Forced Induction is Forced Induction...
A widely misunderstood fact among racers is that forced induction is forced induction is forced induction...period. Now, what do I mean by this? I mean, at the end of the day, a turbocharger, a supercharger, and nitrous oxide all serve the same purpose: to raise motors effective compression level. Although they may use different mechanics to do so, they all fundamentally do the same job.
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Well, what is "effective compression?"
Effective compression is the sum of the static compression, plus the additional compression added to the cylinder via some kind of forced induction tool. Static compression is the amount of air inside the cylinder that is compressed. In order to find what the static compression is, e.g. 12.5:1, one must find the ratio of cylinder volume at BDC, and TDC. An equation for effective compression is as follows:
((boost psi / 14.7) + 1) x motor compression = effective compression
So for example: A motor with a 10.0:1 static CR boosting 10psi 10psi/14.7psi = .68……… .68………+ 1 = 1.68……… 1.68………x 10 = 16.8 effective CR
A motor with an effective compression ratio of 16.8:1 is easily daily driven, with proper fuel and ignition upgrades, high-grade gasoline, and tuning.
Well, what does this effective compression have to do with my nitrous, my supercharger, or my turbocharger?
Essentially, as I stated earlier, all forms of forced induction effectively raise your motors compression ratio. Boost is boost, forced induction is forced induction. They all do the same job, they just go about doing so in different ways. There is no one form of forced induction that is inherently safer than any other form. "Safe" "Reliable"...etc., are all words used by manufacturers and retailers to lure you in to their client base. Safety and reliability start, and end, with you.
A motor boosting 14.7psi, with a static compression ratio of 10.0:1 will have an effective compression ratio of 20. (as shown) ((14.7 / 14.7) + 1) 10 = 20 effective CR
Now, can you see how there is no distinction between turbocharger, nitrous (as expressed as a form of boost), and superchargers? Your motor does not make the distinction. All your motor sees is static and effective compression levels.
Since higher effective compression levels are what I am looking to achieve, should I do so with a low static CR and high boost, or a higher static CR and low boost? Or how about a little bit of both?
Low Static CR & High Boost This sometimes seems to be the best way to go, for people who don't study their homework. Of course, this will work for you, but your motor's efficiency will go straight down the toilet. Daily driving duties are going to be a tedious, high RPM fiasco. Low-end power is going to be nil, and there really is no need to sacrifice it for more boost, there are more efficient ways.
Higher Static CR & Lower Boost This way makes more sense than a low static CR & high boost. With a higher static CR, and lower boost, you can make the exact same power as a low static CR motor & high boost, but, you get the added advantage of low-end power. Your compression is with you always, but your boost is not. Since compression is what will ultimately dictate power levels, why not have more useable compression across the powerband? But, there could be a problem with this area. In order for this to be an efficient way to make power, the turbocharger (or other form of forced induction) must be matched to the motor perfectly. Since boost levels will not reach the heavens to achieve the power we are looking for, the turbocharger cannot be too big. Larger turbochargers usually take more time to spool, and usually need more boost to work correctly. For example, a 60-1 on a D16Z6 with, for the sake of argument, 11.0:1 static CR, will not be efficient at all. This particular turbocharger thrives on boost, and needs more than 9psi to "come alive." Conversely, a ball bearing T3 on a D16Z6 with a static compression level of 11.0:1, will be extremely efficient. This turbocharger can work well in a low boost environment.
Higher Static CR & Low Boost (mid-range) Here is where the balance hangs. The best way to go is to follow the route above, and match your forced induction to your motor. Moderate boost is the way to go in street cars. Selecting the most efficient product means understanding your motor, and the prospective products.
On a side note...
Now that you know more about effective compression, you know why forced induction motors are always going to make far more power than naturally aspirated motors. In order for a naturally aspirated motor to make the kind of power a forced induction motor makes, the naturally aspirated motor would have to have a static compression ratio of over 20.0:1 (and in most cases, far more!).
Happy boosting! MatT3T4
