landonew

Why do people want low compression engines for FI apps. I understand that it allows you to run more boost, but why not just keep regular compression and run a little less boost. Its all about putting the most air/fuel into the engine right? I know i am wrong, i just don't know why I am wrong. Sorry for the NEWBIE question

TT632

iTrader: (3)

Your right, putting more air and fuel into your engine to make more power.

The following is an adiabatic comparison (no heat added to the process) to simplify the answer to your question.

With compression alone you are gaining no additional air to increase your power. You are getting a larger pressure gain during the power stroke due to the increased CR(bigger bang). "At best" you could get a 4% gain for each point in compression increase. With that one point in compression increase you are also increasing the air temp in your combustion chamber the same amount as 1 Atm of boost (14.7 psig at sea level). (This is not taking into account blower/turbo efficiency, pumping losses or heat soak). So, for simplification sake, you could gain twice the power with boost due to twice the volume of air being pumped into your engine with the blower vs. a few percent increase with one point in compression gain.

Note to the above: In reality, with blower inefficiencies, the one point in compression gain may be the equivalent to 9 or 10 psig boost. There are also inefficiencies related to increasing your compression that are not taken into account above. You can go into elaborate detail but the above is a good comparison of the differences.

landonew

Ok. So basically CR and PSI are 2 different things. CR is measured at combustion, when the piston has just finished it's compression stroke. So a higher CR ratio is dependent on the combustion chamber of the heads, the head gasket, and the pistons. PSI is measured when the piston is at the bottom of the intake stroke and the cylinder has achieved it's maximum volume. So thus, the amount of air that is used during combustion is determined at this stage. So in an ideal situation, at 14.7PSI, a given engine should make double the horsepower because it is utilzing twice the amount of air(and twice the amount of fuel)?

So with a lower CR, you are essentially alloting for a bigger volume for this increased amount of air/fuel mix. However, w/ FI application wouldn't your CR go up? I mean, CR is simply => the ratio of pressure in the engine:Atmospheric pressure? So if you have twice the amount of air/fuel mixture, wouldn't you need twice as big a combustion chamber to accomplish the same CR as you had before FI? Or is the CR measured/estimated during NA, and not adjusted post FI?

I really apologize for all these newbie questions, i am seriously not well versed with FI stuff (nor internal engine stuff apparently).
one is me

TT632

iTrader: (3)

Actually, it comes down to temperature when you are looking at reducing the compression ratio and adding boost. When you compress a gas (air in this case) you will increase the temperature...Compression ratio or boost (Ideal gas law). If you exceed the critical temperature, you will have problems with pre-ignition, burnt pistons etc.

So, you have a simple equation that is "Ambient temp" + "Temp gain due to Compression" + "Temp from compressor(Boost)" - "Temp reduction due to the evaporation fuel" Not to exceed "Critical temperature". (If you have an intercooler or water meth injection you could also add that in).

You are correct in the pressure being higher. More air molecules being compressed into the same volume dictates this, but compression ratio is a unitless number and initial volume divided by the final volume stays the same regardless of how many air molecules exist in the space.

landonew

Ok, so CR is maximum-total-volume:Volume-at-combustion? I think I understand the thermodynamics now. So by having a lower compression, you are merely decreasing the heat created from the actual compression stroke.

LS1IMPULSE

iTrader: (42)

CR is the amount of volume in the cylinder when the piston is at BDC compared to the volume when the piston is at TDC

One of the many equations to calculate CR is Swept Volume (plus) Compresed Volume (divided by) Compresed Volume

In the picture bellow the Top cylinder show the Compressed Volume above the piston, this volume takes into account:
-Combustion chamber size
-Compressed head gasket thickness
-Deck Height
-Piston Dome/Dish

The Swept volume is just the bore size

http://rds.yahoo.com/_ylt=A0WTb_njf9...acing/wf25.jpg