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Supercharger vs. Turbo & Compression Ratios
Okay, lately I've been seeing a lot of the same questions being asked over and over again. Being that you're newb's and don't know how to use the search function Rolleyes I've pretty much summed up everything in this one thread. So don't ask any questions till you've read this. Everyone else, feel free to add anything you think is relevant.


The Good Ol' Supercharger vs. Turbocharger vs. Nitrous Debate

Well, what you choose is basically dependant on your preference or funds you've set aside. No matter what people may say, nitrous (notice I said nitrous, not NOS) is just another power adder. Period. It's not really cheating, but rather a simple, relatively cheap way to add power to your engine. General rules of forced induction apply. Keep your engine healthy, feed it the fuel it needs, and it'll work the way you want it to. Basic nitrous systems are probably cheaper than most turbo/supercharger systems by at least hundreds of dollars.

Now for superchargers vs. turbochargers. It still depends on your preference. Most superchargers deliver power practically right off idle, driven by the engine itself by pulleys and belts. Turbochargers, on the other hand, run off spent exhaust gases the engine builds up. Turbochargers don't reach their full power until exhaust gases build up enough. This waiting time is also known as "turbo lag".

So with turbos, you have to worry about turbo lag, but you get more pwoer in the higher end. With superchargers, you get your power right off idle, but not many superchargers can rival the high-end power that a turbo can produce.

...Part II continued later
18th Airborne Corps, Dragon Brigade

1990 180SX

Compression Ratios

What the heck is a compression ratio? Wow, if I had a nickel for every time I heard that...

The amount your engine will compress a fuel mixture depends on how small of a space the mixture was crammed in to. Compression ratio is how much cylinder volume the piston moves from the intake stroke to the compression stroke. Say the piston moves 6" of cylinder volume in the intake stroke and 1" of cylinder volume in the compression stroke. The pison has crammed 6" of cylinder volume into 1" of cylinder volume, a ratio of 6 to 1. This would give you the compression ratio of 6:1.
18th Airborne Corps, Dragon Brigade

1990 180SX

Engine Size (Displacement)

Oh my gosh...there are so many times I've heard this term misused...while we're on the record to set things straight here:

Engine size (not physical dimensions, mind you) is related to piston displacement. Piston displacement refers to the total number of cubic inches of space (liters) in a cylinder when the piston moves from the top of its stroke (TDC) to the bottom (BDC).* Displacement isn't all that complicated. It's just the area of the cylinder multiplied by the total piston travel from TDC to BDC, then multiplied by the number of cylinders.

*=TDC and BDC refers to Top Dead Center and Bottom Dead Center for all you newbsBig Grin.
18th Airborne Corps, Dragon Brigade

1990 180SX
Alirght guys, compression 101:
Pistons are the primary way by which engine builders control static compression ratios. The static compression ratio is the ratio between the cylinder's maximum volume (found at piston BDC [Bottom Dead Center], or the lowest point it reaches in the cylinder) and it's minimum volume (found at piston TDC [Top Dead Center], or the highest point it reaches in the cylinder). Here's the math for all of this in case you want to play around with it sometime...

Static Compression Ratio = (S+C)/ C
Where S= total swept volume and C= chamber volume, and...

Swept volume= (x² × pi) × stroke
Where X= radius of the bore (bore/2), and...

Chamber volume= cylinder head volume + gasket volume + piston volume + crevice volume

So a 10:1 static compression ratio means there is 10 times the cylinder volume at piston BDC than at piston TDC. Now, understand that the piston volume part of the chamber volume equation is what we will be playing with. Now with a piston, you can have three basic sorts of shapes to it's crown (that's the top). You can have dished (also called inverted dome), flat top and domed pistons, here's pictures of each basic type to help you get a handle on these concepts...


Flat Top


So when I say we will be playing with piston volume, let's first understand this can be a positive OR negative amount. A dished piston has positive volume, meaning it actually adds some space to our total chamber volume amount. Conversely, a domed piston not only has no positive volume, but also takes some away from the total chamber volume as it's dome takes up some space that would otherwise be available for air/fuel mixture. So what does this have to do with performance?

What static compression ratio is best to run depends upon many things, such as airflow quantity, airflow quality, heat of the air/fuel mixture before compression, fuel properties, anti-knock ability of the combustion chamber, and a host of other things (it's not important you even understand half of what I just mentioned yet). But understand the whole point to compression is to heat the air/fuel mixture to near it's auto-ignition point, because that's where it will burn the fastest and allow us to most efficiently harness it's power. For an analogy to this all important concept, look at kindling wood for a fire.

With kindling, what you are trying to do is make some heat through very fast burning wood. You aren't going to make the whole fire out of this cheap wood, but you won't ever get a big log burning well without first getting some heat into it. And why, you ask? Because good firewood doesn't burn easily or fast, and you have to get it nearer to it's own kindling point (aka it's auto-ignition point, the temperature at which it automatically begins to burn) to speed up the burning process.

And gasoline is no different really, although it burns much faster from the get-go, it still needs a large amount of extra heat put into it to really get things cooking fast (and you need FAST for high RPM power, where you literally only have a fraction of a second to put pressure on the piston during the power stroke to make power). So how do we do this? By compressing the entire air/fuel mixture, which of course heats it some (everyone who took introductory science should remember that). So how much should we compress (aka heat) the mixture? Well that depends upon all those things I mentioned earlier, plus several more, but in the end most Honda motors tuned properly like an 11-11.5:1 static compression ratio for a safe but powerful amount of heat on premium octane gasoline. You can often go as high as 12.5:1 on a really well built motor with pump gas, but this takes $$ and a great understanding of the engine you're building.

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