[rt]

Anyone care to elaborate? :thumbsup: Just another topic I thought would be good...someoneelse gets to start it though.

 

[CoalShedRacing]

ummm, I'll give it a shot 8)

You need to know 6 things to figure static compression 1. bore 2. stroke 3. deck height 4. combustion chamber volume 5. piston dome volume 6. compressed head gasket thickness

once you know these 6 things, you can figure what the static compression ratio of a motor is going to be, this assumes the piston is at TDC and all the valves are closed.

Dynamic compression is what the engine really sees while its running, the biggest factor that controls dynamic compression is the different profiles of different cams. All things being equal, a stage 2 hotcam equipped motor will have a higher dynamic compression than the same motor running a megacycle x4 cam. Reason is the x4 has much more overlap(the time period, expressed in degrees, that both the intake AND the exhaust vales are open).

I think that covers the basics of it. :crazy:

 

[rt]

thats good stuff! I need to catchup on my calculus for the rest... right? :lol: :3question: :help: :thumbsup:

 

[rt]

does barometric pressure, air temperature, or humidity have any effect?
Example: Riding at 8000ft elevation, isn't the actual air pressure being lower going to make your actual compression lower?

 

[dungbeetle06]

Static compression is when you fart while in bed aka "THE COVERED WAGON" OR "THE DUTCH OVEN".........Dynamic compression is when you get "THE WALKING FARTS" or when you blow one off while in a co-workers office.

 

[rt]

Genius! You really have a way to break it down so we all understand it. I couldn't of done a better job myself! :lol: :lol: :lol:

 

[dungbeetle06]

NOT A GENIUS, BUT I AM A FART SMELLER....I MEAN A SMART FELLER

 

[rt]

Other ideas.....other than the "the dutch oven".... anyone?

 

[CoalShedRacing]

yes, barometric pressure will affect what dynamic compression will be. The key fact to remember is that the limit of dynamic compression that can run on 93 octane is roughly 8:1

 

[rt]

Explain the 8:1 ratio...how'd you go that low? :3question:

 

[CoalShedRacing]

not 8:1 static, but 8:1 dynamic. The static compression ratio assumes the piston is at BDC and then rises to TDC without any of the valves opening at any point thus compressing the entire volume of air, the reality of a running engine (dynamic ratio) is that there is only roughly 65 degrees(slightly different for the different cams) of crankshaft rotation where all the valves are closed; so much of the static compression is bleed off by the valves staying open and letting air escape past them, thus the true dynamic compression is much, much lower than the static value.

 

[rt]

I'm following ya...barely, very interesting though.

 

[DrivenWS6]

Static compression is the area of the cumbustion chamber at BDC compared to it at TDC. But static compression ratio is taking for granted you have 100 volumetric efficiency. As if you you filled that chamber up 100% everytime then that would be your 10:1 ratio or w/e. But as far as dynamic goes your not going to have 100% VE while its running so your dyamic CR isnt going to be 10:1. Granted the more air you can get into the chamber the more chamber pressure and more power you will make and the closer you will get to 100% VE which is what your after with a internal combustion engine. Unless you have a forced induction engine and then you have to figure in boost and you'll be way above your static compression ratio at max boost but thats a whole nother formula for that.

 

[rt]

I understand the whole concept of forced induction motors, That's primarily what I use to deal with. The concept of dc vs sc is (as) like you said a different story,..So the 100% is just another way to introduce the 10:1 as v.efficiency. So technically any naturally aspirated motor running, is never running at 10:1, just a fraction. Coalshed expressed it in terms of 8:1 from a 12:1 motor leading me to believe it was 66% on an average standard motor(for the sake of conversation, actual may vary). So if I have a motor that I compression test at 11:1 (sea level)per say, that is the static compression. 11:1 * 66%=7.26:1 dynamic c. in regards to v.e.????