Hey Legz, compression is exactly what it sounds like, air being compressed, and when it is compressed it heats up. The idea with running a lower CR with forced induction applications is that you can force more and hotter air into the cylinder before it will reach detonation as it is being compressed less in the cylinder. When you compress already hot air it gets hotter so the issue is with lower octane fuels this heat can ignite the fuel before the piston reaches the desired point of ignition, if this occurs very early in the compression stroke you can easily blow parts into nice pieces as you have th entire rotating assembly going one way and one piston and rod being forced the other basically. It has been common to think that running low CR's for forced induction is the way to go but as controls and engine design have advanced this is becoming less and less mainstay. While 9.0:1 CR is still pretty much standard for all FI production vehicles and most modern aftermarket applications there are many engines running 10.0:1-11.0:1 CR with forced induction. The real issue is heat and tune when running FI with higher CR applications. I personally have seen several Toyota 1UZ-FE 4L V8's with a 10.0:1 CR running 12psi or more with the most being twin turbo set up running over 18psi on stock internals I might add - longevitiy is another question.
A lower CR is basically an extra safety margin so you don't loose your $$$ investment. Honestly as long as the pistons are good enough to handle the pressure I see no problem with running pretty much the same boost on 10.0:1 CR as 9.0:1 CR as long as cooling and tune are taken into consideration, once you start going above 10.0:1 CR the tune and sensor data is critical to stay out of the danger zone which is why most stay at this level or below. Dispite this the BMW E92 M3 runs a 12.0:1 CR and there are many supercharged examples running around, usually no higher than 12psi from what I have seen and most running 6-8psi but they are tuned well enough and running good enough fuel that they aren't detonating. The most powerful I have seen was touching 700bhp on the stock engine with standard CR. Can it be done, yes, should it, only if you have the ability, knowledge and a perfect tune.
Turbo lag is a misused term by most, the actual term means the time until the turbo creates positive pressure which it will do pretty much once the throttle is open.
The 'turbo lag' you refer to is the time that it takes or revs that it takes for the turbo to spool to the desired boost pressure. Its a bad term because it is completely dependant on what the desired boost pressure is. You might want 18psi out of a T88 on an SR20 and say you have really bad turbo lag because it takes basically until redline to reach but someone might have the same set up and only want to reach 6psi, same engine, same turbo but one would say they have more turbo lag even though they are both the same. So in response to your question yes you will hit a lower boost level sooner, when counting to 10 do you get to 3 or 7 first? I suggest reading up on forced induction to understand more about turbo sizing, its not how much boost you run but what total system and your goals. You have to figure out how much power you want and when you want it and how you want delivery there is no one easy answer.
In this regard I would rather run a slightly lower CR and slightly more boost, an engine generally makes more power the higher you go up the rev range so I don't see any problem with taking slightly longer to get to your desired boost given that you are on power still, you will still get to the same boost as a higher CR engine within a few hundred rpm but it will simply make more power for the same amount of boost.
Comparing apples to apples a higher CR with the exact same set up in every regard including turbo will spool faster reaching all boost levels at a lower engine rpm, the extent of this is often minimal between single didgit CR changes, ie 9.0:1 to 10.0:1. If you compress the air fuel mixture more (higher CR) which generate more power from the same boost level then you are going to create more exhaust energy meaning the turbo is going to spool faster. You can tune to take more advantage of this but unless your going for all out power the smart person with a lower CR would simply change the exhaust housing to get to their desired boost level quicker. Higher CR with boost = higher efficiency, meaning you will make more power on the same boost. It really varies how much of a power increase you will get based on a number of factors and its really hard to compare unless you have two identical engines with different CR's. What you will get is MORE power at the same boost level and better efficiency regardless.
The number one concern for most people regarding CR is safety, on a stock engine like the 1UZ-FE in the soarers they seem to hold 450rwhp fine and 12-18psi but if you stuff up the tune and kill it they are cheaper than a set of con rods and you bolt everything back on again and try again. If your spending $6000+ on a engine build you generally want a bit more safety so your investment doesn't go BOOM! and leave you with a bunch of scrap metal and not much to show for your money.
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