low compression piston?

[crashed97tsi]

hey guys

I was just reading on low compression pistons and i have a few questions. When you put low compression pistons Into an engine, what's the benefit of doing that? Wouldn't you have the same amount of compression with say 10 or 11:1 pistons running 7 psi that you would if you had say a 8:1 running 14?(I just guessed at the numbers, iff they don't match up sorry) Or is the benefit of low compression getting your turbo into a better efficiency range? If i'm talking out of my ass, I apologize but this has been bothering me all morning

later

 

[larryd]

Most cars that put lower compression pistons in the car are running higher compression then we are from the factory. Ie) the Celica at 11.5:1.

You put in lower compression pistons so you can run more boost and make more power then being at the same compression you were before running less boost. There is a mathematical equation that Ive seen before for determing what compression tor un with x amount of boost to make y power.. Cant recall off top of my head though.

 

[rubbersidedown]

i think a higher compression piston also helps spool your turbo quicker.

 

[larryd]

yeah higher compression means your turbo will spool a lil faster and you can run less boost to make the same power.

 

[crashed97tsi]

cool

so, it would be better to have say 9:1 pistons with a 16G then a car with higher compression pistons with the same turbo? Or would they just run different amounts of boost and just even out

 

[GSX4LIFE]

There is no standard. As boost goes up, so does power. Lower compression does not nec. mean lower power at the same boost level. Not only will lower CR alow higher levels, it will allow relatively high(22-25psi) more safely than 9:1 compression. I run 7.8 on Venolia pistons. 22psi w/ pump gas and 625cc injectors. And Im STILL running a bit rich.

 

[rubbersidedown]

better in what way? the higher comp will spool faster and have a stronger bottom end but the lower comp will have better top end.... i think.. am i correct? anyone?

 

[GSX4LIFE]

The normal person running 9:1 CR on any turbo pushing more than 15psi will most likely go BOOM!

 

[Thalomos]

Originally posted by GSX4LIFE
The normal person running 9:1 CR on any turbo pushing more than 15psi will most likely go BOOM!

Wow, you have no idea what your talking about...

 

[GSX4LIFE]

HAHAHAHA!!! Its a joke dude. Dont take everyhting so literal. But I guess your right, I know nothing.:laugh:

 

[Enigma_Man]

Well, first off you're looking at it the wrong way.

Compression doesn't change depending on turbo boost pressure at all. All compression is, is a ratio of how big the volume of the cylinder is when the piston is down to when the piston is up. IE, if you have a 8:1 ratio, then when the piston is at its lowest point, the volume in the cylinder 8 times larger than when it's at its highest point. Adding boost to an engine doesn't affect this ratio at all. It does affect how much air is crammed into this volume though, but that is different than the size of the space the air is crammed into. Even at 7, 14, or 3000 psi, the cylinder volume is still 8 times larger at the bottom of the piston travel.

Imagine this:
A high compression ratio is like having a lot of gun powder in a small little container. Light it off, and BOOM! a lot of power in a little area. Now imagine taking the same amount of gun powder, and spreading it around on the ground, and lighting that. It's just going to burn semi-quickly, no boom at all. That's a low-compression piston. It's all the same amount of gunpowder, but in a different space.

Okay, got it? Now add this to the equation. The amount of boost you are running is like the amount of gunpowder you use in each situation; the more boost, the more gunpowder. And pretend that instead of blowing stuff up, you want to contain that explosion, and use it. If you pack a lot of gunpowder (high boost) into a very small container (high compression) it's going to explode and break your container (engine damage). So, that is why people run lower compression in a turboed engine. The turbo just adds so much power into the explosion that if the compression ratio is too high, you're going to break something. A lower compression just makes things easier on the engine bits.

Hope that helps some

-Jesse

 

[Thalomos]

Originally posted by GSX4LIFE
But I guess your right, I know nothing.:laugh:

It's ok, i'm still learning too, we can all learn together

 

[crashed97tsi]

Originally posted by Enigma_Man
Well, first off you're looking at it the wrong way.

Compression doesn't change depending on turbo boost pressure at all. All compression is, is a ratio of how big the volume of the cylinder is when the piston is down to when the piston is up. IE, if you have a 8:1 ratio, then when the piston is at its lowest point, the volume in the cylinder 8 times larger than when it's at its highest point. Adding boost to an engine doesn't affect this ratio at all. It does affect how much air is crammed into this volume though, but that is different than the size of the space the air is crammed into. Even at 7, 14, or 3000 psi, the cylinder volume is still 8 times larger at the bottom of the piston travel.

Imagine this:
A high compression ratio is like having a lot of gun powder in a small little container. Light it off, and BOOM! a lot of power in a little area. Now imagine taking the same amount of gun powder, and spreading it around on the ground, and lighting that. It's just going to burn semi-quickly, no boom at all. That's a low-compression piston. It's all the same amount of gunpowder, but in a different space.

Okay, got it? Now add this to the equation. The amount of boost you are running is like the amount of gunpowder you use in each situation; the more boost, the more gunpowder. And pretend that instead of blowing stuff up, you want to contain that explosion, and use it. If you pack a lot of gunpowder (high boost) into a very small container (high compression) it's going to explode and break your container (engine damage). So, that is why people run lower compression in a turboed engine. The turbo just adds so much power into the explosion that if the compression ratio is too high, you're going to break something. A lower compression just makes things easier on the engine bits.

Hope that helps some

-Jesse

Thanks man, that helped alot

 

[CBgst]

In theory wouldn't higher compression make more power than lower compression at the same boost? I'm just asking because say you put a
t-25 on a 10:1 motor at only 11psi would it make power like a bigger turbo. At 11 psi you could keep the turbo in it's efficenty range. I know this probably wouldn't work because there are people that build these engines daily that have probably already tried this. Why wouldn't it work though.

 

[Wilsonman02]

it all makes sense now....i see the light..

 

[dsmstunna]

Originally posted by CBgst
In theory wouldn't higher compression make more power than lower compression at the same boost? I'm just asking because say you put a
t-25 on a 10:1 motor at only 11psi would it make power like a bigger turbo. At 11 psi you could keep the turbo in it's efficenty range. I know this probably wouldn't work because there are people that build these engines daily that have probably already tried this. Why wouldn't it work though.

You got to take air volume in to consideration. The lower compression at higher boost is going to have more air, thus meaning you can add more fuel. at the top of the piston travel a high compression piston at a lower boost level may have the same amount of pressure at the peak of travel as a low compression piston with more boost but, it would have more AV in it.