Guide - Choosing the right supercharger/turbo kit

[The Sentinel]

Originally Posted by King Tut

He said my magic words "centrifugal supercharger" and "linear power". I don't want to waste my time explaining what linear power is again, but I will state that it is not what you get from a centrifugal supercharger.

Here's my dyno graph.... how is that power delivery not linear proportional to RPM?

[Kiske]

Originally Posted by The Sentinel

Here's my dyno graph.... how is that power delivery not linear proportional to RPM?

"...In simple terms, a centrifugal supercharger's boost increases exponentially with engine speed, while a positive displacement supercharger's airflow is linear--with maximum boost occurring very low in the rpm band. That means a Roots or twin-screw blower that delivers, for example, 500 cubic feet per minute (cfm) of air at 2,500 rpm will push 1,000 cfm at 5,000 rpm. A Roots or twin-screw blower makes a small amount of boost whenever the engine is running, while a centrifugal supercharger's boost builds in a non-linear way, much like a turbocharger. As rpm increases, the airflow from the compressor will increase at a faster rate. Because of that, maximum boost is not achieved until the engine's red line, or maximum rpm level. So, the centrifugal "rolls" into its boost, and is generally easier to launch, with a stronger feel through the mid-range and upper rpm levels.

The non-linear airflow delivery also makes the centrifugal supercharger better-suited to drag racing, because the graduated boost application enables an easier launch, with greater power coming on as the rpm increases. Of course, with peak boost not occurring until redline, the blower's effectiveness is not fully realized at lower rpm. In other words, unless you're performing full-throttle launches at every stop light, you're not going to feel the maximum punch of a centrifugal blower. That's not to say centrifugals are weak on the street. We've driven a number of ProCharger-blown '10 Camaros, and they're admirably powerful. But there's no denying that more torque delivered lower in the rpm band would help overcome the Fifth Gens' considerable mass."

Copy paste-ta~~~

[The Sentinel]

Originally Posted by Kiske

"...In simple terms, a centrifugal supercharger's boost increases exponentially with engine speed, while a positive displacement supercharger's airflow is linear--with maximum boost occurring very low in the rpm band. That means a Roots or twin-screw blower that delivers, for example, 500 cubic feet per minute (cfm) of air at 2,500 rpm will push 1,000 cfm at 5,000 rpm. A Roots or twin-screw blower makes a small amount of boost whenever the engine is running, while a centrifugal supercharger's boost builds in a non-linear way, much like a turbocharger. As rpm increases, the airflow from the compressor will increase at a faster rate. Because of that, maximum boost is not achieved until the engine's red line, or maximum rpm level. So, the centrifugal "rolls" into its boost, and is generally easier to launch, with a stronger feel through the mid-range and upper rpm levels."

Copy pasta~~~

That's exactly what the article says.

Centrifugal increases in a linear fashion, directly proportionally to engine speed.

Sounds like we are both saying the same thing, but our use of the term "linear" differs.

[CSG Mike]

Originally Posted by The Sentinel

That's exactly what the article says.

Centrifugal increases in a linear fashion, directly proportionally to engine speed.

Sounds like we are both saying the same thing, but our use of the term "linear" differs.

@King Tut defines linear as a flat (zero slope) torque curve.

You define linear as a flat (straight line) power curve.

Both are linear, in their own right.

[The Sentinel]

Originally Posted by CSG Mike

@King Tut defines linear as a flat (zero slope) torque curve.

You define linear as a flat (straight line) power curve.

Both are linear, in their own right.

Yep. Exactly.

[King Tut]

Originally Posted by The Sentinel

Here's my dyno graph.... how is that power delivery not linear proportional to RPM?

Okay, you guys want to do this again. Let's go back to simple math and use your graph as the example. Here is the old formula for a linear function where slope needs to be constant:

f(x) = ax + b;

f(x) = HP
a = slope
x = RPM
b = intercept which in our case is zero because at 0 RPM you make 0 HP

Now here are two points pulled from your graph:

82KW = a3500RPM + 0;
a = 0.02343

184KW = a6000RPM + 0;
a = 0.03067

The slopes are not equal, which means that the horsepower curve is not linear. When torque becomes a constant or a flat line as @CSG Mike put it, then horsepower increases with RPM at a linear rate and the slope along the horsepower curve will be the same everywhere. That is the definition of linear, and it will not be achieved with an increasing boost/torque curve from a centrifugal supercharger.

[bfrank1972]

Originally Posted by CSG Mike

Efficiency =/= throttle/transient response

^ THIS

This is something I mention over and over - dyno graphs do not depict throttle response. Specifically turbos will have no where near the response of a supercharger - quick throttle response makes a car more fun to drive, feel more powerful, easier to fine tune at the limit, etc. Yes, you cannot beat a well designed turbo kit for efficiency, but even the best factory engineered 'lag free' turbo systems will have a 'soft' throttle response, and typically aftermarket turbo kits will have discernible turbo lag... especially when transitioning from no load to full load in the lower RPM band. At 3000 RPM, until the turbo get's spinning I'd wager even the centrifugal units out there will feel more responsive. The PD units will feel downright beastly - which is why people are often pretty happy with the smaller Sprintex units. On 91 they're only around 240whp or under on a dyno, but they are very responsive and pull pretty hard down low.

