How long can I run a mercruiser 305 (200, alpha drive) at 4500 rpms?

Question

Mercruiser 305, bored 30 over w/alpha 1.65 ratio drive. How long can I run at wide open throttle (4500 RPMs) 4300, 4000 etc.? Recommendations on oil, spark plugs, gas grade, etc.

Answer 1

I've never really experimented with the durability of a 305, but  4300-4500rpm is a safe WOT range.  I'm sure that if you "bounce" the throttle around a bit, say from 3700-WOT,  you should be able to run indefinately.  This, of course, will be determined by the condition of the motor, the load it's pushing, and the limit on your gas credit card.

Use regular gasoline rated 87 octane, non-ethanol is preffered, but not available much anymore.  Any quality 10W30  will do for cooler climates, 25W40 for you lucky southerners. Use a Mercury or comparable filter, and change the oil every 80-100 hrs. Spark plugs, I need the serial# to give you a number, but I prefer NGK.  Keep in mind, the harder you run the boat, the more it'll be broke.  That's just a fact of boating.

Oh yeah, i forgot, because your motor has been overbored, whatever the suggested spark plug is, i'd go one range colder.

Answer 2

Well, it is like any engine, running wide open for extended periods is going to be hard on it, and suck gas like there is no tomorrow. Many people like an 70-80% of WOT as cruising speed. Four your engine that is like 3100 to 3600 rpms. When I am going up and down the lake I cruise at 3000 rpms it seems to be the most fuel efficient, and gets me where I need to go. Additionally, I think my boat could run for days at 3000 rpm, it just purrs right in there.

Answer 3

There's no reason you couldn't make relatively long runs at the throttle setting that you suggest.  However keep in mind that you would be burning 16-18 Gal. per hour.  Your boat and load will be very important in determining speed.

      What I would suggest is that you bring the boat to plane then throttle back slowly and listen to the engine.  You should hear when the strain on the engine is reduced, and to cruise at that speed.   I've made rums of 150 to 240 miles using this method.

Answer 4

The redline on that motor starts at 5000 rpm. you can run a long time at 4500rpm. most boats are governed and the cam in the engine is designed for higher rpm. But if you are going on a long trip, like hours I would keep it around 3500rpm just to reduce wear. I would run strait 30 wieght sincemost likely you wont be in freezing conditions wile boatingIt is best to run premiumin boats since they usually have a little higher compression than usual. As for plugs,run a medium heat densoUridium plug, they burn reliably