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What causes boost to drop off in the top end?

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Hey, so just wondering the causes for boost to drop off in the upper end of the Rev range? Ive seen a few cars now (my R33 for example) where on the Dyno boost climbs quickly to the set PSI and sits nicely for a little bit but then drops off at higher revs, Even tho power is still climbing.

Is it mainly to do with turbo sizing? Too small/big or is it mainly from VE dropping off after peak torque? Any ways to combat this?

This is the usual situation with a stock turbo. Generally the sizing of the turbine wheel and housing are focussed on providing good bottom end performance. There are no free lunches though and in order to achieve fast spool up, the turbine generally becomes a restriction at high rpm. The result is the boost tends to drop off as the rpm climbs.

Put a larger turbo on the engine with a properly matched compressor/turbine that is more in line with your power aims and you will find that it will hold boost pressure much better in the top end.