29-05-2018, 08:43 AM
I dunno if there are any out there curious as to why upward pistons exert so much more force than downward. Some books mention mysterious “angularity of the crank“ with little elaboration. I have explained to other car club members in the past...
Imagine an engine 4 inch stroke (2 inch crank throw) with a conrod just 2 1/8 long. From 90 to 270 deg the piston would barely move but from 270 through 360 to 90 the piston would move near the whole 4 inches up and down. Clearly the acceleration/deceleration at the top half rev is much greater than at the bottom half rev. In the case of the Seven with less extreme conrods the upward force is about 2/3 increased over the downward and as the latter is several hundred times the weight of the parts, increasing with the square of revs, the difference of two up piston assemblies together cf the two down together is very considerable. With the throttle open, on the power stroke the several hundred psi acting on the piston opposes. Although the noise level is much greater, crank at speed on the level may be less stressed than at the same speed downhill with closed throttle.
The topic recalls the exquisite sounds of motor racing from my boyhood. Some of the cars here were prewar GP relics (later sold for millions) with modest brakes and the sound of the double declutch down changes to slow for corners was very much part of the scene. I used to delight in emulating in my Seven until I learned better.... and fitted semi Girling brakes.
Imagine an engine 4 inch stroke (2 inch crank throw) with a conrod just 2 1/8 long. From 90 to 270 deg the piston would barely move but from 270 through 360 to 90 the piston would move near the whole 4 inches up and down. Clearly the acceleration/deceleration at the top half rev is much greater than at the bottom half rev. In the case of the Seven with less extreme conrods the upward force is about 2/3 increased over the downward and as the latter is several hundred times the weight of the parts, increasing with the square of revs, the difference of two up piston assemblies together cf the two down together is very considerable. With the throttle open, on the power stroke the several hundred psi acting on the piston opposes. Although the noise level is much greater, crank at speed on the level may be less stressed than at the same speed downhill with closed throttle.
The topic recalls the exquisite sounds of motor racing from my boyhood. Some of the cars here were prewar GP relics (later sold for millions) with modest brakes and the sound of the double declutch down changes to slow for corners was very much part of the scene. I used to delight in emulating in my Seven until I learned better.... and fitted semi Girling brakes.