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  1. #1

    Default PCM 5.7L HO 315 HP carb conversion to sterndrive, Alpha one?

    Hi all,
    I would have a really good deal on a new old stock PCM 5.7L HO 315 HP carb version, on buying this as a repower to a Alpha stern drive? How hard would such modification be?
    - Exhaust manifolds an risers need to be changed, this should be rather easy.
    - Shift Interrupt system would be needed, don't know how though this will be, possible parts can be moved from old engine?
    - power steering missing from the PCM, that have from the old engine also but pulleys and belt configuration is rather different compared to Mercruiser, wonder how big hassle would be to get the pump fitted?

    Any thoughts or other issues worth considering highly appreciated, thanks

  2. #2
    Join Date
    Dec 2006
    Location
    Chaumont Bay, N.y., USA
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    5,218

    Default Re: PCM 5.7L HO 315 HP carb conversion to sterndrive, Alpha one?

    Ayuh,... You'll need to use the Merc flywheel cover for the rear motor mounts,....
    Any Grease is Better,... Than No grease at All....

  3. #3
    Join Date
    Sep 2010
    Location
    Jersey Shore
    Posts
    4,631

    Default Re: PCM 5.7L HO 315 HP carb conversion to sterndrive, Alpha one?

    What year is the new engine and what year is the current engine?
    Chris
    Dockside Marine Services
    Jersey Shore.
    Mercruiser/Mercury Certified
    Let the insanity begin.

  4. #4

    Default Re: PCM 5.7L HO 315 HP carb conversion to sterndrive, Alpha one?

    Thanks for response so far!

    Correct, need to use the Merc flywheel cover for the rear motor mounts. And in fact the cooling system need to be modified also(if will use the Alpha impeller).

    Don't know 100% as just purchased the boat, but original engine was a 1990 5.7L 260(2bbl). 2009 it got a new marine long block (casting 10243880), new inlet manifold(12 bolt Performer 2604) and 4 bbl Holley carb.
    The PCM(complete engine) is according to serial number built 2008, but still on transport cradle. Would like to use as much as possible from the PCM as it has zero hours(its has a 8 bolt vortec inlet). The 2009 block is cracked , thus the re power issue.



  5. #5
    Join Date
    Sep 2008
    Location
    Maryland - USA
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    Default Re: PCM 5.7L HO 315 HP carb conversion to sterndrive, Alpha one?

    On the shift interrupt circuit - you may be able to reuse the old switch but its possble you'll have to make a custom addition to the wiring harness to get the desired end result....depends on what you have exactly....the little bit of PCM documentation I have only has injected engines in 2008. The issue I see is the ECU is PCM proprietary and I don't see any "shift inputs" on its pinout where they do exist on the Merc 555 ECU pinouts....If there is NO ECU control then it should be a straightforward addition at add the interrrupter...

  6. #6

    Default Re: PCM 5.7L HO 315 HP carb conversion to sterndrive, Alpha one?

    Quote Originally Posted by makomark View Post
    On the shift interrupt circuit - you may be able to reuse the old switch but its possible you'll have to make a custom addition to the wiring harness to get the desired end result....depends on what you have exactly....the little bit of PCM documentation I have only has injected engines in 2008. The issue I see is the ECU is PCM proprietary and I don't see any "shift inputs" on its pinout where they do exist on the Merc 555 ECU pinouts....If there is NO ECU control then it should be a straightforward addition at add the interrrupter...
    Thanks. I now have the manual,don't think there is an ecu. Very little electronics but found Mallory braker less ignition module RP116022. Engine harness is a 10 Pin configuration, wonder if its compatible with merc version or if its totally PCM own design?

  7. #7

    Default Re: PCM 5.7L HO 315 HP carb conversion to sterndrive, Alpha one?

    Quote Originally Posted by X-SAILOR View Post
    Thanks. I now have the manual,don't think there is an ecu. Very little electronics but found Mallory braker less ignition module RP116022. Engine harness is a 10 Pin configuration, wonder if its compatible with merc version or if its totally PCM own design?

    In fact maybe easiest way would be to move the whole thunderbolt IV V8 -22 ignition system from the old merc to the PCM engine?

  8. #8
    Join Date
    Sep 2008
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    Default Re: PCM 5.7L HO 315 HP carb conversion to sterndrive, Alpha one?

    The Mallory distributor should be ok with the original shift interrupt switch....personally, as much as I just love the Thunderbolt IV ignition, can't recommend that swap as the amplifiers (external control module) are hard to find and cost thru the roof.....Also, the timing curve on the Mallory is adjustable ....

    As far as the wires go, just search for an Alpha 350 wiring diagram....then verify you switch works....and then add the two wires.....

    May also consider getting a new coupler while the engine has 'easy access'....

  9. #9

    Default Re: PCM 5.7L HO 315 HP carb conversion to sterndrive, Alpha one?

