Roush vs 427FE vs Keith Craft

[CobraEd]

Originally Posted by eschaider

I am going to suggest that the original statement should have been two engines with similar power curves will accelerate similarily with identical rotating assemblies. Identical rotating assemblies will have the same stroke, rotating assembly weight, diameter and weight placement relative to the crankshaft centerline.

As AL427SBF and/or Cobra ED have already pointed out if you hang a heavy flywheel on one engine and a light flywheel on another identical engine the one with the heavy flywheel will no longer accelerate as quickly as the one with the light flywheel. The same is true if you exchange one rotating assembly for an identical but heavier rotating assembly.

When you remove the rotating assembly from one engine and replace it with a heavier rotating assembly (that provides the same displacement, compression etc) the engine with the now heavier rotating assembly will accelerate more slowly because additional torque is being consumed to accelerate the heavier rotating assembly. Not surprisingly the dyno will also measure less torque being produced because some has already been siphoned off to rotate the now heavier rotating assembly.

When you introduce the variable of different engine designs, even if they result in similar displacements, the ability to noodle this out in our heads is all but non existent. Let me give you an example;

If I choose an engine with 340 gram wrist pins 970 gram pistons and comparably "heavy rods" most observers would say you are looking a a low speed truck engine. In fact they do measure up weight wise similarly. Performance (engine acceleration) is measurably different however. The engine I just described hits it's peak engine speed in literally microseconds after the hit of the throttle — Jerry I am certain already knows what I am talking about.

The reason for the rapid engine acceleration, even under load with this massive rotating assembly, is that the power produced dwarfs any parasitic drag in the engine. If you used this simple comparison, without seeing the actual engine or knowing its power output, you might be tempted to argue heavy rotating assemblies increase engine acceleration. Of course we all know they do not. BTW the engine with those massive rotating assembly components is a fuel motor.

Comparing big block and small block engine acceleration performance can be equally surprising and misleading. The problem is not just the effect of the increased rotational inertia one internal assembly over the other. Instead it is the differences in the intrinsic power production along with the shape of that power curve — one engine architecture vs the other because now we are looking at two different engine designs and implementations. These differences will consistently mislead you and cloud your ability to noodle out the answer in your head.

One thing however is for certain, all things being equal, two identical engines, one of which has a lighter rotating assembly, the engine with the lighter rotating assembly will always out accelerate the engine with the heavier rotating assembly — it is inescapable.


Ed

Wow, great explanation! You took the time and effort to nail it in great detail. I tend to throw out short descriptions because I just assume that everyone will get it. Unfortunately only "most" people get it. You took a lot of time and with a great deal of patience to explain this very very thoroughly. I wish I had as much patience as you. Good for you . . . you get the gold star!


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