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Ntseg Muag!
SRP MY00 Subaru Impreza 2.5RS
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518 Posts
Discussion Starter #1
what does the numbers in the compression ratio mean..like 8:10:1...what does the numbers tell?...if the compression ratio is higher/less does it mean it outputs more/less hp or something.....sorry if this has been ask before but i just want to know everything about my future car before i do any modding.....

PS: PLZ don't flame me.......i post it here cuz if i did it in i-club i know for sure i'll get flame!!!!
 

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Premium Member
Two Mini Coopers!
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10,629 Posts
No flames...

Compression ratio is simple (sorta):

8:1 means that as the piston comes up it compresses 8 units of air into one unit

9:1 is higher compression, which means more air gets compressed into that same one unit

10:1 higher compression (pressure) is about the highest you can run on street gas

11:1 usually requires race gas/high octane gas to operate without detonation (pinging)

A turbo will use a lower compression engine (my Typhoon uses something like 8.01:1, I think the WRX uses something like 8.5:1) because the turbo crams more air into the cylinder, and more air = more compression = heat = detonation.

Does that help? :)
 

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Ntseg Muag!
SRP MY00 Subaru Impreza 2.5RS
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518 Posts
Discussion Starter #3
hey thanks alot!!!!!!

that clearifies just about everything....so if i'm going turbo all i have to do is get low compression pistons and of i'm going NA i have to get high compression rite?...kool...ok thanx again!!!!!!!
 

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MY00 Subaru Impreza 2.5RS BRP
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2,605 Posts
to decrease the compression ratio you could change the head gasket. (cheaper way) No need for new piston heads. I don't think you need higher compression ratio if you are staying NA. I think 00 and newer are 10:1 already. aren't they?

Lue V said:
hey thanks alot!!!!!!

that clearifies just about everything....so if i'm going turbo all i have to do is get low compression pistons and of i'm going NA i have to get high compression rite?...kool...ok thanx again!!!!!!!
 

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Ntseg Muag!
SRP MY00 Subaru Impreza 2.5RS
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518 Posts
Discussion Starter #5
hmm..i never knew that caould lower the sompression ratio. how does it lower the compression ratio?....wow u guys sure know alot bout ur cars..well thanks for the tip iBlueVirus...
 

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2004 Forester XTi
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That's a good reason why the MY99 can handle better boost because it has a slightly lower compresssion ratio and the MY00-01 cannot handle too much boost. The stronger compression engines does create more hp per psi, but the lower compression engine can take more boost depending on how low the compression ratio is.

Those DSM turbo cars run low 8s for their compression ratios. That's why they consider 10 psi low boost for their stock internals, while we consider 10psi too high of boost for our stock internals. That's also a reason why it's not a good idea to boost a honda engine for their high compression engines. Correct me if I'm wrong.
 

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Ntseg Muag!
SRP MY00 Subaru Impreza 2.5RS
Joined
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518 Posts
Discussion Starter #8
yea..guess so...especially with a 1.6 or 1.8 liter....how come no honda owner never though about working on their torque instead of power....those little 1.6 and 1.8 can't output much hp so why don't they just work on the torque...or is it not possible...
 
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