Whats all the sparks from older F1 cars from?

May be...

Right idea, Matt.

In the days without refuelling, the cars' weight would change drastically during the race. To avoid damage to the underbody when the car was heavy with fuel at the start of the race, the teams employed magnesium strips (on the underside of the car between the rear wheels) which would abrade to leave the correct underbody profile once the car was light enough to run in optimum trim.

The sparks are simply from the magnesium strips being rubbed along the ground under acceleration, or when the car went over a bump in the track.

Coming out of the back of the car?

Methinks they are the bargeboards hitting the ground.
the same happened to the newer cars in Singapore due to the uneven track surface.

Yep, they are the magnesium floors of the car hitting the ground. Sparks were a common sight in F1 until 1994 when they introduced the plank that was placed under the car.

I remember seeing Mansell in his Williams Honda at Spa in 1987 sparking like mad!

from the magnesium underbelly of the car.. The strips are now made of wood..verzion online