Logo
Results 1 to 6 of 6
  1. #1
    Join Date
    Jan 2017
    Location
    Salem Oregon
    Posts
    54

    Default question about 5.7 with vortec heads... and pistons?

    the boat is an 1989, the tag on the engine is missing so I cant tell what year it is, but im assuming its the original which is probably an '87 or '88...
    its a 5.7 omc package, and at some point the PO had vortec heads installed.... I have heard the flat top pistons are not correct to use with the vortec heads, so while installing new exhaust and doing some wiring clean up and full tune up, I scoped the pistons to see how they looked (all looked good and normal inside) but I saw the numbers on the pistons "H423NCP".... I cant really tell what these are and am wondering if they are the OEM pistons, or if these are the correct pistons to be used with the vortec heads?....

    the engine is equipped with a Q-jet carb and it runs very smooth without a miss, hiccup or stumble, during idle or during acceleration, or while running at is maximum capability.
    but there is a problem... Its maximum capability isnt very much... it loses all of its get up and go around 2500rpm and will slowly max out at 3100... it achieves the 2500rpm at about 2/3 throttle, and the last third does nothing for it except allow the rpm to creep up to 3100... it doesnt it sound any different at 1/2, 2/3 or full throttle.... I have tried different props (13p-15d and 15p-15d and a 15p-17) but they seem to have the same hole shot and all run out the same no matter which prop is on it..
    the hole shot is fairly decent in my opinion, as its a heavy boat at 5800lbs. it does get up on plane, and runs about 13-14mph but something isnt right with it

    I have checked to make sure the cables are pulling the carb open all the way and changed the fuel filter and checked fuel pressure, checked the dist (electronic module) to make sure the flyweights are not stuck, checked the timing and its all good....
    the timing was at 4BTDC and I moved it to 7BTDC, and there was no change other than it seems to run richer now (although that may be the ethanol free gas that im running now after I drained and cleaned the tank, as opposed to the regular stuff that was in it). it smells like its running richer and the plugs are darker... it was dark brown, now they are black.

    so with the lack of top end performance and everything seeming to be in perfect order, it has prompted me to look at the exhaust... and I found the port side flapper seal to have broken loose from the hinge pin and was stuck in the tube with about a half inch of clearance for the exhaust gasses to get thru... strbd side was working well but showing signs of wear.
    at this time Im still waiting for the new replacement parts to arrive, but im hoping that I have found the source of the running issue....

    as for the pistons, I would like to know if they are the ones that should be used with the vortec heads, and how much increase in HP and torque it could give over the OEM rating of 260hp.... Thank you for any help you can give.
    Last edited by centerline; 08-12-2017 at 10:34 AM. Reason: changed wording..

  2. #2
    Join Date
    Dec 2006
    Location
    Chaumont Bay, N.y., USA
    Posts
    5,145

    Default Re: question about 5.7 with vortec heads... and pistons?

    but there is a problem... Its maximum capability isnt very much... it loses all of its get up and go around 2500rpm and will slowly max out at 3100...
    Ayuh,.... Has it ever run up to 4800 rpms with the prop that's on it now,..??

    If not, try a smaller pitched prop,....
    Any Grease is Better,... Than No grease at All....

  3. #3
    Join Date
    Jan 2017
    Location
    Salem Oregon
    Posts
    54

    Default Re: question about 5.7 with vortec heads... and pistons?

    Quote Originally Posted by bondo View Post
    Ayuh,.... Has it ever run up to 4800 rpms with the prop that's on it now,..??

    If not, try a smaller pitched prop,....
    not sure if it ever ran to full rpm with the 17p prop that came on it, and I havent tried anything smaller than a 13p.... ALL the OMC shops Ive been to says the boat with a stock hp engine should take a 15P, (forum members with this boat claim a 13 works the best), but it had a 17 on it when I got it, AND my engine has had its HP bumped up a little with the vortec heads, so after trying the 17, the 15 and the 13 without any change whatsoever, I am pretty sure it would be pointless to spend money on a 8 or 10p prop just to get the engine to rev higher.... because the engine will rev up in neutral and I am certain that there is a prop with a low enough pitch that will allow the motor to rev to spec, but even if I went that route it still wouldnt be right.... because a boat that was designed to top out around 25mph at 4600rpm would be severely under propped if it only ran at 8mph at 4600...

    As I mentioned in my post (not sure if you read the whole thing) I found a restriction in the exhaust... at this time Im still waiting for parts to arrive, so im anxious to see how much difference a new and proper exhaust makes...
    Last edited by centerline; 08-14-2017 at 11:02 AM. Reason: wording correction

  4. #4
    Join Date
    Sep 2008
    Location
    Maryland - USA
    Posts
    6,764

    Default Re: question about 5.7 with vortec heads... and pistons?

    does the air valve open up when the throttle is wide open, under load??? if that's ok, I'd check to make sure the timing is advancing correctly...

  5. #5
    Join Date
    Jan 2017
    Location
    Salem Oregon
    Posts
    54

    Default Re: question about 5.7 with vortec heads... and pistons?

    Quote Originally Posted by makomark View Post
    does the air valve open up when the throttle is wide open, under load??? if that's ok, I'd check to make sure the timing is advancing correctly...
    I dont know what you mean by the "air valve"... I have a carbureted engine which causes it to have butterflies/throttle plates on the throttle shaft. they open fully, and the shaft for the secondaries also gets activated fully...
    and as for the timing and all else that can be checked and/or adjusted, I explained in my original post... its been checked and is correct....

