Around 12.5:1 is what most people shoot for for the most/safest power.

 

thanks.

does 12.5:1 apply to all turbocharged motors or in general?

 

All motors in general.

 

Effective Compression Ratio
Try that BerinG

 

<blockquote><font size="1" face="Verdana, Arial">quote:</font><hr>Originally posted by Leadfoot/S2:
Effective Compression Ratio
Try that BerinG<hr></blockquote>

Thanks alot dude!

*EDIT* hmm.. site seems to be down.. I'll try later

[ 05-09-2002: Message edited by: BerinG ]</p>

 

Is there an equation of how much a turbocharged engine would raise compression ? depending on how much boost your running for example

 

How can this be right?

compression for running 8 psi of boost is 14.21:1 and for 10 psi, its 15.46:1 .. how the hell can our engine handle this much compression .. its not like we have an s2000 or anything..

I just don't know if this is right or not

 

<blockquote><font size="1" face="Verdana, Arial">quote:</font><hr>Originally posted by BerinG:
How the hell can our engine handle this much compression.<hr></blockquote>

Some don't

 

One of the advantages of turbo over NA, the effective compression ratio only applies while on boost. Boost can also be adjusted easily.

 

<blockquote><font size="1" face="Verdana, Arial">quote:</font><hr>Originally posted by Leadfoot/S2:


Some don't

<hr></blockquote>

Yeah but how can you reduce compression but still have the same kind of power?

Cause that would suck if I ran at like 8 psi, I don't want the engine to blow up

Or what bottom should i have to protect against this high compression? intercooler?!

 

BerinG, It's all in the tuning. There are people on here that trashed thier engine or rings by believing what someone told them. I may be one before it's said and done, but I hope not. Read all you can so you can try to filter out the sh*t!
That chart is for any turbo, if they can do it you can too!

 

<blockquote><font size="1" face="Verdana, Arial">quote:</font><hr>Originally posted by Leadfoot/S2:
BerinG, It's all in the tuning. There are people on here that trashed thier engine or rings by believing what someone told them. I may be one before it's said and done, but I hope not. Read all you can so you can try to filter out the sh*t!
That chart is for any turbo, if they can do it you can too!<hr></blockquote>

That's what I try to do on my spare time

Just don't want to jump into something i'm not "prepared" to do .. (even if I had help) .. I'm 19 yo, I pay for my own car, insurance, school, gas clothes .. anyway, I just don't have alot of money to spare on a new engine and that's why I ask alot of questions before I get into this forced induction club

(I just try NOT to get in the detonation club

)

Thanks for your help Lead .. you've been good help in alot of my questions .. Thanks to everyone else who responded also! (although it wasn't my post originaly

)

 

<blockquote><font size="1" face="Verdana, Arial">quote:</font><hr>Originally posted by BerinG:
How can this be right?

compression for running 8 psi of boost is 14.21:1 and for 10 psi, its 15.46:1 .. how the hell can our engine handle this much compression .. its not like we have an s2000 or anything..

I just don't know if this is right or not<hr></blockquote>


Help. How can the compression ratio change by adding boost? I can understand how combustion pressure would change but not the ratio. Am I be anal here?

 

no not anal

on an NA car, you just have air sucked into the engine and it is not usually more that 1 psi depending on temperature which controlls density.
With a FI car, you have say 8 psi pumped into the cylinder and then it is compressed even more by the piston.....so then you get the higher compression ratio.

That is why people go with dished pistons to bring them down to like 8.x:1 compression and then pump up the boost to like 20+psi

That is why you see these guys with the stock compression ratio and using only like 12psi and seeing over 220hp in the Focus too.

hope that helped

 

<blockquote><font size="1" face="Verdana, Arial">quote:</font><hr>Originally posted by YelloZX3SA:
no not anal

on an NA car, you just have air sucked into the engine and it is not usually more that 1 psi depending on temperature which controlls density.
With a FI car, you have say 8 psi pumped into the cylinder and then it is compressed even more by the piston.....so then you get the higher compression ratio.

That is why people go with dished pistons to bring them down to like 8.x:1 compression and then pump up the boost to like 20+psi

That is why you see these guys with the stock compression ratio and using only like 12psi and seeing over 220hp in the Focus too.

hope that helped<hr></blockquote>

If I were to lower compression to 8.x for example .. and then get boost up to 20 psi .. wouldn't It be the same thing as just leaving the compression high and putting less boost?

Another thing, if I we're to install 8.x pistons and put 20 psi boost.. would I get more power than leaving stock compression and having less boost? And what about the engine .. which engine would last longer with about the same amount of power (but different amount of boost and different compression ratios) ? Would I still have to redo the bottom end if I had like 8.x compression ratio? Or is the piston THE bottom end .. Would it be more reliable/safer? What would be the down side of having lower compression (I know you loose power .. but then again, you gain more with more boost

)

Thanks for the info

 

<blockquote><font size="1" face="Verdana, Arial">quote:</font><hr>Originally posted by BerinG:


If I were to lower compression to 8.x for example .. and then get boost up to 20 psi .. wouldn't It be the same thing as just leaving the compression high and putting less boost?<hr></blockquote>
No.. not the same.. higher compression has a better chance of causing detonation. And with a turbo you are not always under boost.. so those times when your at full boost and raising the effective CR will be less often. Thus less stress on your internals.

