I've been reading around alot and there are alot of ratio's out there, and they've got me all mixed up. If you could clear any of them up for me, or even add some to them for examples it would be greatly appreciated.

There Compression Ration (cr) - ex. 10.0:1 Amount of air to fuel in engine?

Pistons - 9:1 The 9 is the distance from the rod up to the top of the piston? So a 12:1 would mean higher cr, primarily for all motor use where higher compression is necessary for hp gains?

With all motor set-ups you generally go with higher cr (13:1) and longer rods (12:1) I assume. This would reduce the size of the engine keeping everything else constant?

But with forced induction, generally a lower cr (ex. 8:1) and smaller rods (ex. 9:1) are best?

I feel like my examples of the ratio's are way of. But if you could fix what I'm trying to say.

 

Done some looking around at howstuffworks.com and here on this forumn, but not much luck.

ratio's in regards to compression in enigne = air to fuel ratio?
12:1 (best suited for all motor) pistons will have a much higher cr than a 9:1 (more suited for turbo set-up)?

 

C'mon guys, I know there's tons of you out there that know all about this stuff.

 

ttt

 

Um, actually everything you wrote is wrong..

Air to Fuel ratio is the ratio fo air molecules to gasoline(octane) molecules. 14.7:1 is the stoichiometric ratio, which means at that ratio(14.7 air molecules for every octane molecule) there is exactly enough air molecules to completely burn all the octane molecules.

Compression ratio is the ratio of the cylinder volume when the piston is at bottom dead center divided by the cylinder volume when the piston is at top dead center.

The shape/height of the piston, as well as the combustion chamber in the head are the main factors determining the compression ratio, in a stock engine. There are other things, too, that can be done, like milling the block shorter, milling the head, changing the head gasket thickness, etc..

 

I'm surprised you didn't find it in howstuffworks.com but I'm too lazy to look it up for you.

The compression ratio is strictly geometric. It's a ratio of the entire volume of the cylinder when the piston is at the bottom, vs. when the piston is at the top. This includes the irregularly shaped volume inside the head. CR is the ratio between the two volumes, displacement is the difference between them.

CR = (volume at bottom of stroke) / (volume at top of stroke)

displacement = (volume at bottom of stroke) - (volume at top of stroke)

The ratio of air to fuel is called (are you ready?) the AIR:FUEL ratio. If there's exactly enough air to burn all the fuel, it takes 15.1 pounds of air to burn each pound of gasoline. So a ratio of 15.1 : 1 is called stoichiometric.

BTW - I saw Haggar's post after posting this; but A/F ratio is mass ratio, not number of molecules. Some fuels have different stoichiometry, so for example ethanol wants a ratio of 9.0:1. So maybe some kind of ethanol-blend might want 14.7:1

Rod-to-stroke ratio has more to do with the geometry of the crank & rod. If the stroke is, say 80mm and the connecting rod is, say 100mm; then the R:S ratio is 1.25. So if you wanted to make a large R:S ratio, you just need a really tall crankcase.

Forced-induction engines use a lower compression ratio because the compressor (supercharger or turbocharger) is already doing part of the compression. That doesn't require a different rod-stroke ratio.

 

oops.. yeah, you're right.. thats what I get for posting when I'm sleepy..

Anyway, read up here:

http://www.autoshop101.com/

Specifically read: http://www.autoshop101.com/forms/h55.pdf for learning about air-fuel ratio..

 

Ahh good reading. I'm thinking I'll be spending some time on this site reading. Really appreciate it guys.