight over the intercooler and someone constantly spraying water onto the intercooler. So why would the car be detting, and is there anything I can do about it?

Ian.

 

Ian,

It certainly sounds "load" related, and there are several things that can induce this.

If you have a boost gauge, does it only happen at a certain/max boost, or is it purely "rev" related.

Check the plugs, and coil packs, it's possible that the ignition is breaking down under load.

Also check the solenoid isn't contaminated with oil. This is a common problem. It's normally associated with overboost, but having looked at you graph, there is a slight dip in the boost curve, and may be the cause.

IMO, I'd put my money on det', or a dodgy knock sensor that's fooling the ECU into retarding the ignition.

If you are local to Powerstation (or an alternative RR), ask them to run the car on the RR, using a pair of "det' cans". This will at least comfirm, or eliminate det' as a problem.

Mark.

 

Mark,

Thanks, I will take the car down to Powerstation again and ask to to check for det'ing.

Ian.