Re: Compression ratio vs horspower

Posted by john d on June 19, 2001 at 17:01:08 from (205.188.199.49):

In Reply to: Compression ratio vs horspower posted by Dave t on June 19, 2001 at 15:55:15:

Generally, as compression ratio goes up, horsepower will go up.

Power is how much work the engine can do in a given amount of time. Horsepower is (loosely) based on what a horse (not just any horse...) could do. Power is determined by how much twisting effort (torque) an engine can produce at a given speed, measured in revolutions per minute, or rpm.

As speed (rpms) goes up, horsepower generally does also, until the engine gets to the point where internal friction begins to become a problem, or (more likely) until the engine simply can't breathe efficiently. The torque of an internal combustion engine will increase until the breathing begins to be less efficient, then torque will decrease. Horsepower may continue to increase if the rpms increase faster than the torque decreases.....

Now compression ratio is a part of this picture, but indirectly. The compression ratio is the ratio of how much volume is above the piston when it's at the lowest point of travel in the cylinder, compared with the volume above the piston at the top of the stroke, or when it's at the top of the cylinder. Two factors are major players in this: the length of the stroke, and the volume of space in the cylinder head. Increasing compression ratio will generally result in a torque increase.

The old Farmall H has a pretty low compression ratio by modern standards because there is a lot of volume (space) in the cylinder head to squish the air fuel mixture into when it's compressed. The old Farmalls that came from the factory with heads for burning distillate or kerosene have even lower ratios. The swather, with the same bore and stroke, but more compression should (everything else being equal) kick out quite a bit more power than the H.

Increasing compression ratio increases how tightly the air fuel mixture is squeezed before burning it, and results in a big increase in power produced. Going much over the 8:1 ratio will generally mean having to run a higher octane grade of gasoline to avoid detonation, or uncontrolled burning of the air-fuel mixture. Cylinder head design plays a part in controlling this, as does ignition timing.

Follow Ups:

• Re: Re: Compression ratio vs horspower Dave T 17:17:20 06/19/01 (0)

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