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Compression Ratios... Posted by Walt [Email]  (#2707) [Profile/Gallery]
(more from Walt) on Thu, 12 Apr 2001 01:01:20
In Reply to: Good point, so what is the term for that? n/m, Jeff Cunningham, Wed, 11 Apr 2001 18:08:23 Members do not see ads below this line. - Help Keep This Site Online - Signup |
9:1 would be your mechanical compression ratio.
Simply stated, at BDC you have a fixed cylinder volume:
= volume of cylinder(bore x stroke) + volume of cyl head chamber
That volume gets compressed into another fixed volume (at TDC)
= volume of cyl head chamber
That ratio is expressed as the compression ratio. It doesn't change.
There are other considerations. Ideally, for more power, we want to optimize the amount of air and fuel in the cylinder before ignition. A normally aspirated engine typically may only be able to fill the chamber to 80 or 90 percent of it's capacity. With proper cam selection and intake/exhaust tract Mods, its possible to fill the cylinder a bit over 100 percent.
Add a turbo, you can double that! :)
A turbo's biggest power gains come from its ability to compress air and introduce more OXYGEN into the cylinder. More O2 means more fuel can be burned. A similar result can be obtained using nitrous oxide injection. NO2 adds O2 which can support the burning of more fuel...(which is added with the NO2). More air/fuel= more power!
There's also a TEMPERATURE consideration. Cooler air is more dense, and thus contains more O2 molecules. So the air/fuel content of a mixture at 19 lbs of boost isn't always the same, it changes with temperature. A mixture at 100 degrees is more dense (more air/fuel) than a mixture at 200 degrees.
That's why intercoolers work.
Timing is also important. Justin gives a good explanation below.
Octane ratings of fuels have more to do with burn RATE than anything else. A low octane fuel burns faster than a higher octane fuel. Thats why a low octane fuel will ping. It burns TOO fast and cylinder pressure peaks before TDC. It's also more easily ignited and more prone to detonation, that is, being ignited by something other than the spark plug.
Enough for now, gotta catch some shut eye. Looking foreward to more posts on this subject.....
Walt
'88 SPG
'88 base
'79 Buick Turbo
, Thu, 12 Apr 2001 01:01:20 <-- Viewing This Message
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