filetofit

There has been some ongoing confusion regarding compression ratio, and boost and what's best for boost high or low CR. I'll try and contribute to it.

Boost is related to the concept of volumetric efficiency. At bottom dead center of the pistons stroke, there is a certain volume that can be filled with the gasoline/air charge. This total cvolume is: the displacement volume + head volume+ a little more for head gasket, piston to deck clearance etc. If this total volume were filled with gasoline/air at atmospheric pressure the volumetric efficiency would be 100%. On a turbo charged car of course you can pack the cylinder with pressurized air so VE can be greater than 100%.

The compression ratio is related to a different concept, thermodynamic efficiency. As compression ratio increases, so does the thermal efficiency. Simply put compression increases the temperature of what is being compressed. And higher temperature mixtures, once ignited, can expand more, producing more power. There is a point of diminishing return however and thermodynamic efficiency can never be greater than or equal to 100%. For those who remember high school geometry, thermodynamic effieciency is hyperbolic, asymptotic to 100%.

So in summary boost compression is a different animal than engine compression.

stizzit

Not to mention what I just added to the previous post...

Without saying any of you are wrong, static numbers are just that, static. You can boast about numbers all day long, work the math, and do your thing. When you want to try a real comparision, put two identical cars on the track configured based upon your numbers and see who runs the 1/8 mile faster and the 1/4 mile on a street tune. In the case of lower compression, fine you make 400whp ON THE DYNO, now a days it seems like a standard to make that much, big deal. Your buddy only makes 325whp because he has the dreaded 10.2:1 compression. You guys go out for a night on the town with your ladies and you roll up to a light after the movie, around 1am, no one is around. You both turn to eachother and are thinking the same thing. Light turns green, tires light up, gone, but why did your buddy just slam 2 car lengths on you FROM THE LINE? Oh yeah, thats right, he's got torque, he can spool, he moves. So you keep runnin up the speed, hit 100mph and letoff to avoid speeding into the closly approaching main st. area (lots o pigs). You lost - Oh wait, but your buddy only made 325whp? wtf?
We are talking about Street HP here, nothing more. If you can make 400whp thats all fine and dandy but if it takes 3 years to make it, how is it usefull? A fun test would be to plot HP vs Time for the two motors used in the example above. The one thing everyone must keep in mind is that the turbos WILL be sized differently due to the different boost requirements fitted to the engine.

Also, just a comment, with all this talk about how many pounds per hp, I think some of us forget that the boost pressure is a measure of how much resistance the engine creates to the air being forced into it. Take for example a cocktail straw, one of those little tiny red jobbers. Blow into it as HARD as you can and feel the air comming out, also notice how fuggin' tired your *** is. NOW take one of those HUGE straws that come with the giant pixisticks and cut it to the same length as the red straw, then proceded to blow into it as HARD as you can. You run out of breath much faster eh? but your diaphram isnt killing you and you noticed much more air come out of that straw in a shorter amount of time. Use this and compare it to Horsepower vs Boost pressure. Boost numbers on a 2L b20VTEC with a b16a intake mani and stock valve seats are going to be much greater than the same motor with say a VICTOR X manifold and 1.5mm oversize valve seats matched with a ported IM.

Relating this to IB's motor, yes they only have 13psi but I bet if you measured CFM it would be through the roof on that engine. They probably did an extensive job ensuring the intake tract is unrestricted and they have the proper cam timing. I'm sure there's efficency to play in but for thte moment its left out.

Anyhow, just thought I would point that out! '

Peace

filetofit

My point really wasn't that high or low CRs are good or bad. My point was that the power gain to be had by increasing engine compression is a completely different animal than the power gain to be had by running boost. The former is an example of increasing thermodynamic efficiency the latter is an example of increased volumetric efficiency. The physics are different that's all I'm saying.

MatT3T4

You're absolutely right. The two compressions though, will form together to make a new compression ratio, number, level...however you want to consider it. That is the concept of effective compression, which I have gone way into in previous threads. What level of compression on each side of the spectrum, that you want to start with, will relate how effective, your effective compression will actually be.

Full-Race Geoff

no they dont.

they are not constants, you need to realize that the static compression ratio is a constant, but things change with valve events and then the boost variable adds to that.

lots and lots of variables you are not taking into account.

MatT3T4

Of course there are events that change it. Adding boost throws in a whole new load of crap, and that is a given. Boost changes every second, and it is never completely stable. When you add boost, which is not a constant, to your motor, which does have a constant static compression, the two DO form together, although effective compression is not constant. That too, is a given. I am not saying that effective compression is a constant. However, I am saying that it is to be taken into account, among other things. The variables I am not taking into account ARE givens...octane, timing, etc...people know these things, however, most people weren't aware of the concept of effective compression.