Hitting rev limiter with supercharger

[xsnapshot]

Originally Posted by justatroll

I think you need to rethink your "physics".
If the ignition "Cuts" at 7400, the engine RPMs will continue to climb for a fraction of a second and exceed 7400. that is called inertia.

And the additional rotational inertia of the SC will help the engine to KEEP GOING and overshoot 7400. And since the SC has a belt and possibly internal gearing, the increase in inertia is magnified by the transfer ratio.

That is why it is harder to stop a dump truck on ice than it is to stop a VW beetle. - "Inertia".

Normally hitting the rev limiter is not bad.

Sorry bud but that's wrong. Inertia can not accelerate anything. Inertia is just a measure of energy essentially. You need an applied force to accelerate or decelerate something.

Show me an example of what you are describing. How can an engine increase in RPM? Engine power DRIVES the supercharger. Without additional engine power applied, the supercharger does not accelerate anything.

I'm serious. Show me a link or datalog of ignition cut and RPM's going over the specified rev limit.

You cannot have acceleration without force. Force is generated with engine power.

[GsxrMe]

Ya watch your dash. the TC/SC light flicker as its not your trans... its Time for better tires.

[GsxrMe]

I can't punch first or second with the factory tires. Even third breaks loose. These factory tires won't die!

[CSG Mike]

Originally Posted by xsnapshot

Sorry bud but that's wrong. Inertia can not accelerate anything. Inertia is just a measure of energy essentially. You need an applied force to accelerate or decelerate something.

Show me an example of what you are describing. How can an engine increase in RPM? Engine power DRIVES the supercharger. Without additional engine power applied, the supercharger does not accelerate anything.

I'm serious. Show me a link or datalog of ignition cut and RPM's going over the specified rev limit.

You cannot have acceleration without force. Force is generated with engine power.

The engine's energy input is static, which causes an RPM rise, which is caused by an acceleration, which is caused by jerk.

Even with a power/ignition cut, there is still momentum in the climb of acceleration, and the deceleration force from the drag needs a moment to overcome that momentum.

You do NOT want to be hitting the rev limiter with a HBP JRSC.

Here's some data.

Zoomed out, with a spot highlighted.



Zoomed in to the highlighted spot. Note how the clutch is fully depressed, and the transmission is out of gear (on a very obviously grannied shift), and the gas pedal is completely off, yet the RPMs still momentarily keep rising.

[xsnapshot]

Originally Posted by CSG Mike

The engine's energy input is static, which causes an RPM rise, which is caused by an acceleration, which is caused by jerk.

I'm not sure I follow this. Can you explain this more. Why would the engine's input be static? When you hit rev limit nothing is static. It simply stops. There is no input. And what is jerk?

I see the data you are presenting here, and I'll be happy to admit I'm wrong, but I'd love to understand more about how this happens.

[CSG Mike]

Originally Posted by xsnapshot

I'm not sure I follow this. Can you explain this more. Why would the engine's input be static? When you hit rev limit nothing is static. It simply stops. There is no input. And what is jerk?

I see the data you are presenting here, and I'll be happy to admit I'm wrong, but I'd love to understand more about how this happens.

I think this would be a better place to go, as my ability to explain this is somewhat limited.

https://en.wikipedia.org/wiki/Jerk_(physics)

A slightly longer version, using an example:

- At a constant vehicle velocity going forward, you don't feel any force.
- At a constant acceleration going forward, you feel the force, but the force itself is "constant" and easy to adapt to.
- With a variable acceleration going forward, you'll get thrown around, because you're trying to adapt to a constantly changing force. This is Jerk (change in acceleration). It's also why a lot of people prefer a turbo over a sc; that rapid climb in torque output as the turbo spools, causes a large jerk, while the linear torque output of a SC/NA car has a small jerk.

As acceleration is change in velocity over time, jerk is change in acceleration over time.

Now, lets translate this back into our example.

When you hit the rev limiter, the force causing the acceleration in RPMs is instantly taken away. However, the rate of climb in RPMs, is still present...

[jawn]

When you clutch in, you've decreased the moment of inertia. Angular momentum is conserved. Angular velocity increases. No force or energy needs to be added to the system to cause a rise in angular velocity.

[wise345]

Just use manual mode and paddle shift. It's like a manual minus the fun

[Nickosport]

@CSG Mike coming in with a calculus lesson!

But seriously, calculus has a lot of uses in system dynamics like situation:
Differentiate position with respect to time ->velocity
Differentiate velocity with respect to time ->acceleration
Differentiate acceleration with respect to time ->jerk

Source: Im a Mechanical Engineer, and they beat this sh** into your head in school.

[CSG Mike]

Originally Posted by jawn

When you clutch in, you've decreased the moment of inertia. Angular momentum is conserved. Angular velocity increases. No force or energy needs to be added to the system to cause a rise in angular velocity.

Angular momentum is conserved. When you clutch in, the complete assembly, which is now disconnected, is also still spinning, except in multiple pieces. The sum of the momentum of the multiple pieces, is still the same as the sum of the pieces connected.

[CSG Mike]

Originally Posted by Nickosport

@CSG Mike coming in with a calculus lesson!

But seriously, calculus has a lot of uses in system dynamics like situation:
Differentiate position with respect to time ->velocity
Differentiate velocity with respect to time ->acceleration
Differentiate acceleration with respect to time ->jerk

Source: Im a Mechanical Engineer, and they beat this sh** into your head in school.

I hated calculus with a passion. Then I got into cars, and wished I had actually paid attention in my first and second year (engineering, because the curve was friendlier) physics classes, instead of "doing enough for the grade and understand enough for the MCAT".

Sometimes I wonder if I should have gone into engineering or computer science or a combination of, because I wonder how much deeper an understanding I would have of these interactions with respect to vehicle dynamics.

I'm afraid your post will go over the heads of many people here... :p

[CSG David]

Originally Posted by Nickosport

@CSG Mike coming in with a calculus lesson!

But seriously, calculus has a lot of uses in system dynamics like situation:
Differentiate position with respect to time ->velocity
Differentiate velocity with respect to time ->acceleration
Differentiate acceleration with respect to time ->jerk

Source: Im a Mechanical Engineer, and they beat this sh** into your head in school.

The sad realization that I understand this but the density of information is too great in one post for most people to understand.

[jawn]

Originally Posted by CSG Mike

Angular momentum is conserved. When you clutch in, the complete assembly, which is now disconnected, is also still spinning, except in multiple pieces. The sum of the momentum of the multiple pieces, is still the same as the sum of the pieces connected.

You're right. I don't know why I had a brain fart this afternoon.

[jawn]

Originally Posted by CSG David

The sad realization that I understand this but the density of information is too great in one post for most people to understand.

I mean, all of Newtonian physics can be broken down with calculus. It makes perfect sense and is wonderfully elegant.