Thread: Let's talk compression ratios

I found an old thread listing HP numbers for W10 motors. The thread said .

I could have sworn I read the numbers were 180hp @7psi..If that same w10 t25 kit is put on a roller rocker de motor it should make more power due to the higher compression right?

Just did some quick math (cross multiplication:rofl, it works on paper but just seeing if I'm calculating this right because the numbers seem high...

HP @ 7 psi on a roller rocker:
180 hp/8.5 compression=x/9.8 compression. X=207 hp

HP @ 10 psi on a roller rocker
207 hp/7 psi=x/10 psi. X=296 hp

HP @ 12 psi on a roller rocker
207 hp/7 psi=x/12 psi. X=355 hp

You can't really use psi to calculate power. Power is based on airflow and VE. A T25 will never flow enough air to make 300whp (Not the W10 anyway).

Going up in compression ratio will usually net a % increase in power that will slowly diminish as the turbo reaches its flow capacity. For instance, a 10:1 motor may make 5-10% more power on the same psi as an 8.5:1, up until the turbo maxes out.

Yes.



Next?

the most fun way to increase compression ratio is to bore out your motor..

That doesnt mean that the compresion is gonna go up it means you are going to have more displacment . The compression ratio is all up to you with choice of piston , thickness of headgasket , c.c. of combustion chamber , etc.

it increases displacement AND compression so long as you get a new piston with the connecting rod pin in the same position..

think about how much more air you're going to suck into the chamber on your stroke with a larger bore? when you take an 86 mm stroke (like on our cars) you suck more air in for the length of the entire stroke.. when your piston goes back to the top, it's compressing all that extra air into an area that is also stroked, but not 86 mm long.. so its increasing the compression ratio too.

here's a link where we got pretty deep into compression ratio talk..
http://www.sr20-forum.com/wanted/36201-wtb-knowledges.html

increasing bore ups compression a LOT and increasing stroke only ups compression a little, but BOTH increase compression.

When you buy a larger piston, it is going to be designed to give you the advertised compression ratio, so an 86mm 8.5:1 compression piston would give you 8.5:1 compression, and a 90mm 8.5:1 compression piston would give you 8.5:1 compression.

That said, oversize aftermarket pistons are generally available in whatever compression ratio you want.

that said, when you take the forumula for calculating compression ratio, and assume all other things being equal..


b= cylinder bore (diameter)
s= piston stroke length
V_c = clearance volume

bigger bore = more compression period.
again, assuming all other things being equal, as you can see by the forumla

the way they increase compression with a "higher compression piston" is to change the location of the pin that attaches the piston to the connecting rod:

Ok yes it does flow hold and compress more air but say if i was to take a stock ve 86x86 motor wich i believe is 11:1 compression bored it to a 87x86 and added a set of turbo 8.5:1 compression pistons then i would have bored it and it now has lower compression. I am just saying it doesnt always happen to increase compression .

when you change to lower compression pistons, you'll definately lose compression.. with going to 87 bore and a much lower compression piston (8.5) i'm guessing you wouldn't be any higher than 9.0:1 or something around there.. maybe 8.75:1

I don't know how much the clearance volume is to do that forumula and know for sure.