Does anyone know what the maximum, fairly safe, input shaft rpm is for a Mercruiser Alpha 1 and what is the weak part that will break if I exceed it. I am working on a project that could possibly run 7000 rpm. I have a 1.98:1 ratio but only about 250HP.
Thanks in advance for any help.

going to use a rotary?
I have seen one that was used with a turbocharged 13B and it lived just fine, did use synthetic oil in it.

Yes, I am considering it. I have a supercharged rotary that has 267HP and 187 lb' at 7500 RPM. It performs reasonably well at lower RPM as well. I am trying to decide if I should move forward with the project. It is going in a 15' boat and would not run at full power / RPM very often. I don't want to have stern drive problems and I can't find any information about anyone who has run them at this speed.

Drive sustainability is more about torque than RPM's, to a certain level

I've routinely run a couple of them, and an alpha ss at 6000 sometimes a little above and know people who turn them to 6500+. Especially at lower power levels, I wouldn't see why it would be a problem as long as it's in good shape, you keep it from getting too hot and you use good lube...

If you need it I have a water jacketed aluminum marine exh manifold for a 13B that I wont be needing

I am interested in the water jacketed exhaust. Tell me more including what you would like to get for it please.

Definitely get a drive shower. If it survives long at that rpm I would be very surprised, but I have been surprised before

Can you tell me who made the marine 13B manifold you won't be needing and anything about the 13B project you apparently don't have any more. I may be interested in yours as a spare.

I run a 600hp smallblock on mine. Not a ton of low end torque and I'm easy on the take off. I've turned 6100-6200rpm and not had a problem. This drive also has 950hrs on it.... The lower has a good bit of in/out play but I think its from age not from the power. I've never run extended periods at that rpm but I have run for a few minutes in the high 5k rpms. I regularly run 4800-5000rpms for 5-10min and have turned over 4500 for 40-45min. I don't have a drive shower but I do plan on adding one just to help the upper gears should i plan to run longer. This spring after I pulled the top cap the gears still looked really good.

Torque, hard launches and re-entry are drive killers.