[CSG Mike]

Originally Posted by King Tut

Okay, you guys want to do this again. Let's go back to simple math and use your graph as the example. Here is the old formula for a linear function where slope needs to be constant:

f(x) = ax + b;

f(x) = HP
a = slope
x = RPM
b = intercept which in our case is zero because at 0 RPM you make 0 HP

Now here are two points pulled from your graph:

82KW = a3500RPM + 0;
a = 0.02343

184KW = a6000RPM + 0;
a = 0.03067

The slopes are not equal, which means that the horsepower curve is not linear. When torque becomes a constant or a flat line as @CSG Mike put it, then horsepower increases with RPM at a linear rate and the slope along the horsepower curve will be the same everywhere. That is the definition of linear, and it will not be achieved with an increasing boost/torque curve from a centrifugal supercharger.

Well, if we're going to get all mathematical here...

If you do some regression analysis, a straight line is a far better fit than a curve, assuming we ignore the part up top where timing is pulled.

[King Tut]

Originally Posted by CSG Mike

Well, if we're going to get all mathematical here...

If you do some regression analysis, a straight line is a far better fit than a curve, assuming we ignore the part up top where timing is pulled.

Let's approach this another way. Let's make a linear equation based off the peak horsepower point and then see where points should fall on it compared to the graph:

f(x) = ax + b;

f(x) = HP
a = slope
x = RPM
b = intercept which in our case is zero because at 0 RPM you make 0 HP

212KW = a7100RPM + 0
a = 0.02986

So our simplified linear horsepower curve will look like this:

HP = .02986 * RPM

Let's plug in the values I used earlier:

HP = .02986 * 3500RPM
HP = 104.5 KW

HP = .02986 * 6000RPM
HP = 179.2 KW

Linear values = 104.5 KW and 179.2 KW
Graph values = 82 KW and 184 KW

So the 3500 RPM point is 22 KW off what it should be if linear power was being delivered. It isn't there because there is only 3 PSI of boost at that point where there is 11 PSI of boost at the horsepower peak. I am not saying that isn't a nice looking graph. It is a nice graph. I just don't like people saying a centrifugal supercharger delivers linear power.

[CSG Mike]

Originally Posted by King Tut

Let's approach this another way. Let's make a linear equation based off the peak horsepower point and then see where points should fall on it compared to the graph:

f(x) = ax + b;

f(x) = HP
a = slope
x = RPM
b = intercept which in our case is zero because at 0 RPM you make 0 HP

212KW = a7100RPM + 0
a = 0.02986

So our simplified linear horsepower curve will look like this:

HP = .02986 * RPM

Let's plug in the values I used earlier:

HP = .02986 * 3500RPM
HP = 104.5 KW

HP = .02986 * 6000RPM
HP = 179.2 KW

Linear values = 104.5 KW and 179.2 KW
Graph values = 82 KW and 184 KW

So the 3500 RPM point is 22 KW off what it should be if linear power was being delivered. It isn't there because there is only 3 PSI of boost at that point where there is 11 PSI of boost at the horsepower peak. I am not saying that isn't a nice looking graph. It is a nice graph. I just don't like people saying a centrifugal supercharger delivers linear power.

Lets replace the Vortech supercharger graph with a Rotrex supercharger graph



2k = ~50hp
3k = ~100hp
4k = ~155hp
5k = ~220hp
6k = ~290hp
7k = ~350hp



And for a FA20...



While 0rpm = 0 hp, the reasoning behind your math is flawed. Take any hp graph, and if you extrapolate 0 rpm, it will be negative hp.

Going back to a linear regression-ish analysis, using this graph.

2k = ~40hp
3k = ~85hp Δ45hp
4k = ~125hp Δ40hp
5k = ~165hp Δ35hp
6k = ~200hp Δ35hp
7k = ~230hp Δ30hp

I mean, literally, hold a piece of paper up to the graphs, and compare the line to the edge of the paper...

[King Tut]

Originally Posted by CSG Mike

I mean, literally, hold a piece of paper up to the graphs, and compare the line to the edge of the paper...

We know that a lot of dyno curves end up looking like that. Is it somewhat close to linear, sure. Is this dyno plot more linear?



The real thing we are discussing is how flat the torque curve is. A turbo can get the boost up and produce a flatter wider torque curve.

[CSG Mike]

Originally Posted by King Tut

We know that a lot of dyno curves end up looking like that. Is it somewhat close to linear, sure. Is this dyno plot more linear?



The real thing we are discussing is how flat the torque curve is. A turbo can get the boost up and produce a flatter wider torque curve.

My point is that you are saying linear power is a literally flat (as in no slope) torque curve, while he's saying linear power is a straight (constant slope) torque curve.

[King Tut]

Originally Posted by CSG Mike

My point is that you are saying linear power is a literally flat (as in no slope) torque curve, while he's saying linear power is a straight (constant slope) torque curve.

The issue is that torque curves are used along with RPM to calculate the power so if the torque curve is not flat then by default the power curve becomes less linear as it introduces a variable into the equation instead of being a constant. It doesn't matter how constant the slope of the torque curve is as to the linearity of the power curve because it is still a variable instead of a constant.

[CSG Mike]

Originally Posted by King Tut

The issue is that torque curves are used along with RPM to calculate the power so if the torque curve is not flat then by default the power curve becomes less linear as it introduces a variable into the equation instead of being a constant. It doesn't matter how constant the slope of the torque curve is as to the linearity of the power curve because it is still a variable instead of a constant.

You guys are using different definitions of "Flat"!