Low compresion (meaning)

[SIObsesst]

Got the noob question for ya. What do they mean by forged pistons having low compresion?

Is that better performance or does that make your engine more suitable for a turbo?:confused:

[matt]

well forged pistons dont always have low compression.
forged refers to the manufacturing process used to make the piston.
you can have forged pistons in any motor really they are juts stonger than regular/factory pistons which i presume would be cast (i stand to be corrected on that)

http://www.ozhonda.com/forum/showthread.php?t=27465
Toda for example make forged high compression pistons for various engines

you are right tho generally if you were going to turbo a high comp motor you would lower the compression of the engine

[EuroDude]

It means that the pistons produce less compression in the cylinders. Turbo's dont work well with high compression so you need lower compression pistons.

Heres some good info
http://en.wikipedia.org/wiki/Turbocharger
http://en.wikipedia.org/wiki/Internal_combustion_engine

[Twincam16]

It means that the pistons produce less compression in the cylinders. Turbo's dont work well with high compression so you need lower compression pistons.

Heres some good info
http://en.wikipedia.org/wiki/Turbocharger
http://en.wikipedia.org/wiki/Internal_combustion_engine

Not that they wont work well, just inefficiently.

By rule of thumb, the higher the compression in the chamber, the more power you will produce. A turbo works by substantially increasing the pressure inside the chamber by forcing in air, this is why you lower the compression on a high compressed NA car so that there is less strain on the pistons and rest of engine.

The downside is, adding low-comp pistons reduces low down reliability/drivability. This is why low comp are suited to turbocharged setups as they allow you to raise the power/torque by feeding more boost (after your revs get through lag - this is another story).

It is possible to run standard (high comp) pistons with a turbo setup, however it is cruicial to choose your package wisely, and get a very good tune, there is more room for error and in turn detonation when error is made.

Forged pistons dont only have low comp though. There are also High Comp pistons, for higly worked NA engines (eg. B16A - 4AG20v etc), these are used to pressurise the mixture of fuel/air in the chamber to create a bigger bang. Bigger bangs mean higher figures and in turn a faster car.

Its all about the Big Bang Baby... :thumbsup:

[SIObsesst]

Damn... twincam16 you ur stuff, thanks for that.That really helped me out now i know what to search for, i jus want to turn my d16a8 into a hardcore n/a engine.

[CUL8R]

if u want to tune into hard NA u want HIGH compression

[saxman]

I don't know what you're all saying....


turbos LOVE high compression motors. High comp + turbo = way way better than low comp + turbo.

You are, however, much more limited by how much boost you can run and the quality of tune you can safely run.


Think of it like this...

high comp turbo= way more power per psi, more off boost power, better response, etc
low comp turbo=way less power per psi, less off boost power, less response, but the ability to run a lot more psi to try to make up for the above mentioned problems.

[kraiye]

more boost on lower comp engines isnt just to 'make up' for not having higher comp. it allows even more air and fuel to enter the chamber and at an already greater comperession. this higher pressure entering the cylinder also in effect makes it even easier for the piston to retract and draw air in resulting in increased power (because its not working as hard to draw air into the cylinder).
if your after smaller boost then high comp is good but if your after lots of psi and higher revs then low comp is good
i dunno how sound this is and i know the figures are prolly meaningless but... lets say you force 10psi into 11:1 comp ratio. the final compression being 11x10 = 110.
now force 20psi into 9:1 ratio and u get 20x9=180
therefor the lower comp ratio actually results in a higher comp at the end of the compression stroke.
=) prolly a load of bs and if someone shuts me down i'll delete this crap
my non-professional opinion/theory anyway :)

[saxman]

What you're trying to consider there is the difference between static compression and effective compression, but you're not quite going about it properly.

The correct equation for effective compression is
E = C((B / 14.7) + 1)
Where E is effective compression, C is static compression(the C/R of the motor), and B is boost.

For your 10 psi/11:1 set up, your effective compression would be 18.48.
For your 20 psi/9:1 set up, your effective comp would be 21.24.

So yes, a 20 psi/9:1 set up would result in greater power given all else being equal, but honestly, a more accurate comparison to a 10 psi/11:1 set up would be something like a 15psi/9:1 set up. At about this point, the effective comp ratio is about the same. It takes 150% the boost to make up the same power under boost, but you're still left with less power down low, more lag, and poorer throttle response.

[kraiye]

^^ :thumbsup: u rock! i was hoping someone would set this straight :)

personally i'd prefer a higher comp ratio. just trying to say there is a good purpose why some people opt for lower comp when turboing

i dunno, maybe for drag where you're always on the power and lag and throttle response arent such an issue