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im no expert, but compression has to due with the pistions and is defined as a ratio; Swept Volume (SW) + Top Dead Centre (TDC) Volume) divided by TDC Volume- (SW+TDC\TDC) where TDC is Head Volume + Gasket Volume + Deck Volume + Dish/Dome Volume and SW- is pi x Bore Diameter x Bore Diameter x Stroke divided by 4
yea it gets complicated fast, In higher compression engines there is more pressure wich means more power, but also more strss on the internals. By lowering the compression you loose power but relieve some engine stress, so often times people will lower the compression on an engine when going Fi. the loss of power by using a lower compression is made up and far exceeded by the turbo or super charger. compression is not bad for turbos its bad for your engine internals. you could raise compression and turbocharge for large power gains, but expect to pay a preimum for internals that can handle that kinda pressure.

btw: do a serch on google for piston compression or engine compression you shoud find some good stuff
 

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this has been debated many MANY times over at probetalk and some turbo gods have all agreed that it is best to run the highest compression ratio with the highest boost possible.

it's the whole reason why imported engines respond so well to boost. the high compression ratios. if you lower the compression ratio a bunch then up the boost, your car is slow as balls until the turbo finally spools.... which isn't good for daily driving at all and not always the best for track racing either. if you keep the stock compression ratio and just run moderate boost, you still have plenty of power at every RPM.

tried and true.....
-if u want the best power/driveability and a long-lasting turbo system, run high compression with the highest boost possible.
-if you want to put more strain on your turbo, have it wear out faster, lose all of your bottom end power, and be able to say "hey, i'm running 22 psi" then drop the compression and up the boost.
 

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SlickSix311 said:
this has been debated many MANY times over at probetalk and some turbo gods have all agreed that it is best to run the highest compression ratio with the highest boost possible.

it's the whole reason why imported engines respond so well to boost. the high compression ratios. if you lower the compression ratio a bunch then up the boost, your car is slow as balls until the turbo finally spools.... which isn't good for daily driving at all and not always the best for track racing either. if you keep the stock compression ratio and just run moderate boost, you still have plenty of power at every RPM.

tried and true.....
-if u want the best power/driveability and a long-lasting turbo system, run high compression with the highest boost possible.
-if you want to put more strain on your turbo, have it wear out faster, lose all of your bottom end power, and be able to say "hey, i'm running 22 psi" then drop the compression and up the boost.
There is some truth to this but this is not all entirely accurate. Lets start with basics. Compression. Compression is exactly that, compression. On a 4 stroke engine the 2nd stroke is the compression stroke. This is when the engine compresses the air in one cylinder so when it is ignited the force of the explosion is enough to force the piston down and rotate the crank. That was the short version. Now, the higher the compression, the higher the power output of the explosion. Thus applying more torque to the crank. On an NA engine raising the compression as high as possible is the only way to get the full potential out of the engine. Theoretically, running the highest compression possible as well as the highest boost possible will yield the best results. While this is the way to achive the most horse power it is in most cases impossible, because of the strength of the engine internals. Running anything above 11.0:1 CR starts to effect pump gas driveability. The reason most people and manufacturers use around 8.5:1 CR (which is relatively low) on turbo'd cars is because a lower CR is more tolerant of improper tunning, while maintaining moderate boost levels, around 10psi. Boost not only raise your effective displacement (useing compressed air) but also raises your effective compression. At 8.5:1 CR on 10psi the effective compression is 14.3:1. The only problem is that you have to wait until the turbo spools up to reach that power output.

SlickSix311 said:
tried and true.....
-if u want the best power/driveability and a long-lasting turbo system, run high compression with the highest boost possible.
-if you want to put more strain on your turbo, have it wear out faster, lose all of your bottom end power, and be able to say "hey, i'm running 22 psi" then drop the compression and up the boost.
Basically it's all preferance.
high CR + low boost = lots of NA power, faster spool time less top end and requires carefull tuning.

low CR + high boost = less NA power, slower in lower RPM's, slow spool time, lots of top end and less tuning.

high CR + high boost = lots of NA power, fast spool time, lots of top end, extremely carefull tuning and less engine and tanny life.
 

