1937 Mercedes-Benz M125 5.7-litre eight
1937 Mercedes-Benz M125 5.7-litre eight: The F-series engine officially known as the M125. An in-line eight, Daimler-Benz cylinder construction of welded sheet-steel water jackets around forged-steel cylinders was used, built as 2 4-cylinder blocks with integral heads and bolted to an aluminum crankcase, 4 valves per cylinder equally spread at an included angle of 70 degrees, pivoted fingers were interposed between cam-lobe and valve stem.
A 1-piece crankshaft with I-section con-rods that had split big ends carrying 2-rows of 8mm diameter rollers by 10mm long. Split Main bearings with 10mm diameter rollers varying in length for each main bearing from 18mm downwards.
The engine was built first with 5 main bearings and also with 9 main bearings. 9 mains were ultimately standardized after a failure in early testing.
In at least 1 engine, the rod journals closest to the clutch were made with bigger diameter. In 1936 a Hirth build-up crank was also tested for 12 hours on dyno and for 250 miles on Autobahn before 1 of its joining bolts broke.
Hirth supplied 6 complete crank and rods assemblies to suit the 9-main-bearing version but these were never seriously committed too because they were 13 per-cent weaker in torsion.
Mahle aluminum pistons. Twin up-draught carburetors were pressured by a roots-type supercharger with rotors 106mm diameter and 240mm long. Mounted vertically at front.
The M125 weighed 223kg. With the manifold fed from the front end , one problem consistently troubled the M125, the 2 rear spark-plugs in the 2 rear cylinders always ran oily or wet, they had to be replaced with a hotter plug, the Bosch 420, while the others cylinders used a colder 450 plug.
The M125 set new highs for horsepower output, on the normal fuel blend used for racing the M125 engines leaving the 2 sheerer test benches were producing between 550 and 575BHP towards the end of 1937.
Stroke/bore ratio 1.09:1.
Compression ratio 8.9:1.
Con-rod length 167mm.
Rod/crank radius ratio 3.3:1.
Main journal 63mm.
Rod journal 66mm.
Inlet valve 39mm.
Exhaust valve 39mm.
Inlet opens 15 degrees BTDC.
Inlet closes 42 degrees ABDC.
Exhaust opens 30 degrees BBDC.
Exhaust closes 3 degrees ATDC.
Inlet pressure 1.86Atm.
Peak power 575BHP@5500rpm.
Piston speed corrected 17.7m/s.
Peak torque 969Nm@2000RPM.
Peak bmep 304psi.
BHP per litre 103.4BHP per litre.
Engine weight per BHP 0.39kg per BHP.
...not because they are easy, but because they are hard!