Mercruiser Alpha 1 question from newbie
 

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.

Amazing guess
 

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

water jacketed exhaust for 13B
 

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:coolcowboy:

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.

Thanks!
 

Thanks to everyone who gave me input on this. I feel a little more confident that the out drive will hold up fairly well.

I am on a chain of lakes in northern Wisconsin and I don't anticipate running WOT for more than a minute or two, if at all. The boat came with a Mercruiser 90, a 90HP Renault Gordini engine and a Mercruiser drive. I repowered with a 120 and got better low end torque, but it still wasn't enough for my taste (or anybody's really). The torque curve on the rotary favors higher RPM, so I think that running a lower pitch prop will minimize this somewhat undesirable characteristic. will also serve to limit the top speed. I toyed with using a SBC with a lot of aluminum parts, but I think it is still a little heavy for the 15" runabout. The lack of low end torque just may be the key to making the drive last, and the prop will never clear the water on this chain.

If anyone else has some additional input I would really love to hear it. I realize that this boat has only slightly more power than the starter motor on some of the boats on this site, and it is not really off shore. Thanks for humoring me.

I'm glad you asked this question John-

I'm building a mild 383 SBC for my 1984 Formula 242 and am wondering about the "real world" capabilities of the Alpha 1. With good oil, a drive shower, and no hard launches or sustained speeds I think things will be good.