| | | quote | Originally posted by Will:
No. The page in question specifies "bucking and surging at cruise". Do you cruise at idle speed? I've been told that LT1/T56 cars surge at cruise because the plenum is undersized for the engine. This effect *might* be made worse by a lighter flywheel, BUT remains fundamentally a tuning problem with the engine, NOT a function of the lighter flywheel. The lighter flywheel would probably have almost no effect because the gear ratio in question is so extremely tall.
With a given flywheel inertia, a larger displacement engine will spool down faster than a smaller engine due to greater internal friction and pumping losses.
If you allow the RPM to drop THAT low during a shift, you're driving the car wrong.
Both cars cut BETTER sixty foot times with the light flywheel than the heavy one. This blows away the idea that heavy flywheels are better for launching. The 1/8 mile MPH dropped, but the amount is small (<0.5 mph) and could easily be the result of inconsistent shifting. The 1/8 ET *improved*, so I would call the modification a performance increase. As DV said, performance past the 1st corner is lightweight all the way. The only question was the interaction of 60 ft times with flywheel mass. DV subsequently showed that 60' times can be IMPROVED with the right techniques and a light flywheel.
When you evaluate "bang for buck", consider the weight of the clutch also. In a Fiero, the clutch weighs as much as the flywheel, so the total assembly weight is pushing 30#. Going from a 14# flywheel to a n 8# flywheel brings the assembly weight down to 24#, which is not such a big drop from 30. The situation is even worse with the Mustang, as that large diameter clutch probably weighs 20# or more. Thus the stock assembly is ~44# and the lighter assembly ~31#.
The engine also has its own internal MOI in the crankshaft and rotating assembly. That weighs over 50#, but can not be directly compared to flywheel weights do to significant differences in geometry. LIke the internal resistance of a battery, the internal MOI of an engine is basically inescapable. Once all the different MOI's are found and addressed, the change in flywheel weight is really NOT that big a change in the engine's way of running it every night |
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I appreciate exacting tolerances and saying what you mean, but I believe you're dipping into symantics a little Will, "bucking and surging" can be just as general a term as lugging and chugging depending on the speaker. I believe the overall point is an unfavorable change in driveability in that regard. Excessive rpm drop between shifts can also be considered an impractically light flywheel for the application as opposed to driving the car wrong particularly if it's a daily street driver as opposed to a competition vehicle. To avoid it you either have to shift like lightning or actively keep the rpm up between shifts and that can get old fast in routine driving.
DV also addressed the issue of the offline launch being a function of traction and clutch ability since you can store more energy for launch in the heavier flywheel, I wouldn't be surprised if the 60 ft gain is the result of not being able to fully harness all of the energy available from the heavy flywheel due to traction limitations, the closer the launch rpm to the limit, the less room there is for the lighter flywheel to benefit accelerating off the line and at some point it would disappear altogether as the lighter flywheel would run out of the needed time to recover. So the test only conditionally blows away the idea of heavy flywheels being better for launching. Traction and gearing rule here.
It's still important here to acknowledge the power of the test engines Will, in this context the test engines producing what amounts to small gains in all out driving suggests that cutting the power by more than half when applying the theory to a 3800 V6 producing nowhere near the torque, power curves and driving expertise it took to get the test results, will probably yield an imperceptable performance gain if any at all aside from possibly manic depressive shifting that might cause you to think the car is faster. The biggest saving in time will be between shifts which will have to be speeded up to avoid any excessive rpm drop that might occur.
It's just not as simple as going to a lighter flywheel that can potentially result in a driveability nightmare that far exceeds any potential .01 sec gains between shifts, it's important to stress that. Lighter maybe better, the question is how light is worth the potential side effects.
The lightweight flywheel on a V8 swap was unpleasant to me, and the 5 spd getraggedy would not allow me to shift it fast enough to make things a little better and the dives on low gear decels were just as annoying. I didn't drive the car from every stop light as if I were running an 1/8 or 1/4 mile so a heavier flywheel would have made me much happier.