  6. #6
    Join Date
    Mar 2009
    Location
    Portland, Oregon,
    Posts
    9,091

    Default Re: question about 5.7 with vortec heads... and pistons?

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

    Quote Originally Posted by centerline View Post
    the boat is an 1989, the tag on the engine is missing so I cant tell what year it is, but im assuming its the original which is probably an '87 or '88...
    its a 5.7 omc package, and at some point the PO had vortec heads installed.... I have heard the flat top pistons are not correct to use with the vortec heads,
    For Marine use, that would be correct!
    The Vortec's 64cc or 65cc combustion chamber is too small for the F/T pistons.
    The F/T's need the old 76cc chambers.


    so while installing new exhaust and doing some wiring clean up and full tune up, I scoped the pistons to see how they looked (all looked good and normal inside) but I saw the numbers on the pistons "H423NCP".... I cant really tell what these are and am wondering if they are the OEM pistons, or if these are the correct pistons to be used with the vortec heads?
    NO ..... the H423NCP is not the correct piston for use with the Vortec chamber, nor would this piston be OEM for a 1988/89 OMC SBC.
    OEM would have been the General Motors SBC full dished piston..... of which is similar to the H423NCP in design.

    launch this URL > http://www.competitionproducts.com/images/H423NCP.png


    the engine is equipped with a Q-jet carb and it runs very smooth without a miss, hiccup or stumble, during idle or during acceleration, or while running at is maximum capability. but there is a problem... Its maximum capability isnt very much... it loses all of its get up and go around 2500rpm and will slowly max out at 3100... it achieves the 2500rpm at about 2/3 throttle, and the last third does nothing for it except allow the rpm to creep up to 3100... it doesnt it sound any different at 1/2, 2/3 or full throttle.... I have tried different props (13p-15d and 15p-15d and a 15p-17) but they seem to have the same hole shot and all run out the same no matter which prop is on it..
    the hole shot is fairly decent in my opinion, as its a heavy boat at 5800lbs. it does get up on plane, and runs about 13-14mph but something isnt right with it
    I doubt that your hull is up on step at only 13 to 14 mph!

    I have checked to make sure the cables are pulling the carb open all the way and changed the fuel filter and checked fuel pressure, checked the dist (electronic module) to make sure the flyweights are not stuck, checked the timing and its all good....
    How did you "check the timing", and what does the Ignition Advance Curve look like?
    What is your Total Advance and at what RPM is it "Full-In"?


    Setting BASE advance ONLY is NOT a complete means of checking ignition advance!!!!!



    so with the lack of top end performance and everything seeming to be in perfect order, it has prompted me to look at the exhaust... and I found the port side flapper seal to have broken loose from the hinge pin and was stuck in the tube with about a half inch of clearance for the exhaust gasses to get thru... strbd side was working well but showing signs of wear.
    at this time Im still waiting for the new replacement parts to arrive, but im hoping that I have found the source of the running issue....
    Have you performed a cylinder pressure test?
    With all spark plugs removed, cycle each cylinder through 3 compression strokes for each reading!
    Post your results here.


    as for the pistons, I would like to know if they are the ones that should be used with the vortec heads, and how much increase in HP and torque it could give over the OEM rating of 260hp....
    The correct piston profile to be used with the Vortec combustion chamber will be a LCQ style piston with the correct dish volume.
    (see image below)
    Quote Originally Posted by centerline View Post
    I dont know what you mean by the "air valve"... I have a carbureted engine which causes it to have butterflies/throttle plates on the throttle shaft. they open fully, and the shaft for the secondaries also gets activated fully...
    and as for the timing and all else that can be checked and/or adjusted, I explained in my original post... its been checked and is correct....

    With the Rochester Quadrajet, the secondary throttle plates are indeed mechanically linked.
    However, there is an Air Door just above these plates that must see air volume in order to open up.

    An engine in poor health, or an engine that is not in full tune, may cause the Air Door to not open properly.

    Bottom line..... the 5.7L SBC Marine Engine performs best when it incorporates:

    ...... a quench effect combustion chamber.... I.E., correct piston profile for the combustion chamber volume and a .038" to .040" Quench.
    ...... a Static Compression Ratio of no more than approx 9.8:1
    ...... an Ignition Advance that places the LPCP at/near 12 to 14 Degrees ATDC. (approx 32* @ 3,200 rpm)

    .
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

Name:	Quench and Squish area explained 4 .jpg 
Views:	11 
Size:	61.4 KB 
ID:	16307   Click image for larger version. 

Name:	SBC full dished piston NO.jpg 
Views:	10 
Size:	37.5 KB 
ID:	16308  
    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.

Similar Threads

  1. question about 5.7 with vortec heads... and pistons?
    By centerline in forum Marine Transmissions and Reverse Gears
    Replies: 0
    Last Post: 08-09-2017, 10:12 PM
  2. Vortec heads SBC 1987 350 Cu in
    By 87concord in forum Mercruiser Sterndrive Forum
    Replies: 10
    Last Post: 04-26-2010, 06:22 PM
  3. Marine Vortec heads
    By floater1 in forum OMC Sterndrive and Inboard Forum
    Replies: 5
    Last Post: 02-07-2010, 09:53 AM
  4. Ignition Timing with Vortec heads
    By marada1 in forum OMC Sterndrive and Inboard Archive
    Replies: 3
    Last Post: 07-12-2008, 03:38 PM
  5. 1999 maxxum boat is a gen2 vortec have truck 43 vortec the same
    By rvinny in forum Mercruiser Sterndrive Archive
    Replies: 4
    Last Post: 05-21-2006, 08:42 AM

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •