Why are the side housings made of cast iron instead of aluminum like the rotor housings?

There souldn't be a heat problem. The hottest parts on the engine are thr aluminum exhaust ports and spark plug area.

The side housings can still be coated with a material that is friendly to wear.

Structural integrity shouldn't suffer.

So, whats up???

 

Probably cost effectiveness. The rotor housings are aluminum, but the inside is coated w/ high temp/wear metal like cromium something or other by different means in the different years and models (NA vs Turbo.) Plated, plasma sprayed or sleeved I think. The side housings have probably changed as well through the years, I think early ones were just surface hardened through nitriding. You can get aluminum side housings that are plasma sprayed w/ highwear material, but they are lotsa $$$. They reduce engine weight by ALOT so the aviation guys use them.

 

RacingBeat has alum. side housings-front, int. and rear for 13b and rear for 20b. Looks like all 3 would save you about 40-45lbs and cost about $4000. If only I were a millionaire...

 

From a manufacturing and engineer's point of view cast iron is nearly perfect for use in an engine.

It is stable, meaning it does not grow or change shape appreciably when heated up. It is very inexpensive to make, easy to machine, and can be cast into very intricate shapes quite easily. It is porous, which makes it ideal for holding onto a thin layer of lubricating oil. The nitriding process is still being used. It penetrates the cast iron anywhere from .003 to .007 inch. It makes an extremely durable wear surface while maintaining the inherent strength of the underlying cast iron.

Aluminum by itself makes a poor wear surface, especially in a relatively "dirty" environment like a combustion chamber. Many years ago Chevy introduced the Vega with a pure aluminum block. These engines were good for less than 20K miles for the most part. Chevy quickly redesigned the engine with cast iron sleeves and even offered to replace existing customer's engines but by then the bad press and poor public perception had runied the Vega from a marketing point of view.