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Most of what you gents said has alot of accuracy. One of the othe big key factors of adding massive power by adding turbos is the size and number of valves in the engine. If you turbos a 4 cylinder engine with 1-2 valves per cylinder you wouldnt have anywhere near as good of results as with a 4 valve per cylinder engine. This is why the KL engines are so firggin awesome for boost.

That being said, the more air & fuel that can be compressed by the pistons the better the engine breathes as its "boosting". I mean its common sense. When you have a small fire that has burned out into just cinders what happens when you blow on it? The cinders get much hotter and try to burn more. So in a nutt shell if you have a given fire (spark igniting with air and fuel) and you push/compress more air into the fire and ignition and you will get more power delivered from exhaust. I hope that kinda explains it. The only time you run into higher compression being a bad thing with boost is when you have so much air and heat in the combustion chamber that it will pre-ignite and give you an early boom with the spark and that usually sounds like this.... Vrrrroooommmmmm..psssssssss... B A N G !!! :D Been there done that when I overboosted at 15 psi lol. Anyway the point is most engines with compression ratio's above 9.5:1 you can run into serios pre-detonation problems from added heat inside the cylinder walls. Thats all I know, hope it makes sense.

Dan

BTW, sorry so darn long heh :)
 
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All above is true and accurate, there is how do you say, its all up to the user and builder on how they want they cars personality to be under boosted conditions, for example the KL is 9.2:1 stock that means it takes 9.2 parts of air and fuel and compresses them into 1 then kablam a little before top dead center the spark goes of and power is produced by the piston being forced down, a turbo effectively raises compression by forcing more air and hopefully fuel into the cylinder than the motor ever could ingest on its own, however under boost it is next to impossible to determin what your CR ratio is, which is all irrelivent because now I think I'm off topic and rambling.

Ok regroup of things, Stock compression is good for turboing the KL, with higher compression you will see a better powerband on the low and mid side of the rpm scale, the downside is turbos on stock KL's don't usually like low boost pressures of 8psi or less, so the motors happy and the turbo is just ok, if you built the motor and ran higher boost pressure that would be ideal.

The ideal compression ratio for a street driven turbocharged vehicle is 8.5:1, the happy medium of all things, the stock F2T found in the 1st gen probes is low at 7.8:1, turboed from the factory is the reason, on racecars you will see compression ratios of up to 13:1 on turboed cars running high boost, they have the best of both worlds, due to nearly indestructable engines that are rebuilt every few runs and have no life expectancy, but they produce helacious power and they produce power easier due to the higher compression is already making more power than if it were low. COnfused yet.

Ok remember me saying it is the user and builders preferance on how they want the car to act?, ok here is how it goes,

Little turbos spool extremely fast but are just a restriction on the top end.

Big turbos are laggy but are helacious on the top end

Small turbo and low compression, daily driver OEM 1st gen

Properly sized turbo and stock KL compression, decent spool up and will produce boost throughout the powerband

Properly sized turbo and low compression KL, ie 8.5:1, system will be laggier but you will be able to produce more boost and get the turbo happier and more effeicient as indicated by its compressor map

A/r ratios, lower A/R ratios produce boost sooner but are a restriction in the exhaust at high rpm restricted flow and power

Properly chosen A/R ratio, turbo will spool up at a decent rpm, near 3-4k depending on turbo size and will also produce boost at high rpm easily

Overkill A/R ratio, extreme amounts of lag lead to an unhappy turboed KL, I read the post on PT and yes that had to be a diesal turbo, which flow a lot on the turbine side but not all that much on the compressor side

So I think I halfassedly explained the basics of everything and if you have any more questions I can be reached either by PMing or email.
 
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