How do u really change comp. ratio??

[Anonymous]

Ok this is gonna sound ignorant at first but perhaps im just missing something. I always hear about someone changing rods or pistons and milling the head or block to change the compression ratio. This sounded ok to me at first bu tnow that i think about it. Im thinkin its not actually changing the compression ratio. If you have different rods or pistons, all you are really doing is movin the stroke farther up the cylinder. the length of strole stays the same. Also with milling you reduce the chamber volume so your compression ratio is not changed again. So im thinkin the only tru way to change comp. ratio is to change your crank. Im prolly missin somethin and havin a brain burp but its drivin me nuts lol.
thanks all

 

[StrangeRanger]

You're confusing displacement with compression ratio.

Compression ratio is:

CR = (displacement + clearance volume) / clearance volume

By raisnig the pistons in the bore and/or milling the head you are reducing the clearance volume. This is exactly how you raise the compression ratio.

 

[Fiorelli]

Or do like I want to do and remove the head gasket. Boreing and stroking the engine will also increase the compression ratio along with the CID, wish I had the $$$$$ to do that! A lot of peole change there pistions to change there compression ratio a flat top pistion will give you more compression than a dished pistion but not as much as a pistion with a dome.
Anything you can do to reduce the total combustion chamber volume or increase the volume of air you squeze into the combustion chamber will increase your compression.
Even a turbo or blower will increase your compression ratio.
More compression = more power. Till things break or melt.

Jeremy

 

[CZLN6]

Howdy SLV49 and all:

Actual compression ratio is the ratio of the total volume of the cylinder with the piston at bottom dead center, the combustion chamber, compressed head gasket and piston dish, if any, to the same dimensions but with the piston at top dead center. As has been described, many factors effect this ratio, but once fixed, it is a mathematical ratio of these two volume dimension. If you change any of these factors you will change Compression ratio. This is a very mathematical equation.

In general, a point of compression is worth about a 4% power in/decrease, all else being equal.

Theoritical compression ratio is determined by the dynamic relationship of valve timing and power adders such as super chargers, turbo chargers and NOS. They are best measure by cylinder pressure.

Some real world realities;
Higher compression with a stock cam will generate more cylinder pressure than the same compression with a performance cam, because of valves opening earlier.

Compression also generates greater heat. It needs to be manages for best results. Heat is power- to a point.

It is best to include CR in the total over-all plan for your engine.

Adios, David

 

[Anlushac11]

8)

The way I understood it was if you were to hypothetically speaking moved the piston to top dead center, filled the volume there with water and then measured how much volume you had. Now you move the piston to bottom dead center and fill that area with water and measure the volume again. The ratio between the two should be the comtression ratio.

Any time you change the volume at top or bottom you change the ratio between the two.

Thats why different pistons or milling the head or reshaping the combustion chamber can affect the compression ratio, you are changing those volumes and therefore the ratio between the two.

 

[LEROY POLL]

HELLO FELLOW READERS

.... I JUST WANT TO ENTER TO SAY THAT TURBO CHARGERS DO NOT CHANGE THE COMPRESSION RATIO. C/R IS A MACHANICAL RATIO.

.... THE TURBO WILL RAISE THE STATIC PRESSURE IN THE CYL. BUT NOT THE RATIO. IT GIVES MORE VOLUME TO THE CYL... AND RAISES THE STATIC PRESSURE. SEA LEVEL WE GET ABOUT 15 LBS STATIC PPSI. 5000 FEET HIGHER YOU WILL GET LESS THAN 14 LBS STATIC PPSI. THE COMPRESSION RATIO DID NOT CHANGE ...BUT THE STATIC (POUND PER SQ. INCH) PPSI CHANGED AND HAD A DIFFERANT VOLUME RATIO!!!

.... JUST SOMETHING TO THINK ABOUT!!!

LIVE IN GRACE

LEROY POLL