Compression Ratio and Compression ?'s

[Lilsupra]

Whats the safest compression ratio we can run with a turbo? 8.5:1, so we add on more boost say in the range of 12+psi...whats the compression Ratio now...My point is whats the highest the compression ratio that can be reached without blowing gaskets or damaging the GTS internals 14.5:1??? The GTS stock is 11.5:1 compression ratio...how much pressure is that in PSI??

[larryd]

i dont think anyone knows for sure as of yet, but if I had to take a guess Id say no more then 12:1.. which the celica isnt far off stock at 11.5:1.. thats about what youd be running with a 50 shot of nitrous..

[MaasNeotek]

Compression ratio's are just that.... compression ratio's.

Adding boost or Nitrous will not change the compression ratio. It's a mechanical thing. Max volume at peak of intake stroke to min volume at top of compression stroke determines compression.

How much pressure is packed in there is up for discussion however. In the GTS - you are looking at around 170psi at top of compression assuming sea level atmosphere - unmodified (11.5x14.7psi)

Running 6lbs of boost in an SC or Turbo (or Trubo...) you'd see 11.5x20.7psi (240psi) - Larry - what did your compression look like on the GTS cylinders?

I kind of guess that would be topping it out...

[larryd]

i dont remmeber the #s off the top of my head.. the compression was actually bad in my car, i was losnig omcpression ni one of my cylinders.. hence the whole rebuild thing..

but I cant be as technical as you but Im 99% sure that when you add nitrous or some sort of forced induction you are changing your compression ratio..

[MaasNeotek]

Anyone with an intimate knowledge of engine dynamics - please take a read...

I know the the compression ratio is derived mechanically...

Like - http://www.dunegoon.org/compression.html

Theres no question that Turbo/Nitrous/SuperCharger increase the pressure in the cyl at ignition - but not the compression RATIO. It's a mechanical relationship between the maximum volume of the cylinder, and the fully compressed volume immediately prior to detonation...

[Ackbar]

I believe the person is referring the the EFFECTIVE compression ratio, which does change as a function of boost.

Go to this link to calculate effective compression ratios ( http://www.smokemup.com/utilities/calc/eff_cr.cfm ).

Your formula of CR*boost isn't familar to me... honestly, I'm not sure if it even makes much sense. Well, at the very least those numbers don't seem to be representative of anything. While effective CR can be, hence much more useful number.

Now back to EFFECTIVE CR, typically (of course every engine is different!) people try not to exceed 14:1-15:1 effective CR. Although, personally, I feel that can be exceeded by a large amount as long as EGT's as well as fuel considerations are kept in mind. The main concerns of high boost levels on high CR engines are the threat of blowing a head gasket and detonation. I would recommend not exceeding 16:1 EFFECTIVE CR without prior knowledge that over 16:1 EFFECTIVE CR would be safe.

Originally posted by MaasNeotek
Compression ratio's are just that.... compression ratio's.

Adding boost or Nitrous will not change the compression ratio. It's a mechanical thing. Max volume at peak of intake stroke to min volume at top of compression stroke determines compression.

How much pressure is packed in there is up for discussion however. In the GTS - you are looking at around 170psi at top of compression assuming sea level atmosphere - unmodified (11.5x14.7psi)

Running 6lbs of boost in an SC or Turbo (or Trubo...) you'd see 11.5x20.7psi (240psi) - Larry - what did your compression look like on the GTS cylinders?

I kind of guess that would be topping it out...

[Ackbar]

Well... depending on altitude your atmospheric "boost" is ~14.7psi. So if you're boosting then the pressure of air you're putting into the engine is 14.7psi + boost. I guess that's what you're talking about. CR can not be converted to pressure, it's just a ratio (depending on mechanical components of an engine).

Originally posted by Lilsupra
Whats the safest compression ratio we can run with a turbo? 8.5:1, so we add on more boost say in the range of 12+psi...whats the compression Ratio now...My point is whats the highest the compression ratio that can be reached without blowing gaskets or damaging the GTS internals 14.5:1??? The GTS stock is 11.5:1 compression ratio...how much pressure is that in PSI??

[tirechirp]

You'll see more than 170psi when compression checking an 11.5:1 engine because the air heats up as it's compressed, which causes additional pressure. You'd be more likely to see 170psi on an engine with 8.5-9:1.

[MaasNeotek]

Ackbar et al;

Thanks for the clarification. I knew that 'Compression ratio's were a mechanical function- and that there was working process for determining compression at ignition.. thanks for the leadup to effective compression!

That might answer a WHOLE load of questions!

Cheers!

[Kit99bar]

wow where'd all you smart guys come from?! :)

[Ackbar]

Well... I don't know if you're referring to me... but generally, I lurk here until I see something wrong and then I post. Since I've got some "secret plans" of my own for my Camry, I have a pretty good idea of Toyota's and forced induction. You'll probably see me post more as the topic of superchargers and turbos get brought up more. AND (more importantly) my roommate has a GT-S 6-spd and I'm trying to get some mods together so he can get his car up and competing with mine. =)

BTW- If you couldn't figure, my car is not stock.

[spaldam]

I've always know Compression ratio to be a mechanical thing, but I also know that people like to come up with weird ideas like "EFFECTIVE CR" to try an explain things in a way that more easily relates to something they understand better then the more complex way it should be explained such as will psi (which doesn't compare with CR, and thus the reason for making up EFFECTIVE CR so you can theoretically compare it to mechanical CR).

Anyway, it really doesn't matter that much as long as your comparing apples to apples, and everyone understands what your talking about.