More turbo chatting with our friendly London resident:
This chat covers compression ratios, volumetric efficiency, and other things.
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Mystic511: Effective compression ratio = static compression ratio x (1 + boost/14.7)^1/2
MUR1X: yes
Mystic511: alright, you have some 'splainin to do.
MUR1X: :)
MUR1X: when you boost a motor it actually raises the compression ratio
MUR1X: under boost of course
MUR1X: ok- I guess I need to break out in some explanation
MUR1X: here
MUR1X: use this calc for something easier:
MUR1X: http://www.smokemup.com/utilities/calc/eff_cr.cfm
MUR1X: do yo know what the actual compression ratio stands for?
MUR1X: that would be the first start
Mystic511: compression ratio is versus atmospheric pressure?
MUR1X: yes sort of
MUR1X: I imagine a more specific explanation is in order
Mystic511: yea
MUR1X: the key word here is compression
MUR1X: just like it says
MUR1X: you take so much air and fuel
MUR1X: and you compress it as much as possible
MUR1X: the static ratio is the one set by the bore/stroke deck height etc
MUR1X: the effective ratio is what is really happening
MUR1X: when you cram extra air into the engine - ala forced induction - you have more air to compress
MUR1X: it has to compress this air in the EXACT same space as the air it had before
MUR1X: because you are taking more air into the same volume
Mystic511: right
MUR1X: you have now raised the compression ratio
MUR1X: which is the effective compression ratio
MUR1X: the compression ratio tells you nothing of the quality of air you are getting or how efficient the engine is
MUR1X: ve
MUR1X: volumetric efficiency
MUR1X: I found a technical definition as well
MUR1X: the ratio of the volume of the cylinder at the beginning of the compression stroke (when the piston is at BDC) to the volume of the cylinder at the end of the compression stroke (when the piston is at TDC)
MUR1X: lower compression ratios help avoid fuel autoignition - aka as knock which is BAD for our ringlands
MUR1X: The higher the compression ratio, the higher the air temperature in the cylinder at the end of the compression stroke. Higher compression ratios, to a point, lead to higher thermal efficiencies and better fuel economies.
MUR1X: The problem is with that knock we try to avoid in a gasoline engine
Mystic511: ah.
MUR1X: TECHNICALLY, running a higher compression ratio is possible if you can get the tuning DEAD ON
MUR1X: this is what Gary was preaching some time ago

MUR1X: REALISTICALLY
MUR1X: you will blow something
MUR1X: the tuning window become smaller the higher the compression
MUR1X: the stock pistons with boost are highly succeptable to detonation
MUR1X: by lowering the static compression ratio you are buying yourself a larger window to work with which means longer lifespan in my mind
Mystic511: yea
Mystic511: 10 psi on stock engine with turbo gasket... save for rebuild
Mystic511: woohoo
Mystic511: get some colder plugs too
MUR1X: it can be done
MUR1X: another way to save from knock is the crane
MUR1X: retard timing
MUR1X: it makes a difference

MUR1X: oh.
MUR1X: keep in mind the compression ratio does not take in to account parts of fuel or air
MUR1X: the compresion ratio is a total volume of whatever is in the cylinder
MUR1X: things like intercoolers give you more dense air
MUR1X: dense air= more air
MUR1X: packed into the cylinder
MUR1X: thus the need for more fuel of course
MUR1X: this can have an effect on VE
MUR1X: my other thought is that on a small turbo it will spool fast enough to not require a high CR
Mystic511: 16g and low comp should work really well together
MUR1X: I have 8.8:1
MUR1X: so does Armond
MUR1X: I am starting to wonder how low the TTI gasket lowers compression though
MUR1X: I did the math wrong on that one
Mystic511: they said 1 point lower
MUR1X: it does look like agood gasket simply because it is a thicker version of the stock MLS


