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Results 1 to 8 of 8
  1. #1
    Join Date
    Jun 2009
    Location
    New Plymouth,Taranaki,New Zealand
    Posts
    282

    Wink GM Chev 4 cyl 2.5 litre mercruiser motor,about 1985

    Hi,can anybody tell me the RPM for maximum torque and RPM for maximum Horsepower,please?I read that these WOT is between 4200-4600,but would like to know optimum revs.with a big bladed 19' prop,Im getting 31MPH at 4000 RPM.any advice greatly appreciated.

  2. #2
    Join Date
    Mar 2008
    Location
    Manalapan, nj, usa
    Posts
    3,727

    Default Re: GM Chev 4 cyl 2.5 litre mercruiser motor,about 1985

    You need to be between 4200-4600 rpm if you can't reach that then go down in pitch of prop to 17 i bet it will be perfect for you.

    You do understand that wot is for short periods. If you run all day everyday at wot then you will be looking for a new motor.

    If that happens look for a 140 / 3.0 my 2.5 went and i dropped in a 140 / 3.0 and it's like night and day those extra 20 hp.
    Last edited by chiefalen; 02-04-2011 at 05:37 AM.

  3. #3
    Join Date
    Dec 2006
    Location
    Chaumont Bay, N.y., USA
    Posts
    4,154

    Default Re: GM Chev 4 cyl 2.5 litre mercruiser motor,about 1985

    ,Im getting 31MPH at 4000 RPM.
    Ayuh,... You're lugging the motor,...
    Drop down to a 17" pitch prop, before you burn a hole in a piston...
    Any Grease is Better,... Than No grease at All....

  4. #4
    Join Date
    Jun 2009
    Location
    New Plymouth,Taranaki,New Zealand
    Posts
    282

    Default Re: GM Chev 4 cyl 2.5 litre mercruiser motor,about 1985

    As a mechanic I do understand about WOT and "lugging",.Im experimenting with different props ,and knowing the max torque RPMs,would help me to pick a cruising ,or optimum revs..Maximum HP might be at 4400 RPM,but max torque could be about 3500rpm,depending on the cam grind.Those figures of 31MPH at 4000rpm,were just an indication of the max speed Ive had out of this boat so far.Apparantly the prop I had on was an early model big bladed 15 inch,[now worn down to about 14 "]and after talking to a prop technician,he suggested a 4 bladed 18 inch pitch prop,about 13 7/8 dia.Its an 18ft Fi_Glass Warrior.So what revs do you suggest for max fuel efficiency and engine longevity?pl

  5. #5
    Join Date
    May 2007
    Location
    Kawarthas
    Posts
    3,140

    Default Re: GM Chev 4 cyl 2.5 litre mercruiser motor,about 1985

    Once you get the right prop, the best cruise rpm (planing) for that engine is going to be around 3500-3700 rpm. There's nothing like a fuel flow meter to be 100% sure... altitude, fuel octane and other factors may make each boat a bit different, but you'll definitely be in the neighbourhood of peak torque at 3500-3700.

    Best overall fuel economy and longevity will be had at 800 to 1000 rpm (displacement speeds, not planing)

  6. #6
    Join Date
    Mar 2008
    Location
    Manalapan, nj, usa
    Posts
    3,727

    Default Re: GM Chev 4 cyl 2.5 litre mercruiser motor,about 1985

    3000-3500 after you get it dialed in for recommended wot 4200-4600.

    Most will tell you 2/3 of wot for cruising.

  7. #7
    Join Date
    Jun 2009
    Location
    New Plymouth,Taranaki,New Zealand
    Posts
    282

    Default Re: GM Chev 4 cyl 2.5 litre mercruiser motor,about 1985

    Thank you for that,thats the info I needed.I would love to put a 3.0 litre motor in its place,but they are $7900NZ over here,new.second hand ones are hard to get.Im quite happy with its performance,and now that I know its torque range,I can tune the boat accordingly.

  8. #8
    Join Date
    May 2007
    Location
    Kawarthas
    Posts
    3,140

    Default Re: GM Chev 4 cyl 2.5 litre mercruiser motor,about 1985

    Quote Originally Posted by budgie14 View Post
    now that I know its torque range,I can tune the boat accordingly.
    tune it how?

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