Why are sheet metal engines not popular?

Question

What are the reasons engines use casting and not stamping when manufacturing blocks and heads? There were couple of stamped engines before but none of them took off. https://www.macsmotorcitygarage.com/1946-49-crosley-cobra-the-sheet-metal-engine/ this for example claims the strength of the sheet metal engine was good enough and that only problem was corrosion from antifreeze. On an aircooled engine the corrosion wont be a problem either, for eample German SLA 16 engine which had sheet metal block and was aircooled. So what could be the reasons that made casting so much more favourable for manufacturing engine blocks and heads than stamping?

Answer 1

There are probably many reasons, though unless someone finds an article or information directly stating, anything you get for an answer here is going to be opinion or conjecture. The best I can offer is an educated guess.

The biggest reason to my thinking of why cast solids are used over sheet metal is stability. Even the engine you show which was produced by Crosley still used a cast crank case which was bonded to the upper sheet metal parts. Without stability, parts are going to wear out fast. Sheet metal is going to have a lot of flex where the cast parts are very stable overall.

The article states sheet metal strength is "good enough". Good enough means the engine will run when built. Good enough doesn't mean the engine will run for a 100k miles. Nothing in the article states the longevity of the engine, but I would bet an owner wouldn't get a great amount of service life from it. The biggest problem with sheet metal is stress risers are going to occur more readily to where you're going to see cracks forming and failure occurring much sooner then with a cast counterpart.

As an aside, the reason why I used the figure of "100k miles" earlier is because around the time of when Crosley was building these vehicle until the 80's and 90's, the litmus for longevity of an engine/vehicle was around 100k miles. Today's engines/vehicles can be expected to run 200-300k miles if the owner keeps up with maintenance.

I didn't see in the article where it stated what the manufacturing costs of the sheet metal engine would cost as compared to one which has cast parts. While it is probably cheaper overall to produce the sheet metal engine, due to how it is built, it would require complete replacement versus a rebuild. If the engine doesn't last as long and would require more frequent replacement, overall the cost of ownership could be higher and therefore would not be as appealing as having a cast version.

The last thing I can think of is engines with cast parts were well in use by the time Crosley started producing the sheet metal engines. It could be the sheet metal engines never came into favor because the manufacturing process for cast parts was fully entrenched in the way manufacturers were doing business. The manufacturers may not have wanted to change due to retooling cost concerns or the like. A similar situation happened in the 1980's when video cassette recorders (VCRs) were all the rave. Sony had a VCR technology they sold under the name of Betamax. Betamax was considered a far superior format for video recording and playback at the time, yet the VHS format won the war. I don't know if the reason Betamax didn't reign supreme was because of politics or cost or what, but the bottom line is, the format died and VHS continued on. The Crosley sheet metal engine could have died for the same type of reasons.

Which leads to the reasons it probably isn't used today. As I stated earlier, stability is probably a reason it never really caught on. When it was being produced back then, static compression ratios (CR) were probably in the 7-8:1 range. This is a relatively mild CR as compared to the 10.5-11.5:1 you'd see in most every naturally aspirated gas fired engine you'd see produced today. Also consider how it would work if turbocharged. The combustion pressures on the sheet metal would most likely tear it apart. IMHO, sheet metal just could not stand up to the performance demands which would be put on it today.