Flywheel

All internal combustion engines need to have a flywheel. The fact that the explosive forces inside the cylinders are brief and irregular means that there is NOT a consistent torque acting to turn the crankshaft. A heavy enough flywheel smooths out the irregularity. It also has another effect which will be noted momentarily. Early cars had VERY heavy flywheels. Whether hand-cranked or with electric starters, that aided the starting of engines, as it permitted variations in how much gasoline had gotten into each cylinder, by allowing ANY cylinder which fired to increase the spinning speed so that the other cylinders could start behaving correctly. Before around 1954, all cars had stick transmissions. That meant that when the clutch was pushed in, the engine could run with no external load. A very heavy flywheel had the added benefit of keeping the engine from blowing itself apart if the gas pedal was pushed all the way to the floor with the clutch released. The flywheel's Rotational Inertia was designed to be enough where the engine had to take many seconds of being floored without load, before the engine might rev up above its redline speed. At that time, engines were underpowered and also built like tanks, so they really rarely could rev up fast enough to do themselves damage anyway. Manufacturers LIKE if their new vehicles do not self-destruct!

In the 1950s and 1960s, muscle cars started being manufactured. In general, the manufacturers chose to install very heavy (thick) flywheels on their vehicles, such that the public would not be likely to over-rev any of their vehicles and get bad Public Relations. But they installed essentially identical but thinner flywheels in vehicles that were considered high-performance. Why? No FUNCTIONAL reason, actually. The thinner flywheels allowed the engines to run rougher, a disadvantage to the general public. Finally getting to the point here! The thinner flywheel had less Rotational Inertia (I) which meant that TORQUE created by an engine which had the clutch disengaged WOULD REV UP FASTER! If a moderately noisy exhaust system/muffler like a glass-pack was used, the SOUND of the engine revving up unexpectedly fast SOUNDS like the engine is really powerful! It's quite an interesting change, and the sound effects are quite impressive! Conveniently, both Ford and General Motors (and I assume Chrysler) used essentially identical flywheels in nearly all their vehicles for many years. Back then, when friends would bring their cars to me to improve them, they rarely had enough money to buy the big carburetors and improved intake manifolds and exhaust headers and camshafts to ACTUALLY make their cars hotter. I did not have to charge them too much to replace the stock (thick) flywheel with an identical one that was from a performance car (i.e., thinner) and also replace the stock muffler with a glasspack. When they would first sit in their car and rev it up, they were always amazed that it was still their car! Their IMPRESSION was that it sounded far more powerful! The glasspack muffler was so that when they were driving (in other words, the clutch was engaged and the engine was loaded), the fact that the engine was actually no more powerful would not be obvious, the louder exhaust distracting their attention.

Now, there IS a down-side to using a thinner flywheel, which I discovered one day back then. A VERY cute girl kept insisting on sitting in my (severely modified) car. She talked me into letting her start the engine, with the car inside my garage. I had installed a VERY heavy duty clutch, and I was pretty sure that she could never have pushed it down to do any shifting, so the car was not going to go anywhere. But the crazy girl pushed the gas pedal to the floor and kept it there! With the lighter flywheel and very powerful engine, it revved up very fast to speeds which seemed likely to destroy itself. Fortunately, I was sitting right there and I grabbed the ignition key and turned it off, which was certainly the only reason I did not have lots of expensive engine parts all over the garage! I never again allowed any girl to start the engine of any of my hotter cars! Piston Ring gap If you have ever rebuilt an engine, you know that the instructions regarding ensuring a proper ring gap always has exclamation points! Why? Here's why. When you rebuild the engine, all the metal is cold. Nearly all kinds of metal EXPANDS when it gets heated, which includes engine rings. The engine block never gets much chance of getting very hot because of the cooling system. So we have a situation where the cylinder does NOT significantly increase in diameter, while the rings inside it are exposed to extremely hot (well over 3,000°F) gases. So the rings expand, in all dimensions, a little bit. The important dimension is LENGTH, where when the engine is hot, the rings get LONGER, and the specified piston ring gap is based on the Thermal Expansion Coefficient of the metal of the rings and the expected maximum temperature they will be exposed to under hard and fast use of the engine. Notice an interesting detail, which no one else will ever tell you, and which has such minimal effect that it is never noticed, but the engine actually becomes slightly more efficient, that is, las less blowby leakage past the rings, after it has gotten up to operating temperature, than it does when the engine is still cold and the ring gaps are at their greatest. So IF you did not provide the specified ring gap, then when the engine got hot, the expanding metal of the rings has nowhere to go! The softer metal of the piston generally loses this battle, and the engine either seizes up or comes apart, both making for VERY bad days!