Page 1 of 2 1 2 LastLast
Results 1 to 10 of 17
  1. #1

    Join Date
    Feb 2006
    Location
    Massachusetts
    Posts
    214

    Compression Questions

    I've researched A LOT about this, and here's what I have so far...

    The heads I have are R&D stage I.
    #1 47.5cc 124psi
    #2 46.5cc 125psi
    #3 49cc 120psi

    This seemed too low to me, and to R&D as well. Their stage II heads have the following
    #1 46cc
    #2 45cc
    #3 46.5cc

    As compression is power, I am having the heads milled a bit to increase the compression. They are being milled .025", which is the thickness of the stock gasket, which should decrease the volue by 3cc. So the head should now yield
    #1 44.5cc
    #2 43.5cc
    #3 46cc

    It's a bit more than the R&D stage II heads. My question - how much cranking compression can you safely run on pump gas? ADA rates it as high as 160psi, but Riva rates it at 150psi.

    Eric


  2. #2
    unless I'm missing something in ur post here, aren't the stage 1 heads from RnD or Riva supposed to be like 150 psi?

  3. #3
    ABBOTT's Avatar
    Join Date
    Jul 2005
    Location
    Birmingham, AL / Lay Lake
    Posts
    3,736
    +1
    96
    YES they should be. These compression numbers you just gave are the same as a stock boat's compression with stock heads.

    If that is an accurate compression that you just gave us, then all you have are some pretty heads. At that psi, you may as well put your stock head back on (it's stronger anyway).

    I hope that you can find out why the compression isn't there like it should be. It could be a vaccume leak in the jugs or the case too---(worst case scenario), but i wouldn't run it again untill you find the culprit.

  4. #4

    Join Date
    Apr 2006
    Location
    South MS
    Posts
    1,095
    I'm getting 145lbs with the Riva heads which I believe are 45 cc only all three cylinders. Check it with the throttle open and the other 2 plugs in.
    My guess is your number 2 cylinder will not be over 150-155.

  5. #5

    Join Date
    Feb 2006
    Location
    Massachusetts
    Posts
    214

    More confusing results...

    I had a machine shop take .025" (the thickness of a stock 1300 head gasket) off the heads. I picked them up today, and the guy went a bit further, by cc'ing the heads to 43.5cc on all 3!

    The bore is 84mm and the head gasket is 86mm. The heads are now -
    #1 - 84mm
    #2 - 83mm
    #3 - 82mm

    I installed the heads, but I swapped #2 and #3 as I didn't want the PTO cylinder to have the greatest compression. The squish was -
    #1 - .053" (84mm)
    #2 - .046" (82mm)
    #3 - .052" (83mm)

    I ran the compression. Engine cold, Sears Craftsman gauge, spark plugs in the unused holes, booster hooked to the battery. I got -
    #1 - 130 psi
    #2 - 130 psi
    #3 - 140 psi

    Something does not compute. The cranking compression should be sky high. I'm thinking a rebuild is probably necessary...

  6. #6
    Happily Self-Employed WFO's Avatar
    Join Date
    Nov 2005
    Location
    Houston, Texas
    Posts
    12,800
    +1
    449
    The bore is 84mm and the head gasket is 86mm. The heads are now -
    #1 - 84mm
    #2 - 83mm
    #3 - 82mm
    I assume this is the measurement between the squish band? What are your cranking rpms ?

  7. #7

    Join Date
    Feb 2006
    Location
    Massachusetts
    Posts
    214
    Yes, that is the measurement from the outside edge of the squish band. No aftermarket tach, but the RPMs when cranking were more than sufficient with a 50 amp battery booster attached.

    I feel like the Grinch - I've puzzled and puzzed 'til my puzzler was sore, but I've never run across something quite like this before.

  8. #8

    Join Date
    Feb 2006
    Location
    Massachusetts
    Posts
    214
    Yes, that is the measurement from the outside edge of the squish band. No aftermarket tach, but the RPMs when cranking were more than sufficient with a 50 amp battery booster attached.

    I feel like the Grinch - I've puzzled and puzzed 'til my puzzler was sore, but I've never run across something quite like this before.

  9. #9
    Happily Self-Employed WFO's Avatar
    Join Date
    Nov 2005
    Location
    Houston, Texas
    Posts
    12,800
    +1
    449
    Yeah it just doesnt compute. Try a different gauge and (sounds stupid but I gotta say it) tie the throttle wide open.

  10. #10

    Join Date
    Feb 2006
    Location
    Massachusetts
    Posts
    214
    Well, I told my "helper" that I wanted to test the ski again today after work. She said "You don't need MY help - the throttle stays wide open all on its own..."

    So, I will be testing it again today, the RIGHT way. Stay tuned...

    Eric

Page 1 of 2 1 2 LastLast

Thread Information

Users Browsing this Thread

There are currently 1 users browsing this thread. (0 members and 1 guests)

Similar Threads

  1. Building 580 Yellow Engine - Compression Question
    By rob2337 in forum 2-Stroke Performance
    Replies: 2
    Last Post: 08-13-2008, 12:06 PM
  2. SL 750 Compression question?
    By marcporter in forum Polaris Open Discussion
    Replies: 18
    Last Post: 07-22-2008, 04:11 PM
  3. Possibly getting 01 ultra 150.. quick compression question...
    By jcc011185 in forum Kawasaki Open Discussion
    Replies: 10
    Last Post: 07-31-2007, 07:26 PM
  4. compression question !!!
    By neil_p in forum Yamaha PWC Performance (2-stroke)
    Replies: 4
    Last Post: 06-02-2007, 09:13 AM
  5. Compression question
    By christian gpr in forum Yamaha PWC Performance (2-stroke)
    Replies: 75
    Last Post: 04-23-2006, 02:52 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
  •