yeah, i always thought lower compression ratio is better for fi.....usually everybody who seriously goes fi like crazy look for a compression ratio of 8.5:1 ......hondas have higher compressions ratios like 11:1 which greatly increases the na engine setup for high rev engines...thats just how i thought it all worked,lol

 

isn't lower compression ratio only good for turbo'd applications?... wouldn't higher compression ratios help because s/c are motor driven and linear?

 

no, you want low compression to begin with before putting a turbo or s/c on a car. cuz once you add either of those it will raise the compression.

 

actually... the SAE horsepower rating system changed in 2007... scion made up for the power loss by upping the compression ratio.

 

True. It's called forced induction for a reason. You're pretty much cramming air down the engine's throat. As opposed to n/a where it's sucking in the air nice and easily.

 

I was pleasantly suprised when i had the S/C put on my '07. Its alot more fun to drive than stock.

One question, though. I read recently that the compression dosen't change with the S/C. That dosen't sound right, but I'm usually wrong about these things.

 

Compression is determined inside of the engine not the outside. You change compression with the piston. the supercharger changes the Pressure of the air being sucked into the chamber. The compression is how much the pistion compresses that air/fuel inside the chamber. And then the spark ignites it.

 

So the compression doesnt change in the cylinder?

 

Nope. The pressure of the air going in does.

compression is how much the piston COMPRESSES the air inside of the cylinder. On the up stroke in a 4 stroke engine, the fuel/air mixture is compressed to usually to 185PSI then the spark plug ignites, and the piston goes down for the power stroke.

So no the compression of the engine doe not change from outside forces.

 

I think it may be a little misleading to apply a PSI to the compression ratio, beacause that WILL change with forced induction.... it's around 185psi either A under no boost/at idle/when cranking the engine or B on a normally aspirated engine. Under PSI of boost it'd be much higher than 185.... The c/r is a better ay of putting it... For example (just an example don't shoot me on the numbers) if you have an engine with 9.8/1 compression ratio that could hold 9.8 cubic inches of air/fuel mixture it compresses that down to 1 cubic inch(or approximately 1/10th of its original volume in this case).... which happens to (i think) end up around 180psi providing you have good piston rings and all.... but if you are cramming 6psi of air into the cylinder, along with increased fuel and then compressing it 9.8 times... that number of psi is going to go up (if you could even find a way to measure it? ) right? But no, the amount the air is compressed can only be changed with interal changes to the engine (like low/high compression pistons)

Sorry if i made any stupid errors in there, feel free to correct me if i did, and let me know if i was unclear, but i hope i helped someone at least

 

Deffinetly true.
And like you said, i dont know how you would measure cylinder pressure under boost. I used the 185 for a normal cylinder compression test done at idle.
And your right the actual PSI in the cylinder would be more, if its more to begine with.
but since compression is determined in a RATIO, the end result isnt what is measured or important, its still the ratio of compression which wont change from outside forces.

You explained it all much better than me. i couldnt figure away of doing it to explain what i meant.
Thanks man.

 

206 stock s/c is really great numbers. Wonder what a 07 stock s/c auto tranny throws down.

 

Thanks Nick, and no problem always happy to help when I can. Sorry if i threadjacked guys, and yes 206 IS good numbers for a stock s/cer congrats man! welcom to the world of FI

 

I'm sitting here going "right, right" and nodding my head. I got dumb and was thinking the compression ratio changes, of course it dosent.

So when people talk about what pressure they're pushing, is that what their forced induction is pushing? On a stock engine, is the compression 0 psig?

 

Pressure that most people talk about is dealing with a forced Induction car (supercharger or turbo charger)
It is measured in BARS or PSI.