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Thread: Optimum rpm for 307 engine

  1. #1

    Optimum rpm for 307 engine

    Being new to owning a powerboat, I'm looking for what the optimum rpm should be for a 1973 251 Express Cruiser with a 307 engine. I believe max rpm is in 4200-4400 range.

  2. #2
    what engine do you have in yours? 307 Q? if so it is the same as the two of my 251's and the high range is 4400 the gas consuption at that rate is somewhere around 18 gph at that rate
    Aquaholic
    1974 251 express catalina

  3. #3
    by the way I see that you are in door county wi!! I am in appleton if you ever are coming through let me know
    Aquaholic
    1974 251 express catalina

  4. #4
    I'm happy to cruise a ~ 2000 rpm. Easy ride, easy on the motor.
    1983 280 Catalina

    "When in danger, or in doubt.
    Run in circles, scream and shout."

  5. #5
    Don't know the exact model of the 307. It is a rebuild with all new components built be Jasper in Detroit. Only has about 40 hrs on it and. I'm finding that gas mileage appears to be about similar, 2.1, 2.2 gpm whether at 8.9 knots @ 2200 rpm or 19-20 knots @ 3300 rpm.

    Good to know I've got another 251 owner in the area. Might look you up when we're next in Appleton.

  6. #6
    The 251's are planing hulls. You best MPG for the 307/305 is going to be between 2500 and 2800 rpm. 327/350 is best between 2700 and 3000 RPMs. If you have ever looked at a dyno with one of these engines on it, you will see the HP and torque curve are linear and compliment each other in that RPM range, in other words, it is the engines sweet spot. Operating above or below that will kill your range.

  7. #7
    Thanks for the info. Still trying to determine if I have the G series or K series. Engine specs for G series, 200 hp @ 4,400 rpm, have max torque at 276 ft lbs @ 2,900 rpm. And series K has 215 hp @ 4,400 rpm and max torque at 279 ft. Lbs. @ 3,200. So somewhere around 3,000 rpm or a little less should be in the ballpark.

  8. #8
    The only difference between the G & K series engines is the carburetor.. The G series engines were equipped with a two-barrel carburetor, the K series engines were equipped with a four-barrel carburetor..
    CC-John

    1978 251 Catalina Express
    Powered by: PCM 383 CID GM (Bored & Stroked Chevy 350, 330 HP)

    1973 Century Resorter 16
    Powered by: 1972 Chrysler LM318 (225 HP)

  9. #9
    I have a 350 mercruiser in my 251, cruising rpm and mph should be about the same as a 305 I would think. Doesn't it really depend on the prop size and pitch?

  10. #10
    It depends on the horsepower and torque to spin the prop. If you go from a 305 to a 350 you can increase the pitch to take advantage of that whopping 45 cubic inches you just picked up. Likewise, if you go from a 350 to a 305, you would have to decrease the pitch on prop because you would never hit WOT or TS.

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