How hot should an ESC get?

[littlened]

At the weekend I did six laps of a race when my car cut out, then it started going again, then it cut out.

I felt the heat sink on the LRP SPX speed controller, and it was too hot to touch.

I was running a 7.5t novak brushless motor and gearing 78/22.

I noticed when it was working, it felts really slow out of the corners, but built up a decent speed on the straight.

I guessed the 22 pinion was too much, so I've dropped it down to 20, had a mess about in the house, and the ESC is still far to hot to touch, and I can't hold the back of my finger on the motor for more than 2 seconds.

Last time I ran the car it was running a 9.5t brushless motor with a 20t pinion, and it ran fine.

Is the 20t pinion still too much?

I have a little chart I found on the internet that I printed out, and it says to start off with a 23t pinion with a 7.5t motor, so I'm confused as to what I might be doing wrong.