Jackson Racing Supercharger Owners Thread

[sw20kosh]

Originally Posted by Xuningshen

I find it hard to believe that flywheel, crankshaft, whatever other crap is attached, would INSTANTLY stop climbing revs once you cut fuel/spark.

please... please... dont respond to me in analogies.

This is because you are thinking of RPM as distance. RPM are not distance. RPM represents velocity.

You are thinking "the needle is moving through the RPM range so FAST! Surely it wont just stop instantly". The reality is that when the RPM needle stops, it doesn't mean velocity of the crankshaft is 0, it means the acceleration of the crankshaft is 0 and velocity of the crankshaft is whatever it stops at. If the needle stops suddenly at 7,000 rpms, the crankshaft is spinning at 7,000 rpms. It cannot go any faster unless it has a force acting on it (ie combustion).

Simply:

A. Watching the needle sweep through the rpm range means the engine is accelerating (engine speed is changing)
B. When you let off the gas, the needle will stop moving. This means the engine is not accelerating (engine speed is constant).
C. needle will start to come back down. This means the engine is decelerating.

[Xuningshen]

Originally Posted by sw20kosh

This is because you are thinking of RPM as distance. RPM are not distance. RPM represents velocity.

You are thinking "the needle is moving through the RPM range so FAST! Surely it wont just stop instantly". The reality is that when the RPM needle stops, it doesn't mean velocity of the crankshaft is 0, it means the acceleration of the crankshaft is 0 and velocity of the crankshaft is whatever it stops at. If the needle stops suddenly at 7,000 rpms, the crankshaft is spinning at 7,000 rpms. It cannot go any faster unless it has a force acting on it (ie combustion).

Simply:

A. Watching the needle sweep through the rpm range means the engine is accelerating (engine speed is changing)
B. When you let off the gas, the needle will stop moving. This means the engine is not accelerating (engine speed is constant).
C. needle will start to come back down. This means the engine is decelerating.

I'm not thinking of it as distance....

[waterbison777]

Regarding the figure skating analogy...

If we cut her in half while she's spinning, aren't we reducing her effective mass, consequently instantaneously reducing her moment of inertia and providing a means for increased RPM?

[sw20kosh]

Originally Posted by waterbison777

Regarding the figure skating analogy...

If we cut her in half while she's spinning, aren't we reducing her effective mass, consequently instantaneously reducing her moment of inertia and providing a means for increased RPM?

No it has to do with where the mass is in relation to the center of spin. If you have an elephant on a spinning marry-go-round, when the elephant jumps off, the marry-go-round doesn't spin faster.

Now if the elephant is initially on the outer edge of the marry-go-round, and he now moves closer to the center, the marry-go-round will spin faster.

[sw20kosh]

Originally Posted by Xuningshen

I'm not thinking of it as distance....

The thing people are getting confused by is that when you are comparing the:

RPM needle moving up the rpm range

vs.

Ball being thrown up into the air

The RPM needle moving up the rpm range is a representation of acceleration. Acceleration can absolutely become 0 instantaneously. When the throttle plate shuts (when combustion stops) = acceleration becomes 0. Acceleration needs a force and when you take away that force acceleration becomes 0. This is why the needle stops moving instantaneously. Velocity stops increasing.

You can use the ball analogy but you have to take gravity out of the equation. If you are standing in space and you throw a ball towards the moon. The moment it leaves your hand, acceleration becomes 0. This is exactly like taking your foot off the gas and watching the RPM needle stop moving. The velocity of the ball remains equal to the velocity it reached right as it left your hand. The RPM of the motor remains the RPM of the motor right as you let off the gas until other forces (friction) pull (decelerate) the RPMs down. If the ball in space traveling at a certain velocity now entered a cloud of space debris, friction would slow it down too and its velocity would decrease (ball would decelerate just like an engine would),

[waterbison777]

Originally Posted by sw20kosh

No it has to do with where the mass is in relation to the center of spin. If you have an elephant on a spinning marry-go-round, when the elephant jumps off, the marry-go-round doesn't spin faster.

Now if the elephant is initially on the outer edge of the marry-go-round, and he now moves closer to the center, the marry-go-round will spin faster.

I can see that... if the elephant is modeled as an infinitesimal point. Then the moment of inertia is preserved if the elephant is in the center of the merry-go-round and it jumps off.

But what if the elephant is the same diameter as the merry-go-round? We're effectively changing the moment of inertia because mass is being removed where r>0, relative to the CoG...

[Xuningshen]

Originally Posted by sw20kosh

The thing people are getting confused by is that when you are comparing the:

RPM needle moving up the rpm range

vs.......

Isn't acceleration the rate of change? Can an object with mass and inerta who's rate of acceleration is "pretty fast" (I'm so scientific and eloquent) become 0 instantly?

[sw20kosh]

Originally Posted by Xuningshen

Isn't acceleration the rate of change? Can an object with mass and inerta who's rate of acceleration is "pretty fast" (I'm so scientific and eloquent) become 0 instantly?

Yep. Think of throwing a bowling ball down a lane. The ball is accelerating super fast while it is in your hand but once it leaves your hand it's acceleration is 0 and then starts to decelerate due to friction. If there was no friction it would maintain a constant velocity (no rate of change).

[dp1]

Let's derail this thread by writing something about Jackson Racing SC

[bfrank1972]

Originally Posted by Xuningshen

why don't you guys just think about it like an actual engine instead of all these analogies. they're giving me headaches.

for a simple minded guy like myself. I find it hard to believe that flywheel, crankshaft, whatever other crap is attached, would INSTANTLY stop climbing revs once you cut fuel/spark.

please... please... dont respond to me in analogies.

lol, ok ok, maybe it's time for an empirical approach...

[bfrank1972]

Originally Posted by dp1

Let's derail this thread by writing something about Jackson Racing SC

Funny! you're right, sorry guys, just trying to help

[Bramick]

Someone send me the link to the thread that continues this conversation

[0351USMC]

Yeah I think we can end the whole side convo of the physics and mechanics of the engine as it relates to the seconds after you clutch in.

A tame rev limit is in order, so let's put the analogies to rest.

[fasteddy]

Originally Posted by mfd1012

I have always wondered about the time it takes to exhaust the plenum volume with this type of FI design. Is it detectable when driving (throttle response)?

I have the JRSC with the carb tune. I think I have noticed this at high RPM a couple times. I also think Moto Mike probably nailed it with his explanation of the throttle cut style. That sure makes sense to me that the compressed air in the intake system has to go somewhere, unless you are venting it at throttle cut.

Originally Posted by dp1

Let's derail this thread by writing something about Jackson Racing SC

OK, I have about 500 miles on mine now. let me just say WOW! this thing is freaking awesome! As many have said this is exactly what this car needed! The power is incredible!

The thing I was most nervous about the install was the computer flashing. In the end that was the easiest part, took five minutes with no drama.

Car starts, idles drives like stock except for the extra power.

My mileage has not been affected more than 1-2 MPG at highway speeds over 70 MPH, other than that it is the same.

The system is very quiet, which I like. There is some minor whooshing sounds from the blow of valve and a faint whine under hard acceleration. Most average people would not even notice the extra sounds.

I did quite a bit of research and am very happy with my purchase. I would highly recommend this kit.

I bought mine from CSG and both CSG and Jackson have been very helpful with all of my questions before and after the sale.

Pat