I was having a cooling issues during track days. The coolant temperature was going high and the original fans were unable to help while maintaining race speed. When I lower the speed the fans were able to cool the engine temperature.
I thought that may be the problem is that the fans actually stops the natural airflow on higher speeds.
So I removed them completely (the whole frame with fans) and exposed the radiator on direct air. I put a single slim fan ~10inches / 80W behind the radiator connected to "suck" air.
The situation got worst. When the car is standing still and engine running idle the new fan were unable to maintain the temperature. Also when racing on slow tracks there were massive overheating due to low speed and high engine load. Only on higher speed straights there were positive cooling effect.

I experimented with some obstacles around the radiator to help directing the front air only through radiator. This helped a bit, but not satisfying.

One more thing to mention - my radiator is not original (two years ago it got broken). Currently it's alloy cheap brand made in China.

So I need some advises.
Should I go for another more powerful slim fan from behind? Should I look for original radiator? Should I revert to original setup... And what more I can do cool the engine

Thanks,
Georgi

Yes, engine has been recently built and the system has been flushed.
Thermostat also is working, it's been changed also.
May be you are correct I should change my chinese radiator to Nissens one. I'll look for local dealer.

After rebuild I also have additional problem, small leaking of oil into the the coolant. So usually on the morning I open the radiator cap to see whats going on, and there is no any pressure.

Unfortunately it is not the heat exchanger. I have swapped it because I hoped that it is the reason, but this didn't help. I think the reason are the head bolts - last time I didnt ordered new. Old ones looked fine and shorter than 112mm. Now I have ordered new bolts and gasket again
However this is separate story - I don't think that oil leaking has effect on cooling.