    Quote Originally Posted by makomark View Post
    The Mallory distributor should be ok with the original shift interrupt switch....personally, as much as I just love the Thunderbolt IV ignition, can't recommend that swap as the amplifiers (external control module) are hard to find and cost thru the roof.....Also, the timing curve on the Mallory is adjustable ....

    As far as the wires go, just search for an Alpha 350 wiring diagram....then verify you switch works....and then add the two wires.....

    May also consider getting a new coupler while the engine has 'easy access'....
    Great advice, thanks. Very good information regarding the adjustable timing curve! Its interesting that on on the original merc 5.7 260 hp 2 bbl the boat was pitched to have WOT at 4400 and sweet spot cruising was 3500 rpm. On this PCM version, vortec heads 4 bbl WOT range is 4800-5200. So even if there is more power 315 hp (and top speed), guess need to pitch down slightly. Top speed is not at all important but would like to have as much as possible torgue in the range of 3500-3800 RPM for cruise. Wonder if torque curve can be tuned by timing profile? PCM original timing 26 [email protected] .

  10. #10

    Default Re: PCM 5.7L HO 315 HP carb conversion to sterndrive, Alpha one?

    Great advice thanks. Useful information re the adjustable timing profile. On the original 2bb merc 260 the boat was pitched for WOT 4400, sweet spot cruising 3500-3600 rpm. This newer PCM 315 recommended wot is 4800-5200, so even if aiming at WOT 4800 might need to pitch down slightly. Cruising range is important so would like to have top torque at 3500-3800. Wonder how changing the timing profile would affect the torque profile? Not telling that much about profile but PCM timing according to manual= 26 [email protected]

  11. #11
    Join Date
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    Default Re: PCM 5.7L HO 315 HP carb conversion to sterndrive, Alpha one?

    i doubt changing the timing profile would have a major impact on the torque curve....I'd suspect you may move it a bit but you need to be careful with a marine engine....the OEM has a decent investment in establishing the baseline performance and its usually decent based on the parts selected (can profile in particular)...

  12. #12

    Post Re: PCM 5.7L HO 315 HP carb conversion to sterndrive, Alpha one?

    Quote Originally Posted by makomark View Post
    i doubt changing the timing profile would have a major impact on the torque curve....I'd suspect you may move it a bit but you need to be careful with a marine engine....the OEM has a decent investment in establishing the baseline performance and its usually decent based on the parts selected (can profile in particular)...
    OK, Thanks,that make sense to me. Infact I did purchased the PCM engine, got it cheaper than a GM marine long block and this engine is still fully dressed so think it was a good deal. But as it has been standing unused for a such a long time it's also a gamble.... Will start a new thread how to get i safely running on a engine stand after such a long storage time little later( when i have it in my garage).
    Thanks for Your help so far!

  13. #13
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    Default Re: PCM 5.7L HO 315 HP carb conversion to sterndrive, Alpha one?

    If it was "stored properly" you should be in good shape....

  14. #14
    Join Date
    Mar 2009
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    Default Re: PCM 5.7L HO 315 HP carb conversion to sterndrive, Alpha one?

    .............................

    Quote Originally Posted by X-SAILOR View Post
    Great advice thanks. Useful information re the adjustable timing profile.
    Use extreme caution if you begin playing around with the ignition advance curve.

    On the original 2bb merc 260 the boat was pitched for WOT 4400, sweet spot cruising 3500-3600 rpm.
    As long as the hull is up-on-step and then some........, that is just about perfect.

    This newer PCM 315 recommended wot is 4800-5200, so even if aiming at WOT 4800 might need to pitch down slightly
    OK..... understand that the industry is playing around with the numbers. Any Std SBC Marine build that is claiming to produce 315 hp is offering this number at/near 5,200 rpm. We do not operate our cruisers anywhere near 5,200 rpm. If we were to, the engine will live a rather short life!

    Cruising range is important so would like to have top torque at 3500-3800. Wonder how changing the timing profile would affect the torque profile?
    This gets quite involved to explain, so here is the short version:

    Given that all else is designed correctly, our ignition system has the last word re; LPCP.
    LPCP = location of peak cylinder pressure...... of which should be 12 to 14 degrees ATDC.

    A lazy spark lead will hold back LPCP. A lazy LPCP will hinder torque.
    An agressvie spark lead may help the LPCP, but it also puts this engine at risk of Marine Load Ignition Induced Detonation.
    Be very careful if you start playing around with the advance curve!


    Not telling that much about profile but PCM timing according to manual = 26 BTDC @ 4000.

    The PCM SBC will likely be fitted with the GM style Full Dished pistons, of which makes this engine more prone to detonation.
    A conservative number as what you show is safe, however, it is also a bit lazy.

    Go with the OEM ignition curve specs and you will be safe!
    Rick ... aka Ricardo..... AQ series Volvo Penta repair
    Portland, Oregon
    28' SDN F/B w/ twin Volvo Penta 5.7L DP's

    Please... no PMs! Post your questions on the forum.

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