Tom and Vipers
Enthusiast
Keller you are right about friction being "proportional" to power level.
Let's consider coast down. This applies to the dyno roller coasting down or a Viper drive train.
First, if there was no friction, there would be no coast down no matter what the inertia was.
Therefore, 2 out of 3 variables must be known from decel rate, friction, and inertia. You need 2 to solve the equation for the 3rd.
There is no way to tell the difference between reduced friction and increased inertia.
====================================================
Let's consider the Viper/Dyno system:
Dyno Coast Down is Quasi-Static:
If the dyno coast down is quasi-static, i.e., very, very slow, then the added load imposed on the dyno by the Viper drive train would be essentially Viper friction.
Why? Because the rate of decel is very, very slow so that means the inertia terms for the Viper DT are very, very small. But friction remains.
This case is very similar to driving the Viper drive train at a constant speed and noting the torque required to maintain it at a constant speed.
Here we have solved for friction.
__________________________________________
The big problem is if the dyno decel is not quasi static.
As the dyno decel is faster and faster, the added Viper inertia loads become higher and higher.
While the change in dyno decel due to the presence of the Viper DT could be determined, the problem is that now, there is a lot more load going into the Viper DT to cause the decel and hence, friction is increased.
Theoretically, you could put 500 HP into a free-free Viper DT and slow it down from 6000 rpm to 0 in .001 seconds (interesting calc, I didn't make it, but big big HP causes tiny, tiny decel time.)
Clearly, friction during the 500 HP decel would be much greater.
==========================================================
And let's not forget that the inertia of the engine internals is a "parasitic" load just as EVIL as a flywheel! Not to mention the piston/ring/bearing frictions!
A lot of these frictional terms are squares and perhaps higher orders.
==========================================================
MY OPINION, is that the dyno decel is slow relative to the Viper DT decel so that DT friction is being measured.
One way to investigate DT inertia is to do pulls in different gears. Clearly 1st gear would produce the most DT inertia loading and 6th gear would produce the least.
You would not have to do redline pulls to fill out this matrix.
F-body guys always talked about how a carbon fibre driveshaft "makes HP!"
In this case, if you see a difference, that accel rate of the dyno pull is too fast and DT inertia effects are entering.
If you compare accel rates, a high gear Dynojet run is something like 2nd gear in an f-body and perhaps the same in a Viper.
If you had to pick a gear to represent the performance in a 1/4 mile, 3rd or 4th might be more representative regarding 1/4 mile MPH. If you start at 0 or 20 mph in the 1/4, you won't have any effect on trap speed.
Why?
Because you're going 20 mph in the first 50 feet (probably less) and how much trap speed would you tack on if you moved the traps 50 feet? Hardly any. 1 maybe?
============================================================
CONCLUSION: TO GET FLYWHEEL HP:
So to get flywheel HP, you must do a pull which is slow enough so that DT inertia terms are negligible.
And, you do the DT coast down and use this for friction HP which can be added.
Let's consider coast down. This applies to the dyno roller coasting down or a Viper drive train.
First, if there was no friction, there would be no coast down no matter what the inertia was.
Therefore, 2 out of 3 variables must be known from decel rate, friction, and inertia. You need 2 to solve the equation for the 3rd.
There is no way to tell the difference between reduced friction and increased inertia.
====================================================
Let's consider the Viper/Dyno system:
Dyno Coast Down is Quasi-Static:
If the dyno coast down is quasi-static, i.e., very, very slow, then the added load imposed on the dyno by the Viper drive train would be essentially Viper friction.
Why? Because the rate of decel is very, very slow so that means the inertia terms for the Viper DT are very, very small. But friction remains.
This case is very similar to driving the Viper drive train at a constant speed and noting the torque required to maintain it at a constant speed.
Here we have solved for friction.
__________________________________________
The big problem is if the dyno decel is not quasi static.
As the dyno decel is faster and faster, the added Viper inertia loads become higher and higher.
While the change in dyno decel due to the presence of the Viper DT could be determined, the problem is that now, there is a lot more load going into the Viper DT to cause the decel and hence, friction is increased.
Theoretically, you could put 500 HP into a free-free Viper DT and slow it down from 6000 rpm to 0 in .001 seconds (interesting calc, I didn't make it, but big big HP causes tiny, tiny decel time.)
Clearly, friction during the 500 HP decel would be much greater.
==========================================================
And let's not forget that the inertia of the engine internals is a "parasitic" load just as EVIL as a flywheel! Not to mention the piston/ring/bearing frictions!
A lot of these frictional terms are squares and perhaps higher orders.
==========================================================
MY OPINION, is that the dyno decel is slow relative to the Viper DT decel so that DT friction is being measured.
One way to investigate DT inertia is to do pulls in different gears. Clearly 1st gear would produce the most DT inertia loading and 6th gear would produce the least.
You would not have to do redline pulls to fill out this matrix.
F-body guys always talked about how a carbon fibre driveshaft "makes HP!"
In this case, if you see a difference, that accel rate of the dyno pull is too fast and DT inertia effects are entering.
If you compare accel rates, a high gear Dynojet run is something like 2nd gear in an f-body and perhaps the same in a Viper.
If you had to pick a gear to represent the performance in a 1/4 mile, 3rd or 4th might be more representative regarding 1/4 mile MPH. If you start at 0 or 20 mph in the 1/4, you won't have any effect on trap speed.
Why?
Because you're going 20 mph in the first 50 feet (probably less) and how much trap speed would you tack on if you moved the traps 50 feet? Hardly any. 1 maybe?
============================================================
CONCLUSION: TO GET FLYWHEEL HP:
So to get flywheel HP, you must do a pull which is slow enough so that DT inertia terms are negligible.
And, you do the DT coast down and use this for friction HP which can be added.
