What's compression ratio

CitySubie · 04-22-2004, 01:31 PM

I'm new to car mods. Can someone please tell me what this "compression ratio" is? My 2.5rs has a higher compression ratio than the STI but have the same displacment.

I also read somewhere that different pistons for my car can raise the compression ratio from 10.0:1 to 11.0:1. Is that good or bad?

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phattyduck · 04-22-2004, 01:46 PM

If you want to know more about how your car works, read up on www.howstuffworks.com It may not have the exact answer to your question, but a lot of general stuff is covered there.

Compression ratio is simply the ratio of the volume of the cylinder when the piston is at the bottom of its stroke to the volume of the cylinder at the top of its stroke. The displacement of the motor is the total volume swept by the piston during its travel, multiplied by the total number of cylinders. Simple, eh?

-Charlie

CitySubie · 04-22-2004, 02:01 PM

How can I raise or lower the compression ratio? Lower is better for turbo right?

no-coast-punk · 04-22-2004, 02:18 PM

If you don't know what compression ratio is... maybe it's a bit early for you to be delving into turbos. Howstuffworks.com is pretty good

STi-Nismo Dood · 04-22-2004, 02:59 PM

i learned from there...

MeshmeZ · 04-22-2004, 06:26 PM

there are a few ways to change the ratio. you can get low or high compression pistons. i know for hondas they sell lower compression head gaskets which basically lift the head farther off the block, i dont know if something like this is available for the subaru.

lower compression is better for turbo engines for the safety of the engine. turboing a high compression engine can put too much stress on the rods/valves/etc. because you already have high compression and then you are pumping compressed air into the cylinder

and lastly Howstuffworks.com is awesome for learning about a ton of stuff. not just cars.

04-22-2004, 01:31 PM	#1
CitySubie Scooby Newbie Member#: 60099 Join Date: Apr 2004 Location: N.Y.C. Vehicle: 2003 Impreza RS The perty blue one	What's compression ratio I'm new to car mods. Can someone please tell me what this "compression ratio" is? My 2.5rs has a higher compression ratio than the STI but have the same displacment. I also read somewhere that different pistons for my car can raise the compression ratio from 10.0:1 to 11.0:1. Is that good or bad? * Registered users of the site do not see these ads.

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04-22-2004, 01:46 PM	#2
phattyduck Scooby Specialist Member#: 32955 Join Date: Feb 2003 Chapter/Region: SCIC Location: Pasadena, CA Vehicle: '03 WRX Wagon '89 3S-GTE Alltrac Camry	If you want to know more about how your car works, read up on www.howstuffworks.com It may not have the exact answer to your question, but a lot of general stuff is covered there. Compression ratio is simply the ratio of the volume of the cylinder when the piston is at the bottom of its stroke to the volume of the cylinder at the top of its stroke. The displacement of the motor is the total volume swept by the piston during its travel, multiplied by the total number of cylinders. Simple, eh? -Charlie

04-22-2004, 02:01 PM	#3
CitySubie Scooby Newbie Member#: 60099 Join Date: Apr 2004 Location: N.Y.C. Vehicle: 2003 Impreza RS The perty blue one	How can I raise or lower the compression ratio? Lower is better for turbo right?

04-22-2004, 02:18 PM	#4
no-coast-punk Scooby Specialist Member#: 49087 Join Date: Nov 2003 Chapter/Region: RMIC Location: Inside your carnot cycle Vehicle: 1998 RST V8 STi swap '05 R6 '95 BMW V8 hotness	If you don't know what compression ratio is... maybe it's a bit early for you to be delving into turbos. Howstuffworks.com is pretty good

04-22-2004, 02:59 PM	#5
STi-Nismo Dood Scooby Specialist Member#: 4149 Join Date: Feb 2001 Chapter/Region: SCIC Location: Frisco/Diego/LosAngeles Vehicle: MY93 ImprezaRA Sedan GC8.6 Aspen White	i learned from there...

04-22-2004, 06:26 PM	#6
MeshmeZ Scooby Newbie Member#: 58908 Join Date: Apr 2004 Location: San Jose, CA Vehicle: '07 STI silver	there are a few ways to change the ratio. you can get low or high compression pistons. i know for hondas they sell lower compression head gaskets which basically lift the head farther off the block, i dont know if something like this is available for the subaru. lower compression is better for turbo engines for the safety of the engine. turboing a high compression engine can put too much stress on the rods/valves/etc. because you already have high compression and then you are pumping compressed air into the cylinder and lastly Howstuffworks.com is awesome for learning about a ton of stuff. not just cars.