<blockquote><font size="1" face="Verdana, Arial">quote:</font><hr>Originally posted by BerinG:
Another thing, if I we're to install 8.x pistons and put 20 psi boost.. would I get more power than leaving stock compression and having less boost? And what about the engine .. which engine would last longer with about the same amount of power (but different amount of boost and different compression ratios) ? Would I still have to redo the bottom end if I had like 8.x compression ratio? Or is the piston THE bottom end .. Would it be more reliable/safer? What would be the down side of having lower compression (I know you loose power .. but then again, you gain more with more boost

)<hr></blockquote>
Bottom end = Crank, Rods, Pistons (and some would say the block)

Lower compression ALLOWS for higher boost and WILL produce more power. Not more power than high compression at the same boost, but more power than high compression at low boost. With Higher CR engines you WILL hit a wall as to how much boost you can run. Lower the compression.. and you have further you can take it.

Tom would probably say that the engine lasting is REALLY going to depend on your fuel solution. You can do many things (within reason) if you can get enough fuel to accomodate the new air flow!

Maybe some of the other guys can actually give you the mathematical calcs to see what at what ratio effective compression rises under boost. To see why power increases!

 

Its a trade off though. Lower the CR too much and it'll be a dog motor. Higher compression "low" (relatively) boost engines are generally more responsive (and I would think get better cruise mileage etc).

Selecting your CR is going to be a compromise of sorts. There is no "real" answer w/o taking into account all of the factors.

 

I still don't understand how adding boost (or removing for that matter) can increase/decrease the compression ratio. I do see how cylinder pressure changes but not the ratio. What the heck am I missing?

 

<blockquote><font size="1" face="Verdana, Arial">quote:</font><hr>Originally posted by belacyrf:

No.. not the same.. higher compression has a better chance of causing detonation. And with a turbo you are not always under boost.. so those times when your at full boost and raising the effective CR will be less often. Thus less stress on your internals.<hr></blockquote>

Ok but If I run compression of 8:1 for example .. And I get 15psi of boost .. I calculated it and it gives me 16.163265306122447 for Effective Compression .. If I have 9.6:1 (zetec) and put 10 psi of boost, I get 16.130612244897957.. I thought we we're looking for 14.1 (ideal) ? At so, what your saying is even If I have an effective ratio of about the same thing, I will get more power out of the 8:1 static compression with 15 psi, than with 9.6:1 with 10psi of boost ..

But at 8:1 static compression .. my car would run like **** if I didn't have any boost wouldn't it ? Because I'm lowering static compression for the turbo that gives me the extra missing power (and more) but if I don't create any boost .. I'm screwed

What ideal effective compression ratio are we looking for here? And does it matter which static compression your running and boost? What do I NOT want to go over? (numbers)

<blockquote><font size="1" face="Verdana, Arial">quote:</font><hr>Originally posted by belacyrf:

Bottom end = Crank, Rods, Pistons (and some would say the block)

Lower compression ALLOWS for higher boost and WILL produce more power. Not more power than high compression at the same boost, but more power than high compression at low boost. With Higher CR engines you WILL hit a wall as to how much boost you can run. Lower the compression.. and you have further you can take it.

Tom would probably say that the engine lasting is REALLY going to depend on your fuel solution. You can do many things (within reason) if you can get enough fuel to accomodate the new air flow!

Maybe some of the other guys can actually give you the mathematical calcs to see what at what ratio effective compression rises under boost. To see why power increases!<hr></blockquote>

Ok so crank is crank shaft right?, rods and pistons .. hmmm, getting 8.x compression pistons would lower compression but won't really be more durable than the 9.6 I got now right? (unless reworked or something)

And what If I run 94 octane instead of 91 for example .. will it make a difference between both setups? Or will 9.6 require more higher octane fuel? (I guess it will..)


Ok now for the A/F ratio, Project_ZTS said that 12.5:1 was ideal ratio for air and fuel mix .. how come I read 14.7 was ideal?! 14.7:1 would mean Leaner than 12.5:1 right?

There's so much different ratios, how to keep track of all of them

I love you guys, I'm learning so much lately

 

It's compressing when the intake valve is open which does most of its work while the cylinder is heading down. The cylinder heads up while the valve closes. You're increasing the swept(piston down) volume while leaving the compressed (cylinder up) volume the same.

Also remember this is effective compression ratio, and it changes with boost. It is not static compression ratio which is simply the swept volume divided by compressed volume as measured when the engine is disassembled.

 

ahhh, new term

"effective compression ratio"


I'm going to jump to a conclusion this is much different than mechanical compression ratio. Am I getting close?

Thanks guys
Phil

 

Yes. You can change the effective compression ratio even w/o forced induction. Just alter the valve timing and you can end up with slight differences in the effective compression ratio.

 

The 14.7:1 a/f ratio you are talking about is what's know as stochiometric. This is when chemically speaking there is enough oxygen molecules to completely burn all the fuel molecules with nothing left over. This a/f ratio does not produce the most power, you need a richer mixture to achieve max power. This is the 12.5:1

 

<blockquote><font size="1" face="Verdana, Arial">quote:</font><hr>Originally posted by Project_ZTS:
The 14.7:1 a/f ratio you are talking about is what's know as stochiometric. This is when chemically speaking there is enough oxygen molecules to completely burn all the fuel molecules with nothing left over. This a/f ratio does not produce the most power, you need a richer mixture to achieve max power. This is the 12.5:1<hr></blockquote>

Oh cool, thanks for the info

If you change the timing of your sparks, will it affect effective or static compression? i mean, why do we need to reprogram a chip to get best fuel system ..

 

Oh, and something else .. if can I reduce compression with the head or with pistons ? (or both) ..

And if i do one of the do and turbo .. will it have less stress on the rods for example than if i ran 9.6:1 with about the same gain in power (not same amount of boost..)

Who makes stronger rods and stronger pistons .. what would be the FIRST mod to have the bottom end stronger ..

and why do rods break in the first place