Flatspot at 3500RPM

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New Member
Sydney, Australia
My V8 Soarer has always had a slight flatspot at 3500 with WOT but now I have the Map-ECU it's more pronounced. Logging the AFR with a wideband sensor showed it was too rich there (11.5:1) but to get the AFR closer to 12.1-12.5:1 I had to pull so much fuel it was almost the same value as 3000RPM.

Logic tells me that if I put in about as much fuel and air at 3500 as I do at 3000 then I'll roughly get the same power so I think the initally rich AFR is symptomatic of something else which is hindering the amount of air which can get into the cylinders at 3500.

I have Rush headers and an otherwise stock intake (apart from no AFM) so could it perhaps be some kind of pressure wave hindering the cylinders getting filled at those revs? Unfortunately I can't rev to 7000RPM yet to find out if it replicates itself there.... Otherwise something to do with the stock cams perhaps?

I got it dynoed on the weekend and there's a definite dip in the torque curve at 3500.

Ideas anyone?