[MikeK]

I see two parts to answer Mike V.'s question: 1) Noise 2) Longevity

1) Noise
I believe the noise issue is directly related to the pressure wave (vibration) attenuation of the gear material if gear mesh (mis-mesh) issues caused by errant geometry are factored out.

Assuming identical gear mesh geometry, Steel, Cast gray iron, Cast bronze, Cast Al/Si alloy, and Pressure molded phenolic/fabric will all transmit vibration with different levels of attenuation. So who wins?

Back in the 80's I was teaching Foundry practice and metallurgy in a tech school. I had several 10" diameter demonstration bells cast from different metals/alloys that I often used in the lecture hall.

Here's an easy way to look at those materials: Imagine you have those bells made out of those different materials and you are striking them all with the same clapper made from the crank gear material.

Both the steel and bronze bells will ring fairly well, just how well will depend on the specific metallurgy and heat treatment of the material. (Loudest/longest)

The cast Aluminum, if old #12, 319, or 356 will ring but not as well (2nd Loudest/longest)

The cast gray iron (cam gear shown by Vince) will ring but not as loud and for a very short time. One of the great known advantages of cast iron is it's ability to absorb and attenuate vibration. This is why mill and lathe frames are C.I., not steel weldmets. (3rd Loudest/longest) I might add here that Vince's gear with spokes rather than a solid center will further attenuate transmitted vibration.

Pressure molded phenolic with fabric laminations will come in last for performance as a bell. (4th Loudest/longest ring) I never had an actual bell like this, but it surely would go 'thud' when struck.

OK, how would I use this rough, qualitative data? We are looking at sound created from mis-mesh of the gears. If the gear tooth geometry is theoretically perfect, NO vibration is generated and all the materials would be equally silent. As the mis-mesh increases the sound spread between the above materials increases. Conclusion: The gear mis-mesh determines what you hear, and the material determines how much that geometrical gear error vibration will be attenuated.

2a) Longevity (wear)
Assuming a steel crank gear, motor oil, and tooth pressure loading, the phenolic gear should wear fastest. All of the various metals will depend on specific alloy, heat treatment, and surface in their interaction with the crank gear. The cast gray iron will likely be a very long runner, a steel cam gear could grind itself up in 1000 miles if it is the wrong alloy, or go 1,000,000 miles if properly selected. Bronze and Aluminum gears will also be highly dependent on specifically what alloy. I know of one Bronze gear in an RK engine that ground and flaked itself severely in 5000 miles. I have seen an aluminum gear that had silicon crystals so abrasive it severely wore out the crank gear.

2b) Longevity (strength)
What has NOT been addressed here is impact shock delivered to the teeth through the crank as each cylinder fires. The model A has no front damper to attenuate this. A non-counterweighted crank will deliver more firing shock to the cam gear than a weighted one. Basically, it comes down to which material can best withstand a repetitive beating. Here, steel likely wins, as it will distort, molded composition fails, it will crack. The cast iron, bronze, and aluminum gear will depend on alloy and heat treatment to determine the winner.

When you look at the whole problem as posed by Mike V. you have to weigh wives tales, testimonials, and severely lacking materials data. Good Luck. Cogged timing belt, anybody?