Compression Ratio..

[1PhatCX]

just curious, how do compression ratios work? and whats better, higher or lower numbers? like 6.5:1 or 10.9:1? i dunno, fill me in with some info here guys! thank!

[FourthGenHatch]

For N/A engines high compression is usually better. It will give you more power but forces you to use higher octane gas and if compression is to high than your car might not be such a great daily driver.

For Turbo engines low compression is better.

I'm not really sure why these are true, but I know that is the general idea.

[Problem_Si]

depends on the egine setup...

Typically, the higher the compression....its like the motor is working harder. More compression = more power.

But, thats usually something that pertains to NA motors, with built internals. They need the added strenth to handle the extra stress. Thats why well built NA motors rev so high.

Forced induction motors usually run a compression no higher than about 9.5/1 Usually, the lower the compression on a turbo car, the less chance you have of blowing the whole shit to the sky.

This is why I am using a low compression head gasket on my zc before I go turbo. I will be going from whatever stock compression is (like 9.3/1) to 8.5/1....I will lose a few ponies, but I also won't have to worry about running a few extra pounds of boost on race day.

Thats the best way i could describe it.

[Problem_Si]

yea, just like fourthgen said....


Its mostly to do with the fact that anytime you change the geometry of the internals, you are essentially changing the volume of the combustion cylynder.

A thicker head gasket will decrease the amount of compression, by making the overall volume of the cylynder larger.

Same thing applies to domed pistons and valves.

[1PhatCX]

alright guys, that some pretty good insight here,
so if i wanna go turbo, the i could try and get my compression down to like 3.6:1 and it would run awesomly? or what would be around a good compression ratio to use for a turbo on say a ZC?

[SiZ]

Originally posted by 1PhatCX
i could try and get my compression down to like 3.6:1 and it would run awesomly?

I don't think it would run at all.
I think CXs have a pretty low compression ratio as it is.. I assume thats the motor you're thinking about turboing.

[1PhatCX]

no no no no no... not turboing the D15B2... i'm eventually gettin a ZC hopefully before next summer and i wanna turbo that

[Problem_Si]

I don't know what you "should" do, but this is the setup I will be running on my zc.

92-95 Civic (d16z6) Greddy t25 turbo kit
spearco intercooler
cometic low compression head gasket
greddy profec b boost controller
greddy type s bov
apex guages (oil,water,boost)

I will be running a compression of 8.5/1.

With the profec b, you have high/low boost settings. I will be running 4psi for everyday driving.... 7psi when I race, or someone needs to be dealt with Nothing too crazy...hoping to get 200hp outof the setup.

I have heard from many reliable sources that you can go as far a 9psi safely without upgrading injectors or rods.

The main problem Zc's have with turbo is the brittle connecting rods. They have a tendency to break anywhere from 9-12psi. I plan on eventually upgrading to some beefy-er (sp??) rods later.

[SiZ]

8.5 would be pretty good for a ZC.. Then you'll have a little loss in power, but you're safe to run a bit higher boost, and the boost will be making up for the power you lose with a low compression ratio.

[Problem_Si]

Originally posted by SiZ
8.5 would be pretty good for a ZC.. Then you'll have a little loss in power, but you're safe to run a bit higher boost, and the boost will be making up for the power you lose with a low compression ratio.

exactly......


You get a

[HONDABOND]

Are you smoking crack?! 8.5:1... No way dude, you dont want to go under 9.0:1 on any motor or you start having a problem with volumetric efficiancy.... Unless of course you are boosting INSANE amounts and need the space to avoid detonation. (Insane = 25+ PSI with loads of flow)

A Hi-Compression piston does alot of things (Besides making an engine "Work arder") Higher compression pistons work by squishing the fuel harder, they usualy involve a higher dome to take up a little more space in your combustion chamber. The only reason you would need higher octane fuel is because it is not as volatile as lesser octane grades and therfore will not detonate untill a higher flash point is reached. Basicaly, it takes alot more squish to explode 91 oct than it does to explode 89 oct.
Another thing that piston makers do is try and tune the piston to sound wave lengths in an attemp to better Homoginize your air fuel mixture. Plenty of other things are taken into account such as flash points and detonation grooves.
On a normal honda piston, if you detonate... thats pretty much it, I wish I had my old piston to show you. Anyway, the forged pistons are MUCH stronger and lighter.
If you have any further questions, go read a book... Thats were I learned all this shit.