How much power would you expect to loose from high ambient air temps?

Ok so just had a mates car on the dyno to play with the VVT but when we did some base runs we noticed the power was down quite bit from 380-390rwhp down to 315rwhp...

All sensors were working as they should, boost was spot on, afr spot on, base timing check... The ambient temp of the day was about 37 degrees C. When we last did some runs would have been closer to 20 degree.s

Intake air temp was getting to 50 degree's but there is no real aggressive amounts of timing being pulled... 1 degree's at that point...

He had changed tyres to a slightly wider tyre since last time but yea could not thing of why we have lost around 70rwhp... I know the ambient temp would not have helped but I would have only expected 20rwhp max?

On a rolling road tyre choice can make a massive difference, as can tyre pressure and strapping technique. Another thing to look at is the intake temps from the previous runs and also the AFR'S.

I have had a customer come in on my Dynapack for a tune up after some modifications and their initial run was near 30hhp down, it turned out he'd handed in the car with supermarket fuel when it was tuned on premium origionally