A total of 30 firefighters were called to the fire at the Lath Lane scrap yard in Smethwick at about 11pm last night.
They broke through steel gates to the walled compound to find 47 cars, a van and a caravan well alight. No-one was at the scene.
Using water from the Birmingham Canal, they managed to get it under control in about an hour.
The crews from West Bromwich, Smethwick, Oldbury and Dudley eventually left the scene at 1.30am. They returned this morning to check on it.
Watch Commander Mark Sedgley said: "We believe the fire to be arson - it appears it was started in several places in yard."
"It was a fantastic effort by the crews to get the water from the canal, surround the severe fire and stop it so quickly.
"We prevented it spreading to the rest of the site, and any nearby businesses."
This morning, only charred shells of the vehicles remain. About 25 vehicles avoided damage.
In December, the scrap yard was targeted by arsonists who set alight 100 vehicles. The fire was so severe the flames could be seen from the M5.
On May 23, 70 cars were wrecked and again on June 3 six cars were destroyed.
The fires came during a spate of attacks on scrapyards around the Black Country.