MUR1X: does any of what I said make any sense btw?
MUR1X: sometimes explaining things is the most difficult thing
MUR1X: and of course sometimes I might get something wrong by missing something I considered unimportant when it fact it would make the difference
Mystic511: i got it
MUR1X: keep in mind the most basic principle too
MUR1X: the more compressed the air the more powerful the explosion
MUR1X: which pushes the pistons up and down :)
Mystic511: boost affects compression ratio.. lower static compression ratio keeps temp down which prevents detonation
MUR1X: something like that
MUR1X: which also falls into the larger turbos give cooler air theory as well
MUR1X: which is why you can run more boost efficiently on a larger turbo
MUR1X: the air is not as heated
MUR1X: the trade off of course being lag
MUR1X: matching a turbo size to an engine size is EVERYTHING in turbos
MUR1X: and partially a black magic
MUR1X: not only does a large turbo push more air, it pushes more dense air
MUR1X: the idea is just like CR, you have to find the happy medium
MUR1X: simple break down of differences - I am working this out in my head as you go mind you to make sure it is all straight
MUR1X: high compresion = larger explosion
MUR1X: forced induction = more air to compress
MUR1X: find the happy place in between the two and you get a kick ass car :)
MUR1X: that in itself also is the problem with high compression ratios
MUR1X: it does not address VE in terms of how much air it is compressing

MUR1X: you make more power with more boost
MUR1X: you will not lose more than maybe 5-10 hp by lowering the Static compression ratio
MUR1X: you will more than make that up with more boost
MUR1X: especially since you will be compressing a hell of a lot more air at 12psi vs 10psi with relatively the same compression ratio

lol

One thing that i am not 100% sure of that would shoot your theory to hell is that no matter how much air is fed by the turbocharger/supercharger/etc, you can only cram so much air in the cylinders. Is the static compression actually accounting for that *so much* air that is in the chamber?

Or is static compression the amount of air at atmosphere (before boost)?

You have good points, but even though no matter how much you decrease the chance for detonation on our stock internals, the stock rods are known for blowing out and giving way even at STOCK power...

Do you want to chance destroying your block to save a few dollars? In the long term, if you are going boost, there is a HIGH probability that you will want to up the boost. Tell me ONE turbohead here that hasnt upped or want to up their boost? =)

If you are going to spend the time to take the cylinder head off, you might as well as spend some cash on upgraded internals (for less than 600 bux you can get stronger rods and pistons). You CAN swap internals without taking the engine out of your car, ya know ;)

Good topic.

_________________
Vinnie Lima
kiku@asog.net
AIM: Kiku888
1997 Chrysler Sebring "Silver XTC"
I4 420a w/ HRC Stage 2.45 w/ FMIC
14.7@97 mph@6psi
Proud member of the
ASOG BoOsT JuNkIe Crew

>You have good points, but even though no matter how much you
>decrease the chance for detonation on our stock internals,
>the stock rods are known for blowing out and giving way even
>at STOCK power...

In my experience, the only way this has ever happened has been due to oil starvation to the rod bearings, or an assembler error. Don't let monkeys work on your car, and watch ou oil level, should never happen. Just my $.02..

Correct me if I am wrong, but wasn't steve running stock rods with weisco pistons on his rs back before he wrecked (i.e. when he was running 15 or so psi)?

A.D.P.
bring it on............

Steve did use stock rods. So did Stan. Steve ran 15psi on a Star Kit road racing and was harder on his 420A than anyone else I have heard of. It took one ride in his car to see how crazy he was. :)

Kiku, you make a good point, but you are forgetting density and assuming that the VE of the engine is 100% ;)

>>Or is static compression the amount of air at atmosphere (before boost)?

YES

Effective compression is the actual compression that is going on. Static is just that: static and a starting point.

VE for those that ask by the book definition:

volumetric efficiency Thermodynamics. the ratio of the actual mass of gas compressed to the mass of gas occupying piston displacement at the inlet pressure and temperature; used as a measure of engine efficiency.

The static compression is only a basis to start with that specifies the ratio of the volume of the cylinder at the beginning of the compression stroke (when the piston is at BDC) to the volume of the cylinder at the end of the compression stroke (when the piston is at TDC)


It is not a measure of volume but a ratio of the volume compressed.

Damn, homey sure knows his technical shit.

Is there anyone out there boosted, running at least 10psi, for at least 5 months, on stock rods and pistons???

oh yeah, on a daily driven car as well.



_________________
Vinnie Lima
kiku@asog.net
AIM: Kiku888
1997 Chrysler Sebring "Silver XTC"
I4 420a w/ HRC Stage 2.45 w/ FMIC
14.7@97 mph@6psi
Proud member of the
ASOG BoOsT JuNkIe Crew

My freind brian has. His name on here is redturboGS. He doesnt post much though.

I know that Joe ran 12psi daily, for quite some time. He also had a thicker than stock head gasket. As far as only being able to compress the cylinders so much. You could compress 1000psi in them if you wanted. Doesnt mean your connecting rods and pistons would be able to do the actual compressing though.

good info, teach me more.........