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Charlie, you have to remember that when the mushroom tappet first became popular they did provide a significant increase over traditional flat tappet cam profiles of that time...the 1970's! However, technology and the use of computer controlled machine equipment became more affordable and began being used in many automotive aftermarket component companies, especially in the area of camshaft lobe development. In a relatively short time, 10 years or so, companies like Crane and Cam Dynamics and others were able to produce cam profiles that equaled and exceeded what the mushroom tappet were producing. There were tremendous benefits from trying to stay with a traditional flat tappet design compared to a mushroom tappet, that's why so much effort was put into doing it. The mushroom tappet fad fizzled out by the early 1980's.
I'm going to bet that no cam company has ever explored creating any mushroom tappet profiles for a Slant, or would if you asked them since there is no dedicated mushroom lifter for them, and likely no interest. Because of the limitations in the cylinder head capability, a custom flat tappet cam grind will provide the maximum benefits the engine is capable of producing, with the best grind available you could put in a mushroom tappet grind having no measurable gains above the traditional flat tappet.
Now from a truly nostalgic standpoint, a mushroom tappet would really be only that. A cam grinder is going to only provide a best guess as to what specs to put on your mushroom camshaft compared to decades of experience with a full library of lobe profiles for a traditional flat tappet. The cost of exhaustive test could reveal a good profile for you in a mushroom design, but why.....the aggravation of cam swapping with that type of design will make you grow weary of it probably after just the first one...
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I was just "spitballing" about the posibility of using a mushroom tappet setup. So many are considering the posibility of using a Roller setup, which has many problems and expenses in a slant six. Granted a roller would probably offer more benefits, but at a major cost. I was thinking a little less performance, at a much lower cost. Neither setup makes much sense in the kind of